Pharmacodynamics
HuBio 543
September 6, 2007
Frank F. Vincenzi
Learning Objectives
• 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 noncompetitive antagonism
• Spare receptors
• Kd, EC50, pD2, pA2
Typical concentration-effect curve
(plotted arithmetically)
A slide rule (logarithmic scale)
Typical log concentration-effect curve
(graded ‘dose-response’ curve)
Drug (D) - Receptor (R) Interaction
D+R
k1
DR
k2
Kd = ([D] * [R]) / [DR]
= k2/k1
Kd = dissociation constant
k1 = association rate constant
k2 = dissociation rate constant
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)
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)
Binding of a radioligand to tissue samples
Adapted from Schaffhauser et al., 1998
Scatchard analysis of binding of 125iodocyanopindolol
to beta-receptors in human heart
Adapted from Heitz et al., 1983
Acetylcholine (ACh): One drug with different
affinities for two different receptors
(adapted from Clark, 1933)
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).
Properties of an agonist (e.g., ACh)
(on receptors lacking spontaneous activity)
• Accessibility
• Affinity
• Intrinsic activity > 0
Different affinities of related agonist drugs for the same
receptor: Different potencies
(adapted from Ariëns et al., 1964)
Properties of an antagonist
(on receptors lacking spontaneous activity)
• Accessibility
• Affinity
• Intrinsic activity = 0
Pharmacological antagonism in an intact animal
Properties of a partial agonist
(on receptors lacking spontaneous activity)
• Accessibility
• Affinity
• 0 < Intrinsic activity < 1
Theoretical concentration-effect curves for a
full and partial agonist of a given receptor
Multiple receptor conformational states:
How to understand agonists, partial agonists
and antagonists
Simple case: receptor has little or no spontaneous
activity in the absence of added drug
‘inactive’ R
‘active’ R
An agonist binds more tightly to the ‘active’
state of the receptor:
Equilibrium shifts to the active state
A competitive antagonist binds equally tightly
to the ‘inactive’ and active states of the
receptor: No change in equilibrium
A partial agonist binds to both the ‘inactive’
and ‘active’ states of the receptor:
Partial shift of equilibrium
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)
The less simple case: Some receptors are
‘active’ even in the absence of added drug
Inverse agonists bind more tightly to the resting
state of the spontaneously active receptor:
Equilibrium shifts toward the inactive state
Receptor activation by agonists,
inverse agonists, etc.
Newman-Tancredi et al., 1997
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.
Antagonism of acetylcholine by atropine
Adapted from Altiere et al., 1994
Schild plot of antagonism of
acetylcholine by atropine
Adapted from Altiere et al., 1994
Antagonism of acetylcholine by
pirenzepine
Adapted from Altiere et al., 1994
Schild plot: Antagonism of acetylcholine by
two different antagonists
3
atropine
pirenzepine
2
1
0
-10
-9
-8
-7
-6
-5
log [antagonist] (M)
Adapted from Altiere et al., 1994
Different
pA2
values
(affinities)
for different
receptors
of some
clinically
useful
drugs:
The basis of
therapeutic
selectivity
Evidence for the existence of spare receptors
How nature achieves neurotransmitter
sensitivity without a loss of speed:
Spare receptors:
Drug (D) - Receptor (R) Interaction
D+R
k1
DR
k2
Kd = ([D] * [R]) / [DR]
= k2/k1
Kd = dissociation constant
k1 = association rate constant
k2 = dissociation rate constant