1. HuBio 543 September 6, 2007 Frank F. Vincenzi
Pharmacodynamics
HuBio 543
September 6, 2007
Frank F. Vincenzi

2. Learning Objectives
Quantification of drug receptor interactions and responses
Potency
Schild equation and regression
Competitive and non-competitive antagonism
Spare receptors
Kd, EC50, pD2, pA2
Receptors, signal transduction, transmembrane signaling
Agonist, antagonist, partial agonist, inverse agonist, multiple receptor states
Intrinsic activity, efficacy, SAR
Desensitization, up and down regulation

3. Typical concentration-effect curve (plotted arithmetically)

4. A slide rule (logarithmic scale)
In the scale L, numbers show the distance from the origin where the length of rule is assumed to be 1(one).
The scales D and C are related to the fanction y = log x, where the base of it is 10.
In scales D and C, the numbers are placed at points whose distance from the origin is ther logarithm.
The scale L gives the mantissa of the logarithm of the number standing above it in the scale C. For example, log 3.0 = 0.48 by this slide rule.
The scale CI has the same graduation as the scale C, but it is in the reverse order from right to left. A numbers in the scale C gives the inverse of the number located below it in the scale C. For example, the inverse of 3.0 is 0.33 by this slide rule. As in this example, a slide rule requires a rough calculation to determine the number of digits before the dismal point.

5. Typical log concentration-effect curve (graded ‘dose-response’ curve)

6. Drug (D) - Receptor (R) Interactionk1
D + R DR k2
Kd = ([D] * [R]) / [DR] = k2/k1
Kd = dissociation constant
k1 = association rate constant
k2 = dissociation rate constant

7. Several ways to express agonist potency &/or apparent affinity of agonists
EC50 (effective concentration, 50%, M)
Kd (apparent dissociation constant, M)
pD2 (negative log of molar concentration (M)
of the drug giving a response, which
when compared to the maximum,
gives a ratio of 2) (i.e., negative log
of half maximal concentration)

8. The classical concentration-effect relationship and the laws of mass action
Effect = (Effectmax * conc)/(conc + EC50)
In the previous data slide EC50 ~ 3 x 10-9 M
Thus, the apparent Kd of ACh ~ 3 x 10-9 M
IF (NOTE, BIG IF)
EC50 = Kd then
Bound drug = (Bmax * conc)/(conc + Kd)

9. Binding of a radioligand to tissue samples
Adapted from Schaffhauser et al., 1998

10. Scatchard analysis of binding of 125iodocyanopindolol to beta-receptors in human heart
Adapted from Heitz et al., 1983

11. Acetylcholine (ACh): One drug with different affinities for two different receptors
(adapted from Clark, 1933)

12. ACh: Different affinities for different receptors
Muscarinic receptors
EC50 = apparent Kd ~ 3 x 10-8 M, pD2 ~7.5
Nicotinic receptor
EC50 = apparent Kd ~ 3 x 10-6 M, pD2 ~5.5
In these experiments, affinity of ACh for muscarinic receptors is apparently ~100 times greater than for nicotinic receptors. ACh is 100 times more potent as a muscarinic agonist than as a nicotinic agonist. So, when injected as a drug, muscarinic effects normally predominate, unless the muscarinic receptors are blocked. (No problem for nerves releasing ACh locally onto nicotinic receptors, however).

13. Properties of an agonist (e. g
Properties of an agonist (e.g., ACh) (on receptors lacking spontaneous activity)
Accessibility
Affinity
Intrinsic activity > 0

14. Different affinities of related agonist drugs for the same receptor: Different potencies
(adapted from Ariëns et al., 1964)

15. Properties of an antagonist (on receptors lacking spontaneous activity)
Accessibility
Affinity
Intrinsic activity = 0

16. Pharmacological antagonism in an intact animal

17. Properties of a partial agonist (on receptors lacking spontaneous activity)
Accessibility
Affinity
0 < Intrinsic activity < 1

18. Theoretical concentration-effect curves for a full and partial agonist of a given receptor

19. Multiple receptor conformational states: How to understand agonists, partial agonists and antagonists

20. Simple case: receptor has little or no spontaneous activity in the absence of added drug
‘inactive’ R
‘active’ R

21. An agonist binds more tightly to the ‘active’ state of the receptor: Equilibrium shifts to the active state

22. A competitive antagonist binds equally tightly to the ‘inactive’ and active states of the receptor: No change in equilibrium

23. A partial agonist binds to both the ‘inactive’ and ‘active’ states of the receptor: Partial shift of equilibrium

24. Multiple receptor states: How to understand inverse agonists (in this LESS SIMPLE case, the receptor has spontaneous (often called constituitive) activity in the absence of added drug)

25. The less simple case: Some receptors are ‘active’ even in the absence of added drug

26. Inverse agonists bind more tightly to the resting state of the spontaneously active receptor: Equilibrium shifts toward the inactive state

27. Receptor activation by agonists, inverse agonists, etc.
Newman-Tancredi et al., 1997

28. How to quantify drug antagonism
Schild Equation
(C’/C) = 1 + ([I]/Ki)
Schild plot or Schild regression
log(C’/C - 1) vs. log [I]
pA2 = -log([I] giving a dose ratio of 2)
Where [I] = Kd of antagonist at its receptor.

29. Antagonism of acetylcholine by atropine
Adapted from Altiere et al., 1994

30. Schild plot of antagonism of acetylcholine by atropine
Adapted from Altiere et al., 1994

31. Antagonism of acetylcholine by pirenzepine
Adapted from Altiere et al., 1994

32. Schild plot: Antagonism of acetylcholine by two different antagonists3
atropine
pirenzepine 2 1
-10 -9 -8 -7 -6 -5
log [antagonist] (M)
Adapted from Altiere et al., 1994

33. Different pA2 values (affinities) for different receptors of some clinically useful drugs: The basis of therapeutic selectivity

34. Evidence for the existence of spare receptors

35. How nature achieves neurotransmitter sensitivity without a loss of speed: Spare receptors:

36. Drug (D) - Receptor (R) Interactionk1
D + R DR k2
Kd = ([D] * [R]) / [DR] = k2/k1
Kd = dissociation constant
k1 = association rate constant
k2 = dissociation rate constant