Absorption of drugs in pharmacology .ppt

Pharmacokinetics

2. OBJECTIVE
1.Define absorption.
2.Discuss different processes of transport
across cell membrane.
3.Define Fick’sLaw.
4.Enumerate factors affecting drug absorption.
5.What is HendersenHesselbachequation?

3. ABSORPTION
Transport of Drugs
across Cell membrane
DR. MEHTAB MUNIR
Assistant Professor
Pharmacology
Federal Medical College

4. To obtain
Right effect
Of right intensity
At right time
For right duration
With minimum toxicity

5. ABSORPTION
It is the transfer of drug from site of
administration to the blood stream
For intravenous route, Absorption is
complete

6. THE BASIC PROCESSES BY WHICH DRUG
PASSES THROUGH CELL MEMBRANE
Passive diffusion
Filtration
Facilitated diffusion
Active transport
Pinocytosis

7. 1.PASSIVE DIFFUSION
Mainprocessbywhichmostofthedrugsare
absorbed
Drugpassesthroughcellmembrane
fromhigherconcentrationtolower
concentration
Itdependsupon
(a)Absorbingsurfacearea
(b)Lipidsolubility
(c)Concentrationgradient

8. PASSIVE DIFFUSION (cont.)
It follows Fick's law, According to this law
* Greater the concentration gradient greater will be the
drug absorption
* Larger the surface area, greater will be the drug
absorption
* Thicker the medium, lesser will be the drug absorption
* Larger the molecule size, lesser will be the drug
absorption

9. Fick'slaw:
Flux (molecules per unit time) =
Area x Permeability coefficient
(C1-C2) x ------------------------------------------
Thickness

11. 2. FILTERTATION
Drugspassthroughporesofmembrane
bymeansoftheirhydrostaticorosmotic
pressure
Water,ionsandsomelowmolecular
weightsubstancespassthroughthe
pores

13. 3.FACILITATED DIFFUSION
Drugspassthroughcellmembranewiththe
helpofcarrierortransportproteins
Drugsaretransportedalongtheirconcentration
gradient
Selectiveforchemicalstructuresofadrugi.e.carrier
transportonlythosedrugswhichhavespecific
molecularstructure
Doesnotrequireenergy
Onlyfewendogenoussubstanceslikeglucoseand
aminoacidsareabsorbedthroughthisprocess

15. 4. ACTIVE TRANSPORT
Specializedprocessinwhichdrugsmove
acrossthecellmembranewiththehelpof
carrierortransportproteins
Saturableprocess
Worksagainsttheconcentrationgradienti.e.
fromlowtohighconcentration

16. Selectiveforchemicalstructureofadrug
Requiresenergy
Onlyfewdrugsareabsorbedbythisprocess
Renaltubules,choroidsplexusandneuronal
membrane

18. 5.PINOCYTOSIS OR ENDOCYTOSIS
Minormethodbywhichdrugsare
transportedintothecells
Cellsengulfthedrugbythrowing
pseudopodia
Sucrosereachesthecellbythisprocess
Unicellularmicro-organismstakethe
nutrients

20. FACTORS AFFECTING THE
DRUG ABSORPTION
(A)FACTORS RELATED TO THE DRUG
1. Physical state of the drug
2. Disintegration of the drug
3. Molecular size of the drug
4. Solubility of the drug
5. Ionization of the drug

21. (B) FACTORS RELATED TO THE RECIPIENT
1. Absorbing surface area
2. Functional integrity of the absorbing
surface
3. Vascularity of the absorbing surface
4. Time of drug administration
5. Co-administration of other drugs
6. PH of medium

22. pH of the medium and drug
ionization
Unionizedorundissociatedformsofdrugare
lipidsoluble,lesspolarastheyhavenocharge
Ionizedformsarepolarandwatersolubleand
lessabsorbed
pHofmediumplaysimportantroleinionizationof
thedrugandaffectsthedrugabsorption

23. Weakacidsionizelessinacidicmedium,so
arebetterabsorbedandweakbasesare
betterabsorbedinalkalinemedium
Converselyweakacidsareionizedmorein
alkalinemediumandweakbasesionizemore
inacidicmedium,hencepoorlyabsorbed
pkaisthepHatofmediumatwhichbothun-
dissociatedanddissociatedformsareequal
(50%ofthedrugisionized)

24. The Henderson Hasselbach equationrelates the ratio
of protonated and unprotonated weak acid or
weak base to the molecule of pKa and the pH of
medium as follows
(Protonated)
Log-----------------------=pKa -pH
(Unprotonated)
This equation applies to both acidic and basic drugs

25. ObservationshowsthatlowerthepHrelative
topKa,thegreaterwillbefractionofthe
drugintheprotonatedformbecause
unchargedformismorelipidsoluble,more
ofaweakacidwillbeinlipidsolubleformat
acidpH,whilemoreofabasicdrugwillbein
thelipidsolubleformatalkalinepH