Pharmacodynamics (Mechanisn of drug action)

A power point presentation on Pharmacodynamics (what drug does to the body) suitable for undergraduate medical students beginning to study Pharmacology

1. Pharmacodynamics
Dr. D. K. Brahma
Department of Pharmacology
NEIGRIHMS, Shillong

2. What is Pharmacodynamics?
What drugs do to the body when they enter?
Study of action-effect of drugs and dose-effect relationship
Defn.: It is the study of biochemical and physiological effects of drug and
their mechanism of action at organ level as well as cellular level
Also Modification of action of one drug by another drug

3. Drug Action by Physical/Chemical
properties
•Color – Tincture Card co.
•Physical mass – Ispaghula
•Physical form – Dimethicone (antifoaming)
•Smell - Volatile Oils
•Taste - Bitters
•Osmotic action – Mannitol, Magsulf
•Adsorption – Activated Charcoal
•Soothing-demulcent – Soothing agents like calamine
•Oxidizing property – Pot. Permanganate
•Chelation – EDTA, dimercaprol
•Radioactivity - Iodine and others
•Radio-opacity – Barium sulfate
•Chemical properties – Chelating agents (EDTA, dimercaprol)
•Scavenging effect – Mesna (with cyclophosphamide)

4. PRINCIPLES OF DRUG ACTION
-Do NOT impart new functions on any system, organ
or cell
-Only alter the PACE of ongoing activity
•STIMULATION
•DEPRESSION
•IRRITATION
•REPLACEMENT
•CYTOTOXIC ACTION

5. MECHANISM
OF DRUG
ACTION

6. MECHANISM OF DRUG ACTION
•MAJORITY OF DRUGS INTERACT WITH
TARGET BIOMOLECULES:
Usually a Protein
1.ENZYMES
2.ION CHANNELS
3.TRANSPORTERS
4.RECEPTORS

7. 4. Receptors
•Drugs usually do not bind directly with enzymes, channels,
transporters or structural proteins, but act through specific
macromolecules – RECEPTORS
•Definition: It is defined as a macromolecule or binding site
located on cell surface or inside the effector cell that serves
to recognize the signal molecule/drug and initiate the
response to it, but itself has no other function, e.g.
Muscarinic (M type) and Nicotinic (N type) receptors of
Cholinergic system

8. Some Common Terms
•Agonist: An agent which activates a receptor to produce an effect
similar to a that of the physiological signal molecule, e.g. Muscarine
and Nicotine
•Antagonist: an agent which prevents the action of an agonist on a
receptor or the subsequent response, but does not have an effect
of its own, e.g. atropine and muscarine
•Inverse agonist: an agent which activates receptors to produce an
effect in the opposite direction to that of the agonist, e.g. DMCM in
BDZ receptors
•Partial agonist: An agent which activates a receptor to produce
submaximal effect but antagonizes the action of a full agonist, e.g.
opioids
•Ligand: (Latin: ligare – to bind) - any molecule which attaches
selectively to particular receptors or sites (refers only binding or
affinity but no functional change)

9. Evidences of Drug action via receptors
– Historical
1.Drugs exhibit structural specificity of action:
example - Catecholamines
2.Competitive Antagonism: Between agonists and
antagonists (Atropine - M type receptors) – by
Langley
3.Acetylcholine - 1/6000
th
of cardiac cell surface –
maximal effect – by Clark

10. Drug – Receptor occupation theory –
Clark`s equation (1937)
•Drugs are small molecular ligands (pace of cellular function
can be altered)
•Drug (D) and receptor (R) interaction governed by “law of
mass action”
•Effect (E) Is the direct function of the Drug-Receptor complex
•But DR complex may not be sufficient to elicit E (response)
•D must be able to bring a conformational change in R to get E
•Affinity and Intrinsic activity (IA)
D + R DR E (direct function of DR complex)
K1
K2

11. Receptor occupation theory – contd.
•Affinity: Ability to bind with a Receptor
•Intrinsic activity (IA): Capacity to induce functional
change in the receptor
•Competitive antagonists have Affinity but no IA
•Therefore, a theoretical quantity (S) – denoting
strength was interposed
D + R DR S E
K1
K2

12. Definitions redefined
If explained in terms of “affinity and IA”:
•Agonist: Affinity + IA (1)
•Antagonist: Affinity + IA (0)
•Partial agonist: Affinity + IA (0-1)
•Inverse agonist: Affinity + IA (0 to -1)

13. Two-state receptor model

14. Drug-receptor binding and
agonism
•Drug- Receptor:
D
Ri DRa
D
Ri DRa
D
Ri DRa
D
DRi DRa
Full agonist
Partial agonist
Neutral
Inverse agonist

15. Nature of
Receptors
•Not hypothesis anymore – proteins and nucleic acids
•Isolated, purified, cloned and amino acid sequencing done
•Cell surface receptors remain floated in cell membrane lipids
•Non-polar hydrophobic portion of the amino acid remain buried in
membrane while polar hydrophilic remain on cell surface
•Major classes of receptors have same structural motif – pentameric etc.
•But, majority of individual receptor molecules are made up of non-
identical subunits – ligand binding brings about changes in structure or
alignment of subutits
•Binding of polar drugs in ligand binding domain induces conformational
changes (alter distribution of charges and transmitted to coupling domain
to be transmitted to effector domain
•Many drugs act on Physiological receptors – also true drug receptors

16. Receptor Subtypes
•Evaluation of receptors and subtypes – lead to discovery of
various newer target molecules
•Example Acetylcholine - Muscarinic and Nicotinic
–M1, M2, M3 etc.
–NM and NN
–α (alpha) and β (beta) ….
•Criteria of Classification:
–Pharmacological criteria – potencies of selective agonist and
antagonists – Muscarinic, nicotinic, alpha and beta adrenergic etc.
–Tissue distribution – beta 1 and beta 2
–Ligand binding
–Transducer pathway and Molecular cloning

17. Action – effects !
•Receptors : Two essential functions:
•Recognition of specific ligand molecule
•Transduction of signal into response
•Two Domains:
•Ligand binding domain (coupling proteins)
•Effectors Domain – undergoes functional conformational change
•“Action”: Initial combination of the drug with its receptors
resulting in a conformational change (agonist) in the later, or
prevention of conformational change (antagonist)
•“Effect”: It is the ultimate change in biological function
brought about as a consequence of drug action, through a
series of intermediate steps (transducers)

18. The Transducer mechanism
•Most transmembrane signaling is accomplished by a small
number of different molecular mechanisms (transducer
mechanisms)
•Large number of receptors share these handful of
transducer mechanisms to generate an integrated and
amplified response
•Mainly 4 (four) major categories:
1.G-protein coupled receptors (GPCR)
2.Receptors with intrinsic ion channel
3.Enzyme linked receptors
4.Transcription factors (receptors for gene expression)

19. G-protein Coupled
Receptors (GPCR)
•Large family of cell membrane
receptors linked to the effector
enzymes or channel or carrier
proteins through one or more
GTP activated proteins (G-
proteins)
•All receptors has common
pattern of structural organization
•The molecule has 7 α-helical
membrane spanning hydrophobic
amino acid segments – 3 extra
and 3 intracellular loops
•Agonist binding - on extracellular
face and cytosolic segment binds
coupling G-protein
Transducer 1 ….

20. GPCR – contd.
•G-proteins float on the
membrane with exposed domain
in cytosol
•Heteromeric in composition with
alpha, beta and gamma subunits
•Inactive state – GDP is bound to
exposed domain
•Activation by receptor GTP
displaces GDP
•The α subunit carrying GTP
dissociates from the other 2 –
activates or inhibits “effectors”
•β subunits are also important
ɣ

21. GPCR - 3 Major Pathways
1.Adenylyl cyclase:cAMP pathway
2.Phospholipase C: IP3-DAG pathway
3.Channel regulation

22. 1. Adenylyl cyclase: cAMP pathway
PKA Phospholambin
Increased
Interaction with Faster relaxation
Ca++

Troponin
Cardiac
contractility
Other
Functional
proteins

23. Adenylyl cyclase:
cAMP pathway
•Main Results:
–Increased contractility of heart/impulse generation
–Relaxation of smooth muscles
–Lipolysis
–Glycogenolysis
–Inhibition of Secretions
–Modulation of junctional transmission
–Hormone synthesis
–Additionally, opens specific type of Ca++ channel – Cyclic nucleotide
gated channel (CNG) - - -heart, brain and kidney
–Responses are opposite in case of AC inhibition

24. 2. Phospholipase C:IP3-DAG pathway
PKc

25. IP3-DAG pathway
•Main Results:
–Mediates /modulates contraction
–Secretion/transmitter release
–Neuronal excitability
–Intracellular movements
–Eicosanoid synthesis
–Cell Proliferation
–Responses are opposite in case of PLc inhibition

26. 3. Channel regulation
•Activated G-proteins can open or close ion channels
– Ca++, Na+ or K+ etc.
•These effects may be without intervention of any of
above mentioned 2
nd
messengers – cAMP or IP/DAG
•Bring about depolarization, hyperpolrization or Ca ++
changes etc.
•Gs – Ca++ channels in myocardium and skeletal
muscles
•Go and Gi – open K+ channel in heart and muscle and
close Ca+ in neurones

27. G-proteins and Effectors
•Large number can be distinguished by their α-
subunits
G protein Effectors pathway Substrates
Gs Adenylyl cyclase - PKABeta-receptors, H2,
D1
Gi Adenylyl cyclase - PKAMuscarinic M2
D2, alpha-2
Gq Phospholipase C - IP3Alpha-1, H1, M1,
M3
Go Ca++ channel - open
or close
K+ channel in
heart, sm

28. Intrinsic Ion Channel Receptors
•Most useful drugs in clinical medicine act by
mimicking or blocking the actions of endogenous
ligands that regulate the flow of ions through plasma
membrane channels
•The natural ligands include acetylcholine, serotonin,
aminobutyric acid (GABA), and the excitatory amino
acids (eg, glycine, aspartate, and glutamate)
Transducer 2 ….

29. Animation of GPCR - 1
Heart1.exe

30. Animation of GPCR - 2
Heart2.exe

31. Receptors with Intrinsic Ion
Channel

32. Enzyme Linked Receptors
•2 (two) types of receptors:
1.Intrinsic enzyme linked receptors
•Protein kinase or guanyl cyclase domain
1.JAK-STAT-kinase binding receptor
Transducer 3 ….

33. A. Enzyme linked
receptors
•Extracellular hormone-binding domain and a cytoplasmic
enzyme domain (mainly protein tyrosine kinase or serine or
threonine kinase)
•Upon binding the receptor converts from its inactive
monomeric state to an active dimeric state
•t-Pr-K gets activated – tyrosine residues phosphorylates on
each other
•Also phosphorylates other SH2-Pr domain substrate proteins
•Ultimately downstream signaling function
•Examples – Insulin, EGF ------- Trastuzumab, antagonist of a
such type receptor – used in breast cancer

34. B. JAK-STAT-kinase
Binding Receptor
•Mechanism closely resembles that of receptor
tyrosine kinases
•Only difference - protein tyrosine kinase activity is
not intrinsic to the receptor molecule
•Uses Janus-kinase (JAK) family
•Also uses STAT (signal transducers and activators of
transcription)
•Examples – cytokines, growth hormones,
interferones etc.

35. JAK-STAT-kinase Receptors

36. Receptors regulating gene
expression
•Intracellular (cytoplasmic or nuclear) receptors
•Lipid soluble biological signals cross the plasma membrane
and act on intracellular receptors
•Receptors for corticosteroids, mineralocorticoids, thyroid
hormones, sex hormones and Vit. D etc. stimulate the
transcription of genes in the nucleus by binding with specific
DNA sequence – called - “Responsive elements” – to
synthesize new proteins
•Hormones produce their effects after a characteristic lag
period of 30 minutes to several hours – gene active hormonal
drugs take time to be active (Bronchial asthma)
•Beneficial or toxic effects persists even after withdrawal
Transducer 4 ….

37. Receptors of gene expression - Image

38. Receptor Regulation
•Up regulation of receptors:
–In topically active systems, prolonged deprivation
of agonist (by denervation or antagonist) results in
supersensitivity of the receptor as well as to
effector system to the agonist. Sudden
discontinuation of Propranolol, Clonidine etc.
–3 mechanisms - Unmasking of receptors or
proliferation or accentuation of signal
amplification

39. Receptor Regulation – contd.
•Continued exposure to an agonist or intense
receptor stimulation causes desensitization or
refractoriness: receptor become less sensitive to
the agonist
•Examples – beta adrenergic agonist and levodopa
•Causes:
1.Masking or internalization of the receptors
2.Decreased synthesis or increased destruction of the
receptors (down regulation) - Tyrosine kinase
receptors

40. Mechanism of Masking or
internalization
ßARK (beta-adrenergic receptor kinase)
Beta-arrestin

41. Desensitization
•Sometimes response to all agonists which act through different receptors
but produce the same overt effect is decreased by exposure to anyone of
these agonists – heterologous desensitization
•Homologous – when limited to the agonist which is repeatedly activated –
In GPCRs (PKA or PKC) Kinases may also phosphorylate the GPCRs
R+ TransducerHomologous
Ach
Hist
Heterologous

42. Functions of Receptors - Summary
1.To Regulate signals from outside the cell to inside
the effector cell – signals not permeable to cell
membrane
2.To amplify the signal
3.To integrate various intracellular and extracellular
signals
4.To adapt to short term and long term changes and
maintain homeostasis.

43. Non-receptor mediated drug action –
clinically relevant examples
•Physical and chemical means - Antacids, chelating
agents and cholestyramine etc.
•Alkylating agents: binding with nucleic acid and
render cytotoxic activity – Mechlorethamine,
cyclophosphamide etc.
•Antimetabolites: purine and pyrimidine analogues –
6 MP and 5 FU – antineoplastic and
immunosuppressant activity

44. Dose-Response Relationship
•Drug administered – 2 components of dose- response
–Dose-plasma concentration
–Plasma concentration (dose)-response relationship
•E is expressed as
Emax X [D]
Kd + [D]
E is observed effect of drug dose [D], Emax = maximum response,
KD = dissociation constant of drug receptor complex at which
half maximal response is produced
E max

45. Dose-Response Curve
dose
Log dose
%
r
e
s
p
o
n
s
e
%
r
e
s
p
o
n
s
e
100% -

50% -
100% -
50% -
E =
Emax X [D]
Kd + [D]

46. Dose-Response Curve
•Advantages:
–Stimuli can be graded by Fractional change in
stimulus intensity
–A wide range of drug doses can easily be displayed
on a graph
–Potency and efficacy can be compared
–Comparison of study of agonists and antagonists
become easier

47. How we get DRC in vitro
Practically??
•Example: Frog rectus muscle and
Acetylcholine response – in millimeters
–Can compare with a drug being studied for having
skeletal muscle contracting property

48. Potency and efficacy
•Potency: It is the amount of drug required to produce a
certain response
•Efficacy: Maximal response that can be elicited by the drug
R
e
s
p
o
n
s
e
Drug in log conc.
1 2 3 4

49. Potency and efficacy - Examples
•Aspirin is less potent as well as less efficacious than Morphine
•Pethidine is less potent analgesic than Morphine but eually
efficacious
•Diazepam is more potent but less efficacious than
phenobarbitone
•Furosemide is less potent but more efficacious than
metozolone
•Potency and efficacy are indicators only in different clinical
settings e.g. Diazepam Vs phenobarbitone (overdose) and
furosemide vs thaizide (renal failure)

50. Slope of DRC
•Slope of DRC is also important
•Steep slope – moderate increase in dose markedly increase the response
(individualization)
•Flat DRC – little increase in response occurs in wide range of doses
(standard dose can be given to most ptients)
•Example: Hydralazine and Hydrochlorothiazide DRC in Hypertension
Hydralazine
Thiazide
F
a
ll
in

B
P

51. Selectivity
•Drugs produce different effects – not single
•DRC of different effects may be different
•Example – Isoprenaline – Bronchodilatation
and cardiac stimulation – same DRC
•Salbutamol – different (selective
bronchodilatation)

52. Therapeutic index (TI)
•In experimental animals
•Therapeutic Index =
Median Lethal Dose (LD50)
Median Effective dose (ED50)
Idea of margin of safety Margin of Safety

53. Therapeutic index (TI)
•It is defined as the gap between minimal therapeutic effect
DRC and maximal acceptable adverse effect DRC (also called
margin of safety)

54. Risk-benefit Ratioo
•Estimated harm (ADRs, Cost, inconvenience)
Vs
•Expected advantages (relief of symptoms,
cure, reduction of complications, mortality,
improvement of lif etc)

55. Combined Effects of Drugs
•Drug Synergism:
–Additive effect (1 + 1 = 2)
•Aspirin + paracetamol, amlodipine + atenolol, nitrous oxide +
halothane
–Supra-additive effect (1 + 1 = 4)
•Sulfamethoxazole + trimethoprim, levodopa + carbidopa,
acetylcholine + physostigmine
•PABA DHFA THFA

Sulfamethoxazole Trimethoprim
Folate
synthase
Dihydrofolate
Reductase

56. Drug Antagonism
1.Physical: Charcoal
2.Chemical: KMnO4, Chelating agent
3.Physiological antagonism: Histamine and
adrenaline in bronchial asthma, Glucagon
and Insulin
4.Receptor antagonism:
a.Competitive antagonism (equilibrium)
b.Non-competitive
c.Non-equilibrium (competitive)

57. Receptor antagonism - curves
oCompetitive:
o Antagonist is chemically similar to agonist and binds to same receptor
molecules
o Affinity (1) but IA (0), Result – no response
o Log DRC shifts to the right
o But, antagonism is reversible – increase in concentration of agonist
overcomes the block
o Parallel shift of curve to the right side
oNon-competitive:
o Allosteric site binding altering receptor not to bind with agonist
o No competition between them – no change of effect even agonist conc. .is
increased
o Flattening of DRC of agonist by increasing the conc. Of antagonist

58. Receptor antagonism - curves
•Non – equilibrium:
–Antagonists Binds receptor with strong bond
–Dissociation is slow and agonists cannot displace
antagonists (receptor occupancy is unchanged)
–Irreversible antagonism developes
–DRC shifts to the right and Maximal response
lowered

59. Drug antagonism DRC

60. Drug antagonism DRC – non-
competitive antagonism
R
e
s
p
o
n
s
e
Shift to the right
and lowered response
Drug in log conc.
Agonist
Agonist
+ CA (NE)

61. Spare Receptor
•When only a fraction of the total population
of receptors in a system, are needed to
produce maximal effect, then the particular
system is said to have spare receptors
•Example – Adrenaline (90%)

62. Competitive Vs NC antagonism
Competitive
•Binds to same receptor
•Resembles chemically
•Parallel right shift of DRC in
increasing dose of agonist
•Intensity depends on the conc. Of
agonist and antagonist
•Example – Ach and atropine,
Morphine and Naloxone
Noncompetitive
•Binds to other site
•No resemblance
•Maximal response is
suppressed
•Depends only on
concentration of antagonist
•Diazepam - Bicuculline

63. Summary
•Basic Principles of Pharmacodynamics
•Mechanisms of drug action – Enzymes, Ion channels, Transporters and
Receptors with examples
•Definitions of affinity, efficacy, agonist and antagonists etc.
•Drug transducer mechanisms
•GPCR and different GPCR transducing mechanisms – cAMP, Protein kinase
etc.
•Up regulation and down regulation of receptors and desensitization
•Principles of dose response curves and curves in relation to agonist,
competitive antagonist etc.
•Therapeutic index, margin of safety and risk-benefit ratio concepts
•Combined effects of drugs – synergism etc.
•Dose response curve (DRC) – agonist and antagonist

64. Thank you