Integrated Pharmacotherapy I. Drug Targets, Ligands, Receptors, and Mechanisms of Drug Action

Transcription

1 Integrated Pharmacotherapy I Drug Targets, Ligands, Receptors, and Mechanisms of Drug Action Required reading: Chapters 1 and 2, Basic and Clinical Pharmacology, 10th Ed., Katzung BG, McGraw Hill, Edward Fisher, Ph.D., R.Ph. Professor and Associate Dean for Academic Affairs Professor and Associate Dean for Academic Affairs University of Hawaii at Hilo College of Pharmacy fishere@hawaii.edu 1

2 Learning Objectives After attending these lectures, completing the required reading, & studying these handouts, you should be able to describe the basic principles of drug receptors & pharmacodynamics including: The dose-response relationship Signaling mechanisms and drug action The relationship between dose and clinical response 2

3 Ligands Molecules that carry signals for cellular communication Endogenous ligands 3

4 Ligands Endogenous ligands Neurotransmitters Hormones - secreted from endocrine glands Ductless and located in connective tissue Secreted into blood and have effects at distant sites, they are released: In steady amounts (e.g. thyroid hormone) Based on specific cycle» Circadian rhythms (e.g. glucocorticoids)» 4-week cycle female hormones 4

5 Types of Hormones Steroid hormones derived from cholesterol Amino acid derivatives Catecholamines Histamine Serotonin Melatonin Peptides and proteins Neuropeptides (vasopressin, oxytocin) Pituitary hormones (corticotropin, gonadotropins) Gastrointestinal hormones (insulin) Eicosanoids Derived from polyunsaturated fatty acids especially arachidonic acid Prostaglandins, prostacyclins, leukotrienes, thromboxanes 5

6 Types of Hormones Hydrophobic hormones Receptors found in the cell Act by affecting gene expression Examples Sex hormones Androgens and estrogens Adrenal hormones Mineralocorticoids and glucocorticoids Hydrophilic hormones Receptors on cell surfaces Examples Catecholamines Peptide hormones Eicosanoids 6

7 Mechanisms of Hormone Action Activation of enzymes Usually by changing the releative amounts of enzymes that are phosphorylated Modulation of gene expression 7

8 Local Hormones = autocoids Characteristics Transport via blood is not necessary Have local effects Short duration of action Functions Defense Repair Classifications Paracrine act on neighboring cells Synaptic transmission special type of paracrine Autocrine also act on the cell that secreted them Influence a group of cells to act the in the same way Juxtacrine direct contact is necessary Gap junctions 8

9 Local Hormones = autocoids 9

10 Introduction to Pharmacology Pharmacology: the study of substances that interact with living systems through chemical processes. Medical pharmacology therapeutic application Toxicology undesirable effects of chemicals on living systems Pharmacodynamics: the actions of the drug on the body. How the drug effects you Biochemical effects, mechanism of action (MOA) Drug classification Pharmacokinetics: the actions of the body on the drug. How you effect the drug Absorption, distribution, metabolism, & excretion Involved with the time course of the drug in the body 10

11 Introduction to Pharmacology Receptor: part of an organism or cell (macromolecule) that interacts with a ligand (drug, endogenous molecule) causing a chain of biochemical events leading to an observable response. Active states vs. inactive states Drug: any substance that brings about a change in biologic function through its chemical actions. Endogenous synthesized in the body (hormones) Xenobiotics chemicals not synthesized by the body Drug: Pharmacy definition: Articles intended for use in the diagnosis, cure, mitigation, treatment or prevention of disease in man or other animals. 11

12 Introduction to Pharmacology Drug targets biomolecules that have a role in the disease process and are considered to be the site of action for drug therapy (receptors, enzymes, DNA, ion channels, transport proteins) Agonist: a drug that binds to and activates a receptor which brings about an effect. Albuterol β 2 -selective adrenoceptor agonist 12

13 Introduction to Pharmacology Antagonist: a drug that binds to a receptor and prevents agonists from binding; do not activate receptor; blockers. No direct effect, the antagonistic effect results from the prevention of agonist binding and activation of the receptor Atropine antagonist for muscarinic cholinoceptors Curare - antagonist for nicotinic cholinoceptors Generally they are structurally bulky to prevent receptors from going back to active confirmation, and so that they have more sites for receptor interaction 13

14 Introduction to Pharmacology Partial Agonist: a drug that binds to a receptor and activates it, but the effect is not as great as with a full agonist. Are agonists if no full agonist is present; are antagonists if a full agonist is present Pindolol partial β receptor agonist Many drugs that act as competitive antagonists are really partial agonists Inverse Agonist: a drug that binds to a receptor and stabilizes it in the inactive conformation. Constitutively active receptors active without binding to agonist 14

15 Introduction to Pharmacology Regulation of the activity of a receptor with conformationselective drugs a = active state i = inactive state 15 Adapted from: Goodman & Gilman s The Pharmacological Basis of Therapeutics, L.L. Brunton, J.S. Lazo, K.L. Parker (Eds.), 2006 McGraw-Hill Companies

16 Two-state Model 16

17 A Little Background on Drugs A drug is a substance that brings about a change in biologic function through its chemical actions. Sources Endogenous Exogenous (xenobiotics) States Solids Liquids Gases Size Drugs do not create effects they modulate function = Chemicals foreign to the biological system in question Vast majority of drugs have a MW between MW 100 helps achieve selective receptor binding MW 1000 inhibits diffusion-mediated distribution 17

18 A Little Background on Drugs Chirality: stereoisomerism, R or S handedness Over 50% of all useful drugs are chiral In the majority of cases, one enantiomer will have different pharmacodynamics, pharmacokinetics than the other enantiomer Labetalol (2 chiral centers, 4 enantiomers) (S,R) α-blocker (R,R) -blocker (S,S) and (R,S) inactive Eutomer isomer with desired activity Distomer isomer with undesired activity Prototype drug: a member of a drug group that typifies the most important characteristics of the group. There are several thousand drugs that are currently available arranged into ~ 70 groups 18

19 Drug Receptors & Pharmacodynamics Receptors: 1.) Determine the quantitative relations between dose or concentration of drug and pharmacologic effects receptor number in various target tissues 2.) Are responsible for selectivity of drug action Affinity determined by chemical forces that cause drug to bind to the receptor Efficacy change in confirmation toward the active state Intrinsic activity ability to evoke maximal effect after binding 3.) Mediate the actions of both pharmacologic agonists and antagonists receptor classes, subtype, and isoforms Some drug s MOA do not involve receptors: antacids, osmotics. 19

20 Receptor: Class α β Subtype α 1 α 2 Isoforms α 1a α 1b 20

21 Molecular Recognition Physiochemical complementarity Mostly by noncovalent bonding ionic and hydrophobic type interactions are most important Steric complementarity Chiral recognition 21

22 Lock and key Induced fit Models of binding Seen when ligand causes receptor to change conformation from inactive to active 22

23 Regulatory Proteins: Enzymes: Types of Receptors Mediate actions of most drugs and endogenous chemicals (neurotransmitters, hormones, autocoids) Best characterized Usually through inhibition Methotrexate inhibits dihydrofolate reductase Transport Proteins: Digoxin inhibits Na +, K + ATPase Structural Proteins: Colchicine inhibits tubulin Prevents polymerization of microtubules 23

24 Types of Receptors 24

25 Aspects of Drug Receptor Function 1.) Relationship between drug concentration (dose) & pharmacologic response 2.) Signaling mechanisms and drug action 3.) Relationship between dose and clinical response 25

26 Relationship Between Dose & Response Concentration Effect curves = Dose Response curves Hyperbolic curve; concentration-effect reflects concentrationreceptor binding From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), 2007 McGraw-Hill Companies 26

27 Occupation theory Magnitude of pharmacological effect is proportional to percentage of receptors occupied 27

28 Relationship Between Dose & Response E MAX maximal response that can be produced by drug EC 50 concentration of drug that produces 50% maximal effect K d concentration of free drug at which there is 50% maximal binding equilibrium dissociation constant; measure of affinity low K d high affinity slow dissociation high K d low affinity rapid dissociation From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), 2007 McGraw-Hill Companies 28

29 Relationship Between Dose & Response Sigmoidal curve Expands region of low drug concentration Linear mid-portion Compresses higher portion From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), 2007 McGraw-Hill Companies 29

30 Relationship Between Dose & Response Receptor-Effector Coupling: Transduction process between occupancy & drug response Coupling efficiency based on 1.) extent of conformational change (full agonist-full response, partial agonist-partial response) and 2.) biochemical events that transduce occupancy into response Spare Receptors: When maximal response can be elicited by an agonist when not all available receptors are occupied C in the figure demonstrates the concentration of antagonist that causes available receptors to no longer be spare, they are just sufficient to still get maximum response 30 From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), 2007 McGraw-Hill Companies

31 Relationship Between Dose & Response Spare Receptors (cont.): Temporal - when response outlasts binding (GTP binding) Spare in number Excess of β 1 receptors in heart (90% can be blocked and still get maximal response) Sensitivity of a cell or tissue to a particular [ ] of agonist depends on affinity of receptor for agonist & degree of spareness. Spare receptors increase sensitivity to drug. From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), 2001 McGraw-Hill Companies 31

32 Relationship Between Dose & Response Competitive Antagonist: Reversibly competes with agonist for receptor binding Antagonism can be overcome by concentration of agonist Effect is influenced by: Concentration of antagonist Concentration of agonist that is competing for binding to receptors Propranolol vs. Norepi Effects vary widely in individuals due to differences in clearance Exercise can overcome effect From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), 2007 McGraw-Hill Companies 32

33 Relationship Between Dose & Response Irreversible (noncompetitive) Antagonist: Antagonism cannot be overcome by concentration of agonist Not dependent on own rate of elimination Phenoxybenzamine irreversible α adrenoceptor antagonist control hypertension due to pheochromocytoma in case of overdose, cannot competitively block receptor From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), 2007 McGraw-Hill Companies 33

34 Relationship Between Dose & Response Full agonist at single concentration Full agonist at single concentration From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), 2007 McGraw-Hill Companies 34

35 Relationship Between Dose & Response Allosteric Antagonist: Binds to another part of the molecule Chemical Antagonist: A drug may bind to and inactivate another drug Protamine used to counteract heparin Desferrioxamine chelates iron Physiological Antagonist: One type of functional antagonism agonists that oppose via action on a different receptor or system Use of a separate endogenous regulatory pathway Glucocorticoids vs. insulin in controlling blood glucose Effects are less specific & more difficult to control 35

36 Relationship Between Dose & Response Mechanisms of Receptor Antagonism Adapted from: Goodman & Gilman s The Pharmacological Basis of Therapeutics, L.L. Brunton, J.S. Lazo, K.L. Parker (Eds.), McGraw-Hill Companies

37 Relationship Between Dose & Response Mechanisms of Receptor Antagonism Adapted from: Goodman & Gilman s The Pharmacological Basis of Therapeutics, L.L. Brunton, J.S. Lazo, K.L. Parker (Eds.), McGraw-Hill Companies

38 Signaling Mechanisms & Drug Action 1.) Chemical crosses membrane & interacts with intracellular receptor 2.) Ligand-regulated transmembrane enzyme 3.) Receptor tyrosine kinase 4.) Ion channel 5.) Cell surface receptor linked to an effector enzyme by a G protein From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), 2007 McGraw-Hill Companies 38

39 Signaling Mechanisms & Drug Action 1.) Intracellular receptors: Corticosteroids, mineralocorticoids, sex steroids, thyroid hormone Stimulate transcription by binding to response elements Therapeutically hormonal effects are produced after a lag time of 30 min few hours effects can persist for hours days after agonist concentration is 0 (blood levels are not indicative of activity) Nitric Oxide (NO ; gas) stimulates guanylyl cyclase (not shown here) From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), 2007 McGraw-Hill Companies 39

40 Signaling Mechanisms & Drug Action 2.) Ligand Regulatory Transmembrane Enzymes: Ligand activates enzymatic activity of cytoplasmic domain Similar to tyrosine kinases 3.) Receptor Tyrosine Kinases: Ligand activates enzymatic activity of bound protein tyrosine kinase Insulin, epidermal growth factor (EGF), platelet-derived growth factor (PDGF), atrial natriuretic peptide (ANP), transforming growth factor- (TGF- ) Mechanism: autophosphorylation varied substrate phosphorylation Activity outlasts receptor binding Down-regulation agonist continually binding to receptor causes increased endocytosis of receptor Site for drug action for growth factors, cancer 40

41 Signaling Mechanisms & Drug Action EGF receptor typical receptor tyrosine kinase From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), 2007 McGraw-Hill Companies 41

42 Signaling Mechanisms & Drug Action Cytokine receptor From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), 2007 McGraw-Hill Companies 42

43 Signaling Mechanisms & Drug Action 4.) Ligand-Gated Ion Channels: Fastest way; crucial for synapse function Increases conductance of particular ion across membrane Non-specific ion channel Examples nicotinic acetylcholine receptor (Na + ; shown) GABA (Cl - ) excitatory: glutamate, aspartate inhibitory: amino acids glycine and blocked by strychnine From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), 2007 McGraw-Hill Companies 43

44 Signaling Mechanisms & Drug Action First messenger (Hormone) Effector element G protein intermediary Plasma membrane ECF 5.) G Proteins and 2 nd Messengers: GPCRs Serpentine receptors Separate excitation of receptor and activity of effector Receptor (Binding to receptor activates a G protein, the a subunit activates adenylyl cyclase) (Converts) Adenylyl ICF cyclase Second messenger (Activates) (Phosphorylates) Adrenergic amines, serotonin, acetylcholine (M) = phosphate Adapted From: Human Physiology, L. Sherwood (Ed.) 2004 Brooks/Cole 44

45 Signaling Mechanisms & Drug Action Serpentine receptor Duration of action is dependent on how long GTP is bound to G protein Not on affinity of ligand for receptor From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), 2007 McGraw-Hill Companies R = Receptor E = Effector (enzyme or ion channel) 45

46 Extracellular chemical messenger bound to membrane receptor Activated adenylyl cyclase Signaling Mechanisms & Drug Action Signal amplification Amplification (10) Total number of molecules 1 10 Cyclic AMP Activated protein kinase Amplification (100) 1,000 1,000 Phosphorylated (activated) protein (e.g., an enzyme) Products of activated enzyme Adapted From: Human Physiology, L. Sherwood (Ed.) 2001 Brooks/Cole Adapted From: Human Physiology, L. Sherwood (Ed.) 2004 Brooks/Cole Amplification (100) Amplification (100) 100,000 10,000,000 46

47 Signaling Mechanisms & Drug Action G proteins and their receptors and effectors From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), 2004 McGraw-Hill Companies 47

48 Signaling Mechanisms & Drug Action Endogenous ligands and their second messengers From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), 2004 McGraw-Hill Companies 48

49 From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), 2007 McGraw-Hill Co. Signaling Mechanisms & Drug Action Desensitization: responsiveness to agonist, usually reversible; β receptor β-adrenoceptor kinase β -Arrestin 49

50 From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), 2007 McGraw-Hill Co. Signaling Mechanisms & Drug Action Downregulation: number of receptors; β receptor 50

51 Signaling Mechanisms & Drug Action Summary of Receptor Types & Signal Transducers Adapted from: Goodman & Gilman s The Pharmacological Basis of Therapeutics, L.L. Brunton, J.S. Lazo, K.L. Parker (Eds.), 2006 McGraw-Hill Companies 51

52 Signaling Mechanisms & Drug Action Summary of Receptor Types & Signal Transducers Adapted from: Goodman & Gilman s The Pharmacological Basis of Therapeutics, L.L. Brunton, J.S. Lazo, K.L. Parker (Eds.), 2006 McGraw-Hill Companies 52

53 Signaling Mechanisms & Drug Action Summary of Receptor Types & Signal Transducers Adapted from: Goodman & Gilman s The Pharmacological Basis of Therapeutics, L.L. Brunton, J.S. Lazo, K.L. Parker (Eds.), 2006 McGraw-Hill Companies 53

54 Signaling Mechanisms & Drug Action Summary of Receptor Types & Signal Transducers Adapted from: Goodman & Gilman s The Pharmacological Basis of Therapeutics, L.L. Brunton, J.S. Lazo, K.L. Parker (Eds.), 2006 McGraw-Hill Companies 54

55 Signaling Mechanisms & Drug Action camp as a second messenger: Exerts most of its effects by stimulating camp-dependent protein kinases Specificity is dependent on the proteins (enzymes) present Liver phosphorylase kinase vs. glycogen synthase Adipocytes lipase Smooth muscle myosin light chain kinase Inactivated by phosphodiesterase (PDE) caffeine, theophylline are PDE inhibitors From: Basic & Clinical Pharmacology, B.G. Katzung 55 (Ed.), 2007 McGraw-Hill Companies

56 Signaling Mechanisms & Drug Action Ca 2+ and Phosphoinositides: PLC splits PIP 2 into DAG & IP 3 DAG stays in the membrane & activates protein kinase C (there are many types) IP 3 diffuses through cytoplasm & releases Ca 2+ from internal storage vesicles; this promotes Ca 2+ -calmodulin binding which regulates Ca 2+ -dependent protein kinases Inactivation IP 3 - dephosphorylation DAG - phosphorylated or deacylated Ca 2+ - pumps PLC = phospholipase C DAG = diacylglycerol From: Basic & Clinical Pharmacology, B.G. Katzung 56 (Ed.), 2007 McGraw-Hill Companies

57 cgmp: Signaling Mechanisms & Drug Action Only in a few cell types (vascular smooth muscle, intestinal mucosa) Similar to camp, guanylyl cyclase makes cgmp from GTP NO activates guanylyl cyclase cgmp leads to dephosphorylation of myosin light chain relaxation ANP binds to a transmembrane receptor that activates guanylyl cyclase found in membranes cgmp dependent kinases From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), McGraw-Hill Companies

58 Signaling Mechanisms & Drug Action Phosphorylation: Reversible Amplification: records a molecular memory of the pathway that has been activated, dephosphorylation erases the memory Flexible regulation: differing substrate specificities of the multiple protein kinases regulated by 2 nd messengers provide branch points that may be independently regulated Receptor classes & drug development: Structure activity relationships give clues about drug receptors Limited number of regulatory molecules Overall effects are due to receptors Diversity of receptors allows for selectivity of drugs 58

59 Relationship Between Dose & Clinical Response Maximal benefit with minimal toxicity Graded dose-response relationship: Potency EC 50 or ED 50 (dose needed for 50% of drug s effect) dependent on affinity (K d ) and efficiency of coupling response Maximal efficacy limit of the dose-response relationship; important for clinical effectiveness dependent on ability to reach relevant receptors route of administration, absorption, site of action 59

60 Drugs A & B are more potent than drugs C & D The pharmacologic potency of A is less than that of B Drug A has a larger maximal efficacy than drug B Relationship Between Dose & Clinical Response Potency = dose needed for 50% of drug s effect Drugs A, C, & D have equal maximum efficacy which is greater than the maximum efficacy of B A very steep curve implies: cooperative actions of different systems need great majority of receptors to be occupied narrow therapeutic range From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), McGraw-Hill Companies

61 Relationship Between Dose & Clinical Response Quantal dose-response curves: All or none Determine the dose required to produce an effect in a large number of individual patients and plotting the cumulative frequency distribution ED 50 - dose at which 50% of individuals respond (different than ED 50 defined above) TD 50 - dose required to produce a toxic effect in 50% of individuals (LD 50 if death is endpoint) Therapeutic index TD 50 / ED 50 Digoxin narrow Benzodiazepines and antipsychotics - wide The clinically acceptable risk of toxicity depends critically on the severity of the disease being treated. 61

62 Relationship Between Dose & Clinical Response Quantal doseresponse curves From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), 2007 McGraw-Hill Companies 62

63 Relationship Between Dose & Clinical Response Variation in drug responsiveness: Clinical response in individual patients Idiosyncratic infrequently observed in most patients genetic differences in metabolism immunological differences Hypersensitivity true allergy (uncommon) Hyperreactive intensity of effect is increased vs. that in most individuals Hyporeactive intensity of effect is decreased vs. that in most individuals Tolerance responsiveness decreases as a consequence of continued drug administration Tachyphylaxis decreased responsiveness that occurs rapidly after administration of a drug 63

64 Relationship Between Dose & Clinical Response Four mechanisms contribute to variation in drug responsiveness 1.) Alteration in concentration of drug that reaches receptor 2.) Variation in concentration of endogenous receptor ligand 3.) Alteration in number or function of receptors Up-regulation (thyroid hormone increases receptors in heart; antagonists like -blockers also do this) stop antagonist increase in receptor number - response to endogenous ligand (need to wean) Down-regulation stop agonist may have too few receptors to get effective stimulation 64

65 2.) Variation in concentration of endogenous receptor ligand Saralasin weak partial agonist at angiotensin II Angiotensin II is a potent vasoconstrictor What would be its effect on blood pressure up or down? What would be the effect on blood pressure of giving saralasin to a patient with high levels of angiotensin? What would be the effect on blood pressure of giving saralasin to a patient with low levels of angiotensin? 65

66 Relationship Between Dose & Clinical Response Pharmacogenetics: identification of genetic factors that contribute to a particular drug response; may allow the design of the most appropriate individualized pharmacologic therapy for patients 4.) Change in responsiveness distal to receptor Largest and most important class of mechanisms that cause variation in responsiveness to drug therapy Age General health Severity & pathophysiology of disease Wrong diagnosis Compensatory mechanisms (baroreceptor reflex after administration of an anti-hypertensive agent) 66

67 Relationship Between Dose & Clinical Response Clinical selectivity: beneficial vs. toxic effects of drugs: Same receptor-effector mechanism, direct pharmacological extension Same receptor, different effectors / different tissues Different receptor-effector mechanisms From: Basic & Clinical Pharmacology, B.G. Katzung (Ed.), 2001 McGraw-Hill Companies 67

68 Relationship Between Dose & Clinical Response Strategies for lowering adverse effects: Use lowest dose possible Add an adjunctive drug that acts on a different receptor mechanism Anatomical selectivity refine administration to get more drug to site of action 68

69 No drug causes only a single, specific effect 69

70 Drugs are only selective, not specific 70