BMS2043 - drug metabolism 2024.pdf

BMS2043- ANALYTICAL AND CLINICAL BIOCHEMISTRY DRUG METABOLISM DR PENNY LYMPANY [email protected] 28AY04 OBJECTIVES Overview of drug metabolism phase 1 and phase 2 Overview of elimination Kidneys Hepatobiliary system Lungs REFERENCE MATERIAL Introduction to Drug Metabolism - Gibson & Skett Bioch...

BMS2043- ANALYTICAL AND CLINICAL BIOCHEMISTRY DRUG METABOLISM DR PENNY LYMPANY [email protected] 28AY04 OBJECTIVES Overview of drug metabolism phase 1 and phase 2 Overview of elimination Kidneys Hepatobiliary system Lungs REFERENCE MATERIAL Introduction to Drug Metabolism - Gibson & Skett Biochemistry - Berg, Tymoczko & Stryer (or any other general Biochemistry, Physiology or Pharmacology textbook) THE FATE OF DRUGS IN THE BODY Intramuscular Oral Intravenous Site of action Tissue reservoirs Gut Liver Bound Free Drug free Kidney Plasma proteins Excretion Metabolites Plasma Excretion ADME Absorption Distribution Metabolism – anabolism, catabolism Excretion Drug Elimination This Photo by Unknown Author is licensed under CC BY WHY IS THE STUDY OF DRUG METABOLISM & ELIMINATION IMPORTANT? The route of metabolism of a drug can determine its ultimate pharmacological or toxicological activity May impact on dose and frequency FIRST PASS EFFECT Metabolism of drug occurring before entering the systemic circulation Metabolised by: Enzymes in GIT (also not all will be absorbed) Liver < 100% of drug given enters systemic circulation Consequences? Examples – aspirin, GTN, morphine This Photo by Unknown Author is licensed under CC BY-NC DRUG METABOLISM Body naturally acts to remove chemicals (endogenous & exogenous) Removal routes (polar/hydrophilic) Urine Faeces BUT ……. most drugs are non-polar/lipophilic SO ……. we must transform them – liver! DRUG METABOLISM – WHAT AND WHERE? Metabolism –process of converting chemicals to more polar metabolites Primarily in the liver but can occur in all organs TWO PHASES OF DRUG METABOLISM (Functionalisation/ activation) Conjugation Source: Fig 10.1 Rang & Dale’s Pharmacology; 9thEd. Overview of Drug Metabolism Public domain; https://commons.wikimedia.org/wiki/File:Xenobiotic_metabolism.png PHASE 1 REACTIONS Phase 1 reactions are catabolic Oxidation –most important, catalysed by cytochrome P450 enzymes Reduction Hydrolysis Sometimes, the products (metabolites) more toxic or carcinogenic than parent drug Introduction of a reactive group, e.g. hydroxyl group Process known as functionalisation Group acts as a point of attack for conjugating system PHASE 2 REACTIONS Phase 2 reactions are synthetic/’anabolic’ Involves conjugation –attachment of a substituent group Glucuronic acid Sulphate Amino acids Glutathione Acetylation To inactivate products exceptions -active sulphate metabolite of minoxidil (K+channel activator) Phase 2 reactions mainly take place in the liver If drug molecule or Phase 1 product has a suitable ‘handle’, it is susceptible to conjugation Handle -hydroxyl, thiol or amino group DRUG METABOLISM: PHASE 1 ENZYMES DRUG METABOLISM: PHASE 1 Liver important in phase 1 Many drug metab enzymes embedded in smooth ER – microsomal enzymes To interact with these enzymes, drug crosses plasma membrane Polar molecules move less readily unless transport system – polar drugs partly excreted unchanged in urine CYP450 MONOOXYGENASE SYSTEM CYP450 – haem proteins Superfamily Differ in sensitivity to inhibition/induction and specificity of reaction they catalyse Distinct but overlapping substrate specificities CYPs implicated in large number of phase I metabolism CYP450 – SUBSTRATES MIXED-FUNCTION OXIDASE REACTION (CYTOCHROME P450; CYP) NADPH + O2 + RH NADP+ + H2O + ROH Reaction requires substrate, enzyme, molecular oxygen Catalysed by two enzymes: 1. Cytochrome P450 2. NADPH-cytochrome P450 reductase HUMAN LIVER CYTOCHROME P450 Source: Fig. 2.3 Introduction to Drug Metabolism Gordon & Skett EXPRESSION AND REGULATION OF P450 Species differences - Dietary amines – genotoxic products ?consequences Environment – enzyme inducers and inhibitors e.g. grapefruit juice inhibits enzymes, brussels sprouts induce P45. Clinically important Polymorphisms POLYMORPHISMS: CYP3A4 & NIFEDIPINE OXIDATION © Introduction to Drug Metabolism Gordon & Skett CYP2D6 : GENOTYPE/PHENOTYPE VARIABILITY poor metabolizer – little or no CYP2D6 function intermediate metabolizers – metabolize drugs at a rate somewhere between the poor and extensive metabolizers extensive metabolizer – normal CYP2D6 function ultrarapid metabolizer – multiple copies of the CYP2D6 gene are expressed, so greater-than-normal CYP2D6 function occurs OTHER ENZYMES ARE AVAILABLE! Not all drug oxidation involves P450 – Suxamethonium hydrolysed by plasma cholinesterase Ethanol metabolised by alcohol dehydrogenase + CYP2E1 Xanthine oxidase inactivates 6-mercaptopurine MAO – inactivates biologically active amines – e.g. NA, tyramine, 5-HT Hydrolysis – Ester and amide (less readily) bonds susceptible to hydrolytic cleavage Warfarin – reduction of ketone to OH by CYP2A6 DRUG METABOLISM: PHASE 2 ENZYMES PHASE II ENZYMES: CONJUGATION REACTIONS © Introduction to Drug Metabolism Gordon & Skett PHASE II REACTIONS Insertion of a chemical group Glucuronyl Sulphate Methyl Acetyl GLUCURONIDE CONJUGATION R - OH paracetamol, aspirin, morphine R - COOH clofibrate, nicotinic acid R - NH2 dapsone, sulphafurazole, meprobamate R - SH 2-mercapto-benzothiazole Endogenous substrates: bilirubin, steroid hormones, thyroxine and catechols GLUCURONIDE CONJUGATION Glucose 1-phosphate Glycolysis; glycogenesis Enzyme: UDP glucuronosyl transferases Biochemistry, Sixth Edition; Copyright © 2007 by W. H. Freeman and Company GLUCURONIDE CONJUGATION Source: Rang & Dale’s Pharmacology Fig 10.3 GLUTATHIONE (GSH) CONJUGATION: GLUTATHIONE S-TRANSFERASES R-CH2-X + GSH R-CH2-SG + XH Glutathione S-Transferases (GST) Glutathione (GSH; glu-cys-gly) No activation required – electrophilic substrates Slides courtesy of Prof GG Gibson, University of Surrey (modified) Glutathione (GSH) conjugation: Glutathione S-transferases Glutathione (GSH; glu-cys-gly) No activation required – electrophilic substrates Biochemistry, Sixth Edition; Copyright © 2007 by W. H. Freeman and Company STEREOSELECTIVITY Clinically important drugs e.g. warfarin, cyclophosphamide – mixtures of stereoisomers Different pharmacological effects Differences in metabolism & ?pathways Toxicity – linked to one stereoisomer – impact on new drugs INHIBITION OF P450/INDUCTION OF MICROSOMAL ENZYMES P450 inhibition Competitive inhibition – e.g. quinidine (not substrate) Non-competitive inhibition (reversible) – e.g. ketoconazole – complex with Fe3+ form of haem iron of CYP3A4 Mechanism-based – requires oxidation by P450 enzyme e.g. oral contraceptive gestodene – oxidation product (epoxide intermediate of gestodene) binds covalently to enzyme which destroys itself (suicide inhibition) Microsomal enzyme induction Rifampicin, ethanol, carbamazepine - incr. activity of enzymes with repeated admin Carcinogenic chemicals have this effect e.g. benzypyrene Can increase drug tox/carcinogenicity or can be exploited therapeutically ACTIVE DRUG METABOLITES Pro-drugs ?deliberate design – drug delivery e.g. aspirin inhibs platelet function and has anti-inflammatory activity but hydrolysed to salicylic acid (anti-inflammatory not anti-platelet) Metabolites may have similar effects to parent compound e.g. benzodiazepines Toxicity e.g. methanol and ethylene glycol DRUG INTERACTIONS Slow onset of induction, slow recovery, selective induction – clinical issues e.g. graft rejection, seizures, thrombosis, unplanned pregnancy, bleeding >200 drugs – enzyme induction Inducing agent can be substrate for enzymes – tolerance Source: Fig 10.5 Rang & Dale’s Pharmacology ENZYME INHIBITION Enzyme inhibition – slow metabolism, increases action of drugