Biopharmaceutics complete notes

Biopharmaceutics complete notes for Doctor of Pharmacy 4th Professional.
Notes for Pharmacokinetic models also available here in my profile.

1. BIOPHARMACEUTICS &
PHARMACOKINETICS
4
th
PROFESSIONAL

GHULAM MURTAZA HAMAD
4th PROFF. EVENING
PUNJAB UNIVERSITY COLLEGE OF PHARMACY, LAHORE
Reference
Dr. Nadeem Irfan Bukhari Lectures
Shargel – Applied Biopharmaceutics and
Pharmacokinetics, 7
th
edition
MADAN PL – Biopharmaceutics and
Pharmacokinetics, 2
nd
edition
Gilbert S Banker – Modern Pharmaceutics, 4
th

edition

2. GM Hamad

TABLE OF CONTENTS

Contents
1. Definitions and Terminology
2. Gastrointestinal Absorption
3. Biological Half Life and Volume of Distribution
4. Drug Clearance
5. Linear and Non-linear Pharmacokinetics
6. Bioavailability and Bioequivalence
7. Concept of Compartment Models
8. Multiple Dosage Regimen
9. Elimination of Drugs
10. Protein Binding
11. Pharmacokinetic Variations in Disease State
12. Intravenous Infusion
13. Biopharmaceutical Aspects in Developing a Dosage Form
14. In-Vitro-In-Vivo Correlation (IVIVC)

3. GM Hamad

DEFINITIONS AND TERMINOLOGY
BIOPHARMACEUTICS
• “Biopharmaceutics interrelates physicochemical properties of drug,
characteristics of dosage form, and site (route) of administration to the
rate and extent of systemic drug absorption”
• Biopharmaceutics encompasses factors that influence:
1. Stability of drug within drug product
2. Release of drug from drug product
3. Rate of dissolution/release of drug at the absorption site
4. Systemic absorption of drug.
SCHEME DEMONSTRATING THE DYNAMIC RELATIONSHIP BETWEEN THE DRUG,
THE DRUG PRODUCT, AND THE PHARMACOLOGIC EFFECT

• Efficacy (in-vivo performance) and safety of drug product critically
depends on:
­ Physicochemical characteristics of active pharmaceutical
ingredient (API/drug substance)
­ Characteristics of dosage form.
­ Features of the route of administration.
• Sequence of events to elicit a therapeutic effect: (LADMER)
­ Administration (either by an oral, intravenous, subcutaneous,
transdermal, etc. route)
­ Liberation (release) of drug from dosage form in a predictable and
characterizable manner.
­ Absorption (into the blood)
­ Distribution (+ to the site of action)
1

4. GM Hamad

­ Metabolism
­ Excretion
­ Response (Therapeutic effect)
• Studies in biopharmaceutics use both in vitro and in vivo methods.
­ In vitro methods are procedures employing test apparatus and
equipment without involving laboratory animals or humans.
­ In vivo methods are more complex studies involving human
subjects or laboratory animals.
PHARMACOKINETICS
• Pharmacokinetics is the science of the kinetics of drug absorption,
distribution, and elimination (i.e. metabolism and excretion)
• The description of drug distribution and elimination is termed drug
disposition.
• The study of pharmacokinetics involves both experimental and
theoretical approaches.
• The experimental aspect of pharmacokinetics involves the development
of biologic sampling techniques, analytical methods for the
measurement of drugs and metabolites, and procedures that facilitate
data collection and manipulation.
• The theoretical aspect of pharmacokinetics involves the development of
pharmacokinetic models that predict drug disposition after drug
administration.
• The application of statistics is an integral part of pharmacokinetic
studies. Statistical methods are used for pharmacokinetic parameter
estimation and data interpretation ultimately for the purpose of
designing and predicting optimal dosing regimens for individuals or
groups of patients. Statistical methods are applied to pharmacokinetic
models to determine data error and structural model deviations.
CLASSICAL PHARMACOKINETICS
• A study of theoretical models focusing mostly on model development
and parameterization.
POPULATION PHARMACOKINETICS
• A study of pharmacokinetic differences of drugs in various population
groups.
2

5. GM Hamad

CLINICAL PHARMACOKINETICS
• A multidisciplinary approach to individually optimized dosing strategies
for a specific patient based on the patient's disease state and patient-
specific considerations.
• Clinical Pharmacokinetics involves in optimum dosing regimens to
produce desired pharmacologic response in majority of anticipated
patient population.
­ Largely dependent on intra- and inter-individual variations which
results in in either a subtherapeutic or toxic response thus, may
requiring adjustment of the dosing regimen.
• Clinical pharmacokinetics is the application of pharmacokinetic methods
to drug therapy.
PHARMACODYNAMICS
• Pharmacodynamics is the study of the biochemical and physiological
effects of drugs on the body; this includes the mechanisms of drug
action and the relationship between drug concentration and effect.
RELATIONSHIP OF BIOPHARMACEUTICS WITH PHARMACOKINETICS
AND PHARMACODYNAMICS
• Drug action depends upon biopharmaceutics and pharmacokinetics. It
interrelates the blood drug concentration to:
­ Sub-therapeutic (drug concentration below the MEC)
­ Toxic response (drug concentration above the MEC)
­ Onset of drug action
­ Duration of drug action
DRUG ALTERNATIVES
• Alternatives - (of one or more things) available as another possibility or
choice.
1. PHARMACEUTICAL ALTERNATIVES
• Pharmaceutical alternatives - Drug products that contain same APIs but
as different salts/ esters/ complex or forms.
­ Tetracycline phosphate or tetracycline HCl equivalent to 250 mg
tetracycline base.
­ Different dosage forms and strengths within a product line by a
single manufacturer.
3

6. GM Hamad

­ Examples
i. Extended-release and immediate release dosage forms of
same APIs.
ii. Tablet and Capsule containing the same API in the same
dosage strength.
2. THERAPEUTIC ALTERNATIVES
• Therapeutic alternatives - products containing different API that are
indicated for the same therapeutic or clinical objectives.
­ From same pharmacologic class and are expected to have the
same therapeutic effect.
­ E.g., ibuprofen and aspirin
DRUG EQUIVALENTS
• Equivalents - equal in value, amount, function, meaning, etc.
1. PHARMACEUTICAL EQUIVALENTS
• Pharmaceutical (Chemical) equivalents are the products:
­ Having same APIs (same salt or ester)
­ Having same strength, quality and purity
­ Used through the same route
­ They are also therapeutic equivalents
­ They may differ in characteristics such as shape, color, flavor,
scoring configuration, release mechanisms, packaging, excipients,
expiration time, and, within certain limits, labeling.
2. THERAPEUTICAL EQUIVALENTS
• Therapeutic equivalents – Products which are:
­ Pharmaceutical equivalents (same APIs, amount of APIs, dosage
forms, routes)
­ Generic equivalent (having same APIs as the same salt in same
dosage form (but made by a different manufacturer).
­ Same bioavailability (Bioequivalent)
­ Expected to have same clinical effect and safety profile when
administered under the conditions specified in the labeling.
DRUG SUBSTITUTION
• Substitution – act of providing a thing in place of another (always giving
an alternative)
4

7. GM Hamad

1. PHARMACEUTICAL SUBSTITUTION
• Pharmaceutical substitution - Dispensing a pharmaceutical alternative
for a prescribed drug product.
­ E.g. Dispensing of ampicillin suspension in place of ampicillin
capsules.
­ E.g. Tetracycline HCl in place of tetracycline phosphate.
­ Generally, requires the physician's approval.
2. THERAPEUTIC SUBSTITUTION
• Therapeutic substitution - Dispensing a therapeutic alternative in place
of the prescribed drug product.
­ For example, ibuprofen instead of naproxen.
3. GENERIC SUBSTITUTION
• Generic substitution - dispensing a different brand or an unbranded drug
product in place of the prescribed drug product.
­ Substituted drug product must be Pharmaceutical equivalents
(having same API as same salt in same dosage form but is made by
a different manufacturer.
­ E.g. Substitution of Motrin brand of ibuprofen in place of Advil
brand of ibuprofen if permitted by physician.
GENERIC EQUIVALENCE
• These are the same API’s in same salts, same chemical form and dosage
forms, ideally bioequivalent pharmaceutical content.
BIOEQUIVALENCE
• Relationship in terms of bioavailability, therapeutic response, or a set of
established standards of one drug product to another.
BIOEQUIVALENT DRUG PRODUCTS
• This term describes pharmaceutical equivalent or pharmaceutical
alternative products that display comparable bioavailability when
studied under similar experimental conditions.
BIOAVAILABILITY
• Bioavailability means the rate and extent to which the active ingredient
or active moiety is absorbed from a drug product and becomes available
at the site of action. For drug products that are not intended to be
5

8. GM Hamad

absorbed into the bloodstream, bioavailability may be assessed by
measurements intended to reflect the rate and extent to which the
active ingredient or active moiety becomes available at the site of
action.
ABSOLUTE BIOAVAILABILITY
• The absolute bioavailability of drug is the systemic availability of a drug
after extravascular administration compared to IV dosing.
• The absolute bioavailability of a drug is generally measured by
comparing the respective AUCs after extravascular and IV
administration.
• Absolute bioavailability after oral drug administration using plasma data
can be determined as follows:
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[????????????�]
�� / �??????�??????
��
[????????????�]
???????????? / �??????�??????
????????????

• Absolute availability using urinary drug excretion data can be
determined by the following:
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[�
??????
∞
]
�� / �??????�??????
��
[�
??????
∞
]
???????????? / �??????�??????
????????????

RELATIVE BIOAVAILABILITY
• Relative (apparent) bioavailability is the bioavailability of the drug from a
drug product as compared to a recognized standard.
• The availability of drug in the formulation is compared to the availability
of drug in a standard dosage formulation, usually a solution of the pure
drug evaluated in a crossover study.
• The relative bioavailability of two drug products given at the same
dosage level and by the same route of administration can be obtained
using the following equation:
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[????????????�]
�
[????????????�]
�

­ where drug product B is the recognized reference standard. This
fraction may be multiplied by 100 to give percent relative
bioavailability.
• When different doses are administered, a correction for the size of the
dose is made, as in the following equation:
6

9. GM Hamad

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[????????????�]
� / �??????�??????
�
[????????????�]
� / �??????�??????
�

• Urinary drug excretion data may also be used to measure relative
availability, as long as the total amount of intact drug excreted in the
urine is collected. The percent relative availability using urinary
excretion data can be determined as follows:
??????????????????��??????�?????? ??????��????????????��??????????????????�??????=
[�
??????
∞
]
�
[�
??????
∞
]
�

­ where [Du]
∞
is the total amount of drug excreted in the urine.
DRUG ELIMINATION
• Drug elimination refers to the irreversible removal of drug from the body
by all routes of elimination.
• Drug elimination is usually divided into two major components:
­ Excretion
­ Biotransformation
EXCRETION
• Drug excretion is the removal of the intact drug.
• Nonvolatile and polar drugs are excreted mainly by renal excretion, a
process in which the drug passes through the kidney to the bladder and
ultimately into the urine.
• Other pathways for drug excretion may include the excretion of drug
into bile, sweat, saliva, milk (via lactation), or other body fluids.
• Volatile drugs, such as gaseous anesthetics, alcohol, or drugs with high
volatility, are excreted via the lungs into expired air.
BIOTRANSFORMATION
• Biotransformation or drug metabolism is the process by which the drug
is chemically converted in the body to a metabolite.
• Biotransformation is usually an enzymatic process. A few drugs may also
be changed chemically by a nonenzymatic process (e.g. ester hydrolysis).
The enzymes involved in the biotransformation of drugs are located
mainly in the liver.
• Other tissues such as kidney, lung, small intestine, and skin also contain
biotransformation enzymes.
7

10. GM Hamad

DRUG CLEARANCE
• Drug clearance is defined as the fixed volume of fluid (containing the
drug) removed from the drug per unit of time. The units for clearance
are volume/time (e.g. mL/min, L/h).
• Drug clearance is a pharmacokinetic term for describing drug elimination
from the body without identifying the mechanism of the process.
• Drug clearance (also called body clearance or total body clearance)
considers the entire body as a single drug-eliminating system from which
many unidentified elimination processes may occur.
DRUG DISPOSITION
• It refers to the fate of drug after absorption. On reaching bloodstream,
drugs are simultaneously distributed throughout body and eliminated.
• Distribution usually occurs much more rapidly than elimination. Rate of
distribution to tissues of each organ is determined by blood flow
perfusing organs and ease with which drug molecules cross capillary wall
and penetrate cells of particular tissue.
OTHER TERMINOLOGY
REFERENCE LISTED DRUG (RLD)
• Reference listed drug (RLD) - the drug product on which an applicant
relies when seeking approval of an Abbreviated New Drug Application
(ANDA).
­ RLD is generally the brand-name drug that has a full New Drug
Application (NDA).
­ The FDA designates a single reference listed drug as the standard
to which all generic versions must be shown to be bioequivalent.
MULTISOURCE DRUG PRODUCT
• Multisource drug product is a drug product that contains the same
active drug substance in the same dosage form and is marketed by more
than one pharmaceutical manufacturer.
SINGLE SOURCE DRUG PRODUCT
• Single source drug product is a drug product for which patent has not
yet expired or has certain exclusivities so that only one manufacturer
can make it.
8

11. GM Hamad

GASTROINTESTINAL ABSORPTION
INTRODUCTION
• It is defined as “the process of movement of unchanged drug from the
site of administration to the systemic circulation.
• There always present a correlation between plasma concentration of a
drug and the therapeutic response thus, absorption can also be defined
as the “process of movement of unchanged drug from the site of
administration to the site of measurement. i.e. plasma”.
STRUCTURE OF CELL MEMBRANE
• Cell membrane separates living cell from nonliving surroundings.
­ Thin barrier = 8 nm thick
• Controls traffic in and out of the cell
­ Selectively permeable: allows some substances to cross more
easily. than others.
­ Hydrophobic vs hydrophilic
• Made of phospholipids, proteins and other macromolecules.
• Proteins determine membrane’s specific functions.
­ Cell membrane and organelle membranes each have unique
collections of proteins.
• Membrane proteins:
­ Peripheral proteins
▪ Loosely bound to surface of membrane.
▪ Cell surface identity marker (antigens)
• Integral proteins:
­ Penetrate lipid bilayer, usually across whole membrane
­ Transmembrane protein transport proteins
▪ Channels, permeases (pumps)
PHYSIOLOGICAL FACTORS AFFECTING ORAL ABSORPTION
• Passage of drugs across membrane.
9

12. GM Hamad

1. Passive diffusion
2. Pore transport
3. Active transport
4. Facilitated diffusion
5. Pinocytosis
6. Ion pair formation
MECHANISMS OF DRUG ABSORPTION
1. PASSIVE DIFFUSION
CHARACTERISTICS
• Diffusion
­ Movement from high to low concentration.
• Major process for absorption of more than 90% of drugs.
• Non-ionic diffusion.
• Driving force: Concentration or electrochemical gradient.
• Difference in the drug concentration on either side of the membrane.
• Drug movement is a result of kinetic energy of molecules.
FICK’S FIRST LAW OF DIFFUSION
• Expressed by Fick’s first law of diffusion:
­ “The drug molecules diffuse from a region of higher concentration
to one of lower concentration until equilibrium is attained and the
rate of diffusion is directly proportional to the concentration
gradient across the membrane”.
dQ
dt
=
D A K
m/w
h
(C
GIT−C
P)
­ Where,
▪ dQ/dt = rate of drug diffusion (amount/time)
▪ D = diffusion coefficient of the drug
▪ A= surface area of the absorbing membrane for drug
diffusion
▪ Km/w = partition coefficient of drug between the lipoidal
membrane and the aqueous GI fluids
▪ h = thickness of the membrane
▪ (CGIT – Cp) = difference in the concentration of drug in the
GI fluids and the plasma (Concentration Gradient)
­ SINK CONDITION
▪ The passively absorbed drug enters blood, rapidly swept
away and distributed into a larger volume of body fluids.
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13. GM Hamad

▪ Hence, the concentration of drug at absorption site CGIT
is maintained greater than the concentration in the
plasma. Such a condition is called as sink condition for
drug absorption.
▪ Under usual absorption conditions, D, A, Km/w and h are
constants, the term D A Km/w /h can be replaced by a
combined constant P called as permeability coefficient.
▪ Permeability: Ease with which a drug can permeate or
diffuse through a membrane.
▪ Due to sink conditions, the C is very small in comparison
to CGIT.
dQ
dt
= P C
GIT
2. PORE TRANSPORT
• Also known as convective transport, bulk flow or filtration.
• Important in the absorption of low mol. Wt. (less than 100). Low
molecular size (smaller than the diameter of the pore) and generally
water-soluble drugs e.g. urea, water and sugars.
• The driving force for the passage of the drugs is the hydrostatic or the
osmotic pressure difference across the membrane.
• Mechanism – through the protein channel present in the cell membrane.
• Drug permeation through pore transport – renal excretion, removal of
drug from CSF and entry of drug into the liver.
• Rate of absorption via pore transport depends on the number and size
of the pores, and given as follows:
dc
dt
=
N R
2
A ∆C
(η) (h)

• where,
­ dc/ dt = rate of the absorption.
­ N = number of pores
­ R = radius of pores
­ ∆C = concentration gradient
­ η = viscosity of fluid in the pores
­ h = thickness of the membrane
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14. GM Hamad

3. ION-PAIR TRANSPORT
• Responsible for absorption of compounds which ionizes at all pH values.
e.g. quaternary ammonium, sulphonic acids.
• Ionized moieties forms neutral complexes with endogenous ions which
have both the required lipophilicity and aqueous solubility for passive
diffusion.
• E.g. Propranolol, a basic drug that forms an ion pair with oleic acid and is
absorbed by this mechanism.
4. IONIC OR ELECTROCHEMICAL DIFFUSION
• Charge on membrane influences the permeation of drugs.
• Molecular forms of solutes are unaffected by the membrane charge and
permeate faster than ionic forms.
• The permeation of anions and cations is also influenced by pH. Once
inside the membrane, the cations are attached to negatively charged
intracellular membrane, thus giving rise to an electrical gradient.
• If the same drug is moving from a higher to lower concentration, i.e.
moving down the electrical gradient, the phenomenon is known as
electrochemical diffusion.
• Thus, at a given pH, the rate of permeation may be as follows:
­ Unionized molecule > anions > cations.
5. CARRIER MEDIATED TRANSPORT
• Involves a carrier which reversibly binds to the solute molecules and
forms a solute-carrier complex.
• This molecule transverse across the membrane to the other side and
dissociates, yielding the solute molecule.
• The carrier then returns to the original site to accept a new molecule.
• There are two type of carrier mediated transport system.
­ Facilitated diffusion ­ Active transport
I. FACILITATED DIFFUSION
• Facilitated diffusion is a form of carrier transport that does not require
the expenditure of cellular energy.
• Carriers are numerous in number and are found dissolved in cell
membrane.
• The driving force is concentration gradient, particles move from a region
of high conc. to low concentration.
• The transport is aided by integral membrane proteins.
12

15. GM Hamad

• Facilitated diffusion mediates the absorption of some simple sugars,
steroids, amino acids and pyrimidines from the small intestine and their
subsequent transfer across cell membranes.
II. ACTIVE TRANSPORT
• Requires energy, which is provided by hydrolysis of ATP for
transportation.
• More commonly, metabolic energy is provided by the active transport of
Na
+
or is dependent on the electrochemical gradient produced by the
sodium pump, Na
+
/K
+
ATPase (secondary active transport).
• PRIMARY ACTIVE TRANSPORT
­ Direct ATP requirement
­ The process transfers only one ion or molecule and only in one
direction. Hence, called as uniport.
­ E.g. Absorption of glucose.
­ ABC (ATP Binding Cassette) transporters
• SECONDARY ACTIVE TRANSPORT
­ No direct requirement of ATP
­ The energy required in transporting an ion aids transport of
another ion or molecule (co-transport or coupled transport) either
in the same direction or opposite direction.
­ 2 types:
▪ Symport (co-transport)
▪ Antiport (counter transport)
6. ENDOCYTOSIS
• It is a process in which cell absorbs molecules by engulfing them.
• Also termed as vesicular transport.
• It occurs by 3 mechanisms:
­ Phagocytosis
­ Pinocytosis
­ Transcytosis
I. PHAGOCYTOSIS
• Phagocytosis refers to the engulfment of larger particles or
macromolecules, generally by macrophages.
II. PINOCYTOSIS
• It is a form of endocytosis in which small particles are brought to the
cell, forming an invagination.
• These small particles are suspended in small vesicles.
• It requires energy in the form of ATP.
13

16. GM Hamad

• It works as phagocytosis, the only difference being, it is non-specific in
the substances it transports.
• This process is important in the absorption of oil soluble vitamins & in
the uptake of nutrients.
III. TRANSCYTOSIS
• It is the process through which various macromolecules are transferred
across the cell membrane.
• They are captured in vesicles, on one side of the cell and the endocytic
vesicle is transferred from one extracellular compartment to another.
• Generally used for the transfer of IgA and insulin.
FACTORS AFFECTING DRUG ABSORPTION
1. Pharmaceutical factors
A. Physicochemical factors
B. Formulation factors
2. Patient related factors
A. Physiological factor
B. Clinical factor
1. PHARMACEUTICAL FACTORS
A. PHYSICO -CHEMICAL FACTORS
• Drug solubility and
dissolution rate.
• Particle size & effective
surface area.
• Polymorphism & amorphism.
• Salt form of the drug
• Lipophilicity of the drug
• pKa of the drug & pH
• Drug stability
I. DRUG SOLUBILITY AND DISSOLUTION RATE
• Rate determining process in the absorption of orally administered drugs
are:
­ Rate of dissolution
­ Rate of drug permeation through the bio-membrane.
• Hydrophobic: Rate Determination Step → Dissolution
­ E.g: Griseofulvin, spironolactone
• Hydrophilic: Rate Determination Step → Permeation rate limited.
­ E.g: Cromolyn sodium or neomycin
II. PARTICLE SIZE AND EFFECTIVE SURFACE AREA
• Particle size and surface area of a solid drugs are inversely related to
each other.
14

17. GM Hamad

• Hydrophobic drugs → micronization → greater surface area → rapid
dissolution.
­ E.g: griseofulvin, spironolactone
• Some of the Hydrophobic drugs → micronization → decrease in effective
surface area → fall in dissolution rate.
­ Causes
▪ Adsorption of air to surface
▪ Particle reaggregation
▪ Surface charge
­ E.g: aspirin, phenacetin
­ In that case add Surfactants: tween 80, hydrophilic diluents: PEG,
PVP, Dextrose.
III. POLYMORPHISM AND AMORPHISM
• POLYMORPHISM
­ A substance exists in more than one crystalline form, the different
forms are designated as polymorphs and the phenomenon as
polymorphism.
▪ Enantiotropic polymorph: Sulphur
▪ Monotropic polymorph: glyceryl stearate
­ Depending on their relative stability, one of the several
polymorphic forms will be physically more stable than the others.
­ Stable polymorphs
▪ Highest MP
▪ Lowest energy
state
▪ Least aqueous
solubility
­ Metastable polymorphs
▪ Low MP
▪ Higher energy
state
▪ High aqueous
solubility
­ E.g. The vitamin riboflavin exists in several polymorphic forms,
and these have a 20-fold range in aqueous solubility.
• AMORPHISM
­ These drugs can exist with no internal crystal structure.
­ Such drug represents the highest energy state and can be
considered as super cooled liquids and thus have greater
solubility. E.g. Novobiocin.
15

18. GM Hamad

­ Thus, the order of dissolution and hence absorption for different
solid dosage forms is amorphous > meta-stable > stable.
IV. SALT FORM OF THE DRUG
• Salt of weak acid and weak bases have much higher aqueous solubility
than the free acid or base.
• Therefore, if the drug can be given as a salt, the solubility can be
increased, and the dissolution thus can be improved.
V. DRUG pKa, LIPOPHILICITY AND GI pH
pH PARTITION THEORY
• Explains influences of GI pH drug pKa on the extent of drug transfer or
drug absorption (Ka – absorption rate constant).
• The process of absorption of drug compounds of molecular weight
greater than 100 Daltons transported across the bio-membrane by
passive diffusion depend upon the following factors:
­ Dissociation constant of the drug i.e. pKa of the drug
­ Lipid solubility of the unionized drug i.e. Ko/w
­ pH at the absorption site
• The amount of drug that exist in unionized form is a function of
dissociation constant(pKa) of the drug and pH of the fluid at the
absorption site.
• PKa of the drug
­ Dissociation or ionization constant:
▪ pH at which half of the substance is ionized and half is
unionized.
• pH of medium
­ Affects ionization of drugs:
▪ Weak acids → best absorbed in stomach.
▪ Weak bases → best absorbed in intestine.
• pH-partition Hypothesis
­ Unionized Drug: Higher Absorption
­ Ionized Drug: Low Absorption
• FOR WEAK ACIDS
% Drug Ionized=
10
pH
− pKa
1+10
pH
− pKa
X 100
• FOR WEAK BASES
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19. GM Hamad

% Drug Ionized=
10
pKa
− pH
1+10
pKa
− pH
X 100
PREDICTION BASED ON THEORY
Drugs PKa PH / Site of absorption
For Acidic Drugs
Very weak acids
E.g. pentobarbital,
Hexobarbital
> 8
Unionized at all pH values;
Absorbed along the entire
length of GIT.
Moderately weak acids
E.g. aspirin, Ibuprofen
2.5 – 7.5
Unionized in gastric pH and
ionized in intestinal pH; better
absorption from stomach.
Stronger acids
E.g. disodium
cromoglycate
< 2.0
Ionized at all pH values; Poorly
absorbed from GIT.
For Basic Drugs
Very weak bases
E.g. theophylline,
Caffeine
< 5.0
Unionized at all pH values;
Absorbed along entire GIT.
Moderately weak bases
E.g. codeine
5 – 11
Ionized at gastric pH,
unionized
at intestinal pH; better
absorption from intestine.
Stronger bases
E.g. guanethidine
> 11
Ionized at all pH values; Poorly
absorbed from GIT.

VI. LIPOPHILICITY
• Only unionized drug having sufficient lipid solubility is absorbed into
systemic circulation.
• So, drug should have sufficient aqueous solubility to dissolve in the fluids
at the absorption site and lipid solubility high enough to facilitate the
partitioning of the drug in lipoidal membrane and into systemic
circulation.
VII. DRUG PERMEABILITY
• Three major drug properties which affects drug permeability:
­ Lipophilicity
­ Polarity of the drug
­ Molecular size of the drug
VIII. DRUG STABILITY
• Two major stability problems are:
17

20. GM Hamad

­ Degradation of the drug into inactive form.
­ Interaction with one or more component either of the dosage
form or those present in the GIT to form a complex that is poorly
soluble.
B. FORMULATION FACTORS
• Disintegration time
• Manufacturing variables
• Different Oral Dosage forms
• Pharmaceutical ingredients/
Excipients
• Product age and storage
condition
I. DISINTEGRATION TIME
• It Is of particular importance in case of solid dosage forms like tablets
and capsules.
• Rapid disintegration is important in the therapeutic success of solid
dosage form.
• Sugar coated tablets have long disintegration time (DT).
• DT is directly related to the amount of binder present and the
compression force of a tablet.
• After disintegration, granules deaggregate into tiny particles →
dissolution faster.
II. MANUFACTURING VARIABLES
a) METHOD OF GRANULATION
• Wet granulation was thought to be the most conventional technique.
• Direct compressed tablets dissolve faster.
• Agglomerative phase of communition → superior product.
b) COMPRESSION FORCE
• Higher compression force → increased density and hardness →
decreased porosity and penetrability → reduced wettability → in turn
decreased DR.
• Also causes deformation, crushing → increased effective surface area →
increased dissolution rate (DR).
c) INTENSITY OF PACKING OF CAPSULE CONTENTS
• Tightly filled capsules-diffusion of GI fluids → high pressure → rapid
bursting and dissolution of contents.
• Opposite also possible → Poor drug release due to decreased pore size
and poor penetrability of GI fluids.
III. ABSORPTION OF DIFFERENT ORAL DOSAGE FORMS
• Different Types
18

21. GM Hamad

­ Solution
­ Suspension
­ Tablets
­ Capsules
­ Enteric Coated Tablet
­ Powders
• Order of absorption
Solutions > Emulsions > Suspensions > Capsules > Tablets > Coated
Tablets > Enteric Coated Tablet > Sustain Release Tablet
a) SOLUTION
• Aqueous solutions, syrups, elixirs, and emulsions do not present a
dissolution problem and generally result in fast and often complete
absorption as compared to solid dosage forms.
b) SOLID SOLUTIONS
• The solid solution is a formulation in which drug is trapped as a solid
solution or monomolecular dispersion in a water-soluble matrix.
Although the solid solution is an attractive approach to increase drug
absorption, only one drug, griseofulvin, is currently marketed in this
form.
c) SUSPENSIONS
• A drug in a suspension is in solid form but is finely divided and has a
large surface area. Drug particles can diffuse readily between the
stomach and small intestine so that absorption is relatively insensitive to
stomach emptying rate.
• Adjusting the dose to a patient’s needs is easier with solutions and
suspensions than with solid dosage forms. Liquid dosage forms,
therefore, have several practical advantages besides simple dissolution
rate.
• However, they also have some disadvantages, including greater bulk,
difficulty in handling, and perhaps reduced stability.
d) TABLETS AND CAPSULES
• These formulations differ from each other in that material in capsules is
less impacted than in compressed tablets. Once a capsule dissolve, the
contents generally disperse quickly. The capsule material, although
water soluble, can impede drug dissolution by interacting with the drug,
but this is uncommon.
• Tablets generally disintegrate in stages, first into granules and then into
primary particles. As particle size decreases, dissolution rate increases
due to increased surface area.
IV. PHARMACEUTICAL INGREDIENTS/EXCIPIENTS
19

22. GM Hamad

• More the number of excipients in dosage form, more complex it is and
greater the potential for absorption and bioavailability problems.
• Changing an excipient from calcium sulfate to lactose and increasing the
proportion of magnesium silicate, increases the activity of oral
phenytoin.
• Absorption of tetracycline from capsules is reduced by calcium
phosphate due to complexation.
• Most of these types of interactions were reported some time ago and
are unlikely to occur in the current environment of rigorous testing of
new dosage forms and formulations.
• Excipients commonly used:
­ Vehicle
­ Diluents
­ Binders & granulating
agent
­ Disintegrants
­ Lubricants
­ Suspending
agents/viscosity agent
­ Surfactants
­ Bile salts
­ Colorants
V. PRODUCT AGE AND STORAGE CONDITIONS
• Aging and alteration in storage condition changes the physiochemical
properties of a drug which adversely affects bioavailability.
• During storage
­ Metastable form → Stable form
­ Change in particle size
­ Tablet → harden / soften
• E.g.
­ Prednisone tablet containing lactose as a filler, high temp and high
humidity resulted in harder tablet that disintegrated and dissolve
slowly.
2. PATIENT RELATED FACTORS
A. PHYSIOLOGIC FACTORS RELATED TO DRUG ABSORPTION
I. MEMBRANE PHYSIOLOGY
• Nature of Cell Membrane
• Transport Processes
II. GASTERO-INTESTINAL PHYSIOLOGY
a) GASTRIC EMPTYING RATE
• Anatomically, a swallowed drug rapidly reaches the stomach.
20

23. GM Hamad

• Eventually, the stomach empties its contents into the small intestine.
Because the duodenum has the greatest capacity for the absorption of
drugs from the GI tract, a delay in the gastric emptying time for the drug
to reach the duodenum will slow the rate and possibly the extent of drug
absorption, thereby prolonging the onset time for the drug.
• Some drugs, such as penicillin, are unstable in acid and decompose if
stomach emptying is delayed. Other drugs, such as aspirin, may irritate
the gastric mucosa during prolonged contact.
• Gastric emptying rate is faster in case of solution & suspensions than
solid and non-disintegrating dosage forms.
• Factors that influence gastric emptying rate are:
­ Volume of meal
­ Composition of meal
­ Physical state and
viscosity of meal
­ Temperature of meal
­ Gastrointestinal pH
­ Electrolyte and osmotic
pressure
­ Body posture
­ Emotional state
b) INTESTINAL MOTILITY
• Normal peristaltic movements mix the contents of the duodenum,
bringing the drug particles into intimate contact with the intestinal
mucosal cells.
• The drug must have a sufficient time (residence time) at the absorption
site for optimum absorption. In the case of high motility in the intestinal
tract, as in diarrhea, the drug has a very brief residence time and less
opportunity for adequate absorption.
c) DRUG STABILITY IN GIT
• Metabolism or degradation by enzymes or chemical hydrolysis may
adversely affect the drug absorption and thus reduces bioavailability.
• Destruction in gastric acid.
• Generally, a problem with orally administered drugs.
d) INTESTINAL TRANSIT
• Long intestinal transit time is desirable for complete absorption of drug
e.g. for enteric coated formulation and for drugs absorbed from specific
sites in the intestine.
• Peristaltic contraction promotes drug absorption by increasing the drug
membrane contact and by enhancing dissolution especially of poorly
soluble drugs.
21

24. GM Hamad

• Influenced by food, disease and drugs. e.g. metoclopramide which
promotes intestinal transit and thus enhance absorption of rapidly
soluble drugs while anticholinergic retards intestinal transit and
promotes the absorption of poorly soluble drugs.
e) BLOOD FLOW TO GIT
• Once the drug is absorbed from the small intestine, it enters via the
mesenteric vessels to the hepatic-portal vein and the liver prior to
reaching the systemic circulation. Any decrease in mesenteric blood
flow, as in the case of congestive heart failure, will decrease the rate of
drug removal from the intestinal tract, thereby reducing the rate of drug
bioavailability.
• GIT has higher perfusion rate because it is extensively supplied by blood
capillary network.
• Therefore, help in maintaining sink conditions and concentration
gradient for drug absorption by rapidly removing of drug from site of
action.
• Blood flow is important for actively absorption of drugs.
• Highly permeable drugs or drugs that absorbed through pores –GI
perfusion is rate limiting while the drugs with poor permeability GI
perfusion is not important.
• Perfusion increases after meals and persist for few hours, but absorption
is not affected.
f) EFFECT OF FOOD
• The presence of food in the GI tract can affect the bioavailability of the
drug from an oral drug product.
• Digested foods contain amino acids, fatty acids, and many nutrients that
may affect intestinal pH and solubility of drugs. The effects of food are
not always predictable and can have clinically significant consequences.
Some effects of food on the bioavailability of a drug from a drug product
include:
­ Delay in gastric emptying
­ Stimulation of bile flow
­ A change in the pH of GI tract
­ An increase in splanchnic blood flow
­ A changed luminal metabolism of the drug substance
­ Physical or chemical interaction of the meal with the drug product
or drug substance.
22

25. GM Hamad

• The absorption of some antibiotics, such as penicillin and tetracycline, is
decreased with food, whereas other drugs, particularly lipid-soluble
drugs such as griseofulvin and metaxalone, are better absorbed when
given with food containing a high fat content.
• Propranolol plasma concentrations are larger after food than in fasted
subjects. This may be an interaction with the components of food.
III. AGE
• In infants, the gastric pH is high and intestinal surface and blood flow to
the GIT is low resulting in altered absorption pattern in comparison to
adults.
• In elderly persons, causes of impaired drug absorption include altered
gastric emptying, decreased intestinal surface area and GI blood flow,
higher incidents of achlorhydria and bacterial overgrowth in small
intestine.
B. CLINICAL FACTORS
I. DISEASE STATE
• Several disease state may influence the rate and extent of drug
absorption.
• Three major classes of disease may influence bioavailability of drug.
­ GI diseases
­ CVS diseases
­ Hepatic diseases
a) GI DISEASES
• GI Infections
­ Celiac diseases: Characterized by destruction of villi and microvilli.
Abnormalities associated with this disease are increased gastric
emptying rate and GI permeability, altered intestinal drug
metabolism.
• Crohn’s disease
­ Altered gut transit time and decreased gut surface area and
intestinal transit rate.
• GI surgery
­ Gastrectomy may cause drug dumping in intestine, osmotic
diarrhea and reduce intestinal transit time.
b) CVS DISEASES
• In CVS diseases blood flow to GIT decrease causing decreased drug
absorption.
23

26. GM Hamad

c) HEPATIC DISEASES
• Disorders like hepatic cirrhosis influences bioavailability of drugs which
undergoes first pass metabolism.
II. DRUGS
a) ANTICHOLINERGIC
• Anticholinergic drugs in general may reduce stomach acid secretion
Propantheline bromide is an anticholinergic drug that may slow stomach
emptying and motility of the small intestine. Slower stomach emptying
may cause delay in drug absorption
b) METOCLOPRAMIDE
• Metoclopramide is a drug that stimulates stomach contraction, relaxes
the pyloric sphincter, and, in general, increases intestinal peristalsis,
which may reduce the effective time for the absorption of some drugs.
c) ANTACIDS
• Antacids containing aluminum, calcium, or magnesium may complex
with drugs such as tetracycline, ciprofloxacin, and indinavir, resulting in a
decrease in drug absorption.
24

27. GM Hamad

BIOLOGICAL HALF LIFE & VOLUME OF
DISTRIBUTION
HALF LIFE
• The time needed to decrease the body drug level by one half of its initial
level. If the concentration is reduced due to a physiological process, i.e.,
biotransformation then, this half life is called as elimination or biological
half life.
• The half life is the time required for the body to eliminate one half of the
drug which it contains. The elimination half life is a function of both, the
clearance and the volume of distribution of the drug. It is the
characteristic of exponential decay where the time required for a given
fraction of drug to disappear is always the same, regardless of the time
or concentration at which one begins measurements.
• The biological half life is not the time for the response to decline by 50%,
since the requirement for a threshold concentration, latency of drug
response and other factors cause a non-parallelism between blood
concentration and pharmacological response intensity.
• The half life is used to determine the time required for the body to
eliminate by metabolizing, or excretion or by both, one half of the initial
concentration of drug in blood.
OTHER TYPES OF HALF LIFE
• Besides the elimination half life, the other types of half life are as
follows:
ABSORPTION HALF LIFE
• It is the time to reduce drug concentration at absorption site due to
absorption by one half of its initial concentration.
DISTRIBUTION HALF LIFE
• The time required to reduce concentration of drug to half of its initial
concentration due to the distribution of drug is called as distribution half
life.
PHYSICAL HALF LIFE
25

28. GM Hamad

• This half life pertains to the radiopharmaceuticals. Physical half life is the
time to decrease the concentration of a substance to half due to physical
decay. For instance, the radiopharmaceutical decays physically and the
time at which its concentration is reduced to half of its initial
concentration is the physical half life.
EFFECTIVE HALF LIFE
• When a radiopharmaceutical is administered, its concentration decays
due to its physical disintegration as well as due to its biotransformation.
Effective half life combines both the elimination half life and the physical
half life of a radiopharmaceutical.
ZERO AND FIRST ORDER HALF LIFE
• Drug concentration may decay by two modes, first order half life or by
zero order half life.
FIRST ORDER HALF LIFE
• Half life of the first order process is a constant for a given rate process
hence the half life is the time required [C]t (concentration at time t) to
become equal to one half of [C]0 (the initial value at zero time). The first
order half life can be calculated by the formula:
t
½=
0.693
k

• This formula has been derived from the equation of the graph to
describe the first order kinetics. The basis for this equation is Y = mx + b,
where Y is the concentration, m is the slope (or rate constant) of the
curve and b is the y-intercept.
• Log natural (Ln) is included because the blood level time curve is a
straight line on the graph between Ln of concentration versus time.
Ln[C]
t−Ln[C]
0=−kt
This equation can be arranged in the following several ways
−Ln[C]
t=−kt+Ln[C]
0⇛−Ln
[C]
t
[C]
0
=kt⇛Ln
[C]
0
[C]
t
=kt
• By definition half life is the time when the concentration is half of the
initial concentration, thus, at this time, in equation, the [C]t becomes
26

29. GM Hamad

½ [C]0 and t as t½.
Ln
[C]
0
1
2
[C]
0
=kt
½
Rearranging the above equation yields the following equation.
Ln
2[C]
0
[C]
0
=kt
½⇛Ln
2
k
=t
½
• This equation represents one of the ways to calculate the first order half
life using the rate constant determined from a first order graph of the
data. A drug being eliminated by a first order process will have a half life
which is constant and independent of the initial concentration or dose of
the drug.
ZERO ORDER HALF LIFE
• The half life of zero order process is not like that discussed for the first
order process.
• Applying the definition of half life to the zero-order process, equation
yields as:
[C]
t=−kt+[C]
0
• At half life, the [C]t in equation becomes ½[C]0 and the t as t½, thus the
above equation for zero order becomes as following.
[C]
0
2
−[C]
0=−kt
½⇛−
[C]
0
2k
=−t
½⇛t
½=
0.5[C]
0
k

• Where C0 is the initial concentration, k is the rate constant for the zero-
order process. The equation indicates that the half life is not independent
of the initial concentration. Factually, lager the initial concentration, the
greater is the half-life. The difference can be used to distinguish between
zero and first order process by varying the initial concentration (or order)
and measuring the resulting half life.
METHOD OF DETERMINATION
1. Direct graphical method:
­ Half life of a drug can be estimated by direct reading the time
needed for concentration to decrease by one half from any point
on the log concentration time plot.
27

30. GM Hamad

2. From the slope of the terminal log concentration time curve:
­ The slope of the curve is the elimination rate constant. Using the
formula of first order half life, half life can be calculated.







3. Calculation from urine data:
­ Half life is also calculated from the urine data assuming that the
renal clearance of drug is constant, the excretion rate parallels the
plasma concentration. The half life can be calculated by urine
concentration data. But practically, urine concentration data gives
usually poor estimate due to incomplete bladder emptying and
inability to collect samples frequently.
RELATIONSHIP OF ELIMINATION HALF LIFE WITH CLEARANCE, VOLUME OF
DISTRIBUTION
• The clearance and volume of distribution are the two independent
pharmacokinetic parameters which determine elimination half life and
thus, the half life. These two are the dependent parameters.
C??????=K x Vd⇛
C??????
Vd
=k⇛
0.693
t
½

SIGNIFICANCE OF HALF LIFE
1. Reflects the rate of drug elimination, similar to the elimination rate
constant. Drug that have a short half life (i.e., having larger elimination
rate constant) are readily eliminated from the body.
2. Half life is used in deciding the appropriate dosage regimen. Drugs with
shorter half life require frequent dosing during multiple administration.
3. It gives the estimate of the rate of drug removal from the body.
4. It indicates the efficiency of the elimination process. Thus, change in half
life will reflect change in elimination organ functions such as liver
28

31. GM Hamad

biotransformation or the excretion in kidney. It is thus, a prime measure
for the dosage adjustment in disease status.
5. A drug with brief half life requires more frequent dosage that the drug
with long half life.
6. The drug having half-life between 3 to 4 hours are the good candidates
for control release formulations.
7. Drugs reach to steady state concentration in approximately 5 half lives.
8. A drug is completely eliminated after 10 half life and usually negligible
within 7 half lives.
9. Provides the basis for classification of the drugs having ultra-fast, slow
and very slowly disposition drugs. The ultra-fast disposition (UFD) drugs
has half life of less than 1-hour, slow disposition (SD) drugs have half life
of 8-24 hours and that very slow disposition (VSD) drugs have the half
life of more than 24 hours.
FACTORS AFFECTING HALF LIFE
• Half life of a drug is affected by any factor which modifies the drug
metabolism and excretion. The factors can be categorized into patient
related, drug related and combinedly related to the drug as well as to
the patient. Patient related factors include age, genetic, renal
insufficiencies, hepatic insufficiencies, urine pH, gender, nutritional
status, emotional status, hormonal level, body temperature, volume of
distribution, etc.
• Factors related to drug include co-administered drug-drug interaction,
inhibition of drug metabolism, stimulation of drug metabolism via
increased enzyme activity or via enzyme induction, therapy duration
(enzyme exhausted).
COMBINED PATIENT AND DRUG RELATED FACTORS
• Combined patient and drug related factors include the pKa of acidic or
basic drugs, affinity for protein binding, tissue storage, volume of
distribution.
RENAL MALFUNCTION
• The renal clearance is altered if there is any disease, malfunctioning or
insufficiencies of kidney.
• When renal clearance decreases, the half life is increased and vice versa.
HEPATIC DISEASE
29

32. GM Hamad

• Liver is the site of metabolism for most of the drugs. When the liver is
not functioning properly, the rate of drug elimination will be decreased
due to a decreased metabolism leading to increase in half life of the
drug.
URINARY pH
• pH changes leads to altered rate of drug excretion and kidney
reabsorption of the drug. Basic drugs are rapidly excreted through acidic
urine and vice versa. This of course will affect the half life.
AGE
• As rate of metabolism are different in various stage of human age, the
half life obviously varies in different age groups. In children, liver is not
well developed so the metabolism rate is slow. In elder patients, the
system gets exhausted and thus the rate is slow. GFR is also gradually
decreased after the age of 30 years.
AFFINITY FOR PROTEIN BINDING
• Customarily, the drug bound with protein is not available for drug
metabolism and excretion thus leading to an increased half life.
However, drugs can be categorized into restrictively cleared and non-
restrictively cleared drugs.
• The restrictive cleared drugs are also known as binding sensitive drugs.
The bound drugs not able to diffuse through cell membrane, and thus
not able to reach site of metabolism and excretion. Thus for such drugs,
increase in the free drug concentration in the blood will make more drug
available for hepatic extraction as well as for renal excretion.
• Non-restrictively cleared drugs are also known as binding-insensitive
drugs. These drugs are extracted by the liver with greater rate regardless
of bound to protein or free. The elimination half life of such drug is not
significantly affected by a change in the degree of protein binding. The
drug is removed from the plasma binding sites during the circulation
through the liver by inducing a conformation change in the protein,
weakening the process of binding and subjecting the drug to
metabolism. The drugs activity secreted though the renal route are
binding insensitive.
TISSUE STORAGE
30

33. GM Hamad

• Certain drugs have an affinity for adipose tissue and thus are stored for a
prolonged time period leading to an increased half life.
CO-ADMINISTRATION OF DRUG AND DRUG INTERACTION
• Co-administrated drugs may alter the half life of one another drug by:
­ Competing for protein binding i.e., warfarin and phenylbutazone.
­ Competing for metabolizing enzyme if both are metabolized by
the same enzyme
­ Altering the urinary pH
­ Enzyme inhibition - increases the half life
­ Enzyme induction - decrease the half life.
GENETIC FACTORS
• The genetic factors contribute substantially to the larger difference
among the individuals for fate of drug metabolism and clearance of
drug. Thus, half life of drug is affected by genetic variations. Some
individuals are slow acetylators while others are fast acetylators.
VOLUME OF DISTRIBUTION
• Half life is the function of both the clearance and the volume of
distribution of drug may have large clearance but still have a long half
life due to increase value of the volume of distribution.
VOLUME OF DISTRIBUTION (Vd)
• Volume of distribution is defined as the apparent volume available for
the distribution of a drug in body. This parameter indicates the apparent
space as volume in body available to contain drug.
• Since the values of Vd does not have a true physiologic meaning in terms
of an anatomic space, the term “apparent” is used with Vd as prefix. The
Vd represents a volume that must be considered in estimating the
amount of drug in body from the concentration of drug found in the
sampling compartment, i.e. blood.
• Since the drug is not distributed equally in all tissues of the body
(compartments) due to drug’s different affinities to different tissues, the
volume of distribution does not represent a real volume rather
represents a hypothetical volume. This volume relates the amount of
drug in the body to the plasma concentration by the equation:
31

34. GM Hamad

Vd=
Db
Cp

­ Where, Db is the drug in body and Cp is the concentration in
plasma.
• Though the Vd is hypothetical, yet it is influenced by the
physicochemical properties and the affinity of drugs to the blood and
tissues. The drug lipid solubility which dictates the affinity of drug to
tissues and the protein binding, affect Vd.
• Vd is an independent pharmacokinetic parameter which does not
depends on the other pharmacokinetic parameters. Vd provides an
estimate of the drug which does not appear in the plasma or distributed
at tissue level. A very high Vd reflects binding of drug with the tissue
proteins.
RELATIONSHIP OF VD WITH OTHER PHARMACOKINETIC PARAMETERS
• The following equation shows the relationship of Vd with other
pharmacokinetic parameters, such as elimination rate constant and the
clearance.
C??????=k x Vd
SIGNIFICANCE OF Vd
• The Vd is used to calculate a dose of drug required to achieve certain
blood concentration (called as the target concentration and abbreviated
as CT) as:
Dose=C
T x Vd
• The volume of distribution is a distribution parameter which indicates
the extent of distribution. Based on its obtained values, the extent of
drug distribution can be classified as:
­ The widely distributed drugs which show volume of distribution
greater than 0.7 L/Kg.
­ The moderately distributed drug which demonstrates values of
the volume of distribution between 0.3-0.7 L/Kg.
­ Limited distributed drugs having the volume of distribution lesser
than 0.3 L/Kg.
• As has been mentioned that the Vd is not a true physiologic volume,
most of the drugs have an apparent volume of distribution smaller than,
32

35. GM Hamad

equal to or several times more than the body mass. It depends on the
initial plasma concentration.
• Vd is a useful parameter in considering the relative amount of drug in
the vascular and in the extravascular tissues.
• Magnitude of the apparent Vd is a useful indicator for the amount of
drug outside the sampling compartment which is usually blood. The
volume of distribution at steady-state (Vd(ss)) is important for
determining the relevance of changes in the extent of distribution of
drug in the presence of diseases.
ALTERED Vd
• For each drug, the apparent Vd is a constant. In certain pathologic cases
however, the Vd for the drug may be altered if the distribution of the
drug is changed due to the change in the total body water and total
extracellular water. If these increases, (as in the case of edematous
condition), a very larger Vd results for a drug with more water solubility.
EXCESSIVELY LARGER Vd
• Drugs may exhibit very large values of Vd which exceed all the volumes
available in the body, e.g., chloroquine Vd is about 115 L/ kg. Such drugs
show concentration of drug specifically in one or more tissues.
Chloroquine concentrates in liver 1000 times more than in plasma.
• Even wide range of Vd values are expected for the drugs exhibit a non-
uniform distribution in the body with variations due to difference in
their passing through membranes and their lipid/water solubility.
• The highest concentrations of drugs are often present in the kidney,
liver, and intestine which usually reflect the amounts of drug being
excreted.
LARGER Vd
• A larger Vd occurs if the drug is extensively distributed in peripheral
tissues and organs and resulting into a smaller inter-vascular
concentration. This means that the drug with a larger apparent Vd are
more concentrated in extravascular tissues and less concentrated
intravascularly. A Vd between 30 and 50 liters, corresponds to drug
distribution in the total body water.
• Binding of the drug with peripheral tissue or its proteins, results into an
increased Vd. The protein bound drugs showing larger Vd when
33

36. GM Hamad

displaced from the protein, do not show any clinically relevant
consequence.
SMALLER Vd
• If a drug is highly bound to plasma proteins, or remains in the vascular
regions, it will result in a smaller apparent Vd. A value of Vd in the range
of 3-5 liter (in an adult) would indicate that the drug is in the vascular
compartment since this is the value of plasma volume.
• For polar drugs with low lipid solubility, the apparent Vd is generally
small. Protein bound drugs having smaller Vd, produce
pharmacodynamic effects when displaced from plasma protein.
• In two compartment model, Vdss reflects the true distribution volume
occupied by the plasma and the tissue pool when steady state is
achieved. This volume is used to calculate the loading drug dose
necessary to upload the body to a desired plasma drug concentration.
CALCULATION OF VOLUME OF DISTRIBUTION
• The volume of distribution after extravascular administration is
calculated as Vd (area) by the following equation.
V
d(area)=
Dose
AUC .β

­ Where, AUC is total area under the blood level time curve, beta is
rate of elimination.
• The Vd(ss) is the volume of distribution after IV administration and it
represent the volume in which a drug appears to be distributed during
steady-state if drug existed throughout the volume at the same
concentration as in the measured fluid. The Vd(ss) is generally calculated
by non-compartmental approach as:
V
d(ss)=
Dose
IV . AUMC
(AUC)
2

­ Where AUC is the total area under the curve and AUMC is the area
under the first moment of the plasma concentration-time curve.
The value of Vd(ss) is generally smaller than the Vd area.

34

37. GM Hamad
DRUG CLEARANCE
INTRODUCTION
• Synonyms: Systemic clearance, body clearance, total clearance.
• Total clearance represents sum of clearances by the various organs that
contribute to elimination of drug.
C&#3627408473;
Total=C&#3627408473;
Renal+C&#3627408473;
Hepatic+C&#3627408473;
Lungs
• Clearance describes the process of drug elimination from the body or
from a single organ without identifying the individual processes
involved.
• Clearance can be defined as “The volume of fluid cleared of drug from
the body per unit time.”
• Clearance is a proportionality constant describing a relationship
between rate of elimination (as amount per unit time) at a given time
and its corresponding concentration in fluid at that time.
• It may be regarded as, “the volume of blood or plasma (depending upon
the fluid used for drug assay) from which the drug appears to be
removed per unit of time to account for its elimination.”
• Example: clearance considers that a certain portion or fraction (percent)
of the distribution volume is cleared of drug over a given time period.
• Clearance is a pharmacokinetic parameter that describes drug
elimination from a hypothetical well stirred compartment containing
uniform drug distribution.
• For first order elimination process, clearance is constant. Clearance
applies to all elimination rate processes, regardless of the mechanisms
for elimination. It may have values that are not physiological.
• Unit: The unit of clearance are milliliters per minute (ml/min, ml/min.Kg)
or liters per hour (L/h, L/h.Kg).
IMPORTANCE OF CLEARANCE
• Clearance is the one parameter that determines the maintenance dose
rate required to achieve a desired plasma concentration.
Dosing rate=clearance x desired plasma concentration
35

38. GM Hamad
MECHANISIM OF CLEARANCE
• Renal excretion
• Hepatic excretion
• Minor clearance through lungs and skin.
RENAL EXCRETION
• Renal excretion is the major route of elimination for many drugs. For
example, water soluble and low molecular weight drugs, or the drugs
that are slowly bio-transformed by the liver.
• Drug excretion from the kidney involves combination of the following:
­ Glomerular filtration
­ Active tubular secretion
­ Tubular reabsorption
HEPATIC CLEARANCE
• Hepatic clearance is the volume of blood that perfuses the liver which is
cleared of drug per unit time.
• Hepatic clearance involve Biotransformation or Drug Metabolism (Phase
I and Phase II).
CLEARANCE MODELS
• Model Independent
• Model dependent or Compartment Model
• Physiologic Model
MODEL INDEPENDENT
• This is a non-compartmental approach used to calculate Clearance. This
model does not require any assumption for a specific compartment
model.
• Therefore, Clearance can be determined directly from the Plasma-Time
concentration curve using:
&#3627408438;&#3627408473;=∫
&#3627408439;
0
&#3627408438;
??????&#3627408465;&#3627408481;
∞
0

• Since there is no compartment considered, therefore, [AUC]
0
∞
=
∫&#3627408438;
??????&#3627408465;&#3627408481;
∞
0
, Replacing this in the above equation gives:
&#3627408438;&#3627408473;=
&#3627408439;
0
[AUC]
0
∞

36

39. GM Hamad
MODEL DEPENDENT OR COMPARTMENTAL MODEL
• Model Dependent considers compartmental approach and the clearance
is calculated by assuming volume of distribution and elimination rate
constant.
C&#3627408473;
T=&#3627408472;V
??????
PHYSIOLOGIC MODEL
• This model is organ specific. Clearance may be defined as the fraction of
blood volume containing drug that flows through the organ and is
eliminated of drug per unit time. OR
• Clearance is the product of the blood flow (Q) to the organ and the
extraction ratio (ER).
C&#3627408473;
Organ=Q(ER)
• If the drug concentration in the blood (Ca) entering the organ is greater
than the drug concentration of blood (Cv) leaving the organ, then some
of the drug has been extracted by the organ.
• The ER is Ca – Cv divided by the entering drug concentration (Ca), as
shown:
ER=
C
a−C
v
C
a

Substituting in the previous equation
C&#3627408473;
Organ=Q(
C
a−C
v
C
a
)
CALCULATION OF CLEARANCE
• Body clearance (ClB) after E/V route is:
C&#3627408473;
B=
?????? .&#3627408439;??????&#3627408480;&#3627408466;
??????&#3627408456;&#3627408438;

• For ClB after IV route the fraction of dose absorbed (F) is not included in
equation:
C&#3627408473;
B=
&#3627408439;??????&#3627408480;&#3627408466;
??????&#3627408456;&#3627408438;

• For ClB after steady state IV infusion:
C&#3627408473;
B=
k
0
C
ss

37

40. GM Hamad
• If volume of distribution by area calculated from terminal curve (Vdarea)
and rate of elimination (Ke) is known, then ClB is:
C&#3627408473;
B=&#3627408457;&#3627408465;
????????????????????????⨯k
e
RELATIONSHIP OF CLEARANCE WITH HALF-LIFE
• If we know the Clearance, we can determine Half-Life (t½) of a drug
through a simple relation:
C&#3627408473;
T=k⨯Vd
k=
0.693
t
½

Therefore, by substitution
C&#3627408473;
T=
0.693 &#3627408457;&#3627408465;
&#3627408481;
1/2

APPLICATIONS OF CLEARANCE
1. The volume concept is simple and convenient because all drugs are
dissolved and distributed in the fluid of the body.
2. Clearance can be computed reliably.
3. It is used to compute other parameters such as dose.
4. It is pharmacokinetic parameter including drug disposition, drug
elimination and drug excretion.
5. The values of clearance measured based on plasma clearance could be
interpreted physiologically.
PHYSIOLOGICAL INTERPRETATION OF THE DRUG CLEARANCE VALUES
• The systemic clearance may predominantly represent hepatic clearance.
For drugs undergoing extensive biotransformation, systemic clearance
mainly reflects the capacity of liver to metabolize these drugs.
• The clearance exceeding hepatic blood flow indicates the drug is
simultaneously metabolized by liver and extra hepatic site (renal/lungs).
For example: anesthetic propofol is cleared of blood through hepatic
lungs.
• For drugs solely eliminated by glomerular filtration the clearance
expected to reflect the GFR.
• Lower clearance relative to GFR represent renal clearance with drug re
absorption. For example: gentamicin.
38

41. GM Hamad
COMPETING TERMINOLOGIES
• Metabolism and excretion are the competing terminologies of clearance.
EXCRETION
• Excretion is the removal of drug from the body. The unit of excretion are
amount per unit of time. For example: mg/min.
• In excretion the drug may be inactivated but it no longer remains in your
body.
METABOLISM
• Metabolism activates prodrug or inactivates a drug which may remain in
the body.
39

42. GM Hamad

B
Concentration

AUC

LINEAR AND NON-LINEAR
PHARMACOKINETICS
LINEAR PHARMACOKINETICS
• The linear pharmacokinetics is characterized by 1
st
order kinetics in
distribution and elimination kinetics of a drug. The same order of
kinetics is assumed after increase of dose. For example, when the dose
of a drug is doubled, the concentration in blood is doubled.
• Graphically, linear pharmacokinetics can be demonstrated as given in
Figure 1.





• Majority of the drugs follows the linear pharmacokinetics.
NON-LINEAR PHARMACOKINETICS
• Being nonlinear means the effect of increase in one parameter results in
disproportional increase, decrease or no change in the other parameter.
In pharmacokinetics, the area under the curve (AUC), an absorption
parameter when plotted against three graded doses (increased
amounts) of drug it may proportionally increase, decrease or remains
unchanged.
• When the levels of AUC decrease or demonstrates no change with
increase in dose is called as the non-linear pharmacokinetics.
• Nonlinear pharmacokinetics is the deviations from the linear
pharmacokinetic profile of a drug.
A
Figure 1: Linear Pharmacokinetics, A) Concentration increased linearly with increase in
dose and B) Linear correlation between dose and area under the curve (AUC).

40

43. GM Hamad

• A few drugs follows the nonlinear pharmacokinetics where an increase
in dose causes a nonlinear or disproportional change in blood
concentration and thus, area under the curve (AUC).
• Non-linearity leads to a higher or lower than the expected rise in
concentration or AUC with increased dose, as a result of dose-
dependent changes in absorption, distribution and elimination process.
Thus, the nonlinear kinetics is also known as capacity-saturation or
saturation kinetics.
• In non-linear pharmacokinetics, the kinetics of a process dealing with
drug deviates from the first order kinetics, which is the most commonly
followed in pharmacokinetic processes. In terms of kinetics, this
deviation is called as saturation kinetics, Michaelis-Menten of capacity-
limited kinetics.
CAUSES OF NON-LINEARITY
1. Saturation of process
2. Saturation of transporter
3. Saturation of enzyme
4. Pathologic conditions
5. Drug-induced
6. Disproportional activation of endogenous entities, e.g., ATPase, P-pg
PROCESSES INVOLVED IN NON-LINEAR PHARMACOKINETICS
• Nonlinear absorption results from saturation of carrier-mediated
transport causing lower than expected drug concentration with increase
in dose. In absorption, the saturation of pre systemic metabolism in gut
wall results in higher than the expected concentration since greater
proportion of administered dose survives the hepatic metabolism. For
instance, higher doses of salicylates saturate its glycine conjugation in
children.
• There are three categories of drugs which affect P-gp differently.
­ Category I drugs can stimulate P-gp in low concentrations while
inhibit P-gp at higher concentrations.
­ Category II drugs can produce dose dependent activation of
ATPase.
­ Category III can inhibit activity of ATPase.
• The category I can lead to non-liner absorption/excretion/secretion due
to the distribution of P-gp at relevant locations.
41

44. GM Hamad

B A
• Saturation of plasma protein binding causes nonlinear distribution and
occurs when the drug concentration exceeds binding capacity of protein,
an effect is pronounced with the basic drugs that bind to alpha-1 acid
glycoprotein due to lower concentration of this protein than that of the
albumin. However, nonsteroidal anti-inflammatory drugs and valproic
acid show nonlinear protein binding with albumin.
• Drugs saturating absorption or distribution are few and have no
significant effect on clinical dosing.
• Saturation of first pass metabolism in liver, respectively results in higher
than the expected concentration since greater proportion of
administered dose survives metabolism. For instance, higher doses of
salicylates saturate its glycine conjugation in children.
• Nonlinear elimination occurs with saturation of renal or biliary secretion.
Nonlinear elimination occurs for drugs undergoing hepatic metabolism
and taken at higher but within the clinical doses.
• Incremental dose of penicillin causes saturation of a transporter in renal
secretion of the drug leading to its non-linear elimination.
• Phenytoin saturates metabolism at upper range of clinical dose while
ethanol saturate even at lower dose.
INDICATION OF NON-LINEAR PHARMACOKINETICS
• In nonlinear elimination, rather than an exponential (first order) decline
in the plasma concentration, zero order elimination occurs initially
shown by a straight line on the linear plot and convex curve on
semilogarithmic graph.
• Until the concentration of drug falls sufficiently low, the elimination
returns to first order decline because the elimination process will no
longer be saturated (Figure 3).








Figure 3: Michaelis-Menten kinetics

42

45. GM Hamad

A
B
COMPARISON OF MICHAELIS-MENTEN KINETICS TO OTHER KINETICS ORDERS
• The order of kinetics is determined by the graphical method and each
order of kinetics gives a specific profile when drawn as concentration
against time and Ln-concentration against time (or on semilogarithmic
graph paper).
• With linear concentration, the first order is a curved line (Figure 4 A),
zero order kinetics shows a straight line (Figure 4 C) and the Michaelis-
Menten kinetic reflects a rapid decline and then a slow decline due to
saturation of the process (Figure 3 A).
• When the Ln-transformed concentration data is plotted on ordinary
graph (or the semilogarithmic graph is used), the first order becomes a
straight line (Figure 4 B) and zero order gives an upward curve in Figure
4 C.
• For the Michaelis-Menten kinetics, the line with Ln-transformed
concentration yields a slow and then a fast decline (Figure 1 B). The
profile in graph Figure 3 B deviates from linearity (zero order) and then
becomes linear (first order).
INTERPRETATION
• In first order kinetics the rate of decline of drug concentration depends
on the concentration. In zero-order kinetics, the decline is independent
of the drug concentration in the tissue.
• Michaelis Menten kinetics is usually applicable to drug elimination
where the drug elimination depends on the degree of saturation of the
elimination process. In this kinetics, the enzyme involved in drug
biotransformation is activated in a manner dependent on the drug
concentration.







43

46. GM Hamad

C

D
Equation 1
Equation 2 Equation 3





IMPACT OF NON-LINEARITY ON PHARMACOKINETICS
• Due to this complex kinetics, the equations used for linear
pharmacokinetics are not applicable, though the concept of
compartment model is still applicable.
• Application of linear pharmacokinetic models to drug showing nonlinear
pharmacokinetics may lead to large or frequent dosing leading to an
unexpected accumulation of drug.
• Nonlinear pharmacokinetic models require the application of more
complicated, enzyme or saturation (Michaelis-Menten) kinetic theory,
thus non-linear kinetics is also called as Michaelis-Menten kinetics.
PARAMETERS OF NON-LINEAR PHARMACOKINETICS
• It is clinically important to know the dose causing saturation of process
to avoid drug accumulation and toxicity.
• Knowledge of the maximal rate of elimination (Vmax) is important. The Km
is the concentration at the half maximal rate of elimination.
• The rate constant of decline, Vmax is calculated using equation for slope
(m) from the top left straight line on the linear graph. by using the
equation 1:
V
max=−m
• The C0 (y-intercept) is estimated from the straight line at the top left of
the semilogarithmic graph.
• The Kel is calculated from the linear terminal slope of the curve on
semilogarithmic graph.
• The half-life is calculated as: 0.693/Kel.
• The Km and Vd are calculated using the equations 2 and 3:
k
m=
V
max
k
el
Vd=
Dose
C
0

Figure 4: First order kinetics (A-B), and Zero order kinetics (C-D)

44

47. GM Hamad

APPLICATIONS OF PHARMACOKINETICS IN
CLINICAL SITUATIONS
Following are the areas where pharmacokinetic are applied:
• Individualization of drug dosing regimen
­ Individual variations in particular patients.
• Therapeutic drug monitoring
­ Involves monitoring of drug conc. in plasma for optimal drug
therapy.
• Therapeutic window
­ Drugs having 2-3 therapeutic window, these drugs have narrow
therapeutic window
TW=
Max effective conc.
Minimum effective conc.
=
20 µg/ml
8 µg/ml
=2.5µg/ml
• Candidates of therapeutic drug monitoring
• Availability of an assay procedure.
FACTORS INFLUENCING DRUG VARIABILITY
• Variation in drug absorption
• Presence of other drugs
• Drug interactions
• Genetic differences
• Physiological differences
• Pathophysiological condition of different people e.g. Immuno-
compromised patients.
DOSE INDIVIDUALIZATION
• Dose individualization is the determination of a sufficient dose which
delivers a safe drug.
• Concentration that is not toxic not below the minimum effective
concentration.
• Dose individualization is required for the drugs:
­ With narrow therapeutic index
­ Which follow non-linear pharmacokinetics
­ With saturable in any pharmacokinetic process (e.g. metabolism)
45

48. GM Hamad

­ Where a relationship exists between blood concentration and
desired clinical effect
­ Where a relationship exist between blood concentration and
adverse effects.
• Steps to dose individualization are as follows:
­ Dose adjustment
­ Pharmacokinetic studies
­ Empirical dose to patients
THERAPEUTIC DRUG MONITORING
• TDM is individualization of dose to maintain blood drug concentration
within a target therapeutic window. TDM is the measurement of drug
concentration with the aim to adjust a dose to deliver concentration
within safety window. It is also called therapeutic concentration
monitoring.
• Pharmacokinetic is an important component of TDM. TDM is required
for category of drugs under dose individualization.
PROCESS OF THERAPEUTIC DRUG MONITORING
I. DEVELOPMENT OF PLASMA PROFILE IN EACH PATIENT
• Administering a pre-determined dose of drug based on:
­ Manufacturer’s recommendation
­ Patient’s condition
­ Presence of other condition / disease state
­ Previous experience of professional with the drug.
• Collection of blood samples
• Determination of drug conc. in each sample
• Plasma profile and pharmacokinetic model development.
II. OBSERVATION OF CLINICAL EFFECTS OF DRUGS IN PATIENTS
• Incorrect assay methodology
• Problem with patient compliance
III. DEVELOPMENT OF DOSAGE REGIMEN
Following are the methods to design dosage regimen:
• EMPERICAL DOSAGE REGIMEN
­ It is designed by physician based on empirical clinical data,
personal experience and clinical observation.
• INDIVIDUAL DOSAGE REGIMEN
46

49. GM Hamad

­ It is based on the pharmacokinetics of drugs in the individual
patient.
­ Suitable for hospitalized patients.
• DOSAGE REGIMEN BASED ON POPULATION AVERAGES
­ It is based on one of two models
▪ FIXED MODEL
• Population average pharmacokinetic parameters are
used directly to calculate dosage regimens.
▪ ADAPTIVE MODEL
• Based on both population average pharmacokinetic
parameters of the drug as well as patient variable
such as weight.
DOSE ADJUSTMENT IN INFANTS & CHILDREN
• In children pharmacokinetics and pharmacodynamics of most of the
drugs are unknown. There is varied body composition in different age
groups. Different age groups require different doses.
• There is different liver maturity – conjugative enzymes are absent.
• According to FDA guidelines for industry 2000:
Parameter Age
Newborn infant Birth to 28 days
Infant 28 days to 23 months
Young child 2 to 5 years
Older child 6 to 11 years
Adolescent 12 to 18 years
Adult Above 18 years

• Infants have five immature pharmacokinetics:
1. Drug absorption
2. Renal excretion
3. Hepatic metabolism
4. Protein binding of drugs
5. Blood brain barrier
• Kidney function is 30 – 50% lesser than adult based on activity per unit
body weight. Reduced protein binding with albumin. Doses are required
to be adjusted based on half-life by taking into consideration of age and
body surface area.
47

50. GM Hamad

DOSE ADJUSTMENT IN ELDERY PATIENTS
In elderly or Geriatrics (age >65) patients there is:
CHANGES IN DRUG ABSORPTION
• Decline in splanchnic blood
flow
• Altered GI motility
• Increase in gastric pH
• Alteration in gastrointestinal
absorptive surfaces
CHANGES IN DRUG DISTRIBUTION
• Decrease albumin
• Decrease protein binding
• Decreased body fat
• Decreased volume of
distribution
• Decreased muscle mass
CHANGES IN METABOLISIM AND EXCRETION
• Decreased glomerular filtration
• Decreased renal plasma flow
• Decreased cardiac output
• Decreased breathing capacity.
All of the above changes leads to age dependent verified drug concentration
which may lead to therapeutic failure or adverse effect or toxicity.
DOSE ADJUSTMENT IN OBESE PATIENTS
• In obesity, following are the changes:
­ Body weight exceeds ideal body weight by 20%
­ Body mass index (BMI) > 30 – 39.9
­ Greater accumulation of fat tissues compared to muscle tissue
causing lower proportion of total body water.
• These leads to changes in Vd – Vd is increased.
• Altered pharmacokinetic due to distributional changes because of drug
partitioning of drug from aqueous to fat environment. Also, changes in
the liver and cardiovascular conditions. Thus, accumulation occurs at
normal dose.
BMI=[
Weight (lb)
Height (inch)
2
]X 703

BMI=[
Weight (kg)
Height (cm)
2
]X 10,000

48

51. GM Hamad

LEAN BODY WEIGHT
LBW (males) = 50kg + 2.3kg for each inch over 5ft.
LBW (females) = 45.5kg + 2.3kg for each inch over 5ft.
Terms BMI Values
Underweight < 18.5
Normal 18.5 – 24.9
Overweight 25 – 29.9
Obese 30 – 39.9
Extreme obesity > 40

DOSE ADJUSTMENT IN PREGNANCY
• Little information is available in literature.
• Plasma concentration of certain drugs may reduce during pregnancy.
• Example: phenytoin and phenobarbitone.
DOSE ADJUSTMENT IN DISEASE STATES
• Diseases alter the drug concentrations due to change in pharmacokinetic
processes.
­ In renal disease, drug clearance is reduced and the half-life is
prolonged.
­ Liver diseases impairs clearance of drugs which depends upon
bioconversion to more water-soluble compounds.
­ CHF can cause elevated drug levels for drug dependent on hepatic
metabolism for clearance.
49

52. GM Hamad

BIOAVAILABILITY AND BIOEQUIVALENCE
INTRODUCTION
• Bioavailability is the total amount of an intact drug available
systematically after administration of the drug. It is the relative amount
of an administered drug reaching to general circulation and the rate at
which it occurs.
PURPOSE OF BIOAVAILABILITY STUDIES
• Bioavailability studies are performed for both approved active drug
ingredients and therapeutic moieties not yet approved for marketing by
the FDA.
• New formulations of active drug ingredients must be approved by the
FDA before marketing.
• In approving a drug product for marketing, the FDA ensures that the
drug product is safe and effective for its labeled indications for use.
• Moreover, the drug product must meet all applicable standards of
identity, strength, quality, and purity.
• To ensure that these standards are met, the FDA requires bioavailability
/pharmacokinetic studies and, where necessary, bioequivalence studies
for all drug products.
• Bioavailability may be considered as one aspect of drug product quality
that links in-vivo performance of the drug product used in clinical trials
to studies demonstrating evidence of safety and efficacy.
• For un-marketed drugs that do not have full NDA approval by the FDA,
in-vitro and/or in-vivo bioequivalence studies must be performed on the
drug formulation proposed for marketing as a generic drug product.
• Furthermore, the essential pharmacokinetics of the active drug
ingredient or therapeutic moiety must be characterized.
• Essential pharmacokinetic parameters, including the rate and extent of
systemic absorption, elimination half-life, and rates of excretion and
metabolism, should be established after single- and multiple-dose
administration.
• Data from these in-vivo bioavailability studies are important to establish
recommended dosage regimens and to support drug labeling.
50

53. GM Hamad

• In-vivo bioavailability studies are also performed for new formulations of
active drug ingredients or therapeutic moieties that have full NDA
approval and are approved for marketing.
• The purpose of these studies is to determine the bioavailability and to
characterize the pharmacokinetics of the new formulation, new dosage
form, or new salt or ester relative to a reference formulation.
• Clinical studies are useful in determining the safety and efficacy of drug
products.
• Bioavailability studies are used to define the effect of changes in the
physicochemical properties of the drug substance and the effect of the
drug product (dosage form) on the pharmacokinetics of the drug.
• Bioequivalence studies are used to compare the bioavailability of the
same drug (same salt or ester) from various drug products.
• Bioavailability and bioequivalence can also be considered as
performance measures of the drug product in-vivo.
• If the drug products are bioequivalent and therapeutically equivalent,
then the clinical efficacy and the safety profile of these drug products
are assumed to be similar and may be substituted for each other.
TYPES OF BIOAVAILABILIY
1. RELATIVE BIOAVAILABILITY
• Relative (apparent) bioavailability is the extent of absorption (as
measured by AUC) of a drug as compared to a recognized standard or
reference product, both of which given through oral or other route but
not through I/V route. A drug the availability of which is compared is
called the test formulation. The reference or standard drug is usually
with known extent of bioavailability. The relative bioavailability of a drug
can be estimated by blood data or by the urine data.
• Sometimes the relative bioavailability can be used to compare different
conditions as well such as fasting, exercise, etc.
RELATIVE BIOAVAILABILITY BY BLOOD DATA
• In blood data, relative bioavailability is calculated by the comparing AUC
of test and reference drugs. When the test and the reference drug
products given at the same dosage level, the relative bioavailability can
be obtained for the formula:
51

54. GM Hamad

??????&#3627408466;??????&#3627408462;&#3627408481;??????&#3627408483;&#3627408466; ??????&#3627408483;&#3627408462;????????????&#3627408462;&#3627408463;??????????????????&#3627408481;??????=
[??????&#3627408456;&#3627408438;]
??????&#3627408466;&#3627408480;&#3627408481;
[??????&#3627408456;&#3627408438;]
??????&#3627408466;&#3627408467;&#3627408466;&#3627408479;&#3627408466;&#3627408475;??????&#3627408466;

• If the above equation is multiplied by 100, it will give percent availability.
• When different doses are administered, a correction for the size of the
dose is made as given in the formula:
??????&#3627408466;??????&#3627408462;&#3627408481;??????&#3627408483;&#3627408466; ??????&#3627408483;&#3627408462;????????????&#3627408462;&#3627408463;??????????????????&#3627408481;??????=
[??????&#3627408456;&#3627408438;]
??????&#3627408466;&#3627408480;&#3627408481; / &#3627408439;&#3627408476;&#3627408480;&#3627408466;
??????&#3627408466;&#3627408480;&#3627408481;
[??????&#3627408456;&#3627408438;]
??????&#3627408466;&#3627408467;&#3627408466;&#3627408479;&#3627408466;&#3627408475;??????&#3627408466; / &#3627408439;&#3627408476;&#3627408480;&#3627408466;
??????&#3627408466;&#3627408467;&#3627408466;&#3627408479;&#3627408466;&#3627408475;??????&#3627408466;

RELATIVE BIOAVAILABILITY BY URINE DATA
• Urinary drug excretion data may also be used to measure relative
availability whereby the total amount of drug excreted in urine (&#3627408439;
&#3627408482;
∞
) after
administration of test (T) and reference (R) drugs by using the following
formula:
??????&#3627408466;??????&#3627408462;&#3627408481;??????&#3627408483;&#3627408466; ??????&#3627408483;&#3627408462;????????????&#3627408462;&#3627408463;??????????????????&#3627408481;??????=
[&#3627408439;
&#3627408482;
∞
]
??????
[&#3627408439;
&#3627408482;
∞
]
??????

• The percent relative availability using urinary excretion data can be
obtained by multiplying the above equation with 100.
2. ABSOLUTE AVAILABILITY
• Absolute bioavailability is the extent of absorption of a drug after
extravascular administration (e.g., oral, rectal, transdermal,
subcutaneous) as compared to the extent of the drug availability after
administered through I/V route. Thus, absolute bioavailability of drug is
measured by comparing the respective AUCs after extravascular and IV
administration of a drug.
• The absolute bioavailability of a drug using plasma data/ blood data can
be measured as follows:
??????&#3627408463;&#3627408480;&#3627408476;??????&#3627408482;&#3627408481;&#3627408466; ??????&#3627408483;&#3627408462;????????????&#3627408462;&#3627408463;??????????????????&#3627408481;??????=
[??????&#3627408456;&#3627408438;]
&#3627408451;&#3627408450; / &#3627408439;&#3627408476;&#3627408480;&#3627408466;
&#3627408451;&#3627408450;
[??????&#3627408456;&#3627408438;]
???????????? / &#3627408439;&#3627408476;&#3627408480;&#3627408466;
????????????

• Absolute availability using urinary drug excretion data can be
determined by using the following formula:
??????&#3627408463;&#3627408480;&#3627408476;??????&#3627408482;&#3627408481;&#3627408466; ??????&#3627408483;&#3627408462;????????????&#3627408462;&#3627408463;??????????????????&#3627408481;??????=
[&#3627408439;
&#3627408482;
∞
]
&#3627408451;&#3627408450; / &#3627408439;&#3627408476;&#3627408480;&#3627408466;
&#3627408451;&#3627408450;
[&#3627408439;
&#3627408482;
∞
]
???????????? / &#3627408439;&#3627408476;&#3627408480;&#3627408466;
????????????

52

55. GM Hamad

• The absolute bioavailability is also equal to fraction of the dose that is
bioavailable, abbreviated as F. When absolute availability is expressed in
percent, then F = 1 or 100%.
• For drugs given intravascularly or the one which is chemically stable in
gastrointestinal tract, such as by IV bolus injection, F = 1 because all of
the drug is completely absorbed.
• For a drug given through extravascular route or the drugs which undergo
first pass effect, the F is always ≤ 1.
METHODS FOR ASSESSING BIOAVAILABILITY
• Assessment of bioavailability is the estimation of the absorption of drug
after its administration.
• Methods of bioavailability assessment depends on the assumption that
the measurement of the concentration of the drug in a suitable body
fluid (usually blood, plasma, urine or occasionally saliva) over period of
time after administration can be correlated with the clinical efficacy the
drug in treating a given disease condition.
• Term bioavailability encompasses the extent of drug absorption and the
rate of drug absorption thus, the bioavailability of drug from a drug
demands the assessment of the extent and rate of drug absorption of
the drug.
SIGNIFICANCE OF MEASURING BIOAVAILABILITY
• Bioavailability assessment is required for the following situations:
­ Characterization of the pharmacokinetics of new drug molecules
­ All new drug formulation
­ New dosage form of a drug
­ New dosage strength or dosage regimen
­ New salt or ester of a drug
­ New indication of a drug
­ Administration of drug in special population, e.g., pediatrics
­ Change in manufacturing process
­ To determine the safety and efficacy of the drug products
­ A legal requirement from the drug authorities.
1. BIOAVAILABILITY ASSESSMENT BASED ON BLOOD DATA
• Blood data refers to concentration-time data obtained from blood,
plasma or serum after administration of drug. The parameters studied in
this method are peak plasma (or serum, blood) concentration (Cmax),
53

56. GM Hamad

time to reach peak plasma concentration (tmax) and area under the
plasma level time curve (AUC).
PEAK PLASMA CONCENTRATION (Cmax)
• The peak plasma concentration, Cmax represents the maximum
concentration of drug in blood plasma following oral administration of a
drug. This is usually related to dose and rate constant for absorption and
elimination of the drug.
• Usually, a relationship exists between pharmacodynamic effect and the
plasma drug concentration.
• Cmax reflects that a drug is sufficiently absorbed systemically to provide a
therapeutic response.
• Cmax provides warning of possible toxic levels of drugs.
UNITS
• The Cmax is measured in terms of concentration, i.e., µg/ml, ng/ml, etc.
• Although not a unit for rate, Cmax is often used in bioequivalence studies
as a surrogate measure for the rate of drug bioavailability.
ESTIMATION OF Cmax
• Graphical method
­ Cmax can be measure directly observing on the plasma level time
curve.
• Direct method
­ Direct method for estimation of Cmax involves applying formula:
&#3627408438;
&#3627408474;????????????=
??????&#3627408439;
0??????&#3627408462;
&#3627408457;&#3627408465;(??????&#3627408462;−??????)
(&#3627408466;
−??????&#3627408481;
??????????????????−&#3627408466;
−????????????&#3627408481;
??????????????????)
­ Where, F = fraction of dose absorbed, D0 = dose of drug, ka =
absorption rate constant, k = overall rate constant and tmax is the
time for peak plasma concentration.
TIME FOR MAXIMUM CONCENTRATION (Tmax)
• Time to reach maximum concentration (Cmax), tmax is time required to
reach maximum drug concentration after drug administration. At this
point, the absorption is maximum, and the rate of drug absorption
equals the rate of drug elimination. However, drug absorption continues
but at a slower rate after this point.
54

57. GM Hamad

• The tmax is a measure of the rate of drug absorption. A lower tmax value
represents a faster absorption of drug. Usually, two drugs with same
rate (tmax) and extent of absorption (AUC, Cmax), are considered
equivalent.
• In some special cases, where rate of absorption is different, but the
extent is same, the products are considered equivalent.
UNITS OF Tmax
• The tmax is represented in the units of time, i.e., hours, or minutes.
ESTIMATION OF Tmax
• Graphical method
­ Time to reach peak plasma concentration is estimated by direct
reading from the plasma concentration versus time profile.
• Direct method
­ Direct method for estimation of Tmax involves applying formula:
&#3627408481;
max=
2.303
??????&#3627408462;−??????
log
??????&#3627408462;
??????

­ Where ka is the absorption rate constant, k is the overall rate
constant.
AREA UNDER THE CURVE (AUC)
• Area under the curve (AUC0-∞) is the area under the drug plasma level-
time curve from t = 0 to t = ∞ and reflects the total amount of active
drug which reaches the systemic circulation following administration of
drug.
• The drug plasma level-time curve provides the quantitative
measurement for bioavailability. The exact shape of plasma
concentration profile depends on the relative rates of absorption and
elimination and routes of drug administration. Intravenous and
sometimes, intramuscular routes yields an early peak due to the fast or
almost instantaneous absorption. Whereas oral, subcutaneous, rectal,
and other routes demonstrate delayed peaks due to comparatively
slower rates of absorption.
• The AUC is independent of the route of administration and processes of
drug elimination provided the elimination processes do not change. For
many drugs, AUC is directly proportion to their dose. In some cases, the
55

58. GM Hamad

AUC is not directly proportional to the administered dose for all dosage
levels. This is due to the reason that one of the pathways for drug
elimination may become saturated. In such case, the AUC increases
disproportionally to the increase in dose. The drug pharmacokinetic
profile is said to be dose dependent. For the drugs having the dose-
dependent kinetics, the bioavailability assessment is difficult.
• AUC reflects the following phases:
­ Absorption phase in which the absorption is greater than
elimination.
­ Distribution phase which is characterized by absorption ≈
elimination.
­ Elimination phase in which initially absorption rate < elimination
rate, then absorption = 0 and at the end, elimination = 0.
UNITS OF AUC
• AUC is measured in terms of concentration × time, thus, its units are
µg.hr/ml, ng.hr/ml or mg.hr/l
CALCULATION OF AUC0-∞
• Trapezoidal Rule
­ Trapezoidal rule involves the breaking up of the plasma
concentration vs time profile into various trapezoids (small
segments).
­ Calculating the areas of the individual trapezoids and then added
up these areas gives the AUC.
??????&#3627408456;&#3627408438;
0−&#3627408481;&#3627408475; =
&#3627408438;
&#3627408475;−1+ &#3627408438;
1
2
(&#3627408481;
&#3627408475;− &#3627408481;
&#3627408475;−1)
­ Where Cn = Concentration of drug under consideration, Cn-1 =
concentration of drug in plasma prior to that concentration which
is under consideration. While tn and tn-1 are the time of absorption
corresponding to Cn and Cn-1, respectively.
• Direct method
­ Under direct method for calculation of AUC0- is use of certain
formulae which are as follows:
[??????&#3627408456;&#3627408438;]
0
∞
=∫&#3627408438;&#3627408477;&#3627408465;&#3627408481;
∞
0

56

59. GM Hamad

[??????&#3627408456;&#3627408438;]
0
∞
=
??????&#3627408439;
0
&#3627408438;??????&#3627408466;&#3627408462;&#3627408479;&#3627408462;&#3627408475;&#3627408464;&#3627408466;
=
??????&#3627408439;
0
??????&#3627408457;&#3627408465;

­ Where F = dose absorbed; D0 = dose; k = elimination rate
constant; and Vd is the volume of distribution.
2. BIOAVAILABILITY ASSESSMENT BASED ON URINE DATA
• Urinary drug excretion data is an indirect method for estimation of the
bioavailability.
• Bioavailability assessment by urinary data is based upon the assumption
that the appearance of drug or its metabolite(s) in urine is the function
of the rate and extent of absorption. This assumption, however, is valid
only when:
­ Drug and/or its metabolites is extensively excreted in urine.
­ Rate of urinary excretion is proportional to the concentration of
intact drug in blood plasma. This proportionality does not hold if:
▪ The drug and/or its metabolites is excreted by an active
transport process into distal kidney tubules.
▪ The intact drug and/or metabolites is weakly acidic or
weakly basic (i.e., there rate of excretion depends upon
urinary pH.
▪ The excretion rate depends on rate of urine flow.
TOTAL AMOUNT OF DRUG EXCRETED THROUGH URINE (??????
??????
∞
)
• Total amount of drug excreted through urine (&#3627408439;
&#3627408482;
∞
) is also referred to as
the drug ultimately excreted and abbreviated as
??????
&#3627408482;
∞
. The cumulative amount of drug excreted in
the urine is proportional to the total amount of
drug absorbed. A drug absorbed more; more
drug appears in urine.
• A cumulative urinary excretion curve is obtained
by collecting urine samples at known intervals
of time following administration. Enough time is
required to collect the entire absorbed drug in
urine, which is 5 × t½.
• A cumulative urinary excretion curve and the corresponding
plasma concentration-time curve obtained after the
administration of a single dose of a drug by oral route is given in Figure
1-2.
Figure 1
57

60. GM Hamad

• This figure reflects the corresponding plasma
level time plot and the cumulative urinary
drug excretion. The initial A-B reflects the
absorption and the slope of this segment of
urinary excretion curves is related to the rate
of absorption of the drug into blood.
• The total amount of intact drug is excreted in
urine at point C corresponds to the time at
which the plasma concentration of intact drug
is zero and essentially all the drug has been eliminated from
the body. At this point D
u
∞
is obtained.
URINARY EXCRETION RATE (dDu/dt)
• The elimination follows the first order rate
process. The rate of drug excretion depends on
the first order elimination rate constant and the
drug concentration in plasma.
• The plots of plasma level vs time and the rate of
urinary excretion rate vs time are similar as
depicted in figure which indicates the
corresponding plots relating plasma level-time
curve and the rate of urinary drug excretion.
• The maximum rate of drug excretion would be at point B. While
minimum rate of excretion would be at points A and C.
TIME FOR COMPLETE EXCRETION (t
∞
)
• In the Figure 3-4, the slope of the curve
segment AB is related to the rate of drug
absorption whereas point C is related to the
total time required after drug administration
for the drug to be absorbed and completely
excreted (t = ∞).
• The t
∞
is a useful parameter in bioequivalence
studies comparing drug products.
Figure 2
Figure 3
Figure 4
Figure 4
58

61. GM Hamad

3. BIOAVAILABILITY ASSESSMENT BASED ON THE ACUTE
PHARMACODYNAMIC RESPONSE
• In some cases, the quantitative measurement of a drug in plasma or
urine lacks an assay with sufficient accuracy and/or reproducibility.
• For locally acting, non-systemically absorbed drug products, such as
topical corticosteroids, plasma drug concentrations may not reflect the
bioavailability of the drug at the site of action.
• An acute pharmacodynamic effect, such as an effect on forced
expiratory volume, FEV1 (inhaled bronchodilators) or skin blanching
(topical corticosteroids) can be used as an index of drug bioavailability.
• In this case, the acute pharmacodynamic effect is measured over a
period of time after administration of the drug product.
• Measurements of the pharmacodynamic effect should be made with
sufficient frequency to permit a reasonable estimate for a time period at
least three times the half-life of the drug.
• This approach may be particularly applicable to dosage forms that are
not intended to deliver the active moiety to the bloodstream for
systemic distribution.
• The use of an acute pharmacodynamic effect to determine
bioavailability generally requires demonstration of a dose–response
curve.
• Bioavailability is determined by characterization of the dose–response
curve.
• For bioequivalence determination, pharmacodynamic parameters
including the total area under the acute pharmacodynamic effect–time
curve, peak pharmacodynamic effect, and time for peak
pharmacodynamic effect are obtained from the pharmacodynamic
effect–time curve.
• The onset time and duration of the pharmacokinetic effect may also be
included in the analysis of the data.
• The use of pharmacodynamic endpoints for the determination of
bioavailability and bioequivalence is much more variable than the
measurement of plasma or urine drug concentrations.
4. BIOAVAILABILITY ASSESSMENT BASED ON RADIOMETRIC METHOD
• The radiometric method is based on the radioactivity measurement and
involves the administration of radiolabeled drug moiety and determining
the total radioactivity in plasma or urine.
59

62. GM Hamad

• The estimation of relative availability is based upon the area under the
concentration of total radioactivity, apparent drug (or metabolites) in
plasma versus time curve.
• Sometimes, the estimation under this method is based on the
cumulative urinary excretion of total radioactivity.
• The method can also reflect the localization of the drug sat several tissue
level and pattern of the drug distribution.
5. BIOAVAILABILITY ASSESSMENT BASED ON CLINICAL OBSERVATION
• Well-controlled clinical trials in humans establish the safety and
effectiveness of drug products and may be used to determine
bioavailability.
• It is the least accurate, least sensitive, and least reproducible approach
• The FDA considers this approach only when analytical methods and
pharmacodynamic methods are not available.
• Comparative clinical studies have been used to establish bioequivalence
for topical antifungal drug products (e.g. ketoconazole) and for topical
acne preparations.
• For dosage forms intended to deliver the active moiety to the
bloodstream for systemic distribution, this approach may be considered
acceptable only when analytical methods cannot be developed to permit
use of one of the other approaches.
6. BIOAVAILABILITY ASSESSMENT BASED ON IN-VITRO STUDIES
• Drug dissolution studies may under certain conditions give an indication
of drug bioavailability.
• Ideally, the in-vitro drug dissolution rate should correlate with in-vivo
drug bioavailability.
• Dissolution studies are often performed on several test formulations of
the same drug.
• The test formulation that demonstrates the most rapid rate of drug
dissolution in vitro will generally have the most rapid rate of drug
bioavailability in vivo.
BIOEQUIVALENCE STUDIES
• Difference in preclinical response or adverse event may be due to:
­ Difference in PK and/or PD behavior of drug.
­ Difference in bioavailability of drug from drug product.
60

63. GM Hamad

• Bioequivalent drug products that have the same systemic drug
bioavailability will have the same predictable drug response.
• Differences in pharmacodynamics may be due to differences in receptor
sensitivity to the drug.
• Various factors affecting pharmacodynamic drug behavior may include
age, drug tolerance, drug interactions, and unknown pathophysiologic
factors.
• The bioavailability of a drug may be more reproducible among fasted
individuals in controlled studies who take the drug on an empty
stomach.
• When the drug is used on a daily basis, however, the nature of an
individual's diet and lifestyle may affect the plasma drug levels because
of variable absorption in the presence of food or even a change in the
metabolic clearance of the drug. (e.g. theophylline).
BASES FOR DETERMINING BIOEQUIVALENCE
• Bioequivalence is established if the in-vivo bioavailability of a test drug
product (usually the generic product) does not differ significantly (i.e.,
statistically insignificant) in the product's rate and extent of drug
absorption, as determined by comparison of measured parameters (e.g.
concentration of the active drug ingredient in the blood, urinary
excretion rates, or pharmacodynamic effects), from that of the reference
listed drug (usually the brand-name product) when administered at the
same molar dose of the active moiety under similar experimental
conditions, either single dose or multiple dose.
• In few cases, a drug product that differs from the reference listed drug in
its rate of absorption, but not in its extent of absorption, may be
considered bioequivalent if the difference in the rate of absorption is
intentional and appropriately reflected in the labeling and/or the rate of
absorption is not detrimental to the safety and effectiveness of the drug
product.
DRUG PRODUCTS WITH POSSIBLE BIOAVAILABILITY AND
BIOEQUIVALENCE PROBLEMS
• Lack of bioavailability or bioequivalence may be suspected when
evidence from well-controlled clinical trials or controlled observations in
patients of various marketed drug products do not give comparable
therapeutic effects. These drug products need to be evaluated either in-
61

64. GM Hamad

vitro (e.g., drug dissolution/release test) or in-vivo (e.g., bioequivalence
study) to determine if the drug product has a bioavailability problem.
• In addition, during the development of a drug product, certain
biopharmaceutical properties of the active drug substance or the
formulation of the drug product may indicate that the drug may have
variable bioavailability and/or a bioequivalence problem.
• Some of these biopharmaceutical properties include:
­ The active drug ingredient has low solubility in water (e.g., less
than 5 mg/mL).
­ The dissolution rate of one or more such products is slow (e.g.,
less than 50% in 30 minutes when tested with a general method
specified by the FDA).
­ The particle size and/or surface area of the active drug ingredient
is critical in determining its bioavailability.
­ Certain structural forms of the active drug ingredient (e.g.
polymorphic forms, solvates, complexes, and crystal
modifications) dissolve poorly, thus affecting absorption.
­ Drug products that have a high ratio of excipients to active
ingredients (e.g., greater than 5:1).
­ Specific inactive ingredients (e.g., hydrophilic or hydrophobic
excipients and lubricants) either may be required for absorption of
the active drug ingredient or therapeutic moiety or may interfere
with such absorption.
­ The active drug ingredient, therapeutic moiety, or its precursor is
absorbed in large part in a particular segment of the GI tract or is
absorbed from a localized site.
­ The degree of absorption of the active drug ingredient,
therapeutic moiety, or its precursor is poor (e.g., less than 50%,
ordinarily in comparison to an intravenous dose), even when it is
administered in pure form (e.g., in solution).
­ There is rapid metabolism of the therapeutic moiety in the
intestinal wall or liver during the absorption process (first-order
metabolism), so that the rate of absorption is unusually important
in the therapeutic effect and/or toxicity of the drug product.
­ The therapeutic moiety is rapidly metabolized or excreted, so that
rapid dissolution and absorption are required for effectiveness.
62

65. GM Hamad

­ The active drug ingredient or therapeutic moiety is unstable in
specific portions of the GI tract and requires special coatings or
formulations (e.g., buffers, enteric coatings, and film coatings) to
ensure adequate absorption.
­ The drug product is subject to dose-dependent kinetics in or near
the therapeutic range, and the rate and extent of absorption are
important to bioequivalence.
DESIGN AND EVALUATION OF BIOEQUIVALENCE STUDIES
• Bioequivalence studies are performed to compare the bioavailability of
the generic drug product to the brand-name product.
• Statistical techniques should be of sufficient sensitivity to detect
differences in rate and extent of absorption that are not attributable to
subject variability.
• Once bioequivalence is established, it is likely that both the generic and
brand-name dosage forms will produce the same therapeutic effect.
STUDY CONSIDERATIONS
• The basic design for a bioequivalence study is determined by:
­ The scientific questions to be answered.
­ The nature of the reference material and the dosage form to be
tested.
­ The availability of analytical methods.
­ Benefit–risk and ethical considerations with regard to testing in
humans.
• For some generic drugs, the FDA offers general guidelines for conducting
these studies. For example, Statistical Procedures for Bioequivalence
Studies Using a Standard Two-Treatment Crossover Design is available
from the FDA; the publication addresses three specific aspects,
including:
­ Logarithmic transformation of pharmacokinetic data
­ Sequence effect
­ Outlier consideration.
• For bioequivalence studies, the test and reference drug formulations
must contain the pharmaceutical equivalent drug in the same dose
strength, in similar dosage forms (e.g. immediate release or controlled
release) and be given by the same route of administration. Both a single-
dose and/or a multiple-dose (steady-state) study may be required.
63

66. GM Hamad

Before beginning the study, the Institutional Review Board (IRB) of the
clinical facility in which the study is to be performed must approve the
study. The IRB is composed of both professional and lay persons with
diverse backgrounds, who have clinical experience and expertise as well
as sensitivity to ethical issues and community attitudes. The IRB is
responsible for safeguarding the rights and welfare of human subjects.
• The basic guiding principle in performing studies is do not do
unnecessary human research. Generally, the study is performed in
normal, healthy male and female volunteers who have given informed
consent to be in the study. Critically ill patients are not included in an in-
vivo bioavailability study unless the attending physician determines that
there is a potential benefit to the patient. The number of subjects in the
study will depend on the expected inter-subject and intrasubject
variability. Patient selection is made according to certain established
criteria for inclusion into, or exclusion from, the study. For example, the
study might exclude any volunteers who have known allergies to the
drug, are overweight, or have taken any medication within a specified
period (often 1 week) prior to the study.
• Smokers are often included in these studies.
• The subjects are generally fasted for 10 to 12 hours (overnight) prior to
drug administration and may continue to fast for a 2- to 4-hour period
after dosing.
ANALYTICAL METHODS
• The analytical method used in an in-vivo bioavailability or
bioequivalence study to measure the concentration of the active drug
ingredient or therapeutic moiety, or its active metabolite(s), in body
fluids or excretory products, or the method used to measure an acute
pharmacological effect, must be demonstrated to be accurate and of
sufficient sensitivity to measure, with appropriate precision, the actual
concentration of the active drug ingredient or therapeutic moiety, or its
active metabolite(s), achieved in the body.
• For bioavailability studies, both the parent drug and its major active
metabolites are generally measured.
• For bioequivalence studies, the parent drug is measured.
• The active metabolite might be measured for some very high hepatic
clearance (first-pass metabolism) drugs when the parent drug
concentrations are too low to be reliable.
64

67. GM Hamad

REFERENCE LISTED DRUG (RLD)
• For bioequivalence studies, one formulation of the drug is chosen as a
reference standard against which all other formulations of the drug are
compared.
• The reference drug product should be administered by the same route
as the comparison formulations unless an alternative route or additional
route is needed to answer specific pharmacokinetic questions. For
example, if an active drug is poorly bioavailable after oral
administration, the drug may be compared to an oral solution or an
intravenous injection.
• For bioequivalence studies on a proposed generic drug product the
reference standard is the reference listed drug (RLD), which is listed in
Approved Drug Products with Therapeutic Equivalence Evaluations—the
Orange Book and the proposed generic drug product is often referred to
as the "Test" drug product
• The RLD is generally a formulation currently marketed with a fully
approved NDA for which there are valid scientific safety and efficacy
data.
• The RLD is usually the innovator's or original manufacturer's brand-name
product and is administered according to the dosage recommendations
in the labeling.
• Before beginning an in-vivo bioequivalence study, the total content of
the active drug substance in the test product (generally the generic
product) must be within 5% of that of the reference product.
• Moreover, in-vitro comparative dissolution or drug-release studies
under various specified conditions are usually performed for both test
and reference products before performing the in-vivo bioequivalence
study.
EXTENDED-RELEASE FORMULATIONS
• The purpose of an in-vivo bioavailability study involving an extended-
release drug product is to determine if:
­ The drug product meets the controlled-release claims made for it.
­ The bioavailability profile established for the drug product rules
out the occurrence of any dose dumping.
­ The drug product's steady-state performance is equivalent to that
of a currently marketed non-extended-release formulation.
65

68. GM Hamad

­ The drug product's formulation provides consistent
pharmacokinetic performance between individual dosage units.
• A comparison bioavailability study is used for the development of a new
extended release drug product in which the reference drug product may
be either a solution or suspension of the active ingredient or a currently
marketed non-controlled release drug product such as a tablet or
capsule. For example, the bioavailability of an immediate-release drug
product given at a dose of 25 mg every 8 hours is compared to an
extended-release product containing 75 mg of the same drug given once
daily.
• For a bioequivalence study of a new generic extended release drug
product, the reference drug product is the currently marketed extended
release drug product listed as the RLD in the Orange Book and is
administered according to the dosage recommendations in the
approved labeling.
COMBINATION DRUG PRODUCTS
• Generally, the purpose of an in-vivo bioavailability study involving a
combination drug product containing more than one active drug
substance is to determine if the rate and extent of absorption of each
active drug ingredient or therapeutic moiety in the combination drug
product is equivalent to the rate and extent of absorption of each active
drug ingredient or therapeutic moiety administered concurrently in
separate single-ingredient preparations.
• The reference material in such a bioavailability study should be two or
more currently marketed, single-ingredient drug products, each of which
contains one of the active drug ingredients in the combination drug
product. The FDA may, for valid scientific reasons, specify that the
reference material be a combination drug product that is the subject of
an approved NDA.
STUDY DESIGNS
• For many drug products, the FDA, Division of Bioequivalence, Office of
Generic Drugs, provides guidance for the performance of in-vitro
dissolution and in-vivo bioequivalence studies.
• Similar guidelines appear in the United States Pharmacopeia NF.
66

69. GM Hamad

• Currently, three different studies may be required for solid oral dosage
forms, including (1) a fasting study, (2) a food intervention study, and/or
(3) a multiple-dose (steady-state) study.
• Other study designs have been proposed by the FDA. For example, the
FDA published two draft guidelines in October and December 1997 to
consider the performance of individual bioequivalence studies using a
replicate design and a two-way crossover food intervention study.
• Proper study design and statistical evolution are important
considerations for the determination of bioequivalence.
1. FASTING STUDY
• Bioequivalence studies are usually evaluated by a single-dose, two-
period, two-treatment, two-sequence, open-label, randomized
crossover design comparing equal doses of the test and reference
products in fasted, adult, healthy subjects.
• This study is required for all immediate-release and modified-release
oral dosage forms.
• Both male and female subjects may be used in the study.
• Blood sampling is performed just before (zero time) the dose and at
appropriate intervals after the dose to obtain an adequate description of
the plasma drug concentration–time profile.
• The subjects should be in the fasting state (overnight fast of at least 10
hours) before drug administration and should continue to fast for up to
4 hours after dosing.
• No other medication is normally given to the subject for at least 1 week
prior to the study.
• In some cases, a parallel design may be more appropriate for certain
drug products, containing a drug with a very long elimination half-life.
• A replicate design may be used for a drug product containing a drug that
has high intrasubject variability.
2. FOOD INTERVENTION STUDY
• Co-administration of food with an oral drug product may affect the
bioavailability of the drug.
• Food intervention or food effect studies are generally conducted using
meal conditions that are expected to provide the greatest effects on GI
physiology so that systemic drug availability is maximally affected.
• The test meal is a high-fat (approximately 50% of total caloric content of
the meal) and high-calorie (approximately 800–1000 calories) meal.
67

70. GM Hamad

• A typical test meal is two eggs fried in butter, two strips of bacon, two
slices of toast with butter, 4 ounces of brown potatoes, and 8 ounces of
milk.
• This test meal derives approximately 150, 250, and 500–600 calories
from protein, carbohydrate, and fat, respectively
• For bioequivalence studies, drug bioavailability from both the test and
reference products should be affected similarly by food.
• The study design uses a single-dose, randomized, two-treatment, two-
period, crossover study comparing equal doses of the test and reference
products.
• Following an overnight fast of at least 10 hours, subjects are given the
recommended meal 30 minutes before dosing.
• The meal is consumed over 30 minutes, with administration of the drug
product immediately after the meal.
• The drug product is given with 240 mL (8 fluid ounces) of water.
• No food is allowed for at least 4 hours post dose.
• This study is required for all modified-release dosage forms and may be
required for immediate-release dosage forms if the bioavailability of the
active drug ingredient is known to be affected by food (e.g., ibuprofen,
naproxen).
• For certain extended-release capsules that contain coated beads, the
capsule contents are sprinkled over soft foods such as apple sauce,
which is taken by the fasted subject and the bioavailability of the drug is
then measured.
• Bioavailability studies might also examine the effects of other foods and
special vehicles such as apple juice.
3. STEADY STATE/ MULTIPLE DOSE STUDIES
• In a few cases, a multiple-dose, steady-state, randomized, two-
treatment, two-way crossover study comparing equal doses of the test
and reference products may be performed in adults, healthy subjects.
• For these studies, three consecutive trough concentrations (Cmin) on
three consecutive days should be determined to ascertain that the
subjects are at steady state.
• The last morning dose is given to the subject after an overnight fast, with
continual fasting for at least 2 hours following dose administration.
• Blood sampling is performed similarly to the single-dose study.
68

71. NON-COMPARTMENTAL
PHARMACOKINETICS
ISSUES OF COMPARTMENT MODELS
• No precise criteria for determination of number of compartments for
describing disposition.
• Number of compartments and parameters are highly sensitive to
sampling intervals.
• Drug disposition kinetics is needed to be described in detail.
• Non-compartmental model is an alternative approach.
NON-COMPARTMENTAL MODEL
• Does not require any specific compartmental model for body and can be
applied to any pharmacokinetic data.
• Estimates pharmacokinetic parameters without assuming or
understanding any structural or mechanistic properties of
pharmacokinetic behavior of a drug in body. Also estimates PK
parameters from concentration profiles without complicated, and
nonlinear regression processes required for compartmental models.
• Owing to this versatility, approach is a primary pharmacokinetic data
analysis method.
• Derivation of PK parameters is easy, because requires use of simple
algebraic equations.
• Approaches include statistical moment analysis, system analysis, or non-
compartmental recirculatory model.
• Statistical moment approach is more common.
COMPARISON B/W COMPARTMENT AND NON-COMPARTMENT
MODELS
Compartment Model Non-Compartment Model
Require elaborate assumptions to fit
data. E.g. Data follow first order
kinetics.
Do not require assumptions
69

72. Curve fitting of experimental data is
tedious method and require
computers.
Require simple algebraic equations.
No curve fitting and no computers.
Applicable to linear and nonlinear
pharmacokinetics.
Applicable to linear pharmacokinetics
Concentration time profile is
regarded as expressions of exponents
Concentration time profile is
regarded as statistical distribution.
Applicable to most of situations
Particularly useful for clinical PK,
bioavailability, and bioequivalence
studies

STATISTICAL MOMENT APPROACH
• Computation of pharmacokinetics in non-compartmental approach is
based on statistical moment theory, called the moments of a random
variable.
• Plasma concentration is considered as a random variable since it varies
with function of time, have some mean value, where some values would
be higher, while other lower than mean.
• Thus, plasma-concentration time data may be considered as statistical
distribution about a mean value.
• Distribution about a mean can be approximated by statistical moments
(zero and/or first ).
• Statistical moment theory is a mathematical description of a discrete
distribution of data.
• Borrowed from chemical engineering where it is utilized to describe flow
data to describe the kinetics of cholesterol.
STATISTICAL MOMENT THEORY
• Applicable to several disciplines. E.g. Physics as Centre of Mass (a zero
moment)
• Statistical moments calculated from concentration-time data represent
estimates of true probability density function, which describes
relationship between concentration and time.
• Statistical moment theory enables to study time related changes in
macroscopic event, an overall event brought about by constitutive
element involved.
• Example: A dose of tracer molecules may be injected in a tank to track
the transit time (residence time) of molecules in tank.
70

73. • Each tracer molecule is well mixed and distribute non-interactively and
randomly in tank.
• Macroscopic event is an example of drug in body is residence times
shared by groups of molecules.
• Constitutive elements are the molecules. Assumes that drug molecules
are eliminated.
• According to a kinetic function, f(t) = C0e
–kt

• A single molecule, from time t is administered into the body (t = 0) until
it is eventually eliminated (t = tel), is not predictable
­ Because, this individual molecule may be eliminated during first
minute or may reside in body for longer.
­ However, large number of molecules are considered collectively,
their behavior appears much more regular – a concept of mean
residence time
• Collective, or mean time of residence, of all the molecules in the dose, is
called the mean residence time (MRT).
DEFINING MOMENTS
a) μm or m
th
moment = ∫t
m
∞
0
f(t)dt
­ Where f(t) = probability density function, t is time, and m is m
th

moment.
b) When m=0, f(t) is C (i.e., concentration), then zero moment would be: μ0
or zero moment = ∫t
0
∞
0
f(t)dt [AUC]
0
∞
=∫C.
∞
0
dt
c) When m=1, f(t) is C (i.e., concentration), then first moment would be: μ1
or first moment = ∫t
1
∞
0
f(t)dt [AUMC]
0
∞
=∫t.C.
∞
0
dt
d) μ2 or 2
nd
moment ∫t
2
∞
0
f(t)dt defines the variance in distribution.
e) 3
rd
moment defines the skewness of distribution.
f) 4
th
moment defines the kurtosis of distribution.
MOMENTS
• Moment defines a kinetic parameter
­ Zero moment defines AUC
­ First moment defines AUMC
APPLICATIONS OF NON-COMPARTMENTAL APPROACH
• In pk, moment of probability density function or moments from
measured plasma concentration time data.
71

74. • Elements of distribution curve describe distribution of drug molecules
after administration and the residence time of drug molecules in body.
• Principle use of moment curve is calculation of:
­ AUC
­ AUMC
­ Mean residence time (MRT) of a drug in body
­ Mean absorption time
­ Bioavailability
­ Fraction of drug metabolized
­ Systemic clearance
­ Volume of distribution at steady state.
AUC AND AUMC AS MOMENT
• AUC is obtained from a plot of plasma concentration of drug vs time
from zero to infinity by trapezoidal method.
• AUMC is obtained from a plot of product of plasma drug concentration
and time (CXT) vs time from zero to infinity.
• For the last observed sample and infinity:
??????????????????
&#3627408481;
????????????&#3627408480;&#3627408481;−∞=
??????
????????????&#3627408480;&#3627408481;
λ
????????????????????????
&#3627408481;
????????????&#3627408480;&#3627408481;−∞=
??????
????????????&#3627408480;&#3627408481; ??????
????????????&#3627408480;&#3627408481;
λ
+
??????
????????????&#3627408480;&#3627408481;
λ
2

• Clast = last observed conc. at time tlast ,λ is slope of terminal phase of
plasma drug concentration time profile.
MEAN RESIDENCE TIME, MRT
• A dose represents number of drug molecules.
• Dose = 2 mg, MW = 400
• No of molecules = (2X10
-3
g/400) X (6.023 x 10
23
) = 3 X 10
18
Molecules.
• These molecules on administration, spend various amounts of time in
body – time spend by various molecule in body is measurable as
residence time in same way as any statistical
distribution.
• Assume that 20 molecules were placed in a tank.
At 3 min after placing, 5 molecules were
removed, at 10 min, 4 molecules, on 21 min 6
and on 30 min 5 molecules were removed. At 30 min, all of molecules
were removed, = elimination completed.
• MRT of the molecules in tank is simply sum of times that molecules
spend in tank divided by number of molecules placed in in Tank.
72

75. MRT
=
3+3+3+3+3+10+10+10+10+21+21+21+21+21+21+30+30+30+30+30
20

MRT=
(3 x 5)+(10 x 4)+(21 x 6)+(30 x 5)
20
−16.55 min
• Represents the mean of distribution.
• MRT describes average time for all the drug molecules to reside in the
body before elimination.
MRT=
Total residence time for all drug molecules in body
Total number of drug molecules

• Analogous to, t½ (elimination Half-life), i.e., when half of the drug is
eliminated.
• Represent time for 63.2% of drug eliminated when given through IV
route.
• Values are greater when the drug is administered in non-IV route.
• Like half-life, MRT is a function of both, the distribution and elimination.
• MRT calculated as:
MRT=
[AUMC]
0−∞
[AUC]
0−∞

• MRT after IV bolus dose:
MRT
IV=
1
k
10

• Relationship with elimination half-life:
t
½=
0.693
k
10
=0.693 x
1
k
10
=0.693 x MRT
ABSORPTION FROM MRT
• Mean absorption time (MAT) is difference MRT after different modes of
administration.
MAT=MRT
non−IV−MRT
IV
• Absorption half life
t
½ ab=0.693 x MAT
OTHER APPLICATIONS OF MRT
73

76. • Mean Dissolution Time (MDT)
MDT = MRTtest – MRTsoln
• In oral administration
MRToral = MRTiv + 1/Ka
• For evaluation of absorption data
MAT = MRTtest – MRTiv
MAT = 1/Ka
­ Where Ka is first order absorption rate constant.
BIOAVAILABILITY
• Relative bioavailability (Fr) is calculated by comparing zero moments of
test product with a standard or reference product.
F
r=
[AUC]
0−∞ Test
[AUC]
0−∞ Reference

• Absolute bioavailability is expressed by comparing zero moments of
product with oral and IV route of administration.
F=
[AUC]
0−∞ Oral/Dose
Oral
[AUC]
0−∞ IV/Dose
IV

• Bioavailability (F and Fr) of a drug generally refers to the fraction of a
dose administered via a route other than intravenous injection that
reaches the systemic circulation:
F=
D
IV
D
Oral
x
AUC
Oral
AUC
IV

FRACTION OF DOSE METABOLIZED
• Fraction metabolized (Fm)
F
m=
[AUC]
0−∞ Metabolite after oral route
[AUC]
0−∞ Metabolite after IV

CLEARANCE AND VD
• Clearance of a drug is estimated as I/V dose (Div) divided by the AUC
after intravenous bolus administration (AUCiv):
C??????=
D
IV
AUC
IV

74

77. • Volume of Distribution at Steady State (Vdss) is estimated as product of
MRT after intravenous bolus injection (MRTiv) and CI:
VD
ss=MRT
IV.C??????=
AUMC
IV
AUC
IV
⨯
D
IV
AUC
IV

LIMITATIONS OF NON-COMPARTMENTAL APPROACH
• Information regarding plasma drug concentration-time profile is
expressed as an average.
• Generally, not useful for describing the time course of drug in the blood.
• It is applicable only for linear pharmacokinetics.
• MRT application is less developed in pharmacodynamics.
75

78. GM Hamad

MULTIPLE DOSAGE REGIMENS
INTRODUCTION
• After single-dose drug administration, the plasma drug level rises above
and then falls below the minimum effective concentration (MEC),
resulting in a decline in therapeutic effect.
• To maintain prolonged therapeutic activity, many drugs are given in a
multiple-dosage regimen.
GRAPHS


• The plasma level of drugs given in multiple doses must be maintained
within the narrow limits of the therapeutic window (CP above the MEC
and below the MTC) to achieve optimal clinical effectiveness.
MEC
• The minimum concentration of drug needed at the receptors to produce
desired pharmacological effect.
MTC
• Represents the drug concentration needed to just barely produce a toxic
effect.
ONSET TIME
• Onset time corresponds the time required for the drug to reach the
MEC.
Plasma level

Concentration

SIZE OF DOSE
FREQUENCY OF DRUG ADMINISTRATION/
DOSING INTERVAL
76

79. GM Hamad

Cp
(g/ml)
MSC: Maximum Safety Concentration, MEC: Maximum Effective Concentration
INTENSITY
• Intensity of pharmacological effect is proportional to number of
receptors occupied by drug.
DURATION
• Duration of drug action is the difference between onset time and the
time for the drug to decline back to the MEC.
ESTABLISHMENT OF MULTIPLE DOSAGE REGIMEN
• Dosage regimen is established for drug to provide the correct plasma
level without excessive fluctuation and drug accumulation outside the
therapeutic window.






CRITERIA FOR OPTIMUM DOSAGE REGIMEN:
1. The plasma levels of drug given must be maintained within the
therapeutic window.
­ Example: The therapeutic range of theophylline is 10-20µg/L. So,
the best is to maintain the CP around 15µg/L.
2. Should be convenient to the patient,
­ It is difficult to take I.V. injection every ½ hour or one tablet every
2 hour, this lead to poor compliance.
SUPERPOSITION PRINCIPLE
• The superposition principle can be used when all the PK processes are
linear. That is when distribution, metabolism, and excretion (DME)
processes are linear or first order.
• Thus, concentrations after multiple doses can be calculated by adding
together the concentrations from each dose. Also, doubling the dose will
result in the concentrations at each time doubling.
77

80. GM Hamad

PRINCIPLE OF SUPERPOSITION
• The basic assumptions are:
1. The drug is eliminated by first-order kinetics.
2. The pharmacokinetics of the drug after a single dose (first dose)
are not altered after taking multiple doses.
INTRAVENOUS ADMINISTRATION IN MULTIPLE DOSING
DRUG ACCUMULATION
• To appreciate the extent of drug accumulation in the body following
each dose, consider the case of the drug which is administered as a
400mg dose at its biological half-life intervals (dosing interval is equal to
biological half-life of the drug i.e. τ = t½). If the biological half-life of this
drug is 6 hours then the dosing interval τ is also 6 hours. Since t½ = 6
hours, therefore the rate constant of elimination, K is:
&#3627408446;=
0.693
t
½

&#3627408446;=
0.693
6 ℎ&#3627408479;
=0.1155 ℎ&#3627408479;
• The mathematics of drug accumulation as each constant dose is
administered at the fixed dosing interval of 6 hrs. is as follows:
DURING 1
st
DOSE
• Amount administered = 400mg
• Amount eliminated = 200mg
• Amount persisting in the body = 400mg – 200mg = 200mg.
DURING 2
nd
DOSE
• Amount from 1
st
dose = 200mg
• Amount administered = 400mg
• Amount in the body = 600mg (200mg from 1
st
dose and 400mg from 2
nd

dose)
• Amount eliminated = 300mg (100mg from the 1
st
dose and 200mg from
the 2
nd
dose)
• Amount now persisting in the body = 600mg – 300mg = 300mg (100mg
from 1
st
dose and 200mg from 2
nd
dose)
78

81. GM Hamad

DURING 3
rd
DOSE
• Amount from 1
st
two doses = 300mg (100mg from the 1
st
dose and
200mg from the 2
nd
dose)
• Amount administered = 400mg
• Amount in the body = 700mg (100mg from the 1
st
dose, 200mg from the
2
nd
dose and 400mg from 3
rd
dose)
• Amount eliminated = 350mg (50mg from the 1
st
dose, 100mg from the
2
nd
dose and 200mg from 3
rd
dose)
• Amount now persisting in the body = 700mg – 350mg = 350mg (50mg
from the 1
st
dose, 100mg from the 2
nd
dose and 200mg from 3
rd
dose)
CONCLUSION
• Thus, during the 1
st
dose amount of drug in the body ranged between a
maximum of 400mg to minimum of 200mg and during 2
nd
dose the
range was between a maximum of 600mg to a minimum of 300mg,
during the 3
rd
dose the range was between a maximum of 700mg to a
minimum of 350mg.
• Comparing the maximum and minimum amounts of drug in the body
during the first three doses the maximum amounts were 400mg, 600mg
and 700mg showing an increase of 200mg between the 1
st
and 2
nd
dose,
but an increase of 100mg between 2
nd
and 3
rd
dose. Similarly, the
minimum amounts were 200mg, 300mg and 350mg showing an increase
of 100mg between the 1
st
and 2
nd
dose, but an increase of 50mg
between 2
nd
and 3
rd
dose.
• The increase in amount of drug in body becomes successively smaller
with each dose administered. This pattern indicates that with each
successive dose, the maximum amount of drug in body increased 50% of
previous increase.
• For a comprehensive look at the amount of drug in the body during
multiple dosing of similar doses administered at fixed dosing intervals.
Let us consider the elimination and persistence of drug in terms of
percentage of each administered dose.
PERCENT DRUG IN BODY FOLLOWING EACH DOSE
DOSE
PERCENT DRUG IN BODY
PERCENT DRUG PERSISTING FOR EACH DOSE
TOTAL RANGE
I
100 (Total drug administered at each dose)
50 (Amount of conc. from remaining dose)
1OO
50
Max
Min
II 50 + 100 150 Max
79

82. GM Hamad

25 + 50 75 Min
III
25 + 50 + 100
13 + 25 + 50
175
88
Max
Min
IV
13 + 25 + 50 + 100
6 + 13 + 25 + 50
188
94
Max
Min
V
6 + 13 + 25 + 50 + 100
3 + 6 + 13 + 25 + 50
194
97
Max
Min
VI
3 + 6 + 13 + 25 + 50 + 100
2 + 3 + 6 + 13 + 25 + 50
197
99
Max
Min
VII
2 + 3 + 6 + 13 + 25 + 50 + 100
1 + 2 + 3 + 6 + 13 + 25 + 50
199
100
Max
Min
VIII
1 + 2 + 3 + 6 + 13 + 25 + 50 + 100
0 + 1 + 2 + 3 + 6 + 13 + 25 + 50
200
100
Max
Min
IX
0 + 1 + 2 + 3 + 6 + 13 + 25 + 50 + 100
0 + 0 + 1 + 2 + 3 + 6 + 13 + 25 + 50
200
100
Max
Min
X
0 + 0 + 1 + 2 + 3 + 6 + 13 + 25 + 50 + 100
0 + 0 + 0 + 1 + 2 + 3 + 6 + 13 + 25 + 50
200
100
Max
Min
• It shows that approximately 95% of the 1
st
dose is eliminated after about
4 or 5 doses and 1
st
dose is virtually eliminated after about 8 doses, After
8 doses amount of drug appears to reach a plateau, the maximum or
minimum amounts (or conc.) ranging between 200% and 100% of the
dose. This is the time where steady state level is achieved.
FACTORS AFFECTING DRUG CONC. IN BODY DURING MULTIPLE DOSING
• During multiple dosing conc. of drug in the body depends on:
­ Persistence factor
­ Elimination factor
­ Accumulation factor
PERSISTENCE FACTOR
• It represents the fraction of each dose that persists in the body during
multiple dosing. This is the fraction that was not eliminated. Therefore,
the fraction of dose persisting in the body can be determined from the
equation describing the first order rate process because elimination is a
first order process.
• The first order equation describing conc. of drug in body after
administration of a dose is:
C
t=C
0(&#3627408466;
−??????&#3627408481;
)
80

83. GM Hamad

­ Where, Ct = conc. of drug at time t, C0 = conc. of drug at time 0,
K is the first order rate constant of elimination and t is time.
• During multiple dosing, the first order rate constant K is the first order
rate constant of elimination, and t is dosing interval (τ). Substituting t for
τ in above equation we get:
&#3627408438;
&#3627408481;=&#3627408438;
0(&#3627408466;
−(??????
??????)(??????)
)
­ Where,
▪ P = (Ke)(τ)
▪ Ct = C0 e
-p

• Since,
&#3627408438;&#3627408476;&#3627408475;&#3627408464;&#3627408466;&#3627408475;&#3627408481;&#3627408479;&#3627408462;&#3627408481;??????&#3627408476;&#3627408475;=
&#3627408462;&#3627408474;&#3627408476;&#3627408482;&#3627408475;&#3627408481; &#3627408476;&#3627408467; &#3627408465;&#3627408479;&#3627408482;&#3627408468;
&#3627408483;&#3627408476;&#3627408473;&#3627408482;&#3627408474;&#3627408466; &#3627408476;&#3627408467; &#3627408465;??????&#3627408480;&#3627408481;&#3627408479;??????&#3627408463;&#3627408482;&#3627408481;??????&#3627408476;&#3627408475;

&#3627408439;
??????
??????
=
&#3627408439;
0
??????
??????
(&#3627408466;
−????????????
)
­ Where, D is the amount of drug in the body, D0 (= dose) is the
amount of drug in body at time 0, and Vd is apparent volume of
distribution.
Multiplying both sides of above equation by Vd
&#3627408439;=&#3627408439;
0(&#3627408466;
−????????????
)
Rearranging the equation
&#3627408439;
&#3627408439;
0
=(&#3627408466;
−????????????
)
­ Where, the ratio D/D0 represents the fraction of dose in the body
at the end of dosing interval, i.e. this ratio is the fraction of dose
persisting in the body during the dose interval τ. Above equation is
therefore equivalent to:
??????&#3627408466;&#3627408479;&#3627408480;??????&#3627408480;&#3627408481;&#3627408466;&#3627408475;&#3627408464;&#3627408466; &#3627408467;&#3627408462;&#3627408464;&#3627408481;&#3627408476;&#3627408479;=??????=&#3627408466;
−????????????

NUMERICAL # 1
The biological half-life of a drug is 6hr. Calculate the persistence factor if the
dose is administered. (1) Every 6 hours (2) Every 12 hours.
SOLUTION
Since, t½ = 6 hrs, therefore, K = 0.693/ t½ = 0.693/6 hr = 0.1155/hr
81

84. GM Hamad

1. When τ = 6 hrs, the Persistence factor is:
??????&#3627408466;&#3627408479;&#3627408480;??????&#3627408480;&#3627408481;&#3627408466;&#3627408475;&#3627408464;&#3627408466; &#3627408467;&#3627408462;&#3627408464;&#3627408481;&#3627408476;&#3627408479;=??????=&#3627408466;
−????????????

??????=&#3627408466;
−(0.1155)(6)
=&#3627408466;
−(0.693)
=0.5
­ That is 50% of each dose will persist in the body before the next
dose is administered at 6 hours interval.
2. When τ = 12 hrs.
??????&#3627408466;&#3627408479;&#3627408480;??????&#3627408480;&#3627408481;&#3627408466;&#3627408475;&#3627408464;&#3627408466; &#3627408467;&#3627408462;&#3627408464;&#3627408481;&#3627408476;&#3627408479;=??????=&#3627408466;
−????????????

??????=&#3627408466;
−(0.1155)(12)
=&#3627408466;
−(1.386)
=0.25
­ This means that when the dosing interval is 12 hours, 25% of each
dose will persist in the body before the next dose is administered.
LOSS FACTOR
• Loss of drug from the body refers to drug elimination. The fraction of
each dose not persisting in the body represents the fraction of each dose
that is lost (i.e., eliminated from the body). Since the amount of drug lost
from the body (eliminated) plus the amount of drug still in the body
(persistence) is equal to the amount of dose administered, the fraction
of dose lost plus the fraction of dose persisting should be equal to one.
In other words,
&#3627408447;&#3627408476;&#3627408480;&#3627408480; &#3627408441;&#3627408462;&#3627408464;&#3627408481;&#3627408476;&#3627408479;=&#3627408440;=1−??????&#3627408466;&#3627408479;&#3627408480;??????&#3627408480;&#3627408481;&#3627408466;&#3627408475;&#3627408464;&#3627408466; &#3627408441;&#3627408462;&#3627408464;&#3627408481;&#3627408476;&#3627408479;
&#3627408447;&#3627408476;&#3627408480;&#3627408480; &#3627408441;&#3627408462;&#3627408464;&#3627408481;&#3627408476;&#3627408479;=&#3627408440;=1−??????=1−&#3627408466;
−????????????

NUMERICAL # 2
If the biological half-life of a drug is 6 hours, calculate the Loss Factor if
identical doses are administered (1) every 6 hours, (2) every 12 hours.
SOLUTION
Since t½ = 6 hours, therefore, K = 0.693/ t½ = 0.693/6 hr = 0.1155/hr
1. When τ = 6 hrs, the loss factor is:
&#3627408440;=1−&#3627408466;
−??????
=1−&#3627408466;
−(0.1155 x 6)
=0.5
­ Thus, 50% of each dose will be eliminated before the next dose is
administered.
2. When τ = 12 hrs.
82

85. GM Hamad

Equation 1
Equation 2
&#3627408440;=1−&#3627408466;
−??????
=1−&#3627408466;
−(0.1155 x 12)
=0.75
­ Thus, 75 % of each dose will be eliminated before the next dose is
administered.
ACCUMULATION FACTOR
• Accumulation of drug during multiple dosing is due to accumulation of
each successive dose of drug that persists in the body. The accumulation
factor, S, after administration of n doses is determined using the
following ratio:
&#3627408436;&#3627408464;&#3627408464;&#3627408482;&#3627408474;&#3627408482;&#3627408473;&#3627408462;&#3627408481;??????&#3627408476;&#3627408475; &#3627408441;&#3627408462;&#3627408464;&#3627408481;&#3627408476;&#3627408479;=&#3627408454;=
1− &#3627408466;
−&#3627408475;??????
1− &#3627408466;
−??????

• After a large number of doses are administered, i.e., when n becomes
sufficiently large and the value of the exponential term nKτ increases,
the term &#3627408466;
−&#3627408475;????????????
approaches 0. The accumulation factor becomes:
&#3627408454;=
1
1− &#3627408466;
−??????
=
1
&#3627408447;&#3627408476;&#3627408480;&#3627408480; &#3627408441;&#3627408462;&#3627408464;&#3627408481;&#3627408476;&#3627408479;

NUMERICAL # 3
If the biological half-life of a drug is 6 hours and identical doses are
administered every 6 hours, calculate the accumulation factor after (1) 3 doses
and (2) after 20 doses.
SOLUTION
Since t½ = 6 hours, therefore, K = 0.693/ t½ = 0.693/6 hr = 0.1155/hr
1. Accumulation factor after 3 doses: From equation 1
&#3627408454;=
1− &#3627408466;
−&#3627408475;??????
1− &#3627408466;
−??????
=
1− &#3627408466;
−(3)(0.1155)(6)
1− &#3627408466;
−(0.1155)(6)
=
1−0.125
1−0.5
=
0.875
0.5
=1.75
­ The accumulation of drug after 3 doses is (1.75)(Dose) or 175% of
the dose.
2. Accumulation factor after 20 doses: From Equation 1
&#3627408454;=
1− &#3627408466;
−&#3627408475;??????
1− &#3627408466;
−??????
=
1− &#3627408466;
−(20)(0.1155)(6)
1− &#3627408466;
−(0.1155)(6)
=
1−0
1−0.5
=
1
0.5
=2
­ After administration of 20 doses, each dose administered at 6-
hour intervals, the accumulation of the drug will be (2)(Dose) or
200% of the dose.
83

86. GM Hamad

MAXIMUM & MINIMUM PLASMA CONCENTRATION DURING MULTIPLE DOSING
AFTER n
th
DOSE IV
• If the drug follows first-order elimination, then the concentration of drug
in plasma during multiple dosing can be predicted from
the concentration of drug in plasma after the first dose,
as long as the size of each subsequent dose and the
dosing intervals are held constant.
• Figure shows a typical plasma profile after
administration of the first dose. In this profile, the y-
intercept is C0 and the first-order rate constant of
elimination (K) is obtained from the slope of the
straight line.
• The first-order equation for concentration of drug in plasma at any time
t is equation:
C
t=C
0(&#3627408466;
−kt
)
CONCEPT
C
t=C
0(&#3627408466;
−kt
) Where, T = τ, K = ke, P = (ke)(τ) So, C
t= C
0(&#3627408466;
−p
)
FIRST DOSE
• Since plasma concentration immediately after administration of the first
dose (Co) is the maximum plasma drug concentration for the first dose:
&#3627408438;
&#3627408474;????????????1=&#3627408438;
0
• Just before the second dose is administrated (at time = τ), the plasma
drug concentration is the minimum concentration of drug in plasma for
the first dose.
&#3627408438;
&#3627408474;??????&#3627408475;1=&#3627408438;
0&#3627408466;
−??????

SECOND DOSE
• When the second dose is administered, the maximum concentration of
drug in plasma (Cmax2) is the sum of minimum plasma drug concentration
after the first dose (Cmin1) and concentration of drug provided by the
second dose. Since the same dose is given again, the second dose
provides the same concentration of drug as was obtained when the first
dose was administered (i.e., C0):
&#3627408438;
&#3627408474;????????????2=&#3627408438;
0+&#3627408438;
&#3627408474;??????&#3627408475;1
Concentration

84

87. GM Hamad

Substituting the value of Cmin1, from previous equation into this equation
&#3627408438;
&#3627408474;????????????2=&#3627408438;
0+&#3627408438;
0(&#3627408466;
−??????
)
&#3627408438;
&#3627408474;????????????2=&#3627408438;
0(1+&#3627408466;
−??????
)
• Just before administration of the third dose, concentration of drug in
plasma is the minimum plasma concentration after the second dose
(Cmin2). The minimum concentration after the second dose is obtained by
multiplying plasma drug concentration at the time of administration of
the second dose (Cmax2) with the persistence factor
&#3627408438;
&#3627408474;??????&#3627408475;2=(&#3627408438;
&#3627408474;????????????2)(&#3627408466;
−??????
)
Substituting the value of Cmax2, from previous equation into this equation
&#3627408438;
&#3627408474;??????&#3627408475;2=&#3627408438;
0(1+&#3627408466;
−??????
)(&#3627408466;
−??????
)
THIRD DOSE
• When the third dose is given, the maximum concentration in plasma is
minimum concentration after the second dose (Cmin2) plus the
concentration provided by the third dose. Since the same dose is
administered, the third dose provides the same concentration that was
obtained when the first dose was administered (i.e., Co). Therefore,
maximum concentration of drug after the third dose is:
&#3627408438;
&#3627408474;????????????3=&#3627408438;
0+&#3627408438;
&#3627408474;??????&#3627408475;2
Substituting the value of Cmin2, from previous equation into this equation
&#3627408438;
&#3627408474;????????????3=&#3627408438;
0+&#3627408438;
0(1+&#3627408466;
−??????
)(&#3627408466;
−??????
)
&#3627408438;
&#3627408474;????????????3=&#3627408438;
0(1+&#3627408466;
−??????
+ &#3627408466;
−2??????
)
• The minimum concentration of drug in plasma after the third dose is
obtained in a manner similar to one used for determining the minimum
plasma drug concentration after the second dose:
&#3627408438;
&#3627408474;??????&#3627408475;3= (&#3627408438;
&#3627408474;????????????3)(&#3627408466;
−??????
)
Substituting the value of Cmax3, from previous equation into this equation
&#3627408438;
&#3627408474;??????&#3627408475;3=&#3627408438;
0(1+&#3627408466;
−??????
+ &#3627408466;
−2??????
)(&#3627408466;
−??????
)
n
th
DOSE
• The maximum concentration of drug in plasma after administration of n
doses can be expressed as:
85

88. GM Hamad

Equation 1
Equation 2
Equation 1
Equation 2
&#3627408438;
max&#3627408475;=&#3627408438;
0+&#3627408438;
0&#3627408466;
−??????
+&#3627408438;
0&#3627408466;
−2??????
+&#3627408438;
0&#3627408466;
−3??????
+−−−−&#3627408438;
0&#3627408466;
−(&#3627408475;−1)??????

Multiply &#3627408466;
−??????
on both sides to get maximum conc. of drug in plasma after n
doses
&#3627408438;
max&#3627408475;=&#3627408438;
0+&#3627408438;
0&#3627408466;
−??????
+&#3627408438;
0&#3627408466;
−2??????
+&#3627408438;
0&#3627408466;
−3??????
+−−−−&#3627408438;
0&#3627408466;
−(&#3627408475;−1)??????

(&#3627408466;
−??????
)&#3627408438;
max&#3627408475;=&#3627408438;
0&#3627408466;
−??????
+&#3627408438;
0&#3627408466;
−2??????
+&#3627408438;
0&#3627408466;
−3??????
+−−− &#3627408438;
0&#3627408466;
−(&#3627408475;−1)??????
+&#3627408438;
0&#3627408466;
−&#3627408475;??????

Subtracting equation 2 from equation 1, we get
&#3627408438;
max&#3627408475;(−&#3627408466;
−??????
)&#3627408438;
max&#3627408475;=&#3627408438;
0(1−&#3627408466;
−&#3627408475;??????
)
&#3627408438;
max&#3627408475;(1−&#3627408466;
−??????
)=&#3627408438;
0(1−&#3627408466;
−&#3627408475;??????
)
&#3627408438;
max&#3627408475;=
&#3627408438;
0(1−&#3627408466;
−&#3627408475;??????
)
(1−&#3627408466;
−??????
)

• The minimum concentration of drug after administration of n dose can be
expressed as:
&#3627408438;
min&#3627408475;=&#3627408438;
0&#3627408466;
−??????
+&#3627408438;
0&#3627408466;
−2??????
+&#3627408438;
0&#3627408466;
−3??????
+−−−−&#3627408438;
0&#3627408466;
−&#3627408475;??????

Multiply &#3627408466;
−??????
on both sides of equation
&#3627408438;
min&#3627408475;=&#3627408438;
0&#3627408466;
−??????
+&#3627408438;
0&#3627408466;
−2??????
+&#3627408438;
0&#3627408466;
−3??????
+−−−−&#3627408438;
0&#3627408466;
−&#3627408475;??????

(&#3627408466;
−??????
)&#3627408438;
min&#3627408475;=&#3627408438;
0&#3627408466;
−2??????
+&#3627408438;
0&#3627408466;
−3??????
+−−−−&#3627408438;
0&#3627408466;
−&#3627408475;??????
+ &#3627408438;
0&#3627408466;
−(&#3627408475;+1)??????

Subtracting equation 2 from equation 1, we get
&#3627408438;
min&#3627408475;−(&#3627408466;
−??????
)&#3627408438;
min&#3627408475;=&#3627408438;
0&#3627408466;
−??????
− &#3627408438;
0&#3627408466;
−(&#3627408475;+1)??????

&#3627408438;
min&#3627408475;(1−&#3627408466;
−??????
)=&#3627408438;
0&#3627408466;
−??????
(1−&#3627408466;
−&#3627408475;??????
)
&#3627408438;
min&#3627408475;=
&#3627408438;
0&#3627408466;
−??????
(1−&#3627408466;
−&#3627408475;??????
)
1−&#3627408466;
−??????

This equation can also be written as
&#3627408438;
min&#3627408475;=
&#3627408438;
0(1−&#3627408466;
−&#3627408475;??????
)
1−&#3627408466;
−??????
(&#3627408466;
−??????
)
Substituting the value of &#3627408438;
max&#3627408475; in above equation
&#3627408438;
min&#3627408475;=&#3627408438;
max&#3627408475;(&#3627408466;
−??????
)
NUMERICAL
86

89. GM Hamad

Calculate maximum plasma drug concentration after the administration of
fourth dose, if 50mg bolus doses are given intravenously every 4 hours. Given
C0 = 3mg/L and K = 0.2hr.
SOLUTION
Using equation for fourth dose the maximum plasma drug concentration is:
&#3627408438;
max&#3627408475;=
&#3627408438;
0(1−&#3627408466;
−&#3627408475;??????
)
(1−&#3627408466;
−??????
)

&#3627408438;
max&#3627408475;=
3&#3627408474;&#3627408468;/&#3627408473;(1−&#3627408466;
−(4)(0.2/ℎ&#3627408479;)(4)
)
(1−&#3627408466;
−(0.2/ℎ&#3627408479;)(4)
)

&#3627408438;
max&#3627408475;=
3&#3627408474;&#3627408468;/&#3627408473;(1−0.04076)
(1−0.4493)
=5.22&#3627408474;&#3627408468;/&#3627408473;
NUMERICAL
Calculate the maximum and minimum plasma concentration of a drug after
administration of fourth dose, if 50mg bolus dose is given IV every 4 hours,
Volume of distribution of drug is 40L, and the rate constant of elimination is
0.07/hr.
SOLUTION
Since dose = 50mg and apparent volume of distribution = 40L, therefore
&#3627408438;
0=
&#3627408439;&#3627408476;&#3627408480;&#3627408466;
??????&#3627408465;
=
50&#3627408474;&#3627408468;/&#3627408473;
40&#3627408473;
=1.25&#3627408474;&#3627408468;/&#3627408473;
Using equation for fourth dose the maximum plasma drug concentration is:
&#3627408438;
max&#3627408475;=
&#3627408438;
0(1−&#3627408466;
−&#3627408475;??????
)
(1−&#3627408466;
−??????
)

&#3627408438;
max&#3627408475;=
1.25&#3627408474;&#3627408468;/&#3627408473;(1−&#3627408466;
−(4)(0.07/ℎ&#3627408479;)(4)
)
(1−&#3627408466;
−(0.07/ℎ&#3627408479;)(4)
)

&#3627408438;
max&#3627408475;=
1.25&#3627408474;&#3627408468;/&#3627408473;(1−&#3627408466;
−(4)(0.07/ℎ&#3627408479;)(4)
)
(1−&#3627408466;
−(0.07/ℎ&#3627408479;)(4)
)

&#3627408438;
max&#3627408475;=
1.25&#3627408474;&#3627408468;/&#3627408473;(0.6737)
(0.2442)
=3.45&#3627408474;&#3627408468;/&#3627408473;
Using equation for fourth dose the minimum plasma drug concentration is:
87

90. GM Hamad

&#3627408438;
min&#3627408475;=&#3627408438;
max&#3627408475;(&#3627408466;
−??????
)
&#3627408438;
min&#3627408475;=3.45&#3627408474;&#3627408468;/&#3627408473;(&#3627408466;
−(0.07/ℎ&#3627408479;)(4)
)
&#3627408438;
min&#3627408475;=3.45&#3627408474;&#3627408468;/&#3627408473;(0.7558)=2.61&#3627408474;&#3627408468;/&#3627408473;
STEADY-STATE CONCENTRATIONS
• The steady-state concentration of drug in plasma (or the plateau level) is
reached when the values of the maximum concentration of drug in
plasma (Cmax) and the minimum concentration of drug in plasma (Cmin)
become constant for each successive dose. This can happen only when
the equations describing the maximum concentration of drug in plasma
and the minimum concentration of drug in plasma are not affected by it
(the number of dose).
MAXIMUM CONCENTRATION
• The maximum concentration of drug in the plasma at steady-state is the
maximum drug plasma concentration after the administration of a large
number of doses (i.e., Cmax ss, is Cmax n), when n (number of doses
administered) becomes very large. At very large value of n, the
exponential term (e
-np
) in the numerator of equation approaches a value
of zero and numerator now becomes equal to C0 (1 - 0) = C0. Therefore,
equation is reduced to:
&#3627408438;
max&#3627408480;&#3627408480;=
&#3627408438;
0(1−&#3627408466;
−&#3627408475;??????
)
(1−&#3627408466;
−??????
)

&#3627408438;
max&#3627408480;&#3627408480;=
&#3627408438;
0(1)
(1−&#3627408466;
−??????
)

&#3627408438;
max&#3627408480;&#3627408480;=
&#3627408438;
0
(1−&#3627408466;
−??????
)

MINIMUM CONCENTRATION
• The minimum conc. of drug in the plasma at steady-state (Cmin ss) can be
derived when n becomes very large (after administration of a large
number of doses), the exponential term (e
-np
) in the numerator of
equation approaches a value of 0. Therefore, at large values of n, the
numerator in equation is equal to C0 (1 – 0) = C0 e
-p
. Therefore, equation
becomes:
88

91. GM Hamad

&#3627408438;
min&#3627408480;&#3627408480;=
&#3627408438;
0
(1−&#3627408466;
−??????
)
(&#3627408466;
−??????
)
&#3627408438;
min&#3627408480;&#3627408480;=&#3627408438;
max&#3627408480;&#3627408480; (&#3627408466;
−??????
)
EXTRA VASCULAR ADMINISTRATION IN MULTIPLE DOSING
• Extravascular administration, particularly administration by the oral
route is the more popular route used during multiple dosing. Although
accumulation of drug during extravascular administration is very similar
to that found during intravenous administration, the magnitude of
accumulation of drug is not similar in both cases.
• In extravascular administration, immediately after the administration of
the drug dose, the concentration of drug in plasma first increases (due to
absorption), reaches its peak, and then decreases (due to very little
absorption).
• In intravenous administration, the concentration of drug in plasma is at
its peak at the time of administration, but peak concentration in
extravascular administration is usually less than that seen in intravenous
administration, because the entire drug-dose is not placed in the blood
stream all-at-once. The drug appears in plasma because of absorption,
and the magnitude of increase in concentration of drug in plasma
depends upon the rates of absorption, distribution, and elimination of
the drug. Thus, the steady-state concentrations of drug during multiple
dosing following extravascular administration depend on the rates of
absorption, distribution, and elimination of the drug.
• The simplest case is a drug which, when administered extravascularly as
a single bolus dose, confers upon the body the characteristics of one-
compartment model. It is assumed that during multiple dosing, earlier
doses of the drug do not affect pharmacokinetics of subsequent doses,
that is, the processes of absorption, elimination, metabolism, and
clearance, etc., of the drug remain unchanged, and the apparent volume
of distribution of the drug also remains unchanged. it is also assumed
that the entire administered drug dose is completely absorbed (F = 1),
and following absorption, elimination begins according to first-order
kinetics.
• Accumulation of the drug during multiple dosing in extravascular
administration is similar to accumulation of drug during intravenous
administration. The main difference between the two is absorption of
89

92. GM Hamad

the drug. If the drug is absorbed rapidly and completely, the plasma
profiles are very similar except that the graph depicting extravascular
administration exhibits the absorption phase.
• Determination of plateau levels during multiple dosing of extravascularly
administrated dosage forms is similar to the determination of plateau
levels attained during multiple dosing by intravenous administration,
except that during extravascular administration, the rate constant of
absorption (Ka) must be taken into consideration.
• During extravascular administration, the y-intercept B (obtained by back-
extrapolating the terminal linear portion of elimination phase) is used in
place of C0 (the y-intercept used during intravenous administration).
Value of the y-intercept obtained during EV administration is usually less
than the numerical value of the y-intercept of IV administration.
STEADY-STATE CONCENTRATIONS
• Minimum concentration of drug in plasma during a dosing period is the
concentration just prior to administration of the next dose, and if dosing
interval is such that the next dose is administered in the post-absorptive
phase of the previous dose, one can assume that most of the drug from
each dose was absorbed before the next dose was administered.
• Therefore, the rate of drug absorption would not affect the minimum
concentration attained during each dose as well as at the steady-state.
However, since the maximum concentration attained after each dose
occurs at the time of maximum concentration (tmax), and is a complex
function of absorption and elimination rate constants, the utilization of
maximum concentration values after each dose to determine the
maximum plasma concentration at steady-state becomes relatively
complicated. A simpler relationship is given here.
MAXIMUM CONCENTRATION Cmax ss
• The maximum drug plasma concentration at steady-state is calculated
using the equation:
&#3627408438;
max&#3627408480;&#3627408480;=
&#3627408437;
1−&#3627408466;
−??????
(&#3627408466;
−??????
)
­ In this equation, B is the γ-intercept obtained by back-
extrapolating the terminal linear portion of plasma profile to time
zero following the administration of the first dose, R is a factor
90

93. GM Hamad

Equation 1
which accounts for the absorption and elimination rate constants
of the drug. The factor R is calculated using the relationship:
&#3627408453;=&#3627408472;(&#3627408481;
&#3627408474;????????????)
­ tmax is the time of peak concentration after the first dose. For all
practical purposes, if Ka > > > K (i.e., Ka is at least 5 times greater
than K), the plasma-profile shows a distinct and sharp peak
enabling one to read tmax from the graph as a good approximation.
However, if the plasma profile does not exhibit a sharp peak
and/or if it is difficult to read tmax from the graph, it can be
calculated using the equation:
&#3627408481;
&#3627408474;????????????=
&#3627408473;&#3627408475;&#3627408446;
??????−&#3627408473;&#3627408475;&#3627408446;
&#3627408446;
??????−&#3627408446;
=
&#3627408473;&#3627408475;&#3627409148;−&#3627408473;&#3627408475;&#3627409149;
&#3627409148;−&#3627409149;

MINIMUM CONCENTRATION Cmin ss
• The minimum concentration of drug in plasma following extravascular
administration of n doses is described by the equation:
&#3627408438;
min&#3627408475;=(&#3627408437;)[
(1−&#3627408466;
−&#3627408475;??????
)(&#3627408466;
−??????
)
1−&#3627408466;
−??????
−
(1−&#3627408466;
−&#3627408475;??????
)(&#3627408466;
−??????
)
1−&#3627408466;
−??????
]
­ where, Cmin n = minimum concentration of drug in plasma during
the n
th
dose, B = back-extrapolated y-intercept of the terminal
linear phase after the first dose, K = elimination rate constant, τ =
dosing interval, G = Kaτ, and Ka = absorption rate constant.
­ By setting n = 1 in above equation, an expression for the minimum
plasma concentration of drug following the first dose (Cmin 1 can be
obtained. When n = 1, equation becomes:
&#3627408438;
min1=(&#3627408437;)[
(1−&#3627408466;
−??????
)(&#3627408466;
−??????
)
1−&#3627408466;
−??????
−
(1−&#3627408466;
−??????
)(&#3627408466;
−??????
)
1−&#3627408466;
−??????
]
&#3627408438;
min1=(&#3627408437;)(&#3627408466;
−??????
− &#3627408466;
−??????
)
­ When n becomes large (i.e., when steady-state is attained), the
exponential e
-np
and e
-nG
approaches 0, and equation 1 becomes:
&#3627408438;
min&#3627408480;&#3627408480;=(&#3627408437;)[
(&#3627408466;
−??????
)
1−&#3627408466;
−??????
−
(&#3627408466;
−??????
)
1−&#3627408466;
−??????
]
91

94. GM Hamad

Equation 1
Equation 2
­ In the post-absorptive phase, G (the product of Ka and τ) becomes
large, and therefore the exponential term e
-G
approaches 0. Above
equation is then reduced to:
&#3627408438;
min&#3627408480;&#3627408480;=
&#3627408437;
1−&#3627408466;
−??????
(&#3627408466;
−??????
)
FACTORS AFFECTING THE DESIGN OF DOSAGE REGIMEN
1. EFFECT OF DOSE
&#3627408438;
max&#3627408480;&#3627408480;=
&#3627408438;
0
(1−&#3627408466;
−??????
)
&#3627408462;&#3627408475;&#3627408465; &#3627408480;??????&#3627408475;&#3627408464;&#3627408466; &#3627408438;
0=
&#3627408439;
??????
??????
,&#3627408481;ℎ&#3627408466;&#3627408479;&#3627408466;&#3627408467;&#3627408476;&#3627408479;&#3627408466;
&#3627408438;
max&#3627408480;&#3627408480;=
&#3627408439;
??????
??????(1−&#3627408466;
−??????
)

• If a dose D" (instead of the dose D) is administered, and assuming no
change in dosing interval, apparent volume of distribution, and rate
constant of elimination of the drug, then the maximum concentration at
steady-state (Cmax ss”) is given by:
&#3627408438;
&#3627408474;???????????? &#3627408480;&#3627408480;"=
&#3627408439;"
??????
??????(1−&#3627408466;
−??????
)

Dividing equation 1 by equation 2
&#3627408438;
max&#3627408480;&#3627408480;
&#3627408438;
&#3627408474;???????????? &#3627408480;&#3627408480;"
=
&#3627408439;/ ??????
??????(1−&#3627408466;
−??????
)
&#3627408439;"/ ??????
??????(1−&#3627408466;
−??????
)

&#3627408438;
max&#3627408480;&#3627408480;
&#3627408438;
&#3627408474;???????????? &#3627408480;&#3627408480;"
=
&#3627408439;
&#3627408439;"

• Thus, changes in the size of the drug dose administered will
correspondingly increase or decrease the steady-state maximum and
minimum concentration of drug in the plasma.
2. EFFECT OF DOSING INTERVAL
• The effect of dosing interval on the steady-state minimum and
maximum concentrations of drug in the plasma is not as simple and
straight-forward It is complicated.
92

95. GM Hamad

ELIMINATION
ELIMINATION
• Elimination of drug is made possible by their biotransformation and
excretion.
BIOTRANSFORMATION
• Biotransformation is the conversion of a drug to its metabolites. These
metabolites can be inactive, active having same activity as their parent
drug or having different activities.
• A product can be converted into the active drug. The metabolites are
more excretable from the body.
SITES OF BIOTRANSFORMATION
• The biotransformation of a drug can occur at hepatic or non-hepatic
sites. Following are the non-hepatic organs where drugs can be bio-
converted to metabolites. However, liver is the major organ for drug
metabolism.
BRAIN
• A few drugs are metabolized in brain. Example is the levodopa which is
converted into dopamine (active form).
CUTANEOUS TISSUES
• Epidermis can carry out several metabolic reactions including
glucuronide conjugation. There are evidences of cutaneous metabolism
of adrenal steroids, hydrocortisone and flurouracil. Vidarabine (an
antiviral agent) has cutaneous metabolism. First pass drug metabolism in
skin reduces the duration and potency of locally applied drugs.
GASTROINTESTINAL TRACT
• This is also called as the pre-systemic drug metabolism. Drugs can be
conjugated by various enzymes in intestinal epithelium and
consequently this presystemic metabolism cause an incomplete
bioavailability of drugs. Presystemic metabolism of Premarin in
gastrointestinal tract is example.
93

96. GM Hamad

LUNGS
• Lungs are perfused by entire blood supply and the drug in blood is
presented to enzymes in lungs for metabolism. A few drugs are prone to
be metabolized in lungs.
KIDNEYS
• Some drugs are converted to their metabolites in kidney by the action of
angiotensin convertase enzyme.
FIRST PASS EFFECT
• The rapid metabolism of an orally administered drug prior to reaching
the systemic circulation is called the first pass effect.
• Lack of the parent or intact drug in the systemic circulation after oral
administration is the evidence of first pass effect. In such cases, AUC0-∞
after oral administration is lesser than AUC0-∞ after intravenous
administration.
• Examination of the absolute bioavailability, F can be a measure of
hepatic first pass effect.
• The F can be calculated as:
F=
AUC
0−∞ Oral
AUC
0−∞ I/V

• For the drugs undergoing first pass effect have F lower than 1.
• Liver extraction ratio is a direct measure of the first pass effect.
ER=
C
a−C
v
C
a

­ Where, Ca is the drug concentration entering liver and Cv is drug
leaving liver.
HEPATIC BIOTRANSFORMATION
• The pathway of drug biotransformation is divided into two major groups
of reactions:
1. Phase I metabolism
2. Phase II metabolism
• A drug may be exposed to both of the reactions mentioned above and
their consequences may be illustrated as in the following figure:

94

97. GM Hamad









PHASE I METABOLIC REACTIONS
• The phase I reactions occur first and involve a change in the drug
molecules through non-synthetic reactions and thus, also known as the
non-synthetic reaction.
• In these reactions, a drug molecule can be degraded of a functional
group is either introduced or exposed on the drug molecule so as it can
be attacked by phases II enzymes.
• The microsomal enzymes catalyze the phase I reactions. However, if the
phase I reaction induces sufficiently more water solubility in a drug
molecule to permit its excretion through the renal or biliary excretion,
the molecule would not be exposed to the Phase II reactions.
• The phase I reaction include oxidation, reduction and hydrolysis.
OXIDATION
• Oxidation is the most common type of metabolic reaction. It involves the
addition of oxygen or removal of hydrogen from the drug molecule. The
several oxidation reactions occurring in body are:
OXIDATION OF ALKYL CHAIN
• Alkyl compounds, or alkyl side chains of the aromatic drugs with
carboxyl, aldehyde, or amino group undergo oxidation. Examples
include:



Prodrug Active Drug
Active
Metabolite
Inactive
Metabolite
Conjugated
Derivatives
Phase I
Reaction
Phase II
Reaction

↑ Polar
Metabolite
Renal or
Biliary
Excretion
Renal or biliary
excretion
95

98. GM Hamad

OXIDATION OF AROMATIC RING






N OXIDATION





SULFOXIDATION





OXIDATIVE DEAMINATION






96

99. GM Hamad

OXIDATIVE DEALKYLATION





Microsomal as well as nonmicrosomal enzymes are involved in oxidation
reactions. Hydroxylation of aromatic ring, aliphatic hydroxylation, N-oxidation
and sulfoxidation are the reactions catalyzed by microsomal enzymes.
Whereas, dehydrogenation of ethyl alcohol into acetaldehyde, conversion of
hypoxanthine to xanthine, xanthine to uric acid and tyrosine to dopa are
catalyzed by nonmicrosomal oxidases.
REDUCTION
• Reduction is less common than the oxidation and occurs in both,
microsomal as well as in non-microsomal metabolizing systems. These
include:
N-REDUCTION




KETON REDUCTION




HYDROLYSIS
• Hydrolysis is also referred to as the replacement reaction and is
mediated by enzymes estrases and amidases.
97

100. GM Hamad

• The examples of hydrolysis are:
ESTER HYDROLYSIS



AMIDE HYDROLYSIS




PHASE II REACTIONS
• Phase II, synthetic or conjugation reactions may occur with the drug
molecules with exposed or induced polar constituent as a consequence
of Phase I reactions. The phase II reactions offer a mechanism whereby a
functional group of a drug can be blocked by addition of a conjugating
agent.
• The conjugation reactions use conjugating reagents are derived from the
compounds involved in carbohydrate, lipid, fat or protein metabolisms.
The conjugating agents available for such reactions include glucuronic
acid, sulfate, glycine, acetyl CoA, glutathione. The respective high energy
forms of these conjugating agents are uridine diphosphoglucuronic acid
(UDPGA), acetyl co-enzyme (acetyl CoA), 3’-phosphoadenosine 5’-
phosphoslfate (PAPS), or S-adenosylmethionine (SAM). Glucuronidation
and sulfate conjugation are very common phase II reactions that result
in water-soluble metabolites rapidly excreted in bile and or urine.
• Two schemes have been proposed for the phase II reactions. In scheme
A, the conjugating agent, activated with energy combines with the drug
molecule in the presence of an appropriate drug transferase enzyme.
• In scheme B, a drug may be activated to a high energy compound to
react with a conjugating agent in the presence of the conjugating agent-
transferase enzyme. The schemes A and B, with examples can be
illustrated in the following figure:
SCHEME A
98

101. GM Hamad

Conjugating Agent Activated
Conjugating Agent
+
+
Energy
Energy
Drug Transferase
Conjugating Agent Transferase
UDPGA Transferase
Morphine + UDPGA
SCHEME B
The scheme A can be exemplified as:

Benzoic
Acid
Acetyl CoA Benzoyl
CoA
Glycine Transferase
Hippuric
Acid











The scheme B can be exemplified as:


The following are the phase II reactions along with the examples:
GLUCURONIDATION
• Glucuronic acid conjugation is one of the most common route of drug
metabolism. Its significance lies in readily available supply of glucuronic
acid in liver. The glucuronide conjugates are pharmacologically inactive.
• The reaction involves the condensation of drug with the activated form
of glucuronic acid, uridine diphosphate glucuronic acid. This reaction is
catalyzed by glucuronyl transferase in liver. Glucuronidation occurs for
the drugs containing functional groups OH, NH2, SH and COOH. These
reactions include:
ETHER GLUCURONIDATION
• Ether glucuronidation occurs for the drugs with OH group. This reaction
has been mentioned as:


Conjugated Drug
Drug

Drug
Conjugated
Drug

Activated
Drug

Conjugating
Agent

Morphine O’ –
Glucuronide
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102. GM Hamad


ESTER GLUCURONIDATION



SULFATE CONJUGATION
• The sulfate conjugation occurs in the drugs with functional groups of OH
and NH2. The high energy form of sulfate is 3’phosadensin
5’phosphosulfate (PAPS).




ACETYLATION
• Acetylation occurs in the drugs with OH or NH groups. Acetyl CoA is the
high energy form of the conjugating agent.









• Acetylated product is usually less polar than the parent drug and
precipitate in sufficient concentration in kidney tubules causing kidney
damage and crystalluria.
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103. GM Hamad

• Acetylation is a conjugation reaction often implicated in the toxicity of
the drug. The drug metabolized by acetylation include sulfanilamide,
sulfadiazine, procainamide and sulfisoxazole.
METHYLATION
• The conjugating agent in methylation is CH3 from S-
adenosylmethionine. The high energy form of this conjugating agent is
the S-adenosylmethionine (SAM).
• The functional group combined with this conjugating agent are OH and
NH2




AMINO ACID CONJUGATION
• The amino acid conjugation uses the glycine as a conjugating agent. The
high energy form of this conjugating agent is the Coenzyme A thioesters.
In amino acid conjugation, the glycine combines with the drugs having
functional group COOH. The glycine conjugates are known as
Hippurates.
MERCAPTURINE ACID CONJUGATION
• In mercaptopurine acid conjugation, glutathione is the conjugating
agent. Its high energy form is arene oxides or epoxides. The glutathione
combines with the drugs with functional groups including aryl, halides,
epoxies, and arene oxides.
FACTORS AFFECTING BIOTRANSFORMATION
• The rate at which the drug is metabolized determines its intensity and
duration of action. It determines not only the magnitude of
pharmacological effect but frequently also the toxic effects of the drugs.
There are different variables which influence the rate of metabolisms of
drugs including:
1. FACTORS RELATED TO THE PATIENTS
I. SPECIES AND STRAIN FACTORS
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104. GM Hamad

• The rates or the patterns of the metabolism are different in different
species due to the variability in enzyme system or due to the lack of a
certain metabolic pathway entirely in some animals. Cat, in contrast to
the other animals cannot form glucoronoids in significant amounts. Dogs
are unable to acetylate aromatic amine like amphetamine.
• The various metabolic rate of certain drug in different species to species
exemplified as:
­ Procaine is rapidly metabolized in man by plasma esterase
whereas it is extremely slowly hydrolyzed in horse.
­ Hexabarbiturate is rapidly metabolized in mouse (t½ = 19 min.)
but not in rat (t½ = 140 min.), dog (t½ = 260 min.) or man (t½ =
360 min.)
­ Meperidine is rapidly metabolized in dogs, but slowly in monkeys.
II. GENETIC FACTORS
• As the biochemical characteristics are genetically dictated within the
same species there are great individual variations in drug metabolism.
Based on this variation, some individuals are rapid acetylators and other
acetylate at slow rate.
• The rate of acetylation is determined by the microsomal concentration
of S-acetyl CoA which serves as acetyl donor. The concentration of CoA is
genetically controlled. The metabolic rate of the warfarin, dicumarol,
phenytoin, halothane, phenyl butazone and succinyl choline may also be
slow or fast based on the S-acetyl CoA concentration.
III. GENDER FACTOR
• Difference in rate metabolism of certain drugs among male and female is
attributed to the influence of sex hormones, activity of microsomal
enzymes and physiological conditions.
• The microsomal enzyme activity is six times greater in man than in
females. Female rat after puberty metabolizes a variety of drugs such as
barbiturates, narcotics, and sulfonamides at lower rate than the male.
The male rat inactivates the drugs faster because of the greater enzyme
activity of liver microsomes.
IV. AGE FACTOR
• The fetus, newborn, infants and the elderly have been shown to be
deficient in the liver microsomal enzymes necessary for the metabolism
of the variety of drugs. Metabolic enzymes, and pathways are not fully
102

105. GM Hamad

developed in neonates. While in elder people, metabolizing enzymes
systems become exhausted thus, the metabolic rate is highly variable in
Child and elder patients. So, drug dose is carefully modified to meet such
situations.
V. protein binding
• Only the free drug is available for the metabolism. Thus, the greater the
protein bound fraction of the drug, lesser would be its rate of
metabolism.
VI. NUTRITIONAL STATUS
• Starvation and malnutrition depress drug metabolism.
VII. PATHOLOGICAL FACTORS
• The liver is the major site of drug metabolism and thus, if this organ is
dysfunction, drug metabolism is frequently depressed. Jaundice, hepatic
tumors, etc., for instance, depresses the glucuronic acid conjugation and
also the oxidative function of liver microsomes.
2. FACTORS RELATED TO THE DRUG
CO-MEDICATION
• Drugs administered simultaneously may inhibit or stimulate the rate of
biotransformation tending either to increase or reduce the duration of
action, respectively. Following are the effect of drugs administered
simultaneously.
I. INHIBITION OF DRUG METABOLISM
• There are certain drugs that consume or cause the loss of certain
enzymes responsible for metabolism, and thereby prolongation of the
action duration, intensity and also toxicity of the drug.
• Chloramphenicol, and phenylbutazone inhibits the metabolism of
tolbutamide, resulting in an increased hypoglycemic response.
• Chloramphenicol and isoniazid inhibit the metabolism of phenytoin,
resulting in an increased concentration in serum level and thus,
increases anticonvulsant action and possible drug toxicity.
II. STIMULATION OF DRUG METABOLISM
• A number of drugs have been found to stimulate hepatic enzyme activity
resulting in increased rate of metabolism and shortening of the duration
of drug action. The representative compounds which have been
demonstrated to stimulate drug metabolisms are benopyrin which
stimulates the metabolism of aminopyrin, and hexabarbital.
III. INCREASING DRUG METABOLISM VIA ENZYME INDUCTION
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106. GM Hamad

• An increased synthesis of the enzyme due to stimulus is termed as
enzyme induction and the drug which causes this phenomenon are
called enzyme inducers. Many agents cause the increased synthesis of a
hepatic enzyme cytochrom-p-450 which accelerates the metabolism of
other drugs. For example, barbiturates increase the rate of metabolism
of hydrocortisone, digitoxin and phenytoin.
• Many drugs when administered chronically stimulates their own
metabolism as well as the metabolism of other drugs for example,
aminopyrin, chlorpromazine, meprobamate, hexabarbital,
Phenobarbital, phenylbutazone and probenecid tolbutamide.
IV. DISPLACEMENT
• A concurrently administered drug may compete for the protein binding.
Competitive binding displaces the other drug. The rate of metabolism
may be increased for the displaced drug from the protein binding sites.
Phenylbutazone displaces warfarin.
3. MISCELLANEOUS FACTORS
• Following are the factors which modify the metabolic rate of the drugs:
­ Hormonal level
­ Ascorbic acid: deficiency of ascorbic acid decreases the
metabolism of the drugs
­ Accumulation of drugs in fat depots – decreases the rate of
metabolism.
BILIARY EXCRETION OF DRUGS
• Bile juice is secreted by hepatic cells of the liver. It is important in the
digestion and absorption of fats. 90% of bile acid is reabsorbed from
intestine and transported back to the liver for resection.
• Compounds excreted by this route are sodium, potassium, glucose,
bilirubin, Glucuronide, sucrose, Inulin, muco-proteins etc.
• Greater the polarity better the excretion. The metabolites are more
excreted in bile than parent drugs due to increased polarity.
RENAL EXCRETION OF DRUGS
• Renal excretion (excretion of drugs through the kidneys) plays a major
role in the elimination of most drugs, especially those that are water
soluble and/or undergo biotransformation relatively slowly.
• Three events are important with reference to drug excretion through
104

107. GM Hamad

urine:
1. Glomerular Filtration
2. Tubular Secretion
3. Tubular Reabsorption
1. GLOMERULAR FILTRATION
• It Is non-selective, unidirectional process. Ionized or unionized drugs are
filtered, except those that are bound to plasma proteins.
• Driving force for GF is hydrostatic pressure of blood flowing in
capillaries.
• GLOMERULAR FILTRATION RATE: Out of 25% of cardiac output or 1.2
liters of blood/min that goes to the kidney via renal artery only 10% or
120 to 130ml/min is filtered through glomeruli. The rate being called as
glomerular filtration rate (GFR). E.g. insulin, heparin, dextran.
2. TUBULAR SECRETION
• This mainly occurs in proximal tubule. It is carrier mediated process
which requires energy for transportation of compounds against conc.
gradient.
• Two secretion mechanisms are identified:
­ System for secretion of organic acids/anions E.g. Penicillin,
salicylates etc.
­ System for organic base/cations E.g. morphine, mecamylamine
hexamethonium.
• Active secretion is unaffected by change in pH and protein binding. Drug
undergoes active secretion have excretion rate values greater than
normal GFR. E.g. Penicillin.
3. TUBULAR REABSORPTION
• It occurs after the glomerular filtration of drugs. It takes place all along
the renal tubules.
• Reabsorption of drugs indicated when the excretion rate values are less
than the GFR 130ml/min. E.g. Glucose
• TR can be active or passive processes. Reabsorption results in increase in
the half-life of the drug.
MECHANISIM OF RENAL EXCRETION
• For the urine excretion of the drug, the drug is flittered out in bowman
capsule and then it finds way to excrete, if it is not reabsorbed. More
drug would be excreted if the drug is also secreted from blood to loop of
105

108. GM Hamad

henle along with filtered in bowman capsule. The excretion is lesser
when the drug is reabsorbed from nephron to blood.
• For the protein bound drugs, being having larger size, cannot be filtered
out. Only free drug is available for filtration. Besides, protein binding,
there are other parameters which affect drug excretion.
• These factors include drug solubility, urine pH and degree of ionization.
PARAMETERS FOR RENAL EXCRETION
1. Total amount of drug excreted through urine (??????
??????
∞
)
2. Urinary excretion rate (dDu/dt)
3. Time for complete excretion (t
∞
)
ELIMINATION OF DRUGS THROUGH OTHER ORGANS
• Drugs can be excreted from the body through many pathways of
excretion.
• These routes, not necessarily in any particular order, include the
following: kidneys, lungs, saliva, perspiration (sweat), bile, intestines,
hair and skin, and milk.
PULMONARY EXCRETION
• Gaseous and volatile substances such as general anesthetics (Halothane)
are absorbed through lungs by simple diffusion. Pulmonary blood flow,
rate of respiration and solubility of substance effect PE.
• Intact gaseous drugs are excreted but not metabolites. Alcohol which
has high solubility in blood and tissues are excreted slowly by lungs.
SALIVARY EXCRETION
• The pH of saliva varies from 5.8 to 8.4. Unionized lipid soluble drugs are
excreted passively. The bitter after-taste in the mouth of a patient is
indication of drug excreted.
• Some basic drugs inhibit saliva secretion and are responsible for mouth
dryness. Compounds excreted in saliva are Caffeine, Phenytoin,
Theophylline.
MAMILLARY EXCRETION
• Milk consists of lactic secretions which is rich in fats and proteins. 0.5 to
1 liter of milk is secreted per day in lactating mothers. Excretion of drug
in milk is important as it gains entry in breast feeding infants.
• pH of milk varies from 6.4 to 7.6. Free un-ionized and lipid soluble drugs
diffuse passively. Highly plasma bound drug like Diazepam is less
106

109. GM Hamad

secreted in milk. Since milk contains proteins. Drugs excreted can bind to
it.
ADVERSE EFFECTS
• Discoloration of teeth with tetracycline and jaundice due to interaction
of bilirubin with sulfonamides.
• Nicotine is secreted in the milk of mothers who smoke.
GENITAL EXCRETION
• Reproductive tract and genital secretions may contain the excreted
drugs. Some drugs have been detected in semen.
• Drugs can also get excreted via the lachrymal fluid.
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110. GM Hamad

PROTEIN BINDING
INTRODUCTION
• Many drugs interact with plasma or tissue proteins or other
macromolecules, such as melamine or DNA, to form a drug-
macromolecule complex.
• The formation of a drug protein complex is often named as drug-protein
binding.
• Drug protein binding may be a reversible or irreversible process.
IRREVERSIBLE PROTEIN BINDING
• Irreversible protein binding is due to chemical activation of the drug
which then attaches strongly to the protein by covalent bond.
• It may result in certain type of drug toxicity that may occur over a long
time period such as carcinogenesis, or within a short time period as the
formation of reactive chemical intermediates.
REVERSIBLE PROTEIN BINDING
• In reversible protein binding, drug binds with the proteins with weaker
chemical bonds like Hydrogen bonds or Vander Waals forces.
• Most drugs bind with proteins by reversible process.
• Amino acid that compose the protein chain have hydroxyl, carboxyl, or
other sites available for reversible drug interaction.
CHARACTERS OF PROTEIN BOUND DRUG
• The protein bound drugs are a large complex that cannot easily
transverse the cell or capillary membrane and therefore has a restricted
distribution.
• Protein bound drug is pharmacologically inactive, in contrast free or
unbound drug crosses the cell membrane and is therapeutically active.
• Purified protein such as albumin is used to evaluate drug-protein
binding.
• Methods such as equilibrium dialysis and ultrafiltration make use of
semipermeable membrane that separate protein and protein bound
drug from free or unbound drug.
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111. GM Hamad

• By these in vitro methods, the conc. of bound drug, free drug and total
protein may be determined.
IN VITRO METHODS
• Different in vitro methods have advantages and disadvantages in terms
of:
­ Cost
­ Ease of measurement
­ Time
­ Instrumentation
­ Other consideration
FACTORS AFFECTING PROTEIN BINDING
1. THE DRUG
• Physicochemical properties and concentration of the drug in body.
• Increase in lipophilicity, increase extend of binding e.g. cloxacillin 95%
bound, ampicillin 20% bound.
• Acidic/anionic drugs bind to albumin.
• Basic/cationic drugs bind to globulins.
• Neutral/unionized drugs bind to lipoproteins.
• At low concentration, most drugs bound to the proteins, but at higher
concentration more free drugs may be present owing to saturation of
binding sites on protein.
2. THE PROTEIN
• Quality or physicochemical nature of the protein synthesized.
• Concentration of the protein available for drug protein binding.
• Number of binding sites on the proteins.
• Concentration is controlled by variables such as synthesis, catabolism,
distribution between intra and extravascular space and number of
diseases.
3. COMPETITION BETWEEN THE DRUG AND BODY CONSTITUENTS
• Interaction of drugs with free fatty acids.
• Free fatty acid levels are increased during fasting, diabetes, myocardial
infarction etc.
• For example: Interaction of sodium salicylate and bilirubin in neonates.
4. DRUG INTERACTIONS
• Competition of drug by other substances at a protein binding site e.g.
phenylbutazone and warfarin when given simultaneously, warfarin is
displaced by phenylbutazone.
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112. GM Hamad

• Alteration of the protein structure by the drug metabolite that modifies
binding capacity. e.g. aspirin acetylation of albumin modify the binding
capacity of NSAIDs.
5. THE PATHOPHYSIOLOGIC CONDITIONS OF THE PATIENT
• Liver disease: Decrease in albumin conc. due to decreased synthesis.
• Nephrotic syndrome: Due to accumulated metabolites urea and uric
acid, the protein binding may be altered.
• Severe burns: Increased distribution of albumin in ECF.
• Genetic disorders: The quality of the protein may be altered due to
genetic disorders which may result in decreased affinity for drug.
6. SPEED OF INJECTION
• Rapid IV injection may increase the free drug concentration of some
highly protein bound drugs and therefore increase in intensity of its
action.
• It can be attributed to initial saturation of the protein binding sites.
• For example: Diazoxide a hypotensive drug.
­ Injection time: 10 sec dramatic increased hypotensive effect.
­ Injection time: 100 sec smaller hypotensive effect.
MECHANISM OF DRUG PROTEIN BINDING
• Binding of drugs to proteins is generally of reversible & irreversible.
• Reversible generally involves weak chemical bond such as:
­ Hydrogen bonds
­ Hydrophobic bonds
­ Ionic bonds
­ Van der Waal’s forces.
• Irreversible drug binding, though rare, arises as a result of covalent
binding and is often a reason for the carcinogenicity or tissue toxicity of
the drug.
PLASMA PROTEINS INVOLVED IN BINDING OF DRUGS
1. BINDING OF DRUG TO BLOOD COMPONENTS
• Drugs may bind to various macromolecular components in the blood.
­ Albumin
­ Lipoproteins
­ Αlpha1- acid glycoprotein
­ Immunoglobulin (IgG)
110

113. GM Hamad

­ Erythrocytes
I. HUMAN SERUM ALBUMIN (HSA)
• It is a protein with a molecular weight of 65,000 to 69,000 Da that is
synthesized in the liver.
• It is the major component of plasma protein responsible for reversible
drug binding.
• Interstitial fluid albumin concentration is about 60% of that in the
plasma.
• The elimination half-life of albumin is 17-18 days.
• Normally, Albumin concentration is maintained at a relatively constant
level of 3.5% to 5.5% (weight per volume) or 4.5 mg/dL
• Albumin is responsible for maintaining the osmotic pressure of the blood
and for the transport of endogenous and exogenous substances in the
plasma.
• Albumin complexes with endogenous substances such as free fatty acids,
bilirubin and various hormones.
• Many weak acidic drugs bind to albumin by electrostatic and
hydrophobic bonds.
• Weak acidic drugs such as salicylate, phenylbutazone and penicillins are
highly bound to albumin.
• Four different sites on HSA for drug binding:
­ Site I: To this specific site a large population of drugs bind like
Non-Steroidal Anti-Inflammatory Drugs mainly phenylbutazone,
indomethacin, many sulfonamides e.g. sulfamethoxine,
sulfamethizole, and even many anti-epileptic drugs like phenytoin
etc. this site is also called as Warfarin binding site or as
Azapropazone binding site.
­ Site II: This is actually said to be Diazepam binding site.
Benzodiazepines, medium chain fatty acids, ibuprofen,
ketoprofen, etc. bind extensively at the very site. This is so
because due to structural changes the following drugs have high
and specific affinity for the site. At both the sites I & II many drugs
are known to bind.
­ Site III: This very protein site is called as Digitoxin binding site.
­ Site IV: This is referred as Tamoxifen binding site.
• At the sites III & IV very few drugs are known to bind.
111

114. GM Hamad

• Many weak acidic (anionic) drugs bind to albumin by electrostatic and
hydrophobic bonds.
• Weak acidic drugs such as salicylates, phenylbutazone, and penicillins
are highly bound to albumin.
• However, the strength of the drug binding is different for each drug.
II. α₁-ACID GLYCOPROTEIN (AAG)
• Binding by: Hydrophobic bonds
• Molecular weight – 44,000 Da
• Plasma concentration is (0.4% - 1.0%)
• For example: Basic drugs
­ Imipramine
­ Amitriptyline
­ Lidocaine
III. LIPOPROTEINS
• Lipoproteins are those macromolecules present in plasma which
portends a greater capacity of forming hydrophobic bonds. The major
reason attributed to this is the larger lipid content present in them. But
the plasma concentration of lipoproteins is very limited as compared to
that of HSA and AAG.
• Role is circulation of lipid to tissue through blood, similarly, transport
drug.
• Molecular weight ranges from 200,000 – 3,400,000 Da.
• Binding by: Hydrophobic bond; Non- competitive.
• Binding drug dissolves in lipid core.
• For example:
­ Acidic: Diclofenac
­ Basic: Chlorpromazine
­ Neutral: Cyclosporine A
• The 4 classes of lipoproteins are observed depending upon their
variations in density:
­ Very Low-Density Lipoproteins (VLDL)
­ Low Density Lipoproteins (LDL)
­ High Density Lipoproteins (HDL).
­ Chylomicrons
IV. GLOBULIN
• α₁ Globulin (Transcortine / Corticosteroid Binding globulin) binds to
steroids drugs, thyroxin.
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115. GM Hamad

• α₂ Globulin (Ceruloplasmin) binds to vitamin A, D, E, K.
• β₁ Globulin (Transferrin) binds to Ferrous ion.
• β₂ Globulin binds to Carotenoids.
• γ Globulin binds to antigens.
V. BINDING OF DRUG TO BLOOD CELLS
• In blood 40% of blood cells of which major component is RBC (95%). The
RBC is 500 times in diameter as the albumin. The rate & extent of entry
into RBC is more for lipophilic drugs.
• The RBC comprises of 3 components:
­ Hemoglobin: It has a M.W. of 64,500 Dal. Drugs like phenytoin,
pentobarbital bind to hemoglobin.
­ Carbonic anhydrase: Carbonic anhydrase inhibitors drugs are bind
to it like acetazolamide & chlorthalidone.
­ Cell membrane: Imipramine & chlorpromazine are reported to
bind with the RBC membrane.
2. BINDING OF DRUG TO EXTRA-VASCULAR TISSUE PROTEIN
• It increases apparent volume of distribution of drug.
• Localization of a drug at a specific site in body.
• Binding order: Liver › Kidney › Lung › Muscles
TISSUE LOCALIZATION OF DRUGS
• The tissue binding of drugs are also very significant processes occurring
in the body. Unlike HSA, the body tissues constitute 100 times that of
HSA i.e. about 40% of the total body weight. Multiple tissue drug binding
is feasible.
• Tissue drug binding is very essential and vital process as it assists in
enhancing the apparent volume of distribution for drugs as this follows a
direct relation with the ratio of concentration of drug in body to-free or
unbound drug in plasma.
• Also, it results in prolonged duration of action due to increase in half-life
reason being the localization of drug at a specific site in the tissues.
Studies also reveal that a very large population of drugs no matter
acidic, basic or neutral undergoes reversible binding whereas the plasma
protein drug binding exhibits vice-versa.
• The order of binding to extravascular tissues is given as:
Liver > Kidneys > Lungs > Muscle
113

116. GM Hamad

I. Liver: Irreversible binding of drugs like paracetamol and their epoxide-
metabolites to liver tissues results in hepatotoxicity.
II. Lungs: Like imipramine, chlorpromazine and antihistamines
accumulation of drugs like imipramine, desipramine or other drugs in
lungs eventually leads to congestion in heart or may even produce
severe lungs cancer.
III. Kidneys: The protein called as metallothion is widely present in kidneys
which have a tendency to undergo complexation with heavy metals such
as lead, mercury and cadmium. This gradually paves path for the major
renal failures or renal toxicity.
IV. Skin: Many drugs are known to accumulate in skin with subsequent
reaction with melanin which can ultimately result in skin diseases. Drugs
such as chloroquine, phenothiazines are usually involved in this.
V. Eyes: The retinal pigments of the eye also contain melanin. Drugs like
chloroquine are responsible for retinopathy as these drugs interact with
the melanin present in the retinal pigments.
VI. Bones: Bones are made up of calcium and most of the antibiotics mainly
like tetracycline exhibits extensive binding to bones and teeth. The
permanent brown-yellow discoloration of teeth is an adverse effect of
administration of such antibiotics to newly born babies or infants.
Similarly lead also generates similar responses with teeth and bones.
Binding of Drugs to Blood Components and Tissues
Blood Components Drugs
1. Plasma Proteins
(A) Albumin
Site I (Warfarin and azoproxazone
binding site)
Naproxen, Indomethacin, phenytoin
and bilirubin.
Site II (Diazepam binding site) BDZs, Ibuprofen, ketoprofen,
probenecid
Site III (Digitoxin binding site) Digitoxin
Site IV (Tamoxifen binding site) Tamoxifen
(B) α1 – Acid
Glycoprotein / Orosomucoid Imipramine, lignocaine, propranolol
(C) Lipoproteins
VLDL Acidic: Diclofenac
LDL Neutral: Cyclosporin A
HDL Basic: Chlorpromazine
(D) Globulins
114

117. GM Hamad

α1 – Globulin/Transcortin/Corticoid
Binding Globulin (CBG)
Cortisone, Prednisone, thyroxin and
vitamin B12
α1 – Globulin / Ceruloplasmin Vitamin A, D, E and K and Cupric ions
β1 Globulin / Transferrin Ferrous ions
β2 – Globulin Carotenoids
γ – Globulin Antigens
2. Blood Cells
Hemoglobin Phenytoin, Pentobarbital
Carbonic anhydrase CA Inhibitors e.g. acetazolamide
Cell Membrane Imipramine and chlorpromazine
3. Tissues
Liver Epoxides of PCM, CHCl3, CCl4
Lungs Chlorpromazine, Anti-histamine
Kidney Heavy metals (Pb, Hg, Cd)
Skin Chloroquine, Phenothiazine
Eye Chloroquine, Phenothiazine
Hair Arsenic, Chloroquine, Phenothiazine
Bone Tetracycline
Fats Thiopental, DDT
Nucleic Acid Chloroquine and Quinacrine

SIGNIFICANCE OF DRUG PROTEIN BINDING
I. DRUG TARGETING
• Targeted drug delivery is the major research area in this era of medical
and life sciences.
• The binding of drugs to lipid containing proteins called as lipoproteins is
effectively utilized for controlled and site-specific drug delivery.
• For this the hydrophilic drug moiety are of great priority.
• The major application of delivering the drug at a predetermined rate is
highly beneficial in treatment and appreciable management of cancer
therapies.
• The best requirement of site-specific drug delivery is revealed in cancer
treatment with that of estramustine.
• Thus, binding of suitable antineoplastic agents like vincristine or
vinblastine with the LDL is desirable to prevent the normal cells from
damage caused by the administered drug.
• The protein drug binding also portends an advantage of efficient drug
distribution, absorption and finally prolonged duration of action for
longer treatment of chronic diseased conditions.
115

118. GM Hamad

II. ABSORPTION
• The absorption equilibrium is attained by transfer of free drug from the
site of administration to the systemic circulation. Following the
equilibrium, the process may stop.
• However, binding of the absorbed drug to plasma proteins decreases
free drug concentration thus sink condition and concentration gradient
are re-established which now act as driving force for further absorption.
III. DISTRIBUTION
• A protein bound drug in particular does not cross the BBB, the placental
barrier, the glomerulus.
• Thus, protein binding decreases the distribution of drugs.
IV. METABOLISM
• Protein binding decreases the metabolism of drugs and enhances the
biological half-life.
• Only unbound fraction gets metabolized.
• E.g. Phenylbutazone and Sulfonamide.
V. ELIMINATION
• Only the unbound drug is capable of being eliminated.
• Protein binding prevent the entry of drug to the metabolizing organ
(liver) and to glomerulus filtration.
• E.g. Tetracycline is eliminated mainly by glomerular filtration.
VI. SYSTEMIC SOLUBILITY OF DRUG
• Lipoprotein act as vehicle for hydrophobic drugs like steroids, heparin,
oil soluble vitamin.
VII. DRUG ACTION
• Protein binding inactivates the drugs so sufficient concentration of drug
cannot be build up in the receptor site for action. E.g. Naphthoquinone.
VIII. DIAGNOSIS
• The chlorine atom of chloroquine replaced with radio-labeled I
131
can be
used to visualize melanomas of eye and disorders of thyroid gland.
IX. SUSTAIN RELEASE
• The complex of drug protein in the blood acts as a reservoir and
continuously supply the free drug. E.g. Suramin sodium-protein binding
for anti-trypanosomal action.
116

119. GM Hamad

(Equation 3)
(Equation 2)
(Equation 1)
KINETICS OF PROTEIN BINDING
• The kinetics of reversible drug protein binding for a protein with one
simple binding site can be described by the LAW OF MASS ACTION as
follows: Protein + Drug ⇋Drug−Protein−Complex
[??????]+[??????] ⇌[????????????]
• From this equation of law of mass action, an association constant Ka can
be expressed as the ratio of the molar conc. of the reactants.
Ka=
[????????????]
[??????][??????]

• This equation assumes only one binding site per protein molecule.
ASSOCIATION BINDING CONSTANT
• The extent of the drug-protein complex formed is dependent on the
association binding constant Ka.
• Drugs that are strongly bind to the plasma proteins have a very high Ka
and exist as the drug –protein complex.
• With such drugs, a large dose may be required to obtain a reasonable
therapeutic conc. of free drug.
• Most kinetic studies in vitro use purified albumin as a standard protein
source because this protein is responsible for the major portion of
plasma drug-Protein binding.
• Experimentally, both the free drug [D] and the protein bound drug [PD],
as well as the total protein conc. [P] + [PD], may be determined.
• To study the binding behavior of drugs, a determinable ratio “r” is
defined, as follows:
&#3627408479;=
??????&#3627408476;&#3627408473;&#3627408466;&#3627408480; &#3627408476;&#3627408467; &#3627408465;&#3627408479;&#3627408482;&#3627408468; &#3627408463;&#3627408476;&#3627408482;&#3627408475;&#3627408465;
??????&#3627408476;&#3627408481;&#3627408462;&#3627408473; &#3627408474;&#3627408476;&#3627408473;&#3627408466;&#3627408480; &#3627408476;&#3627408467; &#3627408477;&#3627408479;&#3627408476;&#3627408481;&#3627408466;??????&#3627408475;

• B/c moles of drug bound is [PD] and the total moles of protein is [P] +
[PD]. This equation becomes:
&#3627408479;=
[????????????]
[????????????]+[??????]

• According to the equation (2):
[PD]=K
a [P][D]
• By substitution into equation (3) the following equation is obtained:
117

120. GM Hamad

(Equation 4)
(Equation 5)
(Equation 6)
(Equation 7)
&#3627408479;=
??????
?????? [??????][??????]
??????
??????[??????][??????]+[??????]

• To simplify, multiply above equation by [1/P]
&#3627408479;=
??????
?????? [??????]
1+??????
?????? [??????]

• This equation describes the simplest situation, in which 1 mole of drug
binds to 1 mole of protein in a 1:1 complex.
• This can assume only one independent binding site for each molecule of
drug.
• If there are ‘n’ identical independent binding sites per protein molecule,
then the following equation is used:
&#3627408479;=
&#3627408475;??????
?????? [??????]
1+??????
??????[??????]

• In terms of ??????
??????, which is
1
??????
??????
⁄equation 5 reduces to:
&#3627408479;=
&#3627408475;[??????]
??????
??????+[??????]

• Protein molecule are quite large as compared to drug molecules and
may contain more than one type of binding sites and the drug binds
independently on each binding site with its own association constant.
Then equation 6 expands to:
&#3627408479;=
&#3627408475;
1&#3627408472;
1[??????]
1+??????
1[??????]
+
&#3627408475;
2&#3627408472;
2[??????]
1+??????
2[??????]
+ …………
• Where the numerical subscripts represent different types of binding
sites. The K’s represent the binding constant and the n’s represent the
no. of binding sites per molecule of albumin.
• These equations assume that each drug molecule binds to the protein at
an independent binding site and the affinity of a drug for one binding
site does not influence binding to other sites.
• In reality, drug-protein binding sometimes exhibits a phenomenon of
cooperativity.
• For these drugs, the binding of the first drug molecule at one site on the
protein molecule influence the successive binding of other drug
molecules.
118

121. GM Hamad

• The binding of oxygen to hemoglobin is an example of drug
cooperativity.
EFFECT OF PROTEIN BINDING ON THE APPARENT VOLUME OF
DISTRIBUTION
1. The extent of drug protein binding in the plasma or tissue affects the VD.
2. Drugs with highly bound to plasma proteins have a low fraction of free
drug (fu = unbound or free drug fraction) in the plasma water.
3. The plasma protein bound drug does not diffuse easily and is therefore
less extensively distributed to tissues.
4. Drugs with low plasma protein binding have larger fu generally diffuse
more easily into tissue and have a greater volume of distribution.
5. Apparent volume of distribution is influenced by lipid solubility in
addition to protein binding.
6. Drugs such as furosemide, tolbutamide and warfarin are bound greater
than 90% to plasma proteins have a VD value ranging from 7.7 – 11.2
liters per 70 Kg body weight.
7. Basic drugs such as imipramine, propranolol are extensively bound to
both tissue and plasma proteins have very large VD values.
8. Displacement of drugs from plasma proteins can affect the
pharmacokinetics of a drug in several ways E.g:
A. Directly increase the free drug conc. as a result of reduced binding
in the blood.
B. Increase the free drug conc. that reaches the receptor sites
directly causing a more intense pharmacodynamics or toxic
response.
C. Increase the free drug conc. causing a transient increase in VD.
D. Increase the free drug conc. resulting in more drug diffusion into
tissues of eliminating organs particularly the liver and kidney,
resulting in a transient increase in drug elimination.
9. The effect of drug protein binding must be evaluated carefully before
dosing changes are made.
CLINICAL SIGNIFICANCE OF DRUG-PROTEIN BINDING
1. Most drugs bind reversibly to plasma proteins to some extent.
2. When the clinical significance of the fraction of drug bound is concerned,
it is most important to know whether the study was performed using
pharmacological or therapeutic plasma drug concentration.
119

122. GM Hamad

3. The fraction of drug bound can be changed with plasma drug
concentration and dose of drug administration.
4. In addition, the patient’s plasma proteins conc. should be considered.
E.g. if a patient has a low plasma protein conc. then, for any given dose
of drug, the conc. of free (unbound) bioactive drug may be higher than
anticipated bound drug.
5. The plasma protein concentration is controlled by a no. of variable
including:
­ Protein synthesis
­ Protein catabolism
­ Distribution of albumin b/w intravascular and extravascular space
­ Excessive elimination of plasma protein, particularly albumin.
6. A number of diseases (bodily or mental injury, caused by an external
agent), age, trauma and related circumstances affect the plasma protein
conc. E.g:
­ Liver disease results in decrease in plasma albumin conc. due to
decreased protein synthesis.
­ In nephrotic syndrome an accumulation of water metabolites,
such as urea and uric acid, as well as an accumulation of drug
metabolites, may alter protein binding of drugs.
­ Severe burns may cause an increased distribution of albumin into
the extracellular fluid, resulting in a smaller plasma albumin conc.
­ In certain genetic diseases, the quality of the protein that is
synthesized in the plasma may be altered due to a change in the
amino acid sequence.
­ Both chronic liver disease and renal disease, such as uremia, may
cause an alteration in the quality of plasma protein synthesized.
7. An alteration in the protein quality may be demonstrated by an
alteration in the association constant or affinity of the drug for the
protein.
8. When a highly protein bounded drug is displaced from the binding by a
second drug or agent, a sharp increase in the free drug conc. in the
plasma may occur leading to toxicity. E.g. An increase in the free
warfarin level was responsible for an increase in bleeding when warfarin
was co-administered with phenylbutazone, which competes for the
same protein binding site.
120

123. GM Hamad

9. Albumin has two known binding sites that share the binding of many
drugs.
­ Binding site I is shared by phenylbutazone, sulfonamide,
phenytoin and valproic acid.
­ Binding site II is shared by the semi-synthetic penicillins,
probenecid, medium chain fatty acid and benzodiazepines.
­ Some drugs bind to the both sites.
­ Displacement occurs when a second drug is taken that competes
for the same binding site in the protein as the initial drug.
CONDITIONS CAPABLE OF ALTERING PLASMA PROTEINS
Decreased plasma proteins Increased Plasma Proteins
Albumin
• Burns
• Chronic liver disease
• Nephrotic syndrome
• Chronic renal failure
• Trauma
• Hypothyroidism

α1 Acid Glycoprotein
• Nephrotic syndrome • Myocardial infarction
• Renal failure
• Rheumatoid arthritis
• Trauma
REPRESENTATIVE ACID & BASIC DRUGS DEMONSTRATING MORE
THAN 90% BINDING TO PLASMA PROTEINS
Acidic drugs Basic Drugs
• Aspirin
• Probenecid
• Naproxen
• Warfarin
• Penicillin
• Phenytoin
• Phenylbutazone
• Diazepam
• Propranolol
• Lidocaine
• Quinidine
• Lorazepam
• Nifedipine
• Verapamil
METHODS FOR STUDYING DRUG-PROTEIN BINDING
• Equilibrium dialysis
• Dynamic dialysis
• Diafiltration
• Ultrafiltration
• Gel chromatography • Spectrophotometry
121

124. GM Hamad

PHARMACOKINETIC VARIATIONS IN
DISEASE STATE
• Most common diseases which require dose adjustment and applications
of pharmacokinetics are:
­ Renal disease
­ Hepatic disease
RENAL DISEASE
GRADES OF RENAL IMPAIRMENT
Grade
GFR
(ml/min per 1.73 m⁻²)
Serum Creatinine
(mg/dl)
Normal 120 – 130 0.7 – 1.4
Mild 50 – 80 1.5 – 3.0
Moderate 30 – 50 3.0 – 7.0
Severe < 30 > 7.0

UREMIA
• Uremia is impaired Glomerular filtration.
• Uremia causes accumulation of:
­ Excessive fluid in body
­ Blood nitrogenous products
• Causes of uremia:
­ Disease or trauma to kidney
­ Drug overdose
­ Drug induced e.g. by ACE inhibitor, aminoglycosides,
amphetamine and NSAIDS.
• Diseases causing uremia:
­ Diabetes mellitus
­ Hypertension
­ Glomerulonephritis
­ Polycystic disease
­ Obstruction or
infection in kidney
EFFECT OF RENAL DISEASE ON PHARMACOKINETICS
Uremia effects nearly all PK processes, though main effects is on excretion.
1. Decreased renal drug excretion due to:
­ Reduced GFR
122

125. GM Hamad

­ Reduced active secretions
2. Physiologic and metabolic changes due to disturbance in electrolyte and
fluid balance.
3. Oral Bioavailability is generally unchanged but rate of absorption (ka)
may be changed.
4. Indirectly altered absorption and decreased bioavailability as a result of
disease related changes in GIT motility and pH caused by nauseas and
diarrhea:
­ Mesenteric blood flow may be altered.
­ Bioavailability of drugs (having high first pass effect) increased e.g.
BA of propranolol may be increased.
5. Altered distribution as a result of changes in fluid balance, drug protein
binding in plasma or tissue and total body water:
­ Water soluble drugs (e.g. aminoglycosides) may have altered Vd.
­ Digoxin Vd is lower in renal impairment.
6. Altered protein binding due to accumulation of drugs metabolites and
biochemical metabolites (free fatty acids and urea) which compete for
protein binding sites for active drugs:
­ Plasma protein binding of weak acidic drugs in uremic patients is
decreased.
7. Biotransformation and renal excretion is also altered in renal
impairments.
8. Metabolism of drugs that are metabolized in kidney is generally
impaired.
9. First pass hepatic metabolism is decreased and thus leads to increased
BA.
10. Elimination half-life is generally prolonged due to reduced GF.
11. Reduced total body Clarence.
GENERAL APPROACHES FOR DOSE ADJUSTMENT IN RENAL DISEASE
DOSE ADJUSTMENT BASED ON DRUG CLEARANCE
• Method based on drug clearance try to maintain the desired Average
steady-state plasma drug concentration C
AV
∞
which is:
&#3627408438;
????????????
∞
=
&#3627408441;&#3627408439;
0
&#3627408438;&#3627408473;
&#3627408455;??????

­ Where, F = fraction of drug absorbed, D
0 = dose, ?????? = dosing
interval.
123

126. GM Hamad

• For patients with uremic condition or renal impairment, total body
clearance will change to a new value, Cl
T
u
. Therefore, to maintain the
same desired C
AV
∞
, the dose must be changed to a uremic dose, D
o
u
, or
the dosage interval must be changed to τ
u
, as shown in the following
equation:
&#3627408438;
????????????
∞
=
&#3627408439;
??????
??????
&#3627408438;&#3627408473;
&#3627408455;
??????
??????
??????
=
&#3627408439;
&#3627408476;
??????

&#3627408438;&#3627408473;
&#3627408455;
??????
??????
??????



Rearranging above Equation and solving for D
o
u

&#3627408439;
&#3627408476;
??????
=
&#3627408439;
??????
??????
&#3627408438;&#3627408473;
&#3627408455;
??????
??????
??????
&#3627408438;&#3627408473;
&#3627408455;
??????
??????
??????

If the dosage interval ?????? is kept constant, then the uremic dose D
o
u
is
equal to a fraction (&#3627408438;&#3627408473;
&#3627408455;
??????
&#3627408438;&#3627408473;
&#3627408455;
??????
) of the normal dose, as shown in the
equation:
&#3627408439;
0
??????
=
&#3627408439;
0
N
&#3627408438;&#3627408473;
T
u
&#3627408438;&#3627408473;
T
N

• For IV infusions same desired Css is maintained both for patients with
normal renal function and for patients with renal impairment. Therefore,
rate of infusion, R must be changed to a new value, R
u
for uremic
patient.
&#3627408438;
&#3627408454;&#3627408454;=
??????
&#3627408438;&#3627408473;
&#3627408455;
??????
=
??????
&#3627408475;
&#3627408438;&#3627408473;
&#3627408455;
??????



DOSE ADJUSTMENT BASED ON CHANGES IN ELIMINATION RATE CONSTANT
• Overall Ke that is reduced in uremic patient. To maintain the
&#3627408438;
????????????
∞
assuming VD is same for both normal and uremic patients, then
uremic dose &#3627408439;
0
??????
is fraction (k
u
/k
N
) of normal dose:
&#3627408439;
0
??????
=
&#3627408439;
0
??????
&#3627408472;
??????
&#3627408472;
??????

• Renal clearance is product of Vd and rate constant of renal excretion, i.e.
&#3627408438;&#3627408473;
&#3627408453;
??????
=&#3627408472;
&#3627408453;
??????
&#3627408457;
??????
&#3627408475;

(normal) (uremic)
(normal) (uremic)
124

127. GM Hamad

CREATININE BASED GFR MEASUREMENT
• GFR is the rate at which the substance is filtered from blood into the
urine per unit time.
• The clearance of creatinine is used most extensively as a measurement
of GFR. Creatinine is an endogenous substance formed from creatine
phosphate during muscle metabolism.
• Creatinine production varies with age, weight, and gender of the
individual.
• The following equation is used to calculate creatinine clearance in
mL/min when the serum creatinine concentration is known:
&#3627408438;&#3627408473;
cr=
Rate of urinary excretion of creatinine
Serum concentration of creatinine

&#3627408438;&#3627408473;
cr
&#3627408438;
u&#3627408457; x 100
&#3627408438;
cr x 1440

• Where, Ccr = creatinine concentration (mg/dL), V = volume of urine
excreted (mL), Cu = concentration of creatinine in urine (mg/mL), and Clcr
= creatinine clearance in mL/min.
DOSE ADJUSTMENT FOR UREMIC PATIENTS
• Dose adjustment for drugs in uremic or renally impaired patients should
be made in accordance with changes in pharmacodynamics and
pharmacokinetics of the drug in the individual patient.
BASIS FOR DOSE ADJUSTMENT IN UREMIA
• The loading drug dose is based on the apparent volume of distribution of
the patient.
• The maintenance dose is based on clearance of the drug in the patient.
Uremic dose=
??????
u
??????
N
x normal dose
­ Where, Ku = uremic elimination rate constant, KN = normal
elimination rate constant.
• When the dosage interval ?????? is kept constant, the uremic dose is always a
smaller fraction of the normal dose. Instead of reducing the dose for a
uremic patient, the usual dose is kept constant and the dosage interval ??????
is prolonged according to the following equation:
125

128. GM Hamad

Dosage interval in uremia,??????
u=
??????
N
??????
u
x ??????
N
­ Where, ??????
u is the dosage interval for the dose in uremic patients
and ??????
N is the dosage interval for the dose in patients with normal
renal function.
HEPATIC DISEASE
• Hepatic disease can alter drug pharmacokinetics including absorption
and disposition as well as pharmacodynamics including efficacy and
safety.
• Hepatic disease may include common hepatic diseases, such as alcoholic
liver disease (cirrhosis) and chronic infections with hepatitis viruses B
and C, and less common diseases, such as acute hepatitis D or E, primary
biliary cirrhosis, primary sclerosing cholangitis, and α1-antitrypsin
deficiency.
EFFECTS OF HEPATIC DISEASE ON PHARMACOKINETICS
NATURE & SEVERITY
• Not all liver diseases affect the pharmacokinetics of the drugs to the
same extent.
DRUG ABSORPTION
• Hepatic disease alters absorption
­ GIT dysfunction
­ Biliary obstruction
DRUG DISTRIBUTION
• Altered drug protein binding due to effect on quality and quantity of
albumins and globulins and other plasma proteins.
• Hyperbilirubinemia leads to displacement of highly protein bound drugs.
HEPATIC BLOOD FLOW
• Blood flow changes can occur in patient with chronic liver disease.
INTRINSIC CLEARANCE
• Ability of liver that is clear of blood in the absence of blood flow
limitation and binding to cells or proteins in the blood. It is a measure of
enzyme activity in liver.
126

129. GM Hamad

HEPATIC EXTRACTION RATIO
It is the fraction of drug entering the liver in blood which is irreversibly
removed or extracted during one pass of the blood through liver.
DRUG ELIMINATION
• Drug elimination in the body may be divided into:
­ Fraction of drug excretion unchanged (fe)
­ Fraction of drug metabolized
FRACTION OF DRUG METABOLISED
• Estimated from 1 – fe
• Fraction of drug metabolized may be estimated from ratio of Clh/Cl
• Drugs with low fe values conversely dugs with higher ratio of
metabolized drug are more affected by change in liver function due to
hepatic disease
• The following equation assumes that all metabolism occurs in liver and
all the unchanged drug is excreted in urine:
&#3627408438;&#3627408473;
h=&#3627408438;&#3627408473;(1−&#3627408467;&#3627408466;)
­ Where:
▪ Clh is hepatic clearance
▪ Cl is total body clearance
▪ 1 – fe is fraction of drug metabolized.
ASSESSMENT OF LIVER FUNCTION
• There is no single clinical laboratory test used for assessment of total
liver function.
• Usually a series of clinical laboratory tests are used to detect presence of
liver disease and extent of liver damage.
• Liver function test ,such as ALT and AST, only indicate that the liver has
been damaged, but they do not assess the function of the cytochrome p-
450 enzymes or intrinsic clearance by liver.
DOSE ADJUSTMENT WITH LIVER DISEASE
• Dose adjustment for patient with liver disease is more difficult than for
patient with impaired renal function. Unlike the creatinine clearance for
the kidney, for the liver there is no in vivo surrogate to predict drug
clearance.
127

130. GM Hamad

• Due to lack of such in vivo markers, prediction concerning adjustment in
patient with liver disease can only be made based on kinetic property of
drug in patient with liver disease.
• No endogenous indicator of hepatic disease can be relied on to adjust
dosage regimens. This is because there are several enzyme systems in
liver and not all are affected equally.
• Thus, all drugs may not be affected equally by particular disease.
• Drugs are classified according to their liver extraction ratio (ER) and
listed in monograph as high, moderate and low.
&#3627408440;??????=1−
??????&#3627408456;&#3627408438;
&#3627408476;??????????????????
??????&#3627408456;&#3627408438;
????????????

­ Drug showing high ER values needs to carefully assessed for
dosing regimens.
­ Enzymes dependent drugs are usually given in half doses or
less and then response is monitored.
­ Drugs with flow dependent clearance are avoided and if need
are reduced to 1/10
th
of the conventional doses.
128

131. GM Hamad

INTRAVENOUS INFUSION
INTRODUCTION
• IV solutions may be given either as a bolus dose or infused slowly
through a vein into the plasma at a constant or zero-order rate.
• A single IV bolus dose may rapidly produce the desired therapeutic level
of drug but is ineffective when the plasma or tissue concentration
required to be maintained i.e. a prolonged maintenance of therapeutic
effect is desired.
• The aim of administration of drug as continuous infusion is to maintain a
flat (un-fluctuated) profile of drug concentration in blood. The
intravenous infusion is given as constant amount per unit time thus, the
input is of zero order. After 5 half-lives of the drug, steady state
concentration (Css) is achieved where the rate of drug input equals its
rate of output.
SIGNIFICANCE
• IV infusion allows precise control of plasma drug concentrations to fit
the individual needs of the patient.
• For drugs with a narrow therapeutic window (e.g., heparin), IV infusion
maintains an effective constant plasma drug concentration by
eliminating wide fluctuations between maximum and minimum plasma
concentration.
• The IV infusion of drugs, such as antibiotics, may be given with IV fluids
that include electrolytes and nutrients.
• The duration of drug therapy may be maintained or terminated as
needed using IV infusion.
BLOOD DRUG CONCENTRATION PROFILE OF IV INFUSION
• The blood-drug profile of the IV Infusion is given in Figure below. First
the drug concentration increases, become constant and remain constant
until the infusion is stopped.
• When the infusion is stopped, the conc. decline by the first order
kinetics. The post infusion blood level time curve resembles the
pharmacokinetic profile of IV.
129

132. GM Hamad

• Several factors affect this blood concentration of drug after IV Infusion.
• Increase in the drug concentration in blood depends upon the
elimination half-life and the value of Css, while the rate of increase of
blood concentration depends on clearance and the volume of
distribution (Vd). The Vd also affects the time taken to reach Css.
• Drug concentrations before, on and after achieving Css are usually the
important parameters for infusion which are required to be calculated
for I/V infusions.
• The values of volume of distribution (Vd) and clearance (Cl) are used to
calculate loading and maintenance doses which are usually required in
clinical practice.




CATEGORIES OF DRUGS GIVEN THROUGH IV INFUSION
• The drugs with narrow therapeutic index and short half-life are given
through intravenous infusion to avoid fluctuations in concentration.
• The pulsed infusion is an intermittent infusion used to give drug slowly
to reduce the chances of toxicity and is given within 30-120 minutes.
PHARMACOKINETIC PARAMETERS OF IV INFUSION
NUMBER OF COMPARTMENTS
• The number of compartments is observed from the terminal phase of
the blood level time data of the drug after achieving Css (i.e., from the
Post Css curve). A single linear line shows one compartment model. The
post infusion blood level time curve resembles the pharmacokinetic
profile of IV one compartment model in the above case.
ELIMINATION RATE CONSTANT
• From the post infusion blood level time curve, elimination rate constant
(Ke) is calculated from the terminal slope by using the equation of slope:
k
e=
LnY
2−LnY
1
x
2−x
1

130

133. GM Hamad

HALF LIFE
• The time in which the concentration of the drug reduces to half. It is
calculated by using following formula:
t
½=
0.693
k
e

THE STEADY STATE CONCENTRATION (CSS)
• The Css may directly be estimated from the plasma level time curve when
the concentration curve becomes flatter.
• If the values of clearance is known, Css can be calculated by using
following formula:
C
ss=
IR
C??????

­ Where, IR = infusion rate, Cl = drug clearance
DRUG CLEARANCE
• The Css estimated from plasma level time curve graph is used to calculate
clearance (Cl).
C??????=
IR
C
ss
⨯(1−e
K
e⨯T
inf)
­ Where, Tinf = time for infusion
VOLUME OF DISTRIBUTION (VD)
• The volume of distribution (Vd) is calculated with the following formula:
Vd=
C??????
K
e

PREDICTED CONCENTRATION BEFORE COMPLETION OF CSS (PRE CSS)
• The concentration of drug before achieving Css is calculated by following
expression:
C
t pre Css=
IR
C??????
⨯(1−??????
−k
e⨯T
inf)
1. Where, Ct pre Css = concentration at any time before achieving Css,
IR= Infusion rate, Ke = elimination rate constant.
PREDICTED CONCENTRATION AFTER COMPLETION OF CSS (POST CSS)
• The concentration of drug after achieving Css is calculated by
131

134. GM Hamad

following expression:
C
t post Css=
IR
C??????
⨯(1−??????
−k
e⨯T
inf)⨯(1−??????
−k
e⨯(t−T
inf)
)
1. Where, Ct post Css = drug concentration at any time after achieving
Css ,t = time post Css ,Ke = elimination rate constant; Tinf = time for
infusion.
APPLICATIONS OF IV INFUSIONS
• Examples of clinical applications to maintain effectiveness and to control
the toxicity of short half-life drugs that need dosage individualization
include the following:
1. Insulin in hyperglycemic coma
2. Heparin in acute thrombosis
3. Lignocaine in acute arrhythmia
4. Isosorbide dinitrate – coronary vasodilation
5. Adrenergic agonists to maintain the BP and Cardiac output
6. Diazepam for antiepileptic control
7. Aminophylline to maintain bronchodilatation/ respiration in
newborn.
132

135. GM Hamad

BIOPHARMACEUTICAL ASPECTS IN
DEVELOPING A DOSAGE FORM
PRODUCT DESIGN
• Drugs are not generally given as pure chemical substances, but these are
formulated into finished dosage form (drug products) for example:
tablets, capsules, ointments etc. before being administered to the
patients for therapy.
FORMULATED DRUG PRODUCTS
• It includes the active drug substances plus selected ingredient
(excipients) that make up the dosage form.
• The drug products are designed to deliver the drug for local or systemic
effect.
COMMON DRUG PRODUCTS
• It includes:
­ Liquids (emulsion, suspension, syrup)
­ Tablets, capsules (as solid dosage form)
­ Injectable
­ Suppositories
­ TDDS
­ Topical products (creams, ointments and lotions)
BIOPHARMACEUTICS AND RATIONALE FOR DRUG PRODUCT DESIGN
• It is the study of the in-vivo impact of the physicochemical properties of
the drugs and drug products on (after) the drug delivery to the body
under normal or pathological conditions.
• The branch of pharmaceutical sciences that deals with the study of
relationship between physicochemical properties of drug in a dosage
form and therapeutic response observed after its administration.
BIOAVAILABILITY
• It is of primary concern in biopharmaceutics and defined as, “The rate
and the extent of active drug that becomes available at the site of
action.”
133

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• The changes in the bioavailability affect the changes in
pharmacodynamics and toxicity of the drugs.
AIM OF BIOPHARMACEUTICS
• The aim is to adjust the delivery of the drug from the drug product in
such a manner as to provide optimal therapeutic activity and safety for
the patients.
BASIS OF BIOPHARMACEUTICAL STUDIES
• Biopharmaceutical studies are allowed for rational design of the product
and it is based on:
1. Physical and chemical properties of the drug substance.
2. Route of drug administration (oral, topical, injectable, implant,
transdermal patch etc.)
3. The desired pharmacodynamic effects (i.e. immediate or
prolonged effect)
4. Toxicological properties of the drug.
5. Safety of the excipients.
6. Effects of the excipients and dosage form on or after the drug
delivery.
7. Pharmacogenetics or Pharmacogenomics (It is the area of study
that deals and concerned with unusual drug response related to
the genetic or hereditary factors).
CONSIDERATION IN DESIGN OF DRUG PRODUCT
• Considerations in the design of a drug product to deliver the active drug
with the desired bioavailability characteristics and therapeutic objectives
include:
1. The physicochemical properties of the drug molecule.
2. The finished dosage form (e.g., tablet, capsule, etc.)
3. The nature of the excipients in the drug product.
4. The method of manufacturing.
5. The route of drug administration.
DRUG USED FOR LOCAL ACTIVITY
• The drug intended for local activity are designed to have a direct:
­ Pharmacodynamics action without affecting the other body
organs.
­ These drugs may be applied to skin, nose, eyes, muscle
membranes, buccal cavity, throat, rectum and even vagina.
134

137. GM Hamad

DIFFERENT LOCAL ROUTES
• A drug intended for local activity may be given:
­ Orally
­ Intravaginally
­ Intranasally
­ Into the urethral tract
­ Into the ear and eyes
DRUGS USED FOR LOCAL ACTION
• Anti-infective
• Antifungal
• Antacids
• Local anesthetics
• Astringents
• Antihistamines
• Corticosteroids.
FACTORS WHICH MUST BE CONSIDERED
• Route of drug administration is very important in drug product design.
1. FOR VAGINA
• The design of vaginal (tablet) formulation for the treatment of fungal
infection must use the ingredients compatible with the vaginal
anatomy and physiology.
2. OPHTHALMIC ROUTE
• An eye medication must require special biopharmaceutical
considerations i.e.
­ pH
­ Isotonicity
­ Sterility
­ Local irritation to
cornea
­ Concern for the
systemic absorption.
3. EXTRAVASCULAR ROUTE (IM)
• In Intramuscular injection the following factors must be considered:
­ Local irritation
­ Drug dissolution
­ Drug absorption form the intramuscular site.
• Following are the factors that affect systemic absorption from
extravascular site:
­ Anatomic and physiologic properties of the site.
­ The physiochemical properties of the drug and drug product.
4. INTRAVASCULAR ROUTE
• If the drug is given by an IV route for e.g. IV injections, the systemic
drug absorption is considered complete or 100% bioavailability
because the drug is placed directly into the general circulation.
5. SPECIAL MEDICAL DEVICE
135

138. GM Hamad

• In some cases, the drug product is designed in a specialized medical
device or packaging components.
• For example: a drug solution or suspension may be formulated to work
with a nebulizer or inhaler for the administration of the drug into the
lungs.
• Following are the factors that influence drug release from nebulizer:
­ Physical characteristics of the nebulizer.
­ The formulation of the drug product.
­ Can influence the droplet particles and the spray pattern that the
patient receives upon inhalation of the drug product.
IMPORTANCE OF ROUTE OF ADMINISTRATION
• By choosing the route of administration and carefully designing the drug
product, the bioavailability of the active drug can be varied from rapid
and complete absorption to slow sustained rate of absorption or even
no absorption depending on the therapeutic objectives.
• The rate of the drug release from the product and the rate, and the
extent of the drug absorption are important in determining:
­ Distribution of the drug
­ Onset of action
­ Intensity of pharmacological action
­ Duration of the drug action
IMPORTANCE OF BIOPHARMACEUTICAL CONSIDERATIONS
• The biopharmaceutical consideration determines the ultimate dose and
dosage form of a drug product e.g. dosage for a topical drug product
(ointment) is often expressed in concentrations or a percentage of the
active drug in the formulation (for example 0.5% hydrocortisone
ointment)
• The biopharmaceutical studies must be performed to ensure that the
drug product does not irritate, cause allergic response or allow
significant systemic absorption.
• The dosage for the systemic absorption is given on the bases of the mass
such as milligram or gram.
• The therapeutic dose may be based on the weight or surface area of the
patient and the therapeutic doses are expressed as mass per unit
bodyweight (mg/ kg) or mass per unit surface area (mg/ m
2
)
136

139. GM Hamad

RATE LIMITING STEPS IN DRUG ABSORPTION
• For solid oral immediate release of drug product (tablet, capsule) the
rate process includes:
­ Disintegration of drug products of subsequent release of the drug.
­ Dissolution of the drug in an aqueous environment.
­ Absorption across the cell membrane into systemic circulation.
SLOWEST STEP IN RATE LIMITING STEP
• In the process of drug disintegration, dissolution and absorption, the
rate at which drug reach the circulatory system is determined by the
slowest step in the sequence.
• The slowest steps in the series of kinetic process are called as the rate
limiting step.
• For the drugs that have very poor aqueous solubility the rate at which
drug dissolves (dissolution) is the slowest step therefore exerts the rate
limiting effect on the drug bioavailability.
• For the drugs that have high solubility dissolution rate is rapid and the
rate at which the drug crosses the cell membrane is the slowest or rate
limiting step.
• For a drug to exert its biological effect, it must be:
­ Transported by the body fluids
­ Traverse the required biological membrane barriers
­ Escape widespread distribution to unwanted areas
­ Endure metabolic attack
­ Penetrate in adequate concentration to the sites of action
­ Interact in a specific fashion
­ Causing an alteration of cellular function.
PHARMACEUTICAL FACTORS AFFECTING DRUG BIOAVAILABILITY
• The type of the drug product (e.g. solutions, suspensions, suppositories,
tablets, and capsules)
• The nature of the excipients in the drug product
• Physicochemical properties of the drug molecule
• The route of administration

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140. GM Hamad

• Pharmaceutical factors affecting drug bioavailability include:
­ Disintegration
­ Dissolution and solubility
1. DISINTEGRATION
• According to USP it can be defined as; “the state in which no residue of
the tablets except the fragments of insoluble coating material or
capsule shell remaining on the screen of the test apparatus in the soft
mass and have no firm core.”
• For immediate release solid oral dosage form the drug product must
disintegrate into small particles and release the drug. USP has
established an official disintegration test.
• According to BP specification of disintegration:
­ Simple tablet should disintegrate in not more than 15 mins
­ Film coated tablets should disintegrate in not more than 30 mins
­ Sugar coated should disintegrate in not more than 1 hour
­ Enteric coated should disintegrate in not more than 2 – 3 hours
2. DISSOLUTION AND SOLUBILITY
• Dissolution is “a process by which a solid drug substance becomes
dissolve in a solvent”. OR it is “a process in which solid substance
solubilizes in a given solvent i.e. mass transfer from the solid surface to
liquid phase”.
• Solubility is “the mass of solute that dissolve in the specific mass or
volume of solvent at a given temperature”.
• Solubility is a static property whereas dissolution is a dynamic property.
• The drug dissolution in an aqueous medium is an important prior
condition for the system absorption.
• Thus, dissolution test may be used to predict the bioavailability and may
be used to determine formulation factors that affect drug bioavailability.
• Dissolution test is required for all the US, FDA approved solid oral drug
products.
DIFFERENCE BETWEEN DISSOLUTION AND SOLUBILITY
Dissolution Solubility
Dissolution rate is defined as the
amount of solid substance that goes
into solution per unit time under
standard conditions of temperature,
No. of parts of solute dissolved in
one part of solvent.
Absolute solubility is defined as the
maximum amount of solute dissolved
in a given solvent under standard
138

141. GM Hamad

pH, solvent composition and
constant solid surface area.
conditions of temperature, pressure
and pH
It is a dynamic process. It is a static process.

EXPRESSION OF SOLUBILITY
Descriptive term
Parts of solvent required for 1 part
of solute
Very soluble Less than 1 part
Freely soluble 1 – 10 parts
Soluble 10 – 30 parts
Sparingly soluble 30 – 100 parts
Slightly soluble 100 – 1000 parts
Very slightly soluble 1000 – 10,000 parts
Practically insoluble More than 10,000 parts

DRUG DISSOLUTION PROCESS

DISSOLUTION TEST AND NOYES AND WHITNEY EQUATION
• Noyes – Whitney studied the rate of dissolution of the solid drugs.
• According to their observations the process of the drug dissolution at
the surface of the solid particles forming a saturated solution around the
particles that is called as “stagnant layer.”
• The dissolved drug in the saturated solution known as “stagnant layer”
diffuses to the bulk of the solvent from regions of high drug
concentration to the regions of low drug concentrations.
• The equation is as follows:
&#3627408517;
&#3627408516;
&#3627408517;
&#3627408533;
=
&#3627408491;??????
??????
(&#3627408490;
&#3627408532;−&#3627408490;)
139

142. GM Hamad

• Where,
­ dc/ dt = rate of drug dissolution at time t
­ D = Diffusion rate constant
­ A = Surface are of particles
­ Cs = Concentration of the drug (equal to the
solubility of the drug) in the stagnant layer
­ C = Concentration of the drug in bulk solvent
­ h = Thickness of stagnant layer.
DIFFUSION LAYER MODEL OR FILM THEORY

FACTORS AFFECTING DISSOLUTION
• The rate of dissolution dc/dt is the rate of drug dissolved per unit of time
and expressed as concentration change in the dissolution fluid.
• The equation shows that the dissolution in the flask may be affected by
the physicochemical characteristics of the drug, the formulation, the
solvent.
• Drug in the body, particularly in the GIT, is considered to be dissolving in
an aqueous environment.
• Permeation of drug across the gut wall is affected by the ability of the
drug to diffuse (D) and to partition between the lipid membrane. A
favorable partition coefficient (k oil/water) will facilitate drug
absorption.
• The temperature of the medium, agitation rate also affects the rate if
drug dissolution (37℃, peristaltic movement).
• Other factors include:
­ The physical and chemical properties of the API.
­ Nature of the excipients.
­ Method of manufacture.
Dissolution of a solid
drug particle in a
solvent
140

143. GM Hamad

PHYSICOCHEMICAL NATURE OF THE DRUG
• The physical and chemical properties of the drug substance as well as
the excipients are important considerations in the design of the drug
product e.g. IV solutions are different to prepare with drugs that have
poor aqueous solubility.
• The drugs that are physically or chemically unstable may require special
excipients coating or manufacturing process to protect the drug form
the degradation.
1. SOLUBILITY, PH & DRUG ABSORPTION
• In designing the oral dosage form the formulator must consider the
natural pH environment of the GIT varies form acidic in the stomach to
slightly alkaline in the small intestine.
• A basic drug is more soluble in acidic media forming a soluble salt, and
acid drug is more soluble in the intestine and forming a soluble salt at
the more alkaline pH.
• The solubility – pH profile gives a rough estimation of the completeness
of the dissolution for a dose of a drug in the stomach or in the small
intestine.
• Solubility may be improved by the addition of an acidic or basic
excipient.
• For example: the solubilization of aspirin may be increased by the
addition of an alkaline buffer.
• In the formulation of controlled release drugs buffering agents may be
added to slow or modify the release rate of a fast dissolving drug.
2. STABILITY, PH & DRUG ABSORPTION
• If drug decomposition occurs by addition of acid or base catalysis, some
prediction of the degradation of the drug in the GIT may be made.
• Example: Erythromycin has pH dependent stability profile. In acidic
medium, as in stomach erythromycin decomposition occurs rapidly
whereas in neutral or alkaline pH the drug is relatively stable.
Consequently, erythromycin tablets are enteric coated to protect against
acid degradation in stomach.
• The dissolution rate of erythromycin powder varied from 100% dissolved
in 1 hour to less than 40% dissolved in 1 hour.
• The slow dissolving raw drug material (API) also resulted in slow
dissolving drug particles.
141

144. GM Hamad

• The dissolution of powdered raw drug material is very useful “in-vitro”
method for predicting bioavailability problems of erythromycin product
in body.
3. PARTICLE SIZE & DRUG ABSORPTION
• The effective surface area of the drug is increased by a reduction in the
particle size because dissolution takes place at the surface of solute
(drug) the greater the surface area more rapid is the rate of dissolution.
• Many drugs are very active when given intravenously but are not very
effective when given orally because of poor oral absorption.
• Example: Griseofulvin and many steroidal drugs with low aqueous
solubility, reduction of particle size by milling to a micronized form has
improved the oral absorption of these drugs.
• Smaller particle size results in an increase in the surface area of the
particles, enhance water penetration into the particles and increase the
dissolution rate.
• For poorly soluble drugs, a disintegrant may be added to ensure the
rapid disintegration of the tablets and release of the particles.
• The addition of surface-active agents may increase wetting as well as
solubility of the drugs which has poor aqueous solubility.
4. POLYMORPHISM, SOLVATES AND DRUG ADSORPTION
• Polymorphism refers to the arrangement of a drug substance in various
crystal forms or polymorphs.
• The term polymorph has been used to describe.
­ Polymorphs itself (Polymorphs are the same chemical structure
but different physical properties, for e.g. solubility, hardness,
density, compression characteristics)
­ Solvates (Solvates are the forms that contains a solvent (solvates)
or water (hydrates))
­ Amorphous forms (The amorphous forms are non-crystalline
forms)
­ Desolvated solvates (Desolvated solvates are the forms that are
made by removing the solvent form the solvates)
• Some polymorphic crystals have much lower aqueous solubility than the
amorphous forms and causing a product to be incompletely absorbed.
­ Example: Chloramphenicol has several crystal forms and when it is
given orally as suspension the drug concentration dependent on
142

145. GM Hamad

the percentage of β–polymorphs in the suspension because the β–
form is more soluble and better absorbed.
• A drug that exists as an amorphous form (non-crystalline form) generally
desorbs more rapidly than the same drug in a more rapid crystalline
form.
­ Example: antibiotic, novobiocin it is 10 times more soluble than its
crystalline form and have similar differences in dissolution rate.
• Some polymorphs are “metastable” and may convert to a more stable
form over time.
• A change in crystal form may cause problems in manufacturing of the
product. E.g. a change in the crystal structure of the drug may cause
cracking in a tablet or even prevent agranulation from being compressed
into a tablet. Reforming of a product may be necessary if a new crystal
form of a drug is used.
HYDRATES
• Water may form special crystals with drugs called “hydrates” e.g.
erythromycin hydrates have quite different solubility compared to the
anhydrous form of the drug.
SOLVATES
• Some drugs interact with solvent during preparation to form crystals it is
called as solvates.
• Example: The ampicillin trihydrate on the other hand less absorbed than
its anhydrous form because of fast dissolution of the anhydrous form of
the ampicillin because of faster dissolution of the latter.
Other Physiochemical Characteristics of the Drug
• Salt form
• Degree of Ionization
• Hygroscopicity
• Liquid / water solubility
5. SALT FORM OF DRUG
• A lot of drugs from natural origin are insoluble in their parent form. So,
they are converted into salt form to render them more soluble. Salt form
of the drug is more soluble and thus enhances the dissolution property
of the drug product.
• EXAMPLE:
143

146. GM Hamad

I. Alkaloids are converted into their salt form e.g. hyoscine
butylbromide.
II. Aspirin, a weak acid when formulated with sodium bicarbonate,
will form a water-soluble salt in an alkaline medium, in which the
drug rapidly dissolves.
6. HYGROSCOPICITY
• Many pharmaceutical compounds and salts are sensitive to water vapor
or moisture. When compounds come in contact with moisture, they
retain the water by bulk or surface adsorption, capillary condensation,
chemical reaction and, in extreme cases, a solution (deliquescence).
Deliquescence is where a solid dissolve and saturates a thin film of water
on its surface. It has been shown that when moisture is absorbed to the
extent that deliquescence takes place at a certain critical relative
humidity, the liquid film surrounding the solid is saturated. This process
is dictated by vapor diffusion and heat transport rates.
• Moisture is also an important factor that can affect the stability of drugs
candidate and their formulations. Sorption of water molecules onto a
candidate drug (or excipient) can often induce hydrolysis.
• In this situation, by sorbing onto the drug-excipient mixture, the water
molecules may ionize either or both of them and induce a reaction.
7. LIPID WATER SOLUBILITY
• It is also known as Partition coefficient. It may give some indication of
the relative affinity of the drug for oil and water. A drug that has high
affinity for oil may have poor release and dissolution from the drug
product.
8. LIMITED WATER
• Water is used up during the degradation reaction, and there is not
enough present to degrade the compound completely. Adequate water:
Sufficient water is present to decompose the compound completely.
• Excess water: This is an amount of water equal to or greater than
amount of moisture necessary to dissolve the drug. As such, this may
decompose drug with time.
144

147. GM Hamad

FORMULATION FACTORS AFFECTING DRUG PRODUCT
1. Excipients are added to a formulation to provide certain functional
properties to the drug and dosage form. These properties are used to
improve:
­ The compressibility of the active drug
­ Stabilize the drug against degradation
­ Decrease gastric irritation
­ Control the rate of the drug absorption from absorption site
­ Increase drug bioavailability
2. Excipient in the drug product may also affect the dissolution kinetics of
the drug, either by altering the medium in which the drug is dissolved or
by reacting with the drug itself.
3. Tablet Lubricants such as magnesium stearate may repel water and
reduce dissolution.
4. Coating Material e.g. shellac, decrease the dissolution rate.
5. Some excipients, such as sodium bicarbonate, may change pH of the
medium surrounding the active drug substance.
­ For example: aspirin, a weak acid when formulated with sodium
bicarbonate will form water soluble salt in an alkaline medium, in
which the drug rapidly dissolves and the term for this process is
“dissolution in a reactive medium”.
6. Excipients in a formulation may interact directly with the drug to form a
water soluble or water insoluble complex.
­ For example: if tetracycline is formulated with calcium carbonate,
an insoluble complex of Catetracylcline is formed, that has a slow
rate of dissolution and poor absorption.
7. Excipients may be added intentionally to the formulation to increase or
decrease the rate and extent of drug absorption.
­ For example: the excipients that increase the aqueous solubility of
the drug, increase the rate of dissolution and drug absorption.
8. Excipients may act as carrier to increase the drug diffusion across the
intestinal wall.
9. Many excipients may retard drug dissolution and thus reduce drug
absorption.
10. Excipients should be pharmacodynamically inert.
11. They change the functionality of the drug from the dosage form. e.g.
excipients used for a tablet.
145

148. GM Hamad

12. For solid dosage forms such as compressed tablets, excipients may
include:
­ Diluents (e.g. lactose)
­ Disintegrants (e.g. starch)
­ Lubricant (e.g. magnesium stearate)
­ Stability agents
­ Other components e.g. binding agents (i.e. starch paste, gelatin
solution)
DISSOLUTION AND DRUG RELEASE TESTING
1. Dissolution and drug release test are in-vitro test that measure the rate
and extent of dissolution and release of the drug substance from a drug
product usually in an aqueous medium under specified conditions.
2. It is an important quality control procedure for drug product and is often
linked to the product performance in-vivo.
3. In-vivo drug dissolution studies are most often used for monitoring the
drug product stability and manufacturing process control.
4. USP-NF sets standards for dissolution and drug release test of most of
drug product.
5. The dissolution method used for particular drug product in vitro relates
to the bioavailability of the drug in vivo.
6. The dissolution method should be able to discriminate change since
formulation of drug product.
7. It is may be used for:
­ Batch to batch drug release
­ Batch to batch drug release infirmity
­ Stability
­ Predict in-vivo performance
­ Scale up and post approval changes (SUPAC)
8. Dissolution test is valuable tool in formulation of drug product.
9. A suitable dissolution method may uncover a formulation problem with
the drug product that could result in a bioavailability problem.
10. Each dissolution method is specific for drug product and its formulation.
11. The dissolution should be able to reflect the changes in the formulation,
manufacturing process physical and chemical characteristics of the drugs
such as: particle size, polymorphs, surface area etc.
12. Once a suitable dissolution test is obtained, acceptable dissolution
criteria are developed for the drug product and its formulation.
146

149. GM Hamad

DISSOLUTION CONDITIONS
• The development of appropriate dissolution test requires the
investigators to try different agitation rates, different mediums
(including volume and pH medium) and different kinds of apparatus.
SIZE AND SHAPE
• Size and shape of dissolution vessels may affect the rate and extend of
dissolution e.g. dissolution vessel may range in size from several ml to
several liters.
• The shape may be round-bottomed or flat, so the tablet might be in
different position in different experiments.
VOLUME
• Usual volume of medium is 50 – 1000 ml.
• Drugs that are poorly soluble may require use of very large capacity
vessel up to 2000 ml to observe significant dissolution.
SINK CONDITION (SOAKING)
• It is a term referred to an access volume of the medium that allows the
solid drug to dissolve continuously.
• If the drug solution becomes saturated, no further net drug dissolution
will take place.
STIRRING RATE AND TEMPERATURE
• The stirring rate is 50 – 75 rpm up to 100 rpm.
• The temperature is about 37℃.
DISSOLUTION MEDIUM
• Commonly used dissolution mediums are:
­ Deaerated water (free from gases e.g. CO2 by boiling)
­ Buffer aqueous solution (pH 4 – 8)
­ Dilute HCl may be used (0.1N HCl)
­ Phosphate buffer
­ Simulated gastric fluid
­ Simulated intestinal juice
• Choice of the media depends on:
­ Nature of drug product.
­ Location in the GIT where the drug is expected to be dissolved.
147

150. GM Hamad

NOTE
• No single apparatus and test can be used for all drug products. Each drug
product must be tested individually with dissolution test that best
correlates to in-vivo bioavailability. Usually the dissolution test will state
that: “Certain percentage of the label amount of the active drug in a
drug product that must dissolve within a specified period of time”
MEETING DISSOLUTION REQUIREMENT
1. USP / NF sets dissolution requirement for many products.
2. The requirements apply both the basket and the paddle methods.
3. “The amount of drug dissolved within a given period of time (Q) is
expressed as a %age of label content”
4. The Q is generally specified in the individual monograph for a drug
product to pass the dissolution test.
5. Three stages (S1, S2, S3) of testing are allowed by USP-NF.
6. Initially 6 tablets or capsules are tested for dissolution test.
7. If dissolution test fails to meet the criteria for S1 then six more tablets
are tested.
8. Dissolution test continues until the dissolution criteria are met or until
the three stages are exhausted.
9. For many products, the passing value for Q is set at:
­ 75% in 45 minutes.
­ Some products require a Q of 85% in 30 minutes.
­ 75% in 60 minutes.
TABLE / CRITERIA FOR ACCEPTANCE
Stage Number Acceptance
S1 6 Each unit is no less than 5%
S2 6
Average Of 12 units (S1 + S2) is equal to or less than Q and
no unit is less than Q1 – 5%
S3 12
Average of 24 units (S1 + S2 + S3) is equal to or greater
than Q1 not more than 2 units are less than Q – 15% and
no unit is less than Q – 25%

<
DISSOLUTION APPARATUS
Apparatus Name Drug Product
Apparatus 1 Rotating basket Tablet, capsule
Apparatus 2 Paddle apparatus Tablet, capsule, suspension
148

151. GM Hamad

Apparatus 3 Reciprocating
cylinder
Extended release drug product
Apparatus 4 Flow cell
Drug product containing low
water soluble drugs
Apparatus 5 Paddle over disc TDDS
Apparatus 6 Cylinder TDDS
Apparatus 7 Reciprocating disc Extended release drug product
Rotating basket Non-USF, NF Extended release drug product
Diffusion cell Non-USF, NF Ointment, cream and TDDs

ROLE OF EXCIPIENTS
Excipient Property in dosage form
Lactose Diluent
Starch Disintegrant, Diluent
Microcrystalline cellulose Disintegrant, Diluent
Steric acid, Mg stearate Lubricant
Talc Glidant, Lubricant
Sucrose (solution) Granulating Agent (Binder)
Hydroxy propyl methyl cellulose Tablet coating agent
Titanium dioxide Combined with dyes as colored coating
Methyl cellulose Coating or granulating agent
Cellulose acetate phthalate Enteric coating agent
BIOPHARMACEUTICAL CONSIDERATION IN DRUG PRODUCT DESIGN
• The active drug should be adequately delivered at proper rate to the
targeted receptor site in order to achieve the desired therapeutic effect.
The drug must traverse the required biological membrane barriers,
escape widespread distribution to unwanted areas, endure metabolic
attack, and cause an alteration of cellular function. Finished dosage form
should show the highest bioavailability with minor or no adverse effects.
1. Pharmacodynamic Consideration
­ Therapeutic Objective
­ Toxic Effects
­ Adverse Reactions
2. Drug Consideration
­ Chemical and Physical Properties of Drug
3. Drug Product Considerations
­ Pharmacokinetics of Drug
­ Bioavailability of Drug
149

152. GM Hamad

­ Route of Drug Administration
­ Desired Dosage Form
­ Desired Dose of the Drug
4. Patient Consideration
­ Compliance and Acceptability of the Drug Product Cost
5. Manufacturing Considerations
­ Cost
­ Stability
­ Quality Control
­ Method of Manufacturing
­ Availability of raw material
PRODUCT DESIGN
• The prime consideration in the design of a drug product is safety and
efficacy.
• The drug product must effectively deliver the active drug at an
appropriate rate and amount to the target receptor site so that the
intended therapeutic effect is achieved.
• The finished dosage form should not produce any additional side effects
or discomfort due to the drug or excipients.
• Ideally, all excipients in the drug product should be inactive ingredients
alone or in combination in the final dosage form.
• The finished drug product should meet the therapeutic objective by
delivering the drug with maximum bioavailability and minimum adverse
effects.
1. PHARMACODYNAMIC CONSIDERATION
• Evaluation of the drug effect in the body and its mechanism of action is
called as Pharmacodynamics. Therapeutic considerations include desired
Pharmacodynamics and pharmacologic properties of the drug, including
the desired therapeutic response and the type and frequency of adverse
reactions to the drug.
I. THERAPEUTIC OBJECTIVE
• Therapeutic objective will influence the type of drug product to be
manufactured.
• A drug used to treat an acute illness should be formulated to release the
drug rapidly, allowing for quick absorption and rapid onset of action. E.g.
Nitroglycerine is formulated in a sublingual tablet for the treatment of
angina pectoris.
• For prophylactic use in the treatment of certain chronic disease such as
asthma, an extended or controlled release dosage form is preferred.
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153. GM Hamad

Advantage: The extended-release dosage form releases the drug slowly,
thereby controlling the rate of drug absorption and allowing for more
constant plasma drug concentrations.
2. DRUG CONSIDERATIONS
• Physicochemical Properties of Drugs:
­ These are the factors or properties that can be controlled or
modified by formulation.
­ The properties influence:
▪ The type of dosage form.
▪ The process for the manufacture of the dosage form.
• Physical Properties of Drugs such as dissolution, particle size and
Crystalline form are influenced by method of processing and
manufacturing.
• If the drug has low aqueous solubility and an IV injection is desired, a
soluble salt of the drug may be prepared.
3. DRUG PRODUCT CONSIDERATIONS
I. PHARMACOKINETICS OF THE DRUG
• Knowledge of pharmacokinetic profile of the drug is important to
estimate the appropriate amount (dose) of drug in the drug product and
a release rate will maintain a desired drug level in the body.
• The therapeutic window determines the desired or target plasma
concentration that will be effective with minimal adverse effects.
• Drug concentration higher than the therapeutic window may cause
more intense pharmacodynamic toxic response for many drugs.
• Drug concentration below therapeutic window may be sub-therapeutic.
• There is a relationship between plasma concentration and therapeutic
response.
• There is a drug concentration below that the drug is ineffective, the
minimum effective concentration (MEC) and above which the drug has
unwanted or adverse effects the minimum toxic concentration (MTC).
• That defines the range in which we attempt to keep the drug
concentration therapeutic range.
• Most people respond to drug concentration in the therapeutic ranges.
• There is always possibility that the range will be different in an individual
patient.
Penicillinase Resistant
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• Penicillinase resistance of many G+ and G- bacteria is due to their
elaboration of penicillin-destroying enzymes called beta-lactamase.
• They are produced by large number of bacteria (e.g. Staphylococci,
Enterococci, Meningococci, Gonococci and various other) that convert
penicillin into inactive.
II. BIOAVAILABILITY OF THE DRUG
• Bioavailability is a pharmacokinetic term that describes the rate and
extent to which the active drug ingredient is absorbed from a drug
product and becomes available at the site of drug action.
• The stability of the drug in GIT including the stomach and intestine is
another consideration.
• Some drugs such as Penicillin G are unstable in the acidic medium of the
stomach.
• The addition of buffering agent in the formulation or the use of enteric
coating on the dosage form will protect the drug from degradation at
low pH.
• Some drugs have poor bioavailability b/c of the first pass effects (pre-
systemic metabolism), more dose (conc.) may be needed be such as in
case of propranolol or alternative rate of drug administration as in case
of insulin.
• If the drug is not absorbed after oral route or a higher dose causes
toxicity then drug must be given by an alternative route of
administration, and a different dosage form such as parenteral drug
product might be needed.
III. DOSE CONSIDERATION
• The size of dose in the drug product is based on the inherent potency of
the drug and its apparent volume of distribution, which determines the
target plasma drug conc. needed for the desired therapeutic effect.
• For some drugs wide variation in the size of the dose is needed for
different patients b/c of large inter subject differences in the
pharmacokinetic and bioavailability of the drug.
• Inter-subject variation to drug therapy to difference in acetylation of
drug is a well-studied example of genetic polymorphism.
• The desired final drug size manufactured is determined by main two
factors: the dose of the drug and the excipients.
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• Patient ability to metabolize certain drugs such as hydralazine,
procainamide, ‘isoniazid’ can be categorized as either ‘fast acetylators’
normal or ‘slow acetylators’.
• It depends on patient’s genetic composition which determines the
activity of the acetylation enzyme N-acetyl transferase.
• Acetylation status determine whether a patient is dosed with a
correspondingly or lower dose compared to normal acetylator.
• Therefore, the drug product must be available in several dose strength
to allow for individualized dosing.
• The size and shape of a solid drug product are designed for easy
swallowing.
IV. DOSE FREQUENCY
• The dosing frequency is related to the clearance of the drug and the
target plasma drug concentration.
• If the pharmacokinetics shows that the drug has a short duration of
action due to a short elimination half-life or rapid clearance from the
body, the drug must be given more frequently.
• To minimize fluctuation in plasma drug concentration and to improve
patient compliance, an extended release drug product may be preferred.
• An extended release product contains 2 or more doses of the drug that
are released over prolonged period.
V. ROUTE OF DRUG ADMINISTRATION:
• Bioavailability of drug: Route of drug administration greatly affect the
bioavailability of the drug which eventually can affects the duration of
the pharmacologic effect.
• Design: The design of a drug must be taken into careful consideration to
achieve intended pharmacologic effects.
• Types: Systemic or local effects.
­ Local effects – application of the drug directly on the site of action
(e.g. eyes, nose, or skin)
­ Systemic effects – administration of the drug into the systemic
circulation and its subsequent action on the specific desired site
(oral, rectal, parenteral, topical (epicutaneous), implant,
transdermal patch)
4. PATIENT CONSIDERATION
• The drug product and therapeutic regimen must be acceptable to the
patient.
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• Poor patient compliance may result from poor product attributes, such
as:
­ Difficulty in swallowing
­ Disagreeable odor
­ Bitter medicine taste
­ Two frequent or unusual dosage requirements
• In recent years, creative packaging has allowed the patient to remove
one tablet, each day from a specially designed package so that the daily
doses are not missed.
5. MANUFACTURING CONSIDERATION
• In the design of a drug dosage form, the pharmaceutical manufacturer
must consider:
­ The intended route of administration
­ The size of dose
­ The anatomic and physiologic characteristics of the administration
site, such as:
▪ Membrane permeability
▪ Blood flow
­ The physicochemical properties of the site, such as:
▪ pH
▪ Osmotic pressure
▪ Presence of physiologic fluids
­ Interaction of the drug and dosage form at the administration site.
­ Product quality and performance.
­ Quality control important components for drug product
manufacturing – dissolution & drug release tests.
­ Stability – long term stability of drug product – critical attribute of
overall product quality.
­ Lower cost.
­ Availability of raw materials.
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IN-VITRO – IN-VIVO CORRELATION (IVIVC)
INTRODUCTION
CORRELATION
• Mathematically and statistically – relationship that exists between two
variables.
• USP - establishment of a relationship between a biological property
/parameter and a physicochemical property of same dosage form.
• FDA - a predictive mathematical model describing relationship between
an in-vitro property of dosage form and its relevant in-vivo response.
IN-VITRO – IN-VIVO CORRELATION
• FDA’s in-vitro and in-vivo properties:
­ In-vitro property - rate or extent of drug dissolution or release.
­ In-vivo response - plasma drug concentration or amount of drug
absorbed.
• With biopharmaceutical standpoint, relationship between any
appropriate in-vitro release characteristics and any in-vivo bioavailability
parameters.
CORRELATION LEVELS
• Based on the ability to correlate, levels of correlation are:
­ Level A
­ Level B
­ Level C
­ Level D
• FDA describes IVIVC Levels A to C.
1. LEVEL A CORRELATION
• Highest category of correlation.
• Represents a point-to-point relationship between in-vitro dissolution
rate and in-vivo absorption rate of drug from dosage form.
­ % drug absorbed (calculated by model dependent techniques such
as Wagner-Nelson procedure or Loo-Riegelman method or by
model-independent numerical convolution or deconvolution.
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­ Better than a single-point approach since it utilizes all of
dissolution and plasma level data for correlation.
• Defines a direct relationship between in-vivo and in-vitro data:
­ In-vitro dissolution rate alone sufficiently determines
biopharmaceutical rate of dosage form.
­ An in- vitro dissolution curve serves as a surrogate for in vivo
performance.
• An excellent quality control procedure since it can predict dosage forms’
in-vivo performance.
• Justifies the following without need of additional human studies:
­ Change in manufacturing site.
­ Change in manufacturing method.
­ Change in raw material supplies.
­ Minor formulation modification.
­ Modification in product strength.
2. LEVEL B CORRELATION
• Utilizes comparison/correlation of parameters computed based on
principles of statistical moment analysis.
• Correlation between Mean In-vitro Dissolution Time MDTvitro to
­ Mean Residence Time (MRT) or
­ Mean In-vivo Dissolution Time MDTvivo
• Uses entire in-vitro and in-vivo data but not considered to be a point-to-
point or profile-to- profile correlation.
• Does not uniquely reflect actual plasma level curves.
• Thus, level B correlation alone cannot be relied on to justify:
­ Formulation modification.
­ Change in manufacturing site.
­ Change in excipient source.
• In-vitro data alone could not be used to justify extremes of quality
control standards.
3. LEVEL C CORRELATION
• One dissolution time point is compared to one mean pharmacokinetic
parameter as:
­ t50% or t90% to AUC, tmax or Cmax.
• Represents a single point correlation.
• Doses do not reflect entire shape of plasma drug concentration curve,
which is a good indicative of performance of modified-release products.
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• Weakest level of correlation.
­ Partial relationship between absorption and dissolution is
established.
• Limited in predicting in vivo drug performance and subjected to the
same caveats as a Level B correlation.
• Useful in the early stages of formulation development when pilot
formulations are being selected.
• Cant’ be used for waiver of an in-vivo bioequivalence study (biowaiver).
MULTIPLE LEVEL C CORRELATION
• Relates one or several pharmacokinetic parameters of interest (Cmax,
AUC,) to amount of drug dissolved at several time points of dissolution
profile.
• Used to justify a biowaiver, provided that correlation has been
established over entire dissolution profile with one or more
pharmacokinetic parameters of interest.
• Relationship should be demonstrated at each time point at same
parameter such that effect on in-vivo performance of any change in
dissolution can be assessed…
­ If a multiple level C correlation is achievable, then the
development of a level A correlation is also likely.
• Should be based on at least 3 dissolution time points covering the early,
middle, and late stages of the dissolution profile.
4. LEVEL D
• A rank order and qualitative correlation.
• Not considered useful for regulatory purposes.
• Not a formal correlation.
• However, serves as an aid in the development of a formulation or
processing procedure.
PARAMETERS USED IN ESTABLISHING CORRELATION
Level In-vitro In-vivo
A Dissolution curve
Input (absorption)
curves
B
Statistical moments:
MDT
Statistical moments:
MRT, MAT, etc.
C
Disintegration time,
Time to have 10,50,90%
Cmax, Tmax, Ka, Time to
have 10,50,90%
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dissolved, Dissolution
efficiency
absorbed, AUC (total or
cumulative)
Key: MDT = mean dissolution time, MRT = mean residence time, MAT = mean absorption time

DISSOLUTION RATE VS ABSORPTION RATE
• If dissolution is rate limiting, a faster dissolution rate may result in a
faster rate of appearance of drug in plasma.
• A correlation between rate of dissolution and Ka may be established.
• A rapid drug dissolution may be distinguished from slower drug
absorption by observation of Tmax.
• Tmax, being easier to measure than ka, thus it is used in correlating
dissolution data.
PERCENTAGE DRUG DISSOLVED VS PERCENTAGE DRUG
ABSORBED
• If a drug is absorbed completely, a linear correlation
may be obtained by comparing % drug absorbed to %
drug dissolved.
Tmax VS PERCENTAGE DRUG DISSOLVED
• Phenytoin sodium demonstrates a linear correlation
between tmax and the percent of drug dissolved.
­ In-vitro–in-vivo correlation between tmax and percent
drug dissolved in 30 minutes. Letters on graph
indicate different products.
Cmax VERSUS PERCENTAGE DRUG DISSOLVED
­ A poorly formulated drug may not be
completely dissolved and released, resulting
in lower plasma drug concentrations.
­ % drug released at any time interval will be greater for a more
bioavailable drug product.
­ Cmax will be higher for drug product that shows highest %
dissolved.
DRUG CONCENTRATION VS PERCENTAGE DRUG DISSOLVED
• Serum concentration of drug at single point can be correlated to % of
drug dissolved at single point.
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­ E.g., aspirin dissolution is rate-limiting step, and its formulations
with different dissolution rates will cause differences in serum
concentration of aspirin by minutes.
IMPORTANCE OF IVIVC
• IVIVC – a main focus of attention of pharmaceutical industry, academia,
research and regulatory sectors.
• IVIVC is an essential component in formulation development and
optimization of dosage form.
• Formulation development is required for a new and existing drug
molecule as a continuous process, integral part of manufacturing and
marketing.
• Development and optimization of formulation is:
­ Time consuming and costly process.
­ Requires modifications in composition, manufacturing process,
equipment and batch sizes.
• Regulatory authorities requires proof that modified formulations are
bioequivalent.
• Bioequivalence is carried out in humans.
• In-vitro and additional human studies are required in:
­ Change in manufacturing site.
­ Change in manufacturing method.
­ Change in raw material suppliers.
­ Minor formulation modification.
­ Modification in product strength.
• Human studies are expensive and has many ethical issues.
• A valid in-vitro test which could reflect bioavailability/ bioequivalence is
an alternative to human study.
­ Valid = discriminative, well correlated and a surrogate.
• IVIVC may eliminate or, at least reduce number of human studies.
• Thus, the main objective of an IVIVC is to serve as a surrogate for in-vivo
bioavailability and to support biowaivers.
• Assists in quality control for scale up and post-approval changes to:
­ Improve formulations.
­ Change production processes.
• In vitro means of assuring that each batch of the same product will
perform identically in-vivo.
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• Reduces the regulatory burden for additional in-vivo experiments, under
certain conditions.
• IVIVC serves as justification for:
­ Biowaivers.
­ Scale-up or post approval changes (SUPAC).
­ Line extensions (e.g., different dosage strengths).
• IVIVC is Well-defined for modified-release drug products.
• IVIVC is more difficult to predict for immediate-release drug products.
• IVIVC demonstrates that predictability of in-vivo performance of a
formulation from its in-vitro dissolution characteristics over a range of
in-vitro dissolution release rates and manufacturing changes.
• IVIVC is useful for establishing upper and lower dissolution specifications
for a solid oral dosage form.
• IVIVC has relationship with dissolution and BCS.
IVIVC RELATION WITH DISSOLUTION
• When a proper dissolution method is chosen, the rate of dissolution may
be correlated to rate of absorption of drug into body.
• USP-NF may list multiple dissolution tests for some of the products.
• Examples:
­ 10 separate dissolution tests for theophylline extended-release
capsules labeled for OD have been enlisted.
­ USP-NF has separate and distinct dissolution test requirements for
two different phenytoin sodium capsule formulations.
­ For extended-release phenytoin capsules, not more than 35%,
between 30% and 70% and not less than 85% of the labeled
amount dissolves in 30 minutes, 60 minutes, and 120 minutes,
respectively.
­ For prompt capsules, release must not be less than 85% of the
labeled amount in 30 minutes.
BIOPHARMACEUTICS CLASSIFICATION SYSTEM (BCS)
• Biopharmaceutics Classification System (BCS) – a drug development tool.
• Allows estimation of contribution of 3 factors, dissolution, solubility and
intestinal permeability in rate and extent of drug absorption from solid
oral dosage forms.
­ Dissolution is process by which drug is released, dissolved and
becomes ready for absorption.
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­ Permeability is ability of drug to permeate through a membrane
into systemic circulation.
RELATIONSHIP OF BCS AND IVIVC
• BCS is a predictive approach to relate certain physicochemical
characteristics of a drug substance and drug product to in-vivo
bioavailability.
• Since predictability is a major function of IVIVC, any system that predicts
in-vivo performance from in-vitro data may be considered an IVIVC.
CHARACTERISTICS OF DRUGS IN BCS
• BCS Class I drug product contains a highly soluble drug substance that is
highly permeable and from which the drug rapidly dissolves from the
drug product.
• BCS Class II – has low solubility and high permeability.
• BCS Class III – has high solubility and low permeability.
• BCS Class IV – has low solubility and low permeability.
BCS and expected IVIVC for immediate release drug products
BCS Class Solubility Permeability IVIVC
I High High
Established (if dissolution is rate
limiting step)
II Low High
Expected (if in-vitro release is
equal to in-vivo release
III High Low
Little or no – since absorption is
rate limiting
IV Low Low Little or no IVIVC is expected
IVIVC FAILURE
• Sometimes a drug fails dissolution test and yet is well absorbed showing
lack of correlation which may be due to:
­ Complexity of drug absorption and the weakness of the
dissolution design.
­ A formulation with fatty components may be subjected to longer
retention in GIT.
­ Inadequately simulation of in-vivo conditions with a simple
dissolution medium, e.g., effect of digestive enzymes or other
indigenous substance.
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