PATHOPHYSIOLOGY AND MOLECULAR BIOLOGY- BASED PHARMACOLOGY MOLECULAR-BASED APPROACHES: RECEPTOR AGONISTS, ANTAGONISTS, ENZYME INHIBITORS

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1 PharmaTrain Cooperative European Medicines Development Course (CEMDC) Budapest, October, 2017 PATHOPHYSIOLOGY AND MOLECULAR BIOLOGY- BASED PHARMACOLOGY MOLECULAR-BASED APPROACHES: RECEPTOR AGONISTS, ANTAGONISTS, ENZYME INHIBITORS Dr. LASZLO G. HARSING, Jr. DEPARTMENT OF PHARMACOLOGY AND PHARMACOTHERAPY SEMMMELWEIS UNIVERSITY BUDAPEST 1089 Budapest, Nagyvárad tér 4., Hungary Tel.: , CONTENTS: 1. The receptor theory of drug action 2. Dose response curves 3. Definition of the intrinsic activity of drugs 4. Agonist drug action Full agonists Partial agonists Invers agonism 5. Antagonist drug action Competitive antagonists Non-competitive antagonists 6. The binding sites for drug action 7. Signal transduction mechanisms coupled to receptors 8. References Appendix. Mathematical characterization of competitive and non-competitive antagonism

2 OUR CUURENT VIEW ON THE MECHANISM OF DRUG ACTION, A MOLECULAR APPROACH 1. THE RECEPTOR THEORY OF DRUG ACTION Pharmacological effect is considered to be the consequence of a reversible reaction between a drug (D) and a reactive entity (receptor, R). For the reaction: k 1 k 3 (Kia) intrinsic activity [D] + [R] -----> [DR*]-----> e 1 e 2 e 3 e n ---->Effect [E] (1) <----- signal transduction k 2 where [D] is concentration of a drug or ligand, (L) [R]: is number of a receptor, [DR*]: is the drug-receptor complex, [R*]: activated receptor (conformation is altered), ionotropic receptor ion channel closed ion channel open metabotropic receptor signal transduction inactive active enzyme activity bound receptor tyrosine kinase inactive form active form [DR*] drug-receptor complex Chemical bounds between drug and the receptor covalent bound between SH -SH -SH-SHionic bound between COO - and -NH 3 + groups hydrogen bound between -COO - and HO -, HO - and HO - gropus Van der Waals forces between aromactic rings or alphatic groups Phe-Phe and CH 3 -CH 3 (-CH n -CH n -) groups These bounds results in affinity of the drug to the receptor

3 k 1 and k 2 are specific constants for the reaction of combination (association) and for reverse reaction (dissociation) and the ratio of two reaction constants (k 2 / 1 ) is itself a constant (Kd) that is called dissociation constant of the drug the ratio of two reaction constants (k 1 / 2 ) is itself a constant (Kd) that is called association constant of the drug k 3 or Kia: is a constant defined as intrinsic activity (see later) e 1. e 2. e 3.. e n is a chain of signal transduction processes example: [DR*] Gs protein activation adenylyl cyclase activation camp production increases protein kinase A (PKA) activation protein phosphorylation opening Ca 2+ channels, closing K + channels etc in heart tissue, positve chronotropic/inotropic effects (signal transduction process for β1 receptors in the heart) [E]: biological effects smooth muscle cell contraction or relaxation, increased or decreased secretion, action potential propagation and activation, inhibition or disinhibition between two neurons, etc DERIVE OF THE DOSE-RESPONSE CURVES If the reaction is reversible and the law of mass action is applicable then association equals dissociation in equilibrium (steady state): k 1 [D][R] = k 2 DR] (2) Solving Equation 2 for the ratio of the two reaction constants (k 2 nd k 1 ) k 2 D] [R] --- = = Kd (3) k [DR] where the ratio of two reaction constants (k 2 / 1 ) is itself a constant (Kd) that is called dissociation constant of the drug; Kd is equal the concentration of drug [D] that is required for 50% occupation of the receptors.

4 Since we calculate with free receptor ([R]free) as a fraction of total receptor ([R]total) in drug action, the following equation can be inscribed: [R]free = [R]total - [DR] (4) now Equation (4) can be substituted into Equation (3): [D] ([R]total - [DR]) = Kd (5) [DR] solving Equation 5 for [DR], a two-step process receiving Equation 7 [DR] [D] = (6) [R]total Kd + [D] [R]total [D] [DR] = (7) Kd + [D] where [DR] is concentration of drug-receptor complex, [R]total is total receptor number, [D] is concentration of drug, and Kd is the dissociation constant of the drug. Considering that drug-receptor complex ([DR]) relates to drug effect (E) then we can substitute [DR] for E; total number of receptors ([R]total) relates to maximal effect (Emax) then we can substitute [R]total for Emax and further, drug dissociation constant (Kd) relates to half maximal effect (ED 50 or EDmax/2) then we can further substitute Kd for ED 50 in Equation 7. Based upon these considerations, we can describe the relationship between the biological effect (E) and the concentration of the drug ([D]) by the use of Equation 8: Emax [D] E = (8) ED 50 + [D]

5 where E is drug effect, Emax is the maximal effect of drug, [D] is concentration of drug, and ED 50 (EDmax/2) is the half-maximal effect. The mathematical expression of the relationship between dose [D] and effect (E) may be represented in graphic form called dose-response curve. If effect (E) is plotted against drug concentration [D], a typical hyperbola curve is obtained. In case of semi-logarithmic transformation, a sigmoid dose-effect curve is revealed; this transformation is generally used to generate of a dose-response curves. 2. DOSE-RESPONSE CURVES In typical sigmoid dose-response curve, the magnitude of effect observed is plotted versus the logarithm of the drug concentration. In case of a sigmoid dose-response curve, potency of a drug is plotted on X axis and efficacy of a drug is plotted on Y axis. There is no effect at [D] = 0 and as [D] is increased, maximal effect (Emax) is approached asymptotically. Moreover, ED 50 is the drug concentration at which drug is half maximally effective (Emax/2). The importance of ED 50 values is to compare activity of a series of agonists. It is used for exploration of structure-activity relationship.

6 The sigmoid dose-response curve is linear between ED 16 and ED 84 and can be characterized by the slope of the curve. 3. DEFINITION OF THE INTRINSIC ACTIVITY OF DRUGS k 1 k 3 (Kia) intrinsic activity [D] + [R] -----> [DR*]-----> e 1, e 2, e 3 e n ---->Effect [E] (1) <----- signal transduction.. k 2 If drug effect (E) relates to drug-receptor complex ([DR]) then E = Kia [DR] (9) where E is drug effect, [DR] equals drug-receptor complex Kia is the intrinsic activity of a drug Thus, intrinsic activity is a proportionality constant that expresses the activated signal transduction process in percent In case intrinsic activity (Kia) of a drug is: Kia = 1 full agonist, = 0 antagonist >0-<1 partial agonist <0 inverse agonist 4. AGONIST DRUG ACTION 4.1. FULL AGONISTS: binds to receptor, it completely activates the signal transduction system attached to the receptor, endogenous ligands of receptors are usually full agonists. Dose-response curves for full agonists A and B, which differ in potency but not in efficacy: exhibit equal Emax values, dose response curves for A and B are parallel 4.2. PARTIAL AGONISTS: binds to receptor, it activates only a fraction of the signal transduction system attached to the receptor most agonist drugs act as partial agonists.

7 Dose response curves for full agonist A and for partial agonist C, which differs in efficacy but not in potency: possess differences in Emax values, dose response curves for A and B are not parallel 4.3. INVERSE AGONISTS (invers agonism): Consider a receptor population that exists in two conformations: active (Ra) and inactive (Ri), receptors fluctuate between active and inactive states, an agonist that preferable binds to the active conformation of the receptor (Ra), will cause activation (positive efficacy), this drug is called agonist an agonist that preferable binds to the inactive conformation of the receptor (Ri), will produce an effect opposite to that of an agonist (negative efficacy) this drug is called inverse agonist agonist and inverse agonist may be full or partial an antagonist has equal affinity for Ra and Ri and will not alter existing equilibrium, an antagonist will block ability of an agonist or inverse agonist to bind to Ra or Ri receptor conformation, will reverse the effect of both agonist and inverse agonist. 5. ANTAGONIST DRUG ACTION 5.1. COMPETITIVE ANTAGONISTS

8 intrinsic activity of the antagonist is 0, it has no biological effect. it does not activate receptor-coupled signal transduction binds to receptor and competes with the agonist for the same binding site, affinity to the binding site is higher than that of the agonist, agonist in higher dose suspends binding of antagonist to the receptor, binding of the antagonist to receptor is usually reversible. In the presence of a competitive antagonist, dose response curve for agonist is shifted to the right in a parallel manner with the same Emax value. Competitive antagonism: comparison of the agonist-induced dose-response curves obtained in presence and absence of a competitive antagonist ED 50 in the presence of the antagonist (B) Dose ratio (DR) = (10) ED 50 in the absence of the antagonist 5.2. NON-COMPETITIVE ANTAGONISTS intrinsic activity of the antagonist is 0, it has no biological effect. binds to receptor and binding sites are different for agonist and antagonist, affinity to the binding site is higher than that of the agonist, agonist in higher dose does not suspend binding of antagonist to the receptor, binding of the antagonist to receptor can be reversible or irreversible In the presence of a non-competitive antagonist, the dose response curve for agonist is shifted to the right with reduced Emax value, the shift is nonparallel. Non-competitive antagonism: comparison of the agonist-induced dose-response curves obtained in absence and presence of a non-competitive antagonist Emax in the absence of the antagonist Emax ratio (EmaxR) = (11) Emax in the presence of the antagonist (B)

9 Dose-response curves for competitive and non-competitive antagonism. Electrically stimulated isolated guinea-pig ileum, contraction was induced by addition of serotonin (5-HT). The competitive 5-HT3 receptor antagonist ondansetron induced a parallel shift of dose-response curve to the right with equal Emax values. The non-competitive 5-HT3 receptor antagonist GYKI induced a non-parallel shift of dose-response curves to the right with decreasing Emax values.. 6. THE BINDING SITES (R) FOR DRUG ACTION k 1 k 3 (Kia) intrinsic activity [D] + [R] -----> [DR*]-----> e 1, e 2, e 3 e n ---->Effect [E] (1) <----- signal transduction

10 .. k 2 The binding site for drug action is usually macromolecules (peptides) and they can be receptors, ion channels, transporter or enzymes. Binding sites for drugs (lgands) can be: 6.1. RECEPTORS Receptors (ionotropic receptors), which form an ion-(cation or anion) channel Glutamate receptors (NMDA, AMPA, kainite receptors) GABA A receptors Glycine receptor Cholinergic nicotinic receptors Serotonin 5-HT 3 receptors Receptors (metabotropic receptors), which are coupled to G proteins (G q /G o, G s, G i ) metabotrop glutamate receptors (mglur 1-7 ) GABA B receptors all 5-HT receptors except 5-HT 3 receptors Cholinergic muscarinic receptors (M 1 -M 5 receptors) Serotonin receptors (5-HT 1, 2, 4, 5, 6, 7 receptors) Receptors coupled to enzyme (tyrosine kinase) activity insulin, growth factors, cytokine receptors Intranuclear receptors steroid receptors, vitamin D, triiodothyronine (T 3 ) 6.2. ION CHANNELS Ionotropic receptors glutamate, glycine, GABA A, nicotinic, 5-HT 3, Ion (Na + and Cl - )-dependent neurotransmitter transporters (dopamine, serotonin, norepinephrine, glycine transporters) Voltage-dependent ion channels for sodium channels, calcium channels, L, N, R, P/Q, T potassium channels, transiens potassium channels, I KR, I KS, ACh/ATP-sensitive potassium channels 6.3. TRANSPORTERS Proton pumps Neuronal vesicular and plasma membrane transporters (VMAT, NET, SET, DAT) Metabolic transporters (for glucose, amino acids etc) PgP proteins, ABC transporters 6.4. ENZYMES in case of neurochemical transmission,

11 synthesis of neurotransmitters tyrosine hydroxylase, DOPA decarboxylase degradation (metabolism) of neurotransmitters monoamine oxydases, MAO catechol-o-methyl transferase, COMT 7. SIGNAL TRANSDUCTION MECHANISMS COUPLED TO RECEPTORS k 1 k 3 (Kia) intrinsic activity [D] + [R] -----> [DR*]-----> e 1, e 2, e 3 e n ---->Effect [E] (1) <----- signal transduction.. k SIGNAL TRANSDUCTION MECHANISMS FOR NEUROTRANSMITTER RECEPTORS Signal transduction mechanisms Ionotropic receptors anionic channels (Cl - permeable pores) membrane hyperpolarization inhibition of neurotransmitter release cationic ion channels (Ca 2+, Na +, K + permeable pores) membrane depolarization increase in neurotransmitter release Metabotropic receptors (coupled to G proteins) Alpha-1 adrenoceptors, Gq protein-coupled receptors activation of phospholipase C (PLC) production of phosphatidyl inositol (PIP 2 ) (1) inositol triphosphate (IP 3 ) formation increase in intracellular Ca 2+ concentrations Biological effects: smooth muscle contraction (a) (2) diacylglycerol (DAG) formation activation of protein kinase C (PKC) protein phosphorylation closing K + channels, membrane depolarisation Biological effects: smooth muscle contraction (b) Alpha-2 adrenoceptors, Gi protein-coupled receptors inhibition of adenylyl cyclase decrease in camp production inhibition of protein kinase A (PKA) decreased protein phosphorylation

12 Biological effects: inhibition of tyrosine hydroxylase inhibition of noradrenaline syntheis and release (a) Beta-1/2, adrenoceptors, Gs protein-coupled receptors smooth muscle cell activation adenylyl cyclase increase of camp production activation of protein kinase A (PKA) protein phosphorylation smooth muscle cell phosphorylated myosin light chain kinase, inactive form dephosphorylated myosin light chain, inactive form Biological effects: smooth muscle relaxation (a) heart tissue opening of voltage-dependent Ca 2+ channels, increase of intracellular Ca 2+ levels Biological effects: positive chronotropic and inotropic effects (b) 7.2. DIFFERENT SIGNAL TRANSDUCTION MECHANISMS COUPLED TO NEUROTRANSMITTER RECEPTORS Adrenoceptors α 1 adrenoceptors (Gq protein coupled) α 1 A α 1 B α 1 C α 1 D receptors IP 3 /DAG IP 3 /DAG IP 3 /DAG IP 3 /DAG increase, stimulation α 2 adrenoceptors (Gi protein coupled) α 2 A α 2 B α 2 C α 2 receptors camp camp camp camp decrease inhibition β 1 adrenoceptors (Gs protein coupled) β 1 β 1 β 3 receptors camp camp camp increase stimulation Dopamine receptors (Gs and Gi protein coupled) D 1 D 5 receptors camp camp increase activation D 2 D 3 D 4 receptors camp camp camp decrease inhibition GABA receptors (ion channel or Gi protein coupled) GABA A GABA B receptors Cl - sensitive camp decrease inhibition

13 Glutamate receptors (ion channel coupled) NMDA AMPA kainate receptors Na +, K +, and Ca 2+ permeable ion channels activation Glutamate receptors (Gq protein coupled) mglur 1 mglur 5 IP 3 /DAG IP 3 /DAG increase stimulation mglur 2, 3, 4, 6, 7 receptors (Gi protein coupled) camp decrease inhibition Histamine receptors H 1 H 2 H 3 receptors Gq protein coupled Gi protein coupled Gi protein coupled IP 3 /DAG increase camp increase camp decrease stimulation stimulation inhibition Cholinergic Muscarinic receptors M 1 M 3 M 5 receptors (Gq protein coupled) IP 3 /DAG increase M 2 M 4 receptors (Gs protein coupled) camp increase Nicotinic receptors Neuromuscular junction Neuronal nicotinic receptors bungarotoxin sensitive insensitive cation (Na + ) permeable ion channels stimulation stimulation activation Opioid receptors (Gi protein coupled) µ σ κ receptors camp camp camp decrease inhibition Serotonin receptors, 5-HT 1 (Gi protein coupled) 5-HT 1A 5-HT 1B 5-HT 1D 5-HT 1E 5-HT 1F receptors camp decrease inhibition Serotonin receptors, 5-HT 2 (Gq protein coupled) 5-HT 2A 5-HT 2B 5-HT 2C receptors IP 3 /DAG IP 3 /DAG IP 3 /DAG increase stimulation Serotonin receptors, 5-HT 3 (ion channel coupled) Serotonin receptors 5-HT 4 5-HT 5 5-HT 6 5-HT 7 receptors (Gs protein coupled)? (Gs protein coupled)

14 camp? camp camp increase stimulation Purinoceptors P 2Y, metabolic receptors (Gi protein coupled) P 2Y1 P 2Y2 P 2Y4 P 2Y5 IP 3 /DAG increase Purinoceptors, P 2X, (ion channel-coupled receptors) P 2X1 P 2X2 P 2X3 P 2X4 P 2X5 P 2X6 P 2X7 cation channel stimulation stimulation 8. REFERENCES The Biochemical Basis of Neuropharmacology J. R. Cooper, F. E. Bloom, R. H. Roth Oxford University Press Molecular Neuropharmacology E. J. Nesler, S.E. Hyman,r. C. Malenka McGraw Hill Basic and Clinical Pharmacology B. Katzung, A. J. T revor McGraw Hill Rang and Dale s Pharmacology H. P. Rang, M. M. Dale, J. M. Ritter, R. J. Flower, G. Henderson Elsevier Cellular and Molecular Neurobiology C. Hammond Academic Press APPENDIX MATHEMATICAL CHARACTERIZATION OF COMPETITIVE AND NON- COMPETITIVE ANTAGONISMS 1. DETERMINATION OF pa 2 VALUE: competitive antagonism ED 50 in the presence of the antagonist (B) Dose ratio (DR) = (10) ED 50 in the absence of the antagonist Let us calculate now Antagonist dose1 (logb1), Antagonist dose2 (log B2), DR1, log(dr1-1) DR2, log(dr2-1)

15 Antagonist dose3 (logb3), DR3, log(dr3-1) when log(dr-1) is plotted against logb antagonist concentrations, the pa 2 value can be obtained. pa 2 is the concentration of an antagonist that induces a rightward shift with a factor of 2. Calculation of pa 2 value. Isolated rat raphe nuclei slices, loaded with [ 3 H]5- HT and the electrically stimulated release of [ 3 H]5-HT was measured. The release was inhibited by the 5-HT 7 receptor agonist 5-CT and was reversed by the 5-HT 7 receptor competitive antagonist SB The pa 2 value of SB was 6.27 The importance of determination of pa 2 value is to compare activity of a series of competitive antagonists It is used for exploration of structure-activity relationship

16 Structure-activity relationship based upon pa 2 value determination; an example for in vitro testing. Compounds with alloberbane skeleton were tested in isolated rat vas deferens preparation. The pa 2 values for alpha-1 and alpha-2 adrenoceptors were determined against phenylephrine and xylazine. 2. DETERMINATION OF pd 2 VALUE: non-competitive antagonism Emax in the absence of the antagonist (B) Emax ratio (EmaxR) = (11) Emax in the presence of the antagonist Let us calculate now Antagonist dose1 (logb1), EmaxR1, log(emaxr1-1) Antagonist dose2 (log B2), EmaxR2, log(emaxr2-1) Antagonist dose3 (logb3), EmaxR3, log(emaxr3-1) For determination of pd 2 values of non-competitive antagonists, log(emaxr-1) is plotted against log(b) antagonist concentration.

17 The importance of determination of pd 2 value is to compare activity of a series of non-competitive antagonists. It is used for exploration of structure-activity relationship. TEST QESTIONS Second messenger(s) involved in intracellular signal transduction is(are): 1. camp x 2. ADH 3. ACTH 4. IP 3 x Which statement(s) is(are) correst: 1. The dose-response curve describes relationship between drug efficacy and potency x 2. Dose-response curves may have a sigmoid shape x 3. Two full agonists have dose-response curve with identical Emax values and different slopes 4. A competitive antagonist will shift the agonist dose-response curve to the left x Which statement(s) is(are) correct: 1. The potency of two agonists can be described with the ratio of their ED5 50 values x 2. Competitive antagonism can be characterized by pa 2 values x 3. Non-competitive antagonism can be characterized by pa 2 values 4. An agonist and its competitive antagonist will bind to the same binding site of the receptor x Which statement(s) is(are) correct: 1. D 2 receptor is an ionotropic dopamine receptor 2. The ion channel of NMDA receptor is permeable for Ca 2+ ions x 3. All types of 5-HT receptors belong to metabotropic receptors 4. Dopamine receptors have two major groups, D 1 - and D 2 -type receptors x Make figure about the dose-response curves of an agonist in the presence and absence of its competitive antagonist