MBBS Stage 1 Pharmacology 2 PDF

Life Sciences & Medicine Fundamental Principles of Pharmacology 2 Professor Ian McFadzean Pharmacology & Therapeutics MBBS Stage 1 – Physiology & Anatomy of Systems After this lecture you should : Kno...

Life Sciences & Medicine Fundamental Principles of Pharmacology 2 Professor Ian McFadzean Pharmacology & Therapeutics MBBS Stage 1 – Physiology & Anatomy of Systems After this lecture you should : Know what a ‘concentration-response’ curve is and be able to draw a 'typical' log-concentration-response curve; Appreciate the role of receptors in cell signalling; Be able to define the terms specificity, affinity, and efficacy when used in the context of drug-receptor interactions; Understand the difference between full agonists, partial agonists and reversible competitive antagonists; Know the effect of a reversible competitive antagonist on the log-concentration response curve for an agonist; Be able to explain the terms KD, pA2 and EC50 The concentration-response relationship Drug effects are “quantified” by studying the relationship between drug concentration (or dose) and the response produced by the drug. This relationship is described by concentration-response curves 100% 100% response response rectangular symmetrical hyperbola sigmoid 0% 0% 0 Drug concentration Log drug concentration A closer look at the (log) concentration-response curve The Emax and the EC50 Emax E is the maximum effect 70 Response (increase in heart rate) max (response) that a drug can (beats per minute) produce e.g. an increase in heart rate of 70 beats per minute The EC is the concentration of a 35 50 drug that produces 50% of the maximum response Indicates the position of the curve Log EC50 on the concentration axis 0 Used to quantify the potency of a Log drug concentration drug So what is a receptor and what does it do? Receptors are protein macromolecules usually inserted across the lipid bilayer of the cell They perform two main functions  Recognition or detection of extracellular molecules  Transduction; having detected the presence of an extracellular molecule they then bring about changes in cell activity They interact with, or bind, certain chemicals e.g. hormones or neurotransmitters with a high degree of specificity Receptors are very fussy about which molecules they bind! Receptors are often name after, or classified, with respect to the drugs they bind  e.g. nicotinic acetylcholine receptors bind the neurotransmitter acetylcholine AND the exogenous drug nicotine We utilise this specificity of interaction between drug and receptor by designing drugs that bind to only certain subtypes of receptor found in different cells of the body In the clinic this leads to drugs with fewer side-effects, i.e. drugs that are highly selective in their action Binding of drug (D) to receptor (R) D+R DR 100% Binding is reversible (in most p cases) A plot of the proportion of 50% receptors occupied (p) vs drug concentration [D] is a rectangular hyperbola 0% [D] Binding of drug (D) to receptor (R) D+R DR 100% p A plot of the proportion of receptors occupied (p) vs log [D] is a symmetrical 50% sigmoid 0% Log [D] Affinity and KD “kay-dee” The affinity of a drug for its receptor is quantified as “the MOLAR concentration of drug required to occupy 50% of the receptors at equilibrium” This concentration of drug is given the symbol KD Drugs with HIGH affinity have a LOW KD, for example, in the micro- or nanomolar range KD is the equilibrium dissociation constant k +1 D+R DR k -1 k+1 and k-1 are rate constants that tell us something about the likelihood of the forward and backward reactions occurring Rate of FORWARD reaction = k+1[D][R] Rate of BACKWARD reaction = k-1[DR] At equilibrium, backward rate = forward rate, so k-1[DR] = k+1[D][R] KD = k-1/ k+1 = [D][R]/ [DR] The KD is a measure of how tightly the receptor holds on to the drug once they come together Receptors are being continually bombarded by lots of “chemicals”. Only those with affinity will “stick”, or bind Drugs with high affinity (low KD) hit the “sweet spot” on the receptor and stay bound for a (relatively) long time i.e. they have a slow “dissociation rate” (k-1 very small) Take time out to make sure you can answer the following questions Where do you find receptors in a cell? What two functions do receptors carry out? How do we measure the affinity of a drug for its receptor? What is the definition of KD ? Many drugs have affinity, but agonists go a step further Many drugs bind to the receptor (i.e. have affinity), occupy it, and do little else AGONISTS however bind and then activate the receptor i.e. the agonist has efficacy After binding, agonists produce a change in the shape of the receptor - a conformational change - that will ultimately lead to a response in a cell or tissue Agonists possess efficacy Activation of receptor (R) by an agonist (A) produces a biological response A+R AR AR* response Efficacy describes the ability of a drug to activate the receptor i.e. the transition AR AR* A fly landing on an elephant’s back! Schematic representation of the human β2 adrenoceptor. Each circle represents an amino acid. This receptor is a transmembrane protein, going back and forth across the membrane seven times, and is round 400 amino acids long Outside the cell (extracellular) Cell membrane Inside the cell (intracellular) A fly landing on an elephant’s back! What is remarkable is that the hormone adrenaline (also called epinephrine) that activates this receptor by causing a conformational change is the size of a single amino acid! There are two broad types of agonist So agonists bind to the receptor (have affinity) and activate it (have efficacy) All naturally occurring neurotransmitters and hormones are agonists e.g. adrenaline, acetylcholine, insulin, dopamine………..many more Agonists can be either partial agonists or full agonists Full agonists have high efficacy and as a result are very effective at activating receptors and producing a biological response Partial agonists have low efficacy and are therefore less effective at activating receptors and less able to produce a biological response Partial vs full agonists (1) 100% Full agonists often produce maximal response whilst activating only a response fraction of the available receptors (p) i.e. there are lots of spare receptors 0% p 100% Partial vs full agonists (2) 100% Partial agonists often fail to produce a full full response despite response occupying all the available receptors partial 0% p 100% Partial vs full agonists (3) 100% Differences can also be seen in the log concentration vs response response curves full partial 0% Log [agonist] A note of caution Whilst it is tempting to conclude that for an agonist, it will produce a 50% response (EC50) when it is occupying 50% of the available receptors (KD), this is NOT usually the case. This because; 1. the overall response to an agonist is determined by both its affinity and its efficacy; receptor occupancy is determined only on affinity 2. there are often many steps between an agonist drug binding and the response we measure. For example, imagine the numerous steps that exist between adrenaline activating its receptors in the heart and the increase in blood pressure this produces Take time out to make sure you can answer the following What do we call drugs that show both affinity and efficacy at receptors? What is the difference between affinity and efficacy? What do we call drugs that have low efficacy? Is the EC50 for an agonist drug always the same as its KD and if not, why not?  Antagonist (n. opponent, adversary) Many clinically useful drugs are antagonists They act to inhibit the effects of a neurotransmitter or another drug Competitive antagonists compete with the agonist for the same site on the receptor molecule, but don’t activate it i.e. have affinity but zero efficacy Can be reversible or irreversible Non-competitive antagonists act at a different site on the receptor or another molecule closely associated with it Reversible competitive antagonists Very important drugs e.g. pancuronium, cetirizine, propranolol Used to inhibit the effects of a neurotransmitter or hormone Their inhibitory effects can be overcome by increasing the concentration of the AGONIST i.e. the blockade is surmountable A parallel shift to the right Reversible competitive response antagonists produce a parallel shift to the right of the AGONIST log concentration vs response curve Log [agonist] A parallel shift to the right Reversible competitive response antagonists produce a parallel shift to the right of the AGONIST log concentration vs response In presence of curve antagonist Log [agonist] A parallel shift to the right Reversible competitive response antagonists produce a parallel shift to the right of the AGONIST log concentration vs response even more curve antagonist Log [agonist] A measure of ANTAGONIST affinity – the pA2 The extent of the shift in the position of the agonist curve produced by the antagonist can be measured using the “dose-ratio” i.e. the ratio of the concentration of agonist producing the same response in the presence and absence of the antagonist The extent of the shift is a measure of the affinity of the ANTAGONIST for the receptor The affinity of an antagonist is quantified using its pA2. This is the negative logarithm of the Molar concentration of antagonist that necessitates that you double the agonist concentration to produce the same response (i.e. produces a dose ratio of 2.0) A worked example The presence of 6.3 x10-9 M propranolol necessitates that you need to double the concentration of adrenaline to produce the same increase in heart rate The log of 6.3 x10-9 is -8.2 The pA2 for propranolol is 8.2 What about irreversible competitive antagonists? These drugs also produce a shift in the agonist log concentration- response response curve, but the shift is not parallel i.e. the inhibition they produce is not overcome by increasing the agonist concentration; it is not surmountable Log [agonist] What about irreversible competitive antagonists? These drugs also produce a shift in the agonist log concentration- response response curve, but the shift is plus antagonist not parallel i.e. the inhibition they produce is not overcome by increasing the agonist concentration; it is not surmountable Log [agonist] What about irreversible competitive antagonists? These drugs also produce a shift in the agonist log concentration- response response curve, but the shift is not parallel i.e. the inhibition they produce is not overcome by increasing the agonist plus more antagonist concentration; it is not surmountable Log [agonist] After this lecture you should : Know what a ‘concentration-response’ curve is and be able to draw a 'typical' log-concentration-response curve; Appreciate the role of receptors in cell signalling; Be able to define the terms specificity, affinity, and efficacy when used in the context of drug-receptor interactions; Understand the difference between full agonists, partial agonists and reversible competitive antagonists; Know the effect of a reversible competitive antagonist on the log- concentration response curve for an agonist; Be able to explain the terms KD, pA2 and EC50