Lecture 2. How Drugs Act - General Principles Pt1 (PDF)

Possible Exam/Quiz Dates Quiz 1: Tuesday, September 24 Quiz 2: TBA Midterm Exam: Tuesday, Nov 19 November 11 – 15 - Midterm Break 1  Pharmacodynamics General principles underlying the interaction of drugs with living systems...

Possible Exam/Quiz Dates Quiz 1: Tuesday, September 24 Quiz 2: TBA Midterm Exam: Tuesday, Nov 19 November 11 – 15 - Midterm Break 1  Pharmacodynamics General principles underlying the interaction of drugs with living systems 2  Learning Objectives Define what are drugs, their classification and their specific action Describe receptors and their classification Explain drug-receptor interactions using the following terms: – Affinity – Efficacy - Potency – Agonists - Antagonist – Partial - Full – Competitive - ……………….? Describe various dose-response curves and the following terms Emax EC50 ED50 Define and describe five types of drug antagonism Differentiate receptor desensitization, tachyphylaxis and tolerance 3  What are drugs? A drug is a chemical applied to a physiological system that affects its function in a specific way (Rang & Dale) A drug is any biological substance, synthetic or non-synthetic, that, when taken into the organism's body, will in some way alter the biological functions of that organism (Wikipedia) A drug is a substance used in the prevention, diagnosis, or treatment of disease (Lange, Pharmacology) 4  Classification of Drugs By pharmacological effect (e.g. analgesics, antipsychotics, antihypertensives, anti-asthmatics, and antibiotics) By chemical structure (e.g. penicillins, barbiturates, opiates, steroids, and catecholamines) By target systems (e.g. cholinergic and adrenergic systems) By target molecule (e.g. anticholinesterases are drugs that act by inhibiting the enzyme acetylcholinesterase). 5 The emergence of pharmacology as a science came when the emphasis shifted from describing what drugs do -- to explaining how they work Paul Ehrlich was the first to recognize that drug action must be explicable in terms of conventional chemical interactions between drugs and tissues. Drug molecules must exert some chemical influence on one or more constituents of cells to produce a pharmacological response. – Mainly, they act ‘by numbers’ (one drop of a solution of a drug at only 10-10mol/l still contains about 3x109 drug molecules) – However, some drugs act with such precision that a single molecule taken up by a target cell is sufficient to kill it (e.g., diphtheria toxin). 6  The molecules in the organism outnumber the drug molecules, and if the drug molecules were merely distributed at random, the chance of interaction with any particular class of cellular molecule would be insignificant. Pharmacological effects, therefore, require, in general, the non-uniform distribution of the drug molecule within the body or tissue, which is the same as saying that: Drug molecules must be 'bound' to particular constituents of cells and tissues in order to produce an effect. These critical binding sites are often referred to as 'drug targets' – ‘Magic bullet’ – term coined by Erlich – ‘Magic bullet’ vs. ‘shotgun’ Most drug targets are protein molecules. Exceptions: antimicrobial, antitumor drugs; mutagenic and carcinogenic agents (interact with DNA); bisphosphonates (bind to calcium salts in bone matrix); biopharmaceutical drugs (e.g., COVID-19 mRNA vaccines) 7  Target Identification and Validation Historically, identification of highly validated targets— often G-protein-coupled transmembrane receptors Target validation ensures that engagement of the target has potential therapeutic benefit Example: identification of the role of the histamine H2 receptor in the control of stomach acid secretion led to the development of cimetidine and ranitidine Cimetidine (Tagamet) Ranitidine (Zantac) 8 Acid secretion plays a key role in multiple clinical conditions: Dyspepsia Peptic ulcer disease As part of Helicobacter pylori eradication therapy Gastroesophageal reflux disease Barrett’s oesophagus Eosinophilic oesophagitis Stress gastritis and ulcer prevention in critical care Gastrinomas and other conditions that cause hypersecretion of acid, including Zollinger–Ellison syndrome. H2 blockers (antagonists) are effective in all these conditions because of the fundamental link between the target, drug effect, and pathophysiology. 9 No matter how safe and innovative a newly developed drug might be, it will not be efficacious if the original pharmacological hypothesis is wrong and modulating the target protein has little or no effect on altering the course of the disease. 