Pharmacodynamics Lecture Notes PDF

Lecture 1 Drug receptor interactions Occupancy and Affinity Chapter 2 Rang and Dale, 7th Ed Chris K...

Lecture 1 Drug receptor interactions Occupancy and Affinity Chapter 2 Rang and Dale, 7th Ed Chris Keating Image result for receptor affinity cartoon c.keating @herts.ac.uk Image result for receptor affinity cartoon  Objectives At the end of this lecture students will, with the benefit of additional reading, be able to: Define “affinity” in terms of agonist receptor interactions Explain what is meant by KD and Bmax in terms of agonist receptor interactions Understand how affinity of drugs is important in terms of clinical use of drugs  Pharmacodynamics: what the drug does to the body Image result for concentration effect curve pharmacology Image result for drug packet cartoon Image result for human body Image result for concentration effect curve pharmacology Image result for concentration effect curve pharmacology Pharmacokinetics: what the body does to the drug Basic drug-receptor theory: two state model This lecture Next lecture Occupancy Activation governed by governed by AFFINITY EFFICACY Drug k+1 A (agonist) + R k-1 AR AR* RESPONSE AR= Receptor occupied; AR*= Receptor activated NOTE: Agonists bind to receptors (affinity) Agonists activate receptors to give a response (efficacy) RESPONSE  Affinity?? Image result for ballroom dancing What is affinity ? Agonists (and antagonists) have affinity Describes the tendency for a ligand to form a stable complex with a receptor. Governed by chemistry (bonds) and the level of fit (shape of ligand) between ligand and receptor “Drugs will not act unless they are bound” Paul Ehrlich Receptor theory: focus on affinity k+1 Drug A + R AGONIST AR k-1 1) AFFINITY What information How can we Why is this can we get from measure this? important? this? Binding curve: Acetylcholine and guinea pig ileum 800 600 ( f m o l/m g p r o t e in ) A g o n is t b o u n d 400 200 0 0.1 1 10 100 1000 lo g [A c h ] What information can we get from this? Langmuir curve measures drug binding to receptor Bmax 800 Bmax measures 600 SATURATION i.e. ( f m o l/m g p r o t e in ) A g o n is t b o u n d Maximum number of binding sites 400 KD measures 200 affinity of binding (HOW AVIDLY THE DRUG KD 0 BINDS TO 0.1 1 10 100 1000 RECEPTOR) lo g [ A c h ] Binding curve showing ACh binding to guinea pig ileum  KD value in terms of occupancy Occupancy (Pa) is the ratio of bound: total receptors Pa= Na Ntotal We can also express occupancy as: Xa Pa= Hill- Langmuir Equation Xa + K D From this equation we can see that the K D equals the concentration of free ligand that occurs when 50% of receptors are bound. IMPORTANT FACT:  Concept of Affinity KD value gives an estimate of affinity A B C Q. Which of these ligands has the highest affinity for the receptor? LOW KD = high affinity; HIGH KD = low affinity Binding curves can give information upon the specificity of ligand binding to receptors Image result for radioligand binding assay receptor selectivity In this example atenolol has greater affinity for which receptor subtype?????? We can say that atenolol has specificity for which receptor subtypes???  Image result for cartoon of huge pills Importance of affinity in the clinic “I’m sorry, but we’ve run out of the high affinity version of your drug” Clinical example Two drugs: Losartan and candesartan Both drugs are angiotensin II receptor type 1 (AT1) blockers Clinically used to treat hypertension. In-vitro extps against AT1 receptors: Candesartan: KD= 0.6 nM. k-1= >60 min. Losartan: KD= 6.2 nM. k-1= 5 min Q: which one has the highest affinity for the AT1 receptor?? Q: Would they both be effective at lowering blood pressure? Q: Which one would likely be used at a LOWER dose in the clinic  A comparison of the efficacy and duration of action of candesartan cilexetil and losartan in truly hypertensive patients Lacourciere and Asmar (1999) American Journal of Hypertension AIM: This study compared the antihypertensive effect of candesartan cilexetil, to that of losartan on ambulatory BP (ABP). RESULTS: Candesartan cilexetil (16 mg) reduced ABP to a significantly greater extent than 100 mg of losartan. CONCLUSION: candesartan cilexetil provides significant dose-dependent reduction in BP in doses ranging from 8 to 16 mg once daily. The differences observed between both agents are most likely attributable to a tighter binding to, and a slower dissociation from, the receptor binding site with candesartan cilexetil. Summary An agonist is a molecule which BINDS to a receptor and ELICITS a response Agonist binding (occupancy) to a receptor can be determined using radioligand binding Binding studies allow the AFFINITY of agonist binding to the receptor to be determined Affinity is measured by KD and its units are molar (M) Binding studies also allow the maximum amount of receptors present in a tissue to be determined (Bmax, and its units are in mol/mg protein)  Drug receptor interactions: Part 2 Agonists Image result for agonism and antagonism Efficacy of drug receptor interactions Receptor occupied, AND activated Measured by bioassays Receptor occupied, but NOT activated An agonist binds to a receptor and also elicits a response The response can be opening of ion channels, activation of GPCRs or activation of an enzyme depending upon the receptor type  Bioassay http://img.photobucket.com/albums/v377/Rennai/organbath-2-1.jpg Organ bath Response of bladder strips to increasing concentrations of carbachol Bioassay: Measures the biological response of a living tissue to a drug/ hormone or other chemical entity. Whole animals: in- vivo Contraction, relaxation etc Tissue/cells: in-vitro  Dose Response curves DR curves quantify response E m ax of tissue to drug 100 Allow estimation of Emax Allow estimation of re s p o n s e (g ) concentration or dose required to produce 50% of E C 50 maximal response (EC50 or 50 ED50) Allow potency to be determined Allow efficacy to be 0 1 10 100 1000 10000 determined lo g [ A g o n is t]  Potency Potency is an index of how much drug must be administered to elicit a desired response. Potent drugs elicit responses at low concentrations compared to less potent drugs. Usually measured by EC50 However, potency has little clinical significance for a given therapeutic effect. A more potent of two drugs is not necessarily clinically superior: you just need to give less of it Low potency is a disadvantage only if the dose is so large that it is awkward or difficult to administer (such as i.v)  Potency: measured by EC50 Which one of the Agonist B following agonists % Agonist A is most potent? Agonist C A B C log10 NOTE: EC50 measure POTENCY. Low EC50: high potency Efficacy When an agonist binds to a receptor it activates the receptor setting off a chain of biochemical events leading to a response. Efficacy is: The ability of an agonist to ACTIVATE the receptor giving rise to a response It is a feature of the receptor:effector mechanism It refers to the maximal response an agonist can achieve in a tissue Efficacy Definition: Efficacy is a measure of an agonist: receptor complex ability to elicit a response i.e. Its ability to ACTIVATE the receptor D C B A Which agonist has the highest efficacy? Does this agonist have maximal efficacy? What about the other agonists: what is their efficacy?  Partial agonists Partial agonists bind to F u ll a g o n is t receptors but can only 100 produce sub-maximal responses compared to full agonists even when all the receptors % re s p o n s e P a r tia l a g o n is t are occupied. 50 Partial agonists can have HIGH AFFINITY Partial agonists have LOW EFFICACY Q: In the presence of a high 0 1 10 100 1000 10000 affinity partial agonist will a full agonist be able to produce lo g [ A g o n is t] a maximum response in the same tissue  Why are partial agonists “partial” Partial agonists could bind to receptor but doesn’t activate them (antagonist?) Two-stage model of receptor activation is misleading and receptor activation is not a simple “off-on” but is graded: partial agonists are unable to cause complete receptor activation Efficacy or potency??? Q: could you achieve the same range of pain relief with the two drugs on graph A? What about graph B? On graph B which drug would give an equivalent level of pain relief at a lower dose? Dose-response curves demonstrating efficacy and potency. A, Efficacy, or “maximal efficacy,” is an index of the maximal response a drug can produce. Efficacy is an important quality in a drug. B, Potency is an index of how much drug must be administered to elicit a desired response. Potency is usually not an important quality in a drug. Some clinically useful full agonists NOTE: Also look at drug glossary on SN too for other examples  Salbutamol (Ventalin®) Beta 2 adrenoceptor agonist used in treatment of asthma  Morphine (Oramorph®) opioid receptor agonist used to relieve moderate and severe pain  Sumatriptan (Imigran®) serotonin (5-HT1) receptor agonist used in treatment of migraine  Ropinirole (Requip®) D2 dopamine receptor agonist used in treatment of Parkinson’s disease. Clinically useful partial agonists NOTE: Also look at drug glossary on SN too for other examples Buprenorphine (Subutex®) opioid receptor partial agonist used to treat opioid dependence Buspirone (BuSpar®) serotonin (5-HT1A) receptor partial agonist used in the treatment of anxiety Inverse agonists: a special case Two state model states that occupied receptor exists in two forms (AR and AR*) binds activates A + R AR AR* But: This oversimplifies receptor pharmacology. Now known that some receptors have low levels of activity in the absence of agonists (R*: constitutive activity) Note: no agonist, but R R* receptor active Some drugs may reduce the number of these constitutively activated receptors at rest: INVERSE AGONISTS Inverse agonists: https://classconnection.s3.amazonaws.com/361/flashcards/228361/png/screen_shot_2014-08-12_at_25624_pm-147CB944AE446F19A9F.png An inverse agonist reduces the effect below baseline. This is because some receptors have low levels of activity in the absence of ligands Many competitive antagonists display inverse agonist activity (cimetidine, atropine) IUP definition: "A ligand that by binding to receptors reduces the fraction of them in an active conformation... if some of the receptors are in the active form in the absence of a conventional agonist". inverse agonist example: Taranabant (experimental drug only) Cannabinoid (CB1) receptor inverse agonist designed for an anti-obesity drug. Failed clinical trials due to CNS side effects, but used experimentally. β-carbolines on GABAA receptors are anxiogenic as opposed to anxiolytic Spare receptors. Binding curve data: lecture 1 Dose response data from organ bath bioassay Why doesn’t EC50= KD? In the example above acetylcholine elicits a ~100% response at approximately 50% occupancy. This is known as receptor reserve (or spare receptors), in which 100% occupancy is NOT required to give a full (100% response). Comparing the binding and response curves Why can these curves sometimes be different? Curve shapes are similar, and both are concentration dependent! The difference occurs in that the response (contraction of muscle) is a DOWNSTREAM process which involves several steps. Activates Increase Smooth ACh Activates Second Cai Muscle binds to G-proteins messengers Contraction receptor Mechanisms underlying the control of smooth muscle contraction ✓The important component here is the production of second messengers which may act as amplifiers of the original signal. ✓Thus a larger response may be elicited with relatively low occupancy. ✓This is important since low levels of neurotransmitters could elicit maximal responses. THIS INCREASES SENSITIVITY OF SYSTEM  Questions Here we have plotted the occupancy-response relationship of five different drugs. We can determine the drugs’ efficacy (their intrinsic activity, IA) and their receptor occupancy. Which drug(s) is/are partial agonists? Which drug has a receptor reserve of 80% Which drug has a receptor reserve of 40% Which drug has no receptor reserve?  Summary  Agonist activity in producing a biological response can be measured by potency and efficacy  Potency measures how much drug is needed to produce a desired response  Efficacy measures the response produced by the activated agonist:receptor complex. Drugs may be full agonists (efficacy =1) or partial agonists(efficacy< 1).  Inverse agonists act to stabilize the resting state of a receptor and decrease constitutive activity  100% occupancy of receptors is not necessarily needed to produce maximal responses: spare receptors or receptor reserve. Pharmacodynamics Lecture 3: Antagonists Pharmacy Practice and Medicines (5LMS2017) Image result for receptor affinity cartoon Antagonist Chris Keating (C107) c.keating @herts.ac.uk Lecture objectives Introduce concept of antagonism and uses in medicine Describe competitive antagonism in terms of receptor interactions with agonist and antagonist Describe quantification of reversible antagonism (Schild analysis) Describe irreversible antagonism & non-competitive antagonism Lecture Outcome: Student to understand the interaction of antagonists with receptors and the effect of this on agonist dose response relationships General classes of Antagonists Chemical: Two substances combine in solution and as a result the effect of the active drug is lost. Commonly called chelating agents. Example: Dimercaprol and infliximab Physiological: Two drugs act on a common pathway but have opposite effects. Example: glucocorticoids and insulin Pharmacokinetic: The rate of degradation of an active drug may be influenced by other drugs which increase its hepatic metabolism *** Competitive: Binds to a receptor (reversibly or non reversibly) blocking agonist binding and hence action of an agonist. Example: granisetron (reversible antag). *** Non-competitive: Binds either to allosteric site on receptor or downstream from receptor activation and blocks component of signal transduction pathway. Example: nifedipine ***, known as Pharmacological antagonism ANTAGONISM  Competitive antagonism 1: Reversible antagonism http://www.automation-drive.com/EX/05-14-11/compantag1.gif Heinz Otto Schild 1906- 1984 Schild Analysis: see pharmacology practical details In competitive antagonism, the antagonist competes with the agonist for occupancy of the receptor Agonist alone Agonist plus antagonist Antagonism is surmountable Increased by increasing agonist plus dose of antagonist agonist Important rules for competitive antagonism In the presence of a competitive antagonist: The agonist dose response curve is shifted to the right Agonist only Agonist + The agonist dose response curve has the same maximal response (Emax) antagonist The agonist dose response curve exhibits a parallel shift The agonist dose response curve has the same form WHY????????? Because the antagonist binds REVERSIBLY to the receptor and COMPETES with the agonist for the binding site of the receptor. How to Quantify the parallel shift of the agonist dose response curve: dose ratios 100 90 80 70 no antag Response 60 DR dose ratio 10-8M 50 10-7 M 40 10-6M 30 10-5 M 20 10 0 Log dose agonist Dose ratio: how may more times the agonist is needed in the presence of the antagonist to achieve the same response DR= EC50 of agonist in presence of antagonist EC50 of agonist in the absence of antagonist Schild plot: measurement of antagonist affinity. Schild Eq: Log10 (DR-1)= log10 XB- log10 KB Schild Equation Schild Plot DR= 1+ XB 4 KB Log10 (DR-1) 3 2 log KB DR= dose ratio XB= antagonist concentration (M) KB= antagonist equilibrium dissociation 1 constant (M) NOTE: the Schild equation states that 0 “the concentration of antagonist that causes a two-fold rightward shift in the -9 -8 -7 -6 -5 agonist dose response curve is equal to Log10 antagonist concentration the KB of that antagonist”!! Try putting some numbers into the equation to see this for yourself Where the line crosses the x-axis refers to an antagonist conc giving rise to a DR=2  Schild analysis: pA2 Describes antagonist affinity (K B) but uses simple numbers pA2=“negative log10 of the molar concentration of 4 pA2= minus this number an antagonist required produce an agonist dose ratio equal to 2” Log10 (DR-1) 3 pA2 = - logKB 2 But only if line is linear with slope = 1 (i.e 1 competitive antagonist) 0 Therefore if KB is equal to 1x10-8M then pA2= ? -9 -8 -7 -6 -5 Log10 antagonist concentration High pA2= HIGH AFFINITY Clinical implications Look at lecture notes throughout year to see how many compounds you are learning are compet antags Extent of antagonist inhibition depends upon endogenous agonist concentrations. This can cause issues in clinical prescribing since these can vary in normal states and disease. CAN YOU THINK OF ANY EXAMPLES Extent of inhibition developed will depend on antagonist concentration. This can also cause issues since metabolism of drugs is patient dependent and some interpatient variability in PK will occur. CAN YOU THINK OF ANY EXAMPLES Competitive antagonism 2: Irreversible antagonism Antagonist: receptor complex Xi + R XiR Compet irreversible antag No reverse reaction 100 Agonist alone % m ax R esponse Agonist + Antagonist 50 0 lo g [A g o n is t] Irreversible Antagonism In the presence of an irreversible antagonist Agonist curves do not have the same form (shape of curve changes) The Emax of the agonist dose response curve reduces The EC50 increases Why????????????????? Antagonist binds permanently to receptor therefore antagonism cannot be surmounted.  Irreversible Antagonism: Effect of Antagonism that methysergide on cannot be 5-HT response in reversed by rabbit smooth washing the tissue muscle i.e. irreversible Irreversible competitive antagonism is time Effect of dependent dibenamine on carbachol response Note: no parallel in rabbit smooth shift and a fall in muscle the maximum response Competitive versus irreversible antagonism COMPETITIVE IRREVERSIBLE COMMON TYPE. LESS COMMON. LOT’S OF MEDICINES ARE VERY FEW MEDICINES ACT THROUGH COMPET ANTAGS THIS MECHANISM Eg: GRANISETRON (5-HT3 rec) Eg: PHENOXYBENZAMINE (α1 adreno) Partial agonists can behave like irreversible antagonists Agonist 100 Agonist 100 % m ax R esponse % m ax R esponse Agonist + Agonist + 50 50 Antagonist partial agonist 0 0 lo g [A g o n is t] lo g [A g o n is t] Non competitive antagonism  Simply blocks a step somewhere between receptor activation and response. It does not compete with the agonist for the receptor site, and so is termed non- competitive Examples: L-type calcium channel blockers (verapamil or nifedipine) Non-competitive Antagonism Agonist 100 Binds to a non-agonist site. Typically blocks signal % m ax R esponse +Antag transduction system 50 Reduces slope and Emax. +Antag Changes EC50 Examples: nifedipine (L-type calcium channel blocker) 0 lo g [A g o n is t] Desensitisation and Tachyphylaxis  When given continuously or repeatedly, the effect of a drug may diminish over time: a phenomenon known as desensitisation and tachyphylaxis  This may be the result of: ✓ Translocation of receptors (internalisation of receptors followed by recycling and degradation) ✓ Change in receptors (phosphorylation, or conformational change) ✓ Exhaustion of mediators (amphetamine depletion of amine stores) ✓ Increased metabolic degradation of the drug ✓ Physiological adaptations  Summary  Competitive Reversible antagonism is characterised by: ✓ a rightward shift in the agonist concentration: response curve without a change in slope or maximum response. ✓ The extent of the shift can be calculated and is known as the dose ratio (DR) ✓ For competitive antagonists, the DR increases linearly with antagonist concentration: and can be used to measure of the affinity of the antagonist for the receptor. ✓ Antagonists are an important class of drugs for use in medicine  Lecture outcomes Using lecture, workshop and practical notes you should now be able to: Explain the two state hypothesis of agonist receptor interactions. Describe the difference between the affinity, efficacy and potency of an agonist. Explain the differences between full, partial and inverse agonists. Explain the general classes of antagonism Define the effect of competitive and irreversible competitive on an agonist dose response curve. Appreciate how we quantify antagonism using the Schild equation and Schild plot.