MD214 Unit 1 Lecture 2 2022 Agonists and Antagonists: Dose-Response Relationships PDF

MD214 Unit 1 Lecture 2: Agonists and Antagonists; Dose-Response relationships Dr Louise Rabbitt MRCPI Email: [email protected] Specialist Registrar, Clinical Pharmacology and Therapeutics, Galway University Hospitals; University of Galway University ofGalway.ie Unit 1, Week 1:...

MD214 Unit 1 Lecture 2: Agonists and Antagonists; Dose-Response relationships Dr Louise Rabbitt MRCPI Email: [email protected] Specialist Registrar, Clinical Pharmacology and Therapeutics, Galway University Hospitals; University of Galway University ofGalway.ie Unit 1, Week 1: Drug Action What are drugs? Sources Classification What are drugs targets? Proteins Others What is the nature of drug interactions? Agonists Antagonists How are drug effects measured? Receptor occupancy Names Receptors Ion channels Enzymes Transporters Full Partial Inverse Biased Competitive Noncompetitive Kd and Bmax Potency Dose response EC50 and Emax Efficacy Agonists and antagonists University ofGalway.ie University ofGalway.ie Learning Outcomes You should be able to: • Describe the concepts of agonists and antagonists • Describe he concept of partial agonists • Describe the two-state receptor model and constitutive receptors • Depict the effects of inverse agonists and biased agonists University ofGalway.ie Types of interaction Receptors are modelled as being in an active or inactive state Agonist: favours the active receptor conformation, binds to a receptor and causes an effect Antagonist: a drug that prevents the agonist-induced activation of the receptor Partial agonists and inverse agonists do not fit neatly into this simple definition of agonist and antagonist University ofGalway.ie Activation governed by efficacy Occupation governed by affinity Drug A (agonist) + R k+ 1 AR k-1 Drug B (antagonist) + R k+ 1 b AR* RESPONSE a BR NO RESPONSE k-1 Source: From Rang and Dale’s Pharmacology Antagonists Classification of antagonists can vary between texts We will use the classification of Rang and Dale, which divides into 2 types: • Competitive antagonists • Non-competitive antagonists University ofGalway.ie Classification of Antagonists Bind elsewhere Bind at agonist site Reversible Irreversible competitive (competitive) Allosteric site Noncompetitive Competitive Antagonists This type of antagonism is at the same binding site of the endogenous ligand Reversible: Most common and most important type of antagonism Irreversible: Covalent bond formation University ofGalway.ie A. Antagonist competes with agonist for receptor binding sites. Increasing concentrations of antagonist progressively inhibit the agonist response. High concentrations of agonist can surmount the effect of the antagonist A. B. B. Antagonist binds with covalent bonds to receptor. Agonist cannot displace antagonist Downloaded from: StudentConsult (on 18 June 2012 12:01 PM) © 2005 Elsevier Reversible competitive antagonist can be out-competed Must add extra agonist to out-compete antagonist Irreversible antagonist stays bound Irreversible antagonist, can’t be displaced because it dissociates only very slowly Non-competitive antagonism • Antagonist blocks at some point other than the receptor binding site • May bind to another site on the receptor (allosteric inhibition) • May block the signal transduction process downstream University ofGalway.ie Allosteric Antagonists are non-competitive Because they don’t bind at the agonist binding site University ofGalway.ie Non-receptor antagonism Chemical antagonism: inactivates agonist by forming a complex with it, e.g. protamine is a basic protein that binds to heparin forming an inactive complex Physiological antagonism: activates or blocks a receptor that mediates a response physiologically opposite to that of the receptor. For example, histamine acts on parietal cells to stimulate gastric acid secretion, whilst the proton pump inhibitor omeprazole blocks this action University ofGalway.ie Other types of agonists University ofGalway.ie • • • • Partial Agonists The two-state receptor model Constitutive receptors Inverse Agonists University ofGalway.ie Partial Agonists • Produce a lower response than full agonists • Reduced response not due to decreased binding affinity, but decreased efficacy • As partial agonists bind to same site, they can reduce the response produced by a full agonist, thus can act as competitive antagonist University ofGalway.ie Partial agonists: a clinical example • Buprenorphine is a partial agonist of m opioid receptors • Produces a submaximal relief of analgesia when compared to morphine • As a result, ought to have less addiction liability than morphine University ofGalway.ie Molecular basis of partial agonism Unknown, but there are 2 theories 1. The partial agonist is a good fit for the receptor binding site, but less able to promote the receptor conformational change leading to transduction 2. The receptor may be in an active and inactive state, and that partial agonists form complexes with both states, whereas full agonists preferentially bind the active form University ofGalway.ie Two-state receptor model Theory to account for fact that • Agonists bind receptors and activate them • Antagonists bind but do not activate Receptor can be in one of two states: • Resting (R) • Activated (R*) Binding an agonist favours the active (R*) state Binding an antagonist favours the resting R state University ofGalway.ie Constitutively Active Rs • Unbound receptor normally in R state • But some receptors have a degree of activity in the unbound state (they are in the R*) state • Constitutively active • For example cannabinoid receptors, benzodiazepine receptors, serotonin receptors • Disease causing mutations can cause a receptor to become constitutively active University ofGalway.ie Inverse agonist A ligand that binds to receptors And thereby reduces the fraction of them in an active conformation, …has biological effects opposite to those produced by an agonist Downloaded from: StudentConsult (on 18 June 2012 12:01 PM) © 2005 Elsevier Constitutively active Rs – Inverse agonists D+R DR D + R* (constitutively active) DR* An inverse agonist binds to a constitutively active receptor and REDUCES its activation Negative efficacy Inverse agonist Low, 2011 Comparison of inverse agonists and competitive antagonists Downloaded from: StudentConsult (on 18 June 2012 12:01 PM) © 2005 Elsevier Examples of Inverse Agonists • A -blocker does not simply “block” the receptor, but further inactivates receptor activity beyond its baseline value, and thus possesses inverse agonist activity, e.g. propranolol • H2 receptor antagonists, such as cimetidine, act as inverse agonists and diminish basal cAMP levels associated with gastric acid secretion University ofGalway.ie Biased agonists • The classical idea of an agonist is that it activates the whole repertoire of signals following activation • “Selectivity” thus based on a specific receptor type/subtype • Partial agonists and inverse agonists still fed into the two-state model • Now, a revision of the theory suggests that receptor coupling results in myriad configurations (states), and that a certain configuration may elicit a specific subset of signalling response • Opens the way for developing biased agonists Violin et al., 2014 University ofGalway.ie AngII type I receptor • Angiotensin II (AngII) type 1 receptor • Signalling through G protein and arrestin-dependent pathways • TRV027 is a selective -arrestin-biased ligand without activating G protein Violin et al., 2014 • This approach may prove useful for treating acute heart failure The m opioid receptor • On-target effects of activating G protein produce analgesia • However activating -arrestin pathway results in adverse effects • TRV130 is a strong agonist of the m opioid receptor but selective for activating the G protein pathway and not the -arrestin pathway • TRV130 has progressed to early clinical trials Violin et al., 2014 Agonists and antagonists: Key concepts 1. Receptor activation can be described with mathematical models 2. Receptor activation is not linear with ligand binding 3. Usually relatively low receptor occupancy is required to elicit relatively high response 4. Receptor mediated activity is affected by the inherent properties of the ligand, the University number of receptors present and the characteristics of the signal transduction ofGalway.ie machinery Agonists and Antagonists: Summary 1. Agonists come in different types, e.g. full, partial agonists, inverse agonists and biased agonists 2. Antagonists can be competitive and non-competitive 3. Agonist action can be explained by the two-state receptor model 4. Constitutive activity is commonplace and helps to explain actions of some currently marketed drugs and the potential for future ones University ofGalway.ie Dose response relationships University ofGalway.ie Dose Response: Overview • What are the consequences of activating a receptor and how can these be measured, at a biochemical and physiological level? • How can such activation responses be measured graphically? • What parameters can be derived from such graphical representations? University ofGalway.ie Occupation governed by affinity Drug A (agonist) + R k+ 1 k-1 A R Activation governed by efficacy  AR* RESPONSE a Source: From Rang and Dale’s Pharmacology Dose-Response Curves • Measure the relation between the concentration of drug and some physiological response (as opposed to binding) • Rise in blood pressure; Contraction /relaxation of smooth muscle University ofGalway.ie Isolated tissues • Whole organs placed in heated chambers, incubated with physiological salt solutions, kept at physiological pH, and perfused with oxygen such that they behaved as in the intact organism • Enabled quantitative measurements to be made University ofGalway.ie Experimental Setup “The guinea pig longitudinal muscle is a great gift to the pharmacologist. It has low spontaneous activity; nicely graded responses; is highly sensitive to a wide range of stimulants; is tough, if properly handled, and capable of hours of reproducible behaviour” (WDM Paton, 1986) The Guinea Pig Ileum Contraction Traces following addition of an agonist Drug Concentration (arbitary units) University ofGalway.ie Contraction Traces following addition of an agonist 0.1 Drug Concentration (arbitary units) University ofGalway.ie Contraction Traces following addition of an agonist 0.1 0.3 Drug Concentration (arbitary units) University ofGalway.ie Contraction Traces following addition of an agonist 0.1 0.3 1 Drug Concentration (arbitary units) University ofGalway.ie Contraction Traces following addition of an agonist 0.1 0.3 1 3 Drug Concentration (arbitary units) University ofGalway.ie Contraction Traces following addition of an agonist 0.1 0.3 1 3 10 Drug Concentration (arbitary units) University ofGalway.ie Contraction Traces following addition of an agonist 0.1 0.3 1 3 10 30 Drug Concentration (arbitary units) University ofGalway.ie Contraction Traces following addition of an agonist 0.1 0.3 1 3 10 30 100 Drug Concentration (arbitary units) University ofGalway.ie Contraction Traces following addition of an agonist 0.1 0.3 1 3 10 30 100 300 Drug Concentration (arbitary units) University ofGalway.ie Contraction Traces following addition of an agonist 0.1 0.3 1 3 10 30 100 300 1000 Drug Concentration (arbitary units) University ofGalway.ie Data generated Concentration (M) Log concentration Response (mm) % Maximal Response University ofGalway.ie Data generated Concentration (M) Log concentration Response (mm) % Maximal Response 1.67 x 10-9 1.33 x 10-9 8.33 x 10-9 1.66 x 10-8 3.32 x 10-8 8.26 x 10-8 1.64 x 10-7 3.23 x 10-7 6.25 x 10-7 1.18 x 10-7 2.11 x 10-6 University ofGalway.ie Data generated Concentration (M) Log concentration 1.67 x 10-9 -8.78 1.33 x 10-9 -8.48 8.33 x 10-9 -8.08 1.66 x 10-8 -7.78 3.32 x 10-8 -7.48 8.26 x 10-8 -7.08 1.64 x 10-7 -6.79 3.23 x 10-7 -6.49 6.25 x 10-7 -6.20 1.18 x 10-7 -5.93 2.11 x 10-6 -5.68 Response (mm) % Maximal Response University ofGalway.ie Data generated Concentration (M) Log concentration Response (mm) 1.67 x 10-9 -8.78 0 1.33 x 10-9 -8.48 3 8.33 x 10-9 -8.08 9 1.66 x 10-8 -7.78 19 3.32 x 10-8 -7.48 31 8.26 x 10-8 -7.08 45 1.64 x 10-7 -6.79 50 3.23 x 10-7 -6.49 55 6.25 x 10-7 -6.20 58 1.18 x 10-7 -5.93 59 2.11 x 10-6 -5.68 59 (Max Response) % Maximal Response University ofGalway.ie Data generated Concentration (M) Log concentration Response (mm) % Maximal Response 1.67 x 10-9 -8.78 0 0% 1.33 x 10-9 -8.48 3 5.0% 8.33 x 10-9 -8.08 9 15.3% 1.66 x 10-8 -7.78 19 32.2% 3.32 x 10-8 -7.48 31 52.5% 8.26 x 10-8 -7.08 45 76.3% 1.64 x 10-7 -6.79 50 84.7% 3.23 x 10-7 -6.49 55 93.2% 6.25 x 10-7 -6.20 58 98.3% 1.18 x 10-7 -5.93 59 100% 2.11 x 10-6 -5.68 59 (Max Response) 100% University ofGalway.ie -7.54 EC50 = 2.88 x 10-8 M -7.54 Dose-Response Curves Can estimate: Potency conc needed to produce 50% response Half-max EC50 or ED50 C=concentration (in vitro); D=dose (in vivo) Efficacy max response Emax State at which signalling is max; more drug will give no additional response University ofGalway.ie Dose-Response • ED50 = dose of drug that produces 50% of its own maximum effect • Related to affinity of drug for receptor • Typically, the lower the ED50, the greater the potency of the drug University ofGalway.ie Biological response Emax 50% ED50 Drug Dose Biological Response Emax = maximal response elicited Emax ED50 = dose of drug required to produce 50% of the maximal response 50% ED50 Log Drug Dose Dose-Response Relationships Measured in a single biological unit Graded One person, one frog, one guinea pig Continuous scale (dose → effect) Relates dose to intensity of effect Population studies Quantal All-or-none pharmacological effect Relates dose to frequency of effect (how often it is present in the population) University ofGalway.ie Graded Dose Response Curves Potency = EC50 Which is more potent? Efficacy is max response Which has more efficacy? So Drug A will give half-maximal response at a lower conc than drug B But both will eventually produce the same max response Potency and Efficacy • Remember • EC50 (or ED50)for Potency • The lower the EC50 the higher the potency • Emax (maximum effect) for efficacy • The higher the Emax the greater the effect Quantal Dose Response • Graded dose response • One individual, increasing drug dose • Quantal dose response • The fraction of the population that responds to a single dose of drug • Each drug dose, many individuals! • Variation between people in response to drug University ofGalway.ie Quantal Dose Response • • • • Describes the dose of a drug that produces a given effect in a population Variation among individuals Effects seen over a range of doses Responses are present/absent • • • Sleep/no sleep Pain/no pain Quantal, not graded University ofGalway.ie Quantal Dose-Response Distribution A. Frequency distribution B. Cumulative frequency distribution ED50 ED50 Potency and Efficacy 1000 800 Potency Efficacy 600 400 200 0 0 2 4 6 8 10 Comparison of ibuprofen and aspirin Ibuprofen Aspirin Which is more potent? Comparison of morphine and aspirin Morphine Aspirin Morphine and aspirin Dose response: Summary • Receptor activation results in intracellular events that can be measured as physiological responses (e.g. muscle contraction) • These responses can be plotted graphically in a similar fashion to that employed for receptor occupancy curves • From these graphs, measures of potency and efficacy can be derived, which enable the properties of drugs to be compared and contrasted University ofGalway.ie Thank you University ofGalway.ie