10  General characteristics of drug targets Druggable genome: a subset of the ∼30,000 genes in the human genome that express proteins able to bind drug-like molecules. Hopkins A.L., Groom C.R. The druggable genome Nature Reviews. Drug Discovery, 9, 2002;1: 727-730 A recent analysis of drug targets points to 667 human proteins and 28 ‘other human biomolecules’ Santos R., Ursu O., Gaulton A., Bento A.P., Donadi R.S., Bologa C.G., et al. A comprehensive map of molecular drug targets Nature Reviews. Drug Discovery 1, 2016;16: 19-34 The key challenge is to identify those targets that play a meaningful role in the disease process 11  PROTEIN TARGETS FOR DRUG BINDING Generally, “target” refers to the biochemical entity to which the drug first binds in the body to elicit its effect. Protein targets include: 1. Receptors 2. Enzymes 3. Carrier molecules (transporters) 4. Ion channels Possible confusion, Re: DRUG RECEPTORS vs TARGETS – Receptors = term used in immunology to describe molecules through which soluble physiological mediators produce their effects (usually cell surface receptors = proteins) – Receptors = term used in pharmacology to describe protein molecules whose function is to recognize and respond to endogenous chemical signals (e.g., acetylcholine receptors, cytokine receptors, steroid receptors) – Other macromolecules (non-protein) with which drugs interact to produce their effects (e.g. DNA, lipids, carbohydrates, salts) are known as drug targets (it is incorrect to call them ‘receptors). 12  Why Protein Targets? A target must be able to discern differences in electronic structure minute enough to be present in a drug molecule. Proteins have the tertiary 3-dimensional structure necessary for a detailed definition of the electronic forces involved in small molecule binding. Signals are initiated through complementary binding of drug molecules to protein conformations that have a physiological purpose in the cell. The act of these molecules binding to the protein will change it, and a pharmacological effect will occur with the change. 13  Pharmacologic targets can be used to modify physiological processes Protein receptors and ion channel targets: chemicals can be used to cause activation, blockade, or modulation; Enzymes and transporter proteins: the main drug effect is inhibition of ongoing basal activity of these targets. 14  Location of Targets Receptors, ion channels and transporter proteins are usually found on the cell surface (exposed to the extracellular space), Enzymes are most often found in the cytosol of the cell (drugs must enter the cell to act on enzymes). Exceptions: nuclear receptors which reside in the cell nucleus. There are other drug targets present in the cell, such as DNA, and chemicals can have physical effects (i.e., membrane stabilization) that can change cellular function. 15  Pharmacological effect on cells Changes in the mechanical function of cells – Cardiac contractility – Contraction of bronchiole smooth muscle Biochemical metabolic effects – Levels of second messengers such as Ca2+ or cAMP Modulation of basal activity – Level of catalytic degradation of cAMP by enzymes such as phosphodiesterase – Rate of uptake of amine neurotransmitters such as norepinephrine and serotonin. 16  Target class distribution for drugs approved up to 2018 Oprea, T. I., Bologa, C. G., Brunak, S., Campbell, A., Gan, G. N., Gaulton, A., et al.. Unexplored therapeutic opportunities in the human genome, Nature Reviews Drug Discovery, 17, 317–332 17 Drug Targets: Specificity/Selectivity of Drug Interactions Selectivity - the ability of a drug to discriminate between related targets (e.g., receptors or enzymes), showing a higher binding affinity for one subtype or isoform). Specificity - If a drug has one effect, and only one effect on all biological systems Selectivity is reciprocal: – individual classes of drugs bind only to certain targets – individual targets recognize only certain classes of drug No drugs are completely specific/selective in their actions !!!!! In many cases, increasing the dose of a drug will cause it to affect targets other than the principal one, and this can lead to side effects (adverse, unwanted effects) 18  Drug Targets: Specificity/Selectivity of Drug Interactions The lower the potency of a drug and the higher the dose needed, the more likely it is that sites of action other than the primary one will assume significance. ‘On-target’ side effects are mediated by the same receptor as the clinically desired effect: for example, constipation and respiratory depression by opioid analgesic drugs ‘Off-target’ side effects are mediated by a different mechanism: antiemetic dimenhydrinate acts on two receptors 19  Drug Targets: Specificity/Selectivity of Drug Interactions Side effects are usually undesirable (e.g. antiemetic dimenhydrinate (marketed under brand names Gravol in Canada; Dramamine in the USA) Diphenhydramine (the primary constituent of dimen- hydrinate) is a competitive H1 receptor antagonist (or inverse agonist?), which is widely distributed in the human brain  anti-emetic effect Antagonism of muscarinic acetylcholine receptors  drowsiness Abusing Dramamine is sometimes referred to as Dramatizing or "going a dime a dozen“ (deliriant) 20 Dimenhydrinate Diphenhydramine (DPH) is an antihistamine medication mainly used to treat allergies. It can also be used for insomnia, symptoms of the common cold, tremor in parkinsonism, and nausea 1,3-dimethyl-8-chloroxanthine stimulant drug of the xanthine chemical class, with physiological effects similar to caffeine 21  Another illustration of fundamental role of receptors: Epinephrine (Adrenaline) – Epinephrine first binds to a receptor protein (β adrenoceptor) – β adrenoceptor serves as a recognition site for adrenaline and other catecholamines – When epinephrine binds to this receptor, a train of reactions is initiated, leading to: Increase in the force of the heartbeat Increase in the rate of the heartbeat – in the absence of adrenaline, the receptor is functionally silent. Adrenaline is a nonselective agonist of all adrenergic receptors 22 Binding and Activation of a Receptor ? Binding = Activation 23  Binding and Activation of a Receptor Binding = Occupation of a receptor by a drug molecule – may or may not result in activation of the receptor – the binding of drugs to receptors can often be measured directly by the use of drug molecules labelled with one or more radioactive atoms (e.g. 3H, 14C or 125I) Activation = the receptor is affected by the bound molecule in such a way as to alter the function of the cell and elicit a tissue response Agonists = drugs that activate receptors Agonists = drugs that affect the receptor in such a way as to alter the function of the cell and elicit a tissue response Binding and activation represent two distinct steps in the generation of the receptor-mediated response by an agonist 24  Binding and Activation of a Receptor Structural and functional studies indicate that receptors exist in at least two conformations, active and inactive, and these are in equilibrium Agonists have a higher affinity for the receptor’s active conformation The equilibrium is pushed to the active state, resulting in receptor activation 25 Binding and Activation of a Receptor 26  Agonists vs. Antagonists Agonists = drugs that affect the receptor in such a way as to elicit a tissue response Antagonists = drugs that antagonize agonists Receptor Antagonists = drugs that may combine with a receptor at the same site as agonist without causing activation, and block the effect of agonists on that receptor Receptor Ligands = Agonists + Antagonists 27  Binding and Activation of a Receptor The distinction between drug binding and receptor activation. Ligand A is an agonist because when it is bound, the receptor (R) tends to become activated, whereas ligand B is an antagonist because binding does not lead to activation. It is important to realize that for most drugs, binding and activation are reversible, dynamic processes. The rate constants k+1, k−1, α, and β for the binding, unbinding, and activation steps vary between drugs. For an antagonist, which does not activate the receptor, β = 0 28 Affinity and the equilibrium constant KA for the binding reaction, KA = k-1/k+1 The equilibrium constant, KA, is a characteristic of the drug and of the receptor It has the dimensions of concentration It is numerically equal to the concentration of drug required to occupy 50% of the sites at equilibrium The reciprocal of KA is a measure of the affinity: The higher the affinity of the drug for the receptors, the lower will be the value of KA 29  The Binding Reaction and Hill- Langmuir Equation ( P A ) (or N A /N tot ) is the proportion of receptors occupied. When the concentration of a drug is equal to KA, then PA equals 0.5; that is, the KA is the concentration of the drug that occupies 50% of the available receptor population. The magnitude of KA is inversely proportional to the affinity of the drug for the receptor. 30  Concentration-effect or dose- response curves Although binding can be measured directly, it is usually the biological response to a drug that is actually measured: increase in heart rate, a rise in blood pressure, contraction or relaxation of a strip of smooth muscle in an organ bath, or the activation of an enzyme These are often plotted as concentration-effect (in vitro) or dose-response curves (in vivo) 31  Dose-Response Curves A characteristic feature of drugs acting on a specific target in a physiological system is that there will be a graded increase in response with an increase in drug concentration (dose). Increasing doses of epinephrine produce increases in heart rate. 32  Dose-Response Curves The increased heart rate as a function of epinephrine concentration. 33  Dose-Response Curves The stimulus input is the amount of fluid entering the vessel from the spigot; the fluid level in the vessel is the cellular response. It is assumed that the cell has a basal activity. As the drug increases the stimulus to the system (increases fluid entry), the fluid level rises (cellular response increases). At some point, the level exceeds the limits of the vessel and escapes through the overflow; this represents the maximal capability of the cell to increase activity. 34  Dose-Response Curves The dose-response relationship given in Slide 33 on a logarithmic scale fit to a semilogarithmic sigmoidal 35 mathematical function. Dose-response curves: linear and semilog 36  Efficacy (Emax) Efficacy (Emax) is the maximum effect that can be expected from this drug Craig W Clarkson, Tulane University School of Medicine 37  Efficacy defines difference between Full and Partial Agonists Efficacy = parameter originally defined by Stephenson (1956) that describes the 'strength' of the agonist-receptor complex in evoking a response of the tissue Efficacy describes the tendency of the drug-receptor complex to adopt the active (AR*) rather than the resting (AR) state. A drug with zero efficacy (e = 0) has no tendency to cause receptor activation and causes no tissue response. A drug with sufficiently high efficacy (max e = 1) is a full agonist Partial agonists lie in between. 