Pharmacodynamics

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

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

3. Typical concentration-effect curve(plotted arithmetically)

4. A slide rule (logarithmic scale)

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

6. Drug(D) - Receptor (R) Interaction k1 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 fromSchaffhauser et al., 1998

10. Scatchard analysis of binding of 125iodocyanopindolol to beta-receptors in human heart Adapted fromHeitz 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., 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 receptorhas 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 antagonists 3 atropine pirenzepine 2 1 0 -6 -5 -10 -9 -8 -7 log [antagonist] (M) Adapted from Altiere et al., 1994

33. DifferentpA2 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) Interaction k1 D + R DR k2 Kd = ([D] * [R]) / [DR] = k2/k1 Kd = dissociation constant k1 = association rate constant k2 = dissociation rate constant