Pharmacology. Biomedical Sciences. Dynamics Kinetics Genetics. School of. Dr Lindsey Ferrie

Transcription

1 Pharmacology Dynamics Kinetics Genetics Dr Lindsey Ferrie MRCPsych Neuroscience and Psychopharmacology School of Biomedical Sciences

2 Dynamics What the drug does to the body What the body does to the drug Kinetics

3 Lecture Outline - Dynamics Agonists Dose response curves Affinity Efficacy Partial/inverse agonists Antagonists Dose response curves Competitive Irreversible (noncompetitive) Learning outcomes Compare the affinity and efficacy of agonists and antagonists on the basis of their dose-response curves.

4 Drugs act at receptors as either agonists or antagonists Agonist: An agonist is a ligand (drug, hormone or neurotransmitter) that combines with receptors to elicit a cellular response e.g. amphetamine Antagonist An antagonist is a drug which blocks the response to an agonist e.g. Reserpine

5 Agonist Receptor Agonist-receptor complex Action Effect Antagonist Receptor Antagonistreceptor complex Effect

6 % Response % Response Dose-Response Curves Concentration effect curve Semi-logarithmic plot of agonist concentration against response Agonist Concentration [Log] Agonist Concentration

7 DOSE-RESPONSE RELATIONSHIPS GRADED Response of a particular system isolated tissue, animal or patient measured against agonist concentration QUANTAL Drug doses (agonist or antagonist) required to produce a specified response determined in each member of a population

8 Ileum tissue contraction (mv) tumour expression / 100,000 (%) Which is an example of a quantal dose response curve? Acetylcholine Concentration (nm) [Log] Herceptin Concentration (mg/kg)

9 % Response Dose-Response Curves Allow estimation of E max 100 E max Allow estimation of concentration or dose required to produce 50% of maximal response (EC 50 or ED 50 ) 50 EC 50 Allow efficacy to be determined Allow potency to be determined [Log] Agonist Concentration

10 Dose-Response Curves Two state hypothesis Drug A (agonist) Occupancy K+1 Activation β + R AR AR* Response K-1 α 1. Affinity 2. Efficacy R = rested state R* = activated state

11 Drug Bound Drug Bound Drug Binding Total Total Non specific = Specific Bmax Drug Concentration Non specific Drug Concentration Saturation is easily measured i.e. maximum number of binding sites (Bmax) BUT difficult to get a measure of how avidly the drug binds affinity (K D )

12 What is the K D telling us? The K D is a physiochemical constant like Avogadros number. The K D is the same for a given receptor and drug combination in any tissue, in any species (as long as the receptor is the same), anywhere in the universe. The K D can therefore be used to identify an unknown receptor. The K D can be used to quantitatively compare the affinity of different drugs on the same receptor.

13 1. Affinity Agonists (and antagonists) have affinity Describes the tendency of the ligand to form a stable complex with the receptor. Determined by the number of bonds and the level of fit between ligand and receptor. Characterised by the equilibrium constant (K A )

14 Affinity Drug A (agonist) Occupancy K+1 + R AR AR* Response K-1 1. Affinity Activation β α If we assume a direct relationship between receptor occupancy and response A lower K A indicates a tighter ligand-receptor interaction (higher affinity) Agonists with high potency tend to have high affinity

15 % Response Affinity - examples 100 A B 50 A = Higher affinity B = Lower affinity EC 50 A = Higher potency B = Lower potency [Log] Agonist Concentration True affinity can only be determined by binding-dose relationships

16 Potency Potent drugs are those which elicit a response by binding to a critical number of receptors at low concentration (high affinity) compared with other drugs acting on the same system with lower affinity. Potency dependent on: Receptor density Efficiency of stimulus-response mechanisms used Affinity of drug Efficacy of drug.

17 2. Efficacy When an agonist binds to a receptor, this induces a conformational change that sets off a chain of biochemical events - an action. Efficacy: describes the ability of an agonist to activate a receptor i.e. to evoke an action at the cellular level is determined by the nature of the receptor-effector system refers to the maximum effect an agonist can produce regardless of dose

18 Efficacy of Agonists Drug A (agonist) Occupancy K+1 + R AR AR* Response K-1 Activation β α 2. Efficacy Full agonist (high efficacy) - AR* very likely produce a maximum response while occupying only a small % of receptors available Partial agonist (low efficacy) - AR* less likely unable to produce a maximum response even when occupying all the available receptors

19 % Response Full and Partial Agonists With full agonists, the maximum response produced corresponds to the maximum response that the tissue can give. A partial agonist is a ligand that combines with receptors to elicit a maximal response which falls short of the maximal response that the system is capable of producing 100 Full agonist Partial agonist 50 Agonist (M3) KA (um) efficacy carbacol [Log] Agonist Concentration McN-A

20 Examples: Partial Agonists Varenicline Nicotine receptor partial agonist for smoking cessation. Aripiprazole Antipsychotic partial agonists at selected dopamine receptors.

21 Over simplification! Two state model predicts that a receptor can exist in two forms AR and AR* Increasing evidence suggests receptors can activate in the absence of ligands i.e. R* (constitutive activity) or change state depending on GPCR function. Ternary complex model (four active states!) ARG AR*G AR AR* RG R*G R R*

22 Inverse agonists Have higher affinity for the AR (inactive) state than for AR* (active) state Many classical competitive antagonists display inverse agonist activity: Cimetidine (H2), pirenzepine (M2), atropine (M) ~85% of competitive antagonists are actually inverse agonists (Kenakin, 2004) Examples; β-carbolines on GABA A receptors anxiogenic rather than anxiolytic

23 Allosteric Modulators Benzodiazepines acting on a GABA A receptor GABA binding site Orthosteric GABA BZ binding site allosteric Clcurrent BZ BZ GABA Cl- Increases K A for GABA Increase efficacy of GABA Clcurrent

24 Allosteric Modulators Positive (PAM) Not active alone but increase affinity and/or efficacy of endogenous agonist Examples: Diazepam Propofol Isoflurane Negative (NAM) Not active alone but decrease affinity and/or efficacy of endogenous agonists Examples: mglur5 dipraglurant???

25 The results shown below were obtained in a comparison of positive ionotropic agents (drugs used in heart failure to increase cardiac contractility). Which statement is correct? 1. Drug A is most effective 2. Drug B is least potent 3. Drug C is most potent 4. Drug B is more potent than Drug C and more effective than Drug A 5. Drug A is more potent than Drug B and more effective than Drug C.

26 Lecture Outline - Dynamics Agonists Dose response curves Affinity Efficacy Partial/inverse agonists Antagonists Dose response curves Competitive Irreversible (noncompetitive) Learning outcomes Compare the affinity and efficacy of agonists and antagonists on the basis of their dose-response curves.

27 General classes of antagonists Chemical Binding of two agents to render active drug, inactive Commonly called chelating agents Example - protamine binds (sequesters) heparin. Physiological Two agents with opposite effects cancel each other out. Example glucocorticoids and insulin Pharmacological Binds to receptor and blocks the normal action of an agonist on receptor responses

28 Efficacy and Antagonists Pure antagonists do not by themselves cause any action by binding to the receptor What effect does this have on efficacy? Drug A (agonist) Occupancy K+1 Activation β + R AR AR* Response K-1 α 1. Affinity 2. Efficacy Full agonist (high efficacy) - AR* very likely Partial agonist (low efficacy) - AR* less likely Antagonist (no efficacy) AR* does not exist

29 Pharmacological antagonism 1. Competitive Binds and prevents agonist action but can be overcome with increased agonist concentration. Causes parallel shift to right of the agonist-response curve 2. Irreversible (non-competitive) Binds and forms irreversible covalent bonds with receptor Causes parallel shift to right of the agonist-response curve and reduced maximal asymptote. 3. Non-competitive Signal transduction rather than receptor effects Downstream responses are blocked (e.g. Ca 2+ influx) Reduces slope and maximum of dose response curve

30 % Response 1. The Competitive Antagonist AGONIST (A) + RECEPTOR (R) AR COMPLEX ACTION ANTAGONIST (D) + RECEPTOR (R) DR COMPLEX NO ACTION 100 Agonist Agonist + Antagonist [Log] Agonist Concentration

31 In the presence of the competitive antagonist Agonist curves have the same form Agonist curves are displaced to the right Agonist curves have the same maximal response The linear portion of the curves are parallel This is because the competitive antagonist binds reversibly with the receptor gives rise to antagonism which can be overcome by an increased concentration of agonist

32 % Response The dose ratio Agonist plus increasing concentrations of competitive antagonist Antagonist 50 Agonist EC50 x Dose Ratio agonist concentration in the presence of an antagonist (x) agonist concentration (dose) in the absence of antagonist (y) y [Log] Concentration

33 Log (r-1) Schild Plot for Competitive Antagonist Schild Equation r -1 = [B] r = dose ratio Kb B = antagonist conc Kb = antagonist dissociation constant Log Kb Schild Plot Log antagonist conc [B] (nmol/l)

34 pa 2 values Describes the activity of a receptor antagonist in simple numbers. the negative logarithm of the molar concentration of antagonist required to produce an agonist dose ratio equal to 2. pa2 = - log Kb Only if relationship is linear and slope of Schild plot = 1 i.e. only if a competitive antagonist.

35 Implications for the clinician The extent of antagonist inhibition depends upon the concentration of the competing agonist Varies in response to normal physical activity as well as disease states. The extent of inhibition depends on the antagonist s concentration. Inter individual differences in metabolism or clearance influence plasma concentrations.

36 % Response 2. The Irreversible Antagonist AGONIST (A) + RECEPTOR (R) AR COMPLEX ACTION ANTAGONIST (D) + RECEPTOR (R) DR COMPLEX NO ACTION 100 Agonist 50 Agonist + Antagonist [Log] Agonist Concentration

37 In the presence of the irreversible antagonist Agonist curves do not have the same form Agonist curves have a reduced maximal response This is because the irreversible antagonist binds irreversibly with the receptor gives rise to antagonism which cannot be overcome by an increased concentration of agonist

38 % Response Irreversible Competitive Antagonism Antagonist 50 Agonist EC50 EC50 EC [Log] Concentration Increased EC50 Duration of effect is related to receptor turnover. Receptor reserves allow parallel shift to right.

39 % Response % Response Weak partial agonists are similar to irreversible antagonists! 10 0 Full Agonist 10 0 Full Agonist 50 Partial Agonist 50 Agonist + Antagonist [Log] Concentration [Log] Concentration

40 Competitive vs. Irreversible Antagonists Competitive common type of antagonism examples include: cimetidine at the H 2 receptor tamoxifen at the oestrogen receptor Irreversible much less common type of antagonism examples include: phenoxybenzamine at the a 1 adrenoceptor

41 % Response 3. The Non-Competitive Antagonist Antagonist 50 Agonist EC50 EC50 EC [Log] Concentration Blocks signal transduction events. E.g. Nifedipine bocks Ca2+ influx Reduces slope and maximal effect.

42 % patients [Log] drug plasma concentration % patients [Log] drug plasma concentration Therapeutic Window/Index (TI) Risk:benefit ratio (TI) = TD50 or LD50 ED50 ED Small TI e.g. Warfarin 10 0 Large TI e.g Penicillin 50 Desired therapeutic effect Unwanted adverse effect 50 Desired therapeutic effect Unwanted adverse effect

43 Summary - dynamics You should now be able to: Explain the differences in format of a dose response curve (graded vs. quantal) Explain the two state hypothesis of agonistreceptor interactions. Describe the difference between the affinity, efficacy and potency of an agonist. Explain the differences between full, partial and inverse agonists.

44 Summary - dynamics You should now be able to: Explain the three general classes of antagonism Define the effect of a competitive, irreversible competitive and non-competitive antagonist on an agonist dose response curve. Appreciate how we quantify antagonism using the schild equation and schild plot. Explain how the risk/benefit ratio is determined with the therapeutic window.