38  Affinity vs. Efficacy Affinity governs the tendency of a drug to bind to the receptors Efficacy governs a drug's tendency to activate the receptor once bound. It is an inherent property of an agonist that determines its ability to produce its biological effect Affinity gets the drug bound to the receptor, and efficacy determines what happens once the drug is bound Agonists will possess high efficacy – Fentanyl selectively binds to and activates the μ-receptor (agonist) Antagonists will, in the simplest case, have zero efficacy – Naloxone - competitive non-selective antagonist at opioid receptors (binding affinity is highest for the μ-opioid) and acts to reverse the effects of most opioid analgesics 39  Potency Depends on two drug factors: affinity (i.e. tendency to bind to receptors) and efficacy (i.e. ability, once bound, to initiate changes that lead to effects). Refers to the concentration (EC50) or dose (ED50) of a drug required to produce 50% of the drug’s maximal effect as depicted by a graded dose-response curve. EC50 equals KA when there is a linear relationship between occupancy and response 40  Potency The location parameter along the concentration axis represents the drug potency. This is usually quantified by the molar concentration that is seen to produce 50% of the maximal response. For the solid line dose- response curve the concentration producing 50% response is 10-6 M; this is reported as the negative logarithm of this value (pEC50), in this case, the pEC50 is 6.0 41  Potency Schematic illustration of the dose-response curves for a series of agonists (A, B, C and D) that have the same efficacy, but differ in terms of their potency. The most potent drug (Drug A) has the lowest EC50 value and is approximately 20-30 fold more potent than Drug D. Craig W Clarkson, Tulane University School of Medicine 42  Efficacy vs Potency Craig W Clarkson, Tulane University School of Medicine 43  Efficacy (Emax) Each drug has essentially the same EC50 value (same potency) 44 Craig W Clarkson, Tulane University School of Medicine  Affinity, Efficacy, Potency Generally, drugs with high potency often have a high affinity for their receptors because they can achieve a significant biological effect at lower concentrations (partly due to their strong binding to the receptor). A drug may have low affinity (binds weakly) but high efficacy (produces a strong response upon binding). If such a drug is an agonist with high efficacy, it can produce a maximal effect even when only a few receptors are occupied. This makes it potent (low EC50) despite its low affinity (high KA). Other situations where high potency does not necessarily mean high affinity: – Receptor Reserve – Allosteric Modulation – Signal Amplification – Pharmacokinetic Factors 45  Dose-response curves (recap) Dose-response curves allow us to estimate: Emax - the maximal response that the drug can produce (efficacy) EC50 or ED50 - the concentration or dose needed to produce a 50% maximal response, parameters that are useful for comparing the potencies of different drugs that produce qualitatively similar effects. Refers to the concentration of a drug that induces a response halfway between the baseline and maximum Although they look similar to the binding curves, concentration-effect curves cannot be used to measure the affinity of full agonist drugs for their receptors because the physiological response produced is not, as a rule, directly proportional to occupancy; also, concentration applied may differ from that at the receptor For a partial agonist, the EC50 value is a good approximation of the affinity for the agonist (Mol Pharmacol. 2016 Feb; 89(2): 297–302.) 46  Dose-response curves If the maximal response to the agonist is lower than the system’s maximal response, then the concentration producing the maximal response approximates the saturation binding concentration of the agonist (to 100% of the available receptors) Then, the EC50 of the curve also approximates the KA for binding 47 (to 50% of the sites).  Spare Receptors (Receptor Reserve) In many cases, the number of receptor sites available exceeds the signaling capacity of the cell. For example, if there is a 10 to 1 ratio of receptor sites to signaling machinery, occupying 10% of the available receptor sites will produce a maximum response. If two different tissues express the same receptor but with different reserve levels, the impact of a set concentration of an agonist may have different physiological outcomes. For most full agonists, a higher receptor reserve typically results in increased apparent potency. Why? What about partial agonists? 48  Literature Rang & Dale’s Pharmacology 10th Edition Terry P. Kenakin, Pharmacology in Drug Discovery and Development, 2nd edition Drug Discovery and Development. Technology in Transition. Edited by Raymond G. Hill and Duncan B. Richards, 3rd edition Benjamin E. Blass, Basic Principles of Drug Discovery and Development Graham L. Patrick, An Introduction to Medicinal Chemistry, 5th edition 49