Lecture 5: Receptor Pharmacology (PDF)

Enantiomers: Mirror Images of the “Same” Molecule “Handedness” is due to the presence of a chiral center Receptors often are...

Enantiomers: Mirror Images of the “Same” Molecule “Handedness” is due to the presence of a chiral center Receptors often are configured to be selective for one enantiomer over the other Enantiomeric pairs often have different Citalopram pharmacokinetics ~200-fold difference in ability to inhibit SERT ~2-fold difference in pharmacokinetics  Affinity, Concentration, and Selectivity The higher concentration of the drug, means less selectivity for targeted receptor, which can activate other not intended receptors Drug Concentration mAChR KD = 100 nM α-adrenergic H R KD = 10 nM SERT KD = 1 nM Higher affinity means that the drug will less likely activate other recpetors, meaning more selective  Agonist Potency Profiles Identify Receptor Subtypes Raymond Ahlquist carried out catecholamine concentration-response analyses of various smooth muscle tissues and cardiac tissue across several species “Alpha adrenergic” potency rank-order – Epinephrine > norepinephrine > α-methyl norepinephrine > isoproterenol – Six alpha subtypes now identified α1A, α1B, α1D, α2A, α2B α2C “Beta adrenergic” potency rank-order – Isoproterenol > epinephrine > α-methyl nor norepinephrine > norepinephrine – Three beta subtypes now identified β1, β2, β3 h ea t lu n g Raymond P. Ahlquist 1914-1983 Epinephrine Norpinephrine α-methyl norepinephrine Isoproterenol (Corbadrine) Potency Equals the Concentration of Agonist that Elicits a Half-maximal Response Lower KD, higher affinity Lower EC50, higher potency 2000 Response Units 1500 Jshjhkijhi 1000 EC50 (50% Effective Concentration) 500 0 -11 -10 -9 -8 -7 -6 log [agonist] (M) What is the relationship between the EC50 and the KD? (potency vs affinity)?  EFFICACY Intrinsic Efficacy, not Clinical Efficacy How good the agonist shows effect (efficacy)  Agonist-stimulated adenylyl cyclase activity in membranes isolated from CHO-cells stably expressing human β-adrenergic receptor subtypes Hoffmann et al (2004) Naunyn-Schmiedeberg’s Archives of Pharmacology, 369 (2), p.151-159 EC50 (μM) = Potency ISO NE EPI Isoproterenol β1 0.07 0.80 1.00 β2 0.06 5.00 0.10 Norepinephrine β3 0.25 2.50 50.00 Epinephrine *Ki (μM) = Affinity ISO NE EPI β1 0.22 3.60 4.00 β2 0.50 26.20 0.74 β3 1.60 4.30 126.00 *Ki is and indirect measure of KD 1 β1: expressed in cardiac tissue β 2: expressed in lung β 3: expressed in adipose tissue Agonists of the Adenosine A3 Receptor Differ in Maximal Effectiveness Fossetta et al (2003) Molecular Pharmacology, 63(2):342-350 Higher the efficacy Lower the efficacy Calcium responses to adenosine-related agonists in hA3 recombinant cells  Drug 1 FULL AGONIST 100 100 % of Biological Response % of Occupied Receptors Biological Dose-Response Curve Receptor Occupancy Curve EC50 ~ KD 0 0 [DRUG]  Drug 2 “SUPER” AGONIST HIGH EFFICACY AGONIST 100 100 % of Biological Response % of Occupied Receptors KD ~ EC90 EC50 KD EC50 < KD 0 0 [DRUG]  Demonstration of Spare Receptors by Irreversible Blockade 2000 Irreversible Antagonist Agonist alone + 1 nM Antagonist Response Units 1500 + 10 nM Antagonist 1000 + 100 nM Antagonist + 300 nM Antagonist 500 + 1 uM Antagonist 0 -11 -10 -9 -8 -7 -6 -5 -4 -3 log [agonist] (M)  Full and Partial Agonists Intrinsic Efficacy and Affinity are Independent  Graphic Comparison of Full and Partial Agonists Antagonism of the full agonist’s maximal effect is the defining characteristic of a low efficacy partial agonist 2000 2000 Response Units Response Units 1500 1500 1000 1000 500 500 0 0 -10 -8 -6 -4 -10 -8 -6 -4 log [agonist] (M) log [agonist] (M) High efficacy, full agonist Low efficacy, partial agonist Full and partial agonists added simultaneously  EFFICACY “Intrinsic Efficacy” When Referring to Receptor Stimulation In some cases, the response to an agonist is not linearly proportional to the number of receptors occupied A maximum effect can be produced by an agonist when occupying only a small proportion of receptors – “High Efficacy Agonist” Some agonist drugs are not capable of stimulating a full biological response, even when occupying 100% of the receptors – “Partial Agonists” Drugs that have no intrinsic efficacy are called ANTAGONISTS – They block the response of agonists or endogenous neurotransmitters and hormones An Inverse Agonist has “Negative Intrinsic Efficacy” Agonists stabilize receptor conformations that couple to G (COMPETITIVE) proteins Competitive antagonists prevent agonists from binding Inverse agonists stabilize receptor conformations that cannot couple to G proteins  Receptors Assume Multiple Conformations Receptor ligands stabilize (“select”) a conformation Need more energy (rare) Most Activation Zero Activation Agonist stabilizes it and keeps it in that conformation, allows to keep signaling Rate of G Protein Activation  Biased Agonists Select Receptor Conformations that Couple to Alternative Signal Transduction Pathways 1 “Classic” agonist Biased agonist Olestrena, pain medication Same receptor Biased agaonism, relive pain, but cause addiction In this hypothetical In this hypothetical example, most agonists example, the biased activate the PLC agonist activate the signaling cascade adenylyl cyclase signaling cascade PLC Adenylyl Cyclase PLC Adenylyl Cyclase  ANTAGONISM 0 efficacy drugs Competitive and Noncompetitive An Antagonist Blocks the Response of an Agonist by Occupying the Receptors Without Stimulating the Receptors A preferred because 1250 Agonist activity High affinity 1000 Antagonist A Response Units 750 Antagonist B A n t ag on is t s Low affinity 500 250 0 Inhibition of agonist activity -12 -11 -10 -9 -8 -7 -6 -5 -4 log [antagonist](M) Competitive Antagonists Occupy the Agonist Binding Site Without Producing an Effect Agonist Competitive Antagonist Inactive Inactive Receptor Receptor  Classic Competitive Antagonism Results in Rightward Parallel Shifts of the Agonist Concentration-effect Curve 2000 Agonist alone + 1 nM Antagonist Response Units 1500 + 10 nM Antagonist 1000 + 100 nM Antagonist A B C D E + 1 μM Antagonist 500 0 -11 -10 -9 -8 -7 -6 -5 -4 -3 log [agonist] (M) A B C D E EC50 (nM) 3 28 270 2537 29280  Noncompetitive Antagonists Interfere with Agonist Binding through a “Distal Binding Site” Receptor Agonist Noncompetitive Antagonist The noncompetitive antagonist changes the receptor conformation so that the agonist binding site is no longer available  Noncompetitive Antagonism Results in a Downward Shift of the Agonist Concentration-effect Curve Rightward/downward shifts also are observed 2000 Agonist alone + 1 nM Antagonist Response Units 1500 + 10 nM Antagonist 1000 + 100 nM Antagonist 500 0 -11 -10 -9 -8 -7 -6 log [agonist] (M)