• Drug-Receptor Interactions

  • Drugs typically exert their effects by interacting with a macromolecule (receptor)

  • Drug-receptor interactions have been important in:

    • New drug development

    • Therapeutic decisions

  • Major roles of receptors:

    • Determines quantitative relationships between drug dose and pharmacological effect.

    • Determines drug action selectivity

    • Mediates antagonist (blocking) as well as agonist (activating) effects

  • Molecular characteristics of receptors:

    • Usually proteins, specifically regulatory proteins -- mediating effects of:

      • Neurotransmitters

      • Autacoids (histamine, serotonin, endogenous peptides, prostaglandins, leukotrienes)

      • Hormones

    • Some receptors are enzymes whiche may be inhibited or occasionally activated by drugs

      • For example,  dihydrofolate reductase  is the receptor and  methotrexate is the drug acting as a receptor inhibitor.

    • Other receptors are transport proteins:

      • For example,  Na/K ATPase (receptor for digitalis glycosides {digoxin (Lanoxin, Lanoxicaps), digitoxin(Crystodigin)}

    • Still other receptors are structural proteins:

      • Tubulin:  receptor for certain anticancer drugs and anti-inflammatory drugs

• Signal Transduction

  • Signal transduction is the process by which extracellular inputs (drug-receptor interactions) leads to intracellular messages that modulate cellular physiology.

  • Molecular mechanisms of signal transduction:

    •  Five basic mechanisms:

      1. Drug crosses the cellular membrane: activates an intracellular receptor

      2. Transmembrane receptor protein: intracellular enzyme activity affected by drug binding to a site on the enzyme that can alter its activity.

      3. Drug- transmembrane receptor protein complex binds and stimulates a second protein, such as a protein tyrosine kinase.

         • A tyrosine kinase enzyme promotes phosphorylation of proteins (at the aminoacid tyrosine site)

      4. Drug binding to a transmembrane ion channel changes the ion channel conductance property--affecting membrane potential

      5. Agonist drug binding to a transmembrane receptor causes stimulation of a GTP-binding signal transducer protein (G protein) -- leading to an increase in intracellular second messenger that results in many secondary intracellular responses.

• Intracellular receptors:

  • lipid-soluble drugs, after crossing the cell membrane barrier, interact with intracellular receptors. Example:nitric oxide (NO)-- stimulates guanylyl cyclase, increasing cGMP levels

  • The agents below bind to DNA response elements that control transcription:

    • Thyroid hormone

    • Corticosteroids

    • Mineralocorticoids

    • Sex steroids

    • Vitamin D

• Drug (ligand)-regulated transmembrane enzymes (may involve receptor tyrosine kinases)

  • Mediate signaling first step by:

    • Insulin

    • Epidermal growth factor (EGF)

    • Platelet-derived growth factor (PDGF)

    • Atrial natriuretc factor (ANF)

• Cytokine receptors

  • Activated by many diverse peptide ligands:

    • Growth hormone

    • Erythropoietin

    • Eome interferons

    • Other growth and differentiation regulators

• Ligand-gated Channels

  • Introduction: Many drugs mimic or block the action of normally occurring (endogenous) agents that effect ion conductance of membrane integrated ion channels.

  • Endogenous ligand include:

    • Acetylcholine

    • γ-amino butyric acid (GABA, inhibitory action)

    • Excitatory amino acids:

      • Glycine

      • Aspartate

      • Glutamate

  • Receptor example: nicotinic acetylcholine receptor:

    • Activation:

      • Acetylcholine binds

      • Rceptor channel opens

      • Na⁺ enters (down its concentration and electrical gradient)

      • Depolarization occurs (EPSP)

    • Other multisubunit ligand-gated examples:

      • Glutamate receptor

      • GABA_A receptor

        • Benzodiazepines (diazepam {Valium} enhance chloride conductance by allosteric modification of the GABA_A receptor

      • Glycine receptor

      • 5-HT₃ receptor

• G proteins and Second Messengers

  • Second messenger effects:

    •  Increases in cAMP

    •  Ca²⁺ concentration changes

    •  Phosphoinositides effects

  • Four steps:

    1. Drug binding

    2. G protein activation (cytoplasmic side)

    3. Activity of effector (ion channel or enzyme) changed

    4. Intracellular second messenger concentration changes

       • cAMP: effector enzyme is adenylyl cyclase, converting ATP to cAMP

       • Adenylyl cyclase activated by a G protein

       • G proteins may be activated by many neurotransmitters and hormones

• Receptor desensitization:

  •  The magnitude of receptors-mediated responses decrease with repeated drug administration.

  •  Desensitization is often reversible.

Bourne, H.R. Drug Receptors and Pharmacodynamics, in Basic and Clinical Pharmacology, (Katzung, B. G., ed) Appleton-Lange, 1998, pp 9-33.

Ross, E.M. Pharmacodynamics In, Goodman and Gillman's The Pharmacologial Basis of Therapeutics,(Hardman, J.G, Limbird, L.E, Molinoff, P.B., Ruddon, R.W, and Gilman, A.G.,eds) TheMcGraw-Hill Companies, Inc.,1996, pp. 29 -41.

• Concentration-Response Relationship

  • Drug effect (assuming the drug acts reversibly with the receptor) is thought proportional to the number of occupied receptors.

  • Drug (D) + Receptor (R) « DR leads to  Effect (equation 1)

  • Observed Drug Effect = (maximal drug effect · [D]) / K_d + [D] (equation 2)

    • where [D] is the free drug concentration;

    • K_d is the dissociation constant for the drug-receptor (DR) complex

    • Equation 2 describes drug potency -- the dependency of drug effect on drug concentration

  • Drug antagonists bind either to the receptor itself or to some component of the effector mechanism to prevent the agonist action.

    • Antagonists themselves have no effect.

    • If the antagonist-mediated inhibition can be overcome by increasing agonist concentration ultimately reaching the same maximal effect, the antagonist is termed competitive.

      • Competitive inhibition is based on reversible binding at receptor sites.

      • With competitive inhibition, the dose-effect curve will be shifted to the right.

      • With competitive inhibition, the maximal drug effect will not be affected.

    • By contrast, a non-competitive antagonist will prevent the agonist from producing a maximal effect (and any agonist concentration)

      • If the antagonist binds at the active site and is a reversible antagonist, the inhibition will be competitive.

      • If the antagonist binds that the active site and is an irreversible antagonist, the inhibition will be noncompetitive.

  Ross, E.M. Pharmacodynamics In, Goodman and Gillman's The Pharmacologial Basis of Therapeutics,(Hardman, J.G, Limbird, L.E, Molinoff, P.B., Ruddon, R.W, and Gilman, A.G.,eds) TheMcGraw-Hill Companies, Inc.,1996, pp. 29 -41.

• Concepts for signaling mechanisms and drug action

  • Intracellular receptors:

    • Lipid-soluble drugs, after crossing the cell membrane barrier, interact with intracellular receptors. Example: nitric oxide (NO)-- stimulates guanylyl cyclase, increasing cGMP levels

    • Numerous agents can bind to DNA response elements, thus controlling transcription.

  • Hormones that act through gene transcription may take thirty minutes to several hours lag time before effect begins and may take a long time to dissipate.

Bourne, H.R. Drug Receptors and Pharmacodynamics, in Basic and Clinical Pharmacology, (Katzung, B. G., ed) Appleton-Lange, 1998, pp 9-33

• Transmembrane Sites of Action

  • Drug (ligand)-regulated transmembrane enzymes (may involve receptor tyrosine kinases)

  • intracellular signaling may be mediated initially by binding substances such as those noted below.

    •  Insulin

    •  Epidermal growth factor (EGF)

    •  Platelet-derived growth factor (PDGF)

    •  Atrial natriuretc factor (ANF)

Bourne, H.R. Drug Receptors and Pharmacodynamics, in Basic and Clinical Pharmacology,(Katzung, B. G., ed) Appleton-Lange, 1998, pp 9-33.

• G Protein Coupling

  • G-protein coupled receptors are involved in signal transduction for:

    • Biogenic amines

    • Eicosanoids

    • Peptide hormones

• G-Protein systems influence other important regulatory molecules, such as:

  • Adenylyl cyclase (cAMP)

  • Phospholipases A₂, C and D.

  • Ca²⁺, K⁺, Na⁺ channels

  • Transport proteins

Ross, Elliott M.: Pharmacodynamics: mechanisms of Drug Action and the Relationship Between Drug Concentration and Effect: In, Goodman and Gillman's The Pharmacologial Basis of Therapeutics,(Hardman, J.G, Limbird, L.E, Molinoff, P.B., Ruddon, R.W, and Gilman, A.G.,eds) The McGraw-Hill Companies, Inc.,1996, pp.29 - 35.

Hoffman, B. B. Adrenoceptor-Activating & Other Sympathomimetic Drugs: in Basic and Clinical Pharmacology, (Katzung, B. G., ed) Appleton-Lange, 1998, p.118-122.

•  Second Messenger Systems: cAMP, Calcium and Phosphoinositides, cGMP

  • cAMP: intracellular second messenger

    • Hormone response mediator:

      • Carbohydrate breakdown (liver)

      • Triglyceride breakdown (fat cells)

      • Conservation of water (renal -- vasopressin)

      • Calcium homeostasis

      • Cardiac chronotropic (rate) and inotropic (contractility) state

      • Adrenal and sex steroids regulation (responding to corticotropin and follicle stimulating hormone)

      • Smooth muscle relaxation

      • Other endocrine/neural effects

    • Specificity:

      • Due to the presence of different protein substrates, associated with different cell types:

        • Liver:

        • Fat cells

        • Smooth muscle

    • Termination of effect:

      • Proteins which were phosphorylated by cAMP dependent processes are dephosphorylated by the action of specific and nonspecific enzymes (phosphatases).

      • cAMP is degraded to 5'-AMP (inactive) by cyclic nucleotide phosphodiesterases.

        • Some pharmacological effects of caffeine, theophylline, and other methylxanthines may be due to competitive inhibition of cAMP degradation

  • Calcium and Phosphoinositides

  • G protein or tyrosine kinase receptor linked

  • Central Steps:

    • Stimulation of phospholipase C

    • Subsequent cascade of steps results in:increased intracellular calcium enhances calcium binding to calmodulin

    • Calmodulin regulates enzyme activities, including calcium-dependent protein kinases.

  • cGMP:

    • cGMP-based signal transduction may be more limited than cAMP-based systems.

    • Intestinal mucosa and vascular smooth muscle:

    • Vascular smooth muscle

• Signaling mechanisms:  interrelationships:

  • Activation of calcium-phosphoinositide and cAMP signaling systems may produce complementary or opposing results:

    • Opposition: vasopressor induced smooth muscle contraction: IP₃-mediated increase in calcium; compounds that cause smooth muscle relaxation often do so by increasing cAMP concentration.

    • Complementary: cAMP and phosphoinositides second messenger systems act both to stimulate hepatic glucose release.

Pappano, A.J. Cholinoceptor-Activating & Cholinesterase-Inhibiting Drugs, In Basic and Clinical Pharmacology, 7th Edition, (Katzung, B.G.,ed) Appleton & Lange, 1998, p. 93-94

Bourne, H.R. Drug Receptors and Pharmacodynamics, in Basic and Clinical Pharmacology,(Katzung, B. G., ed) Appleton-Lange, 1998, pp 9-33

• Nitric Oxide

  • Blood vessel endothelium is required for ACh-mediated smooth muscle relaxation.

  • The endothelial cell layer modulates vessel responsiveness to autonomic and hormonal influences.

  • Endothelial cell elaborate endothelium-derived relaxing factor (EDRF,NO) and a contracting factor.

    • Pharmacological actions of:

      • Serotonin

      • Histamine

      • Bradykinin

      • Purines

      • Thrombin are mediated to some degree by stimulation of NO release.

  • EDRF is nitric oxide.

  • Endothelial-released nitric oxide:

    1. Diffuses into vascular smooth muscle

    2. Increases cGMP

    3. Facilitates vascular smooth muscle relaxation

  Lefkowitz, R.J, Hoffman, B.B and Taylor, P. Neurotransmission: The Autonomic and Somatic Motor Nervous Systems, In, Goodman and Gillman's The Pharmacologial Basis of Therapeutics,(Hardman, J.G, Limbird, L.E, Molinoff, P.B., Ruddon, R.W, and Gilman, A.G.,eds) TheMcGraw-Hill Companies, Inc.,1996, pp.112-137.

   

• Catecholamine Refractoriness

  • Following exposure to catecholamines, there is a progressive loss of the ability of the target site to respond to catecholamines. This phenomenon is termed tachyphylaxis, desensitization or refractoriness.

  • Regulation of catecholamine responsiveness occurs at several levels including receptors, G-proteins, adenylyl cyclase, and cyclic nucleotide phosphodiesterase.

  • Characteristics of refractoriness depends on the nature and extent of involvement of the above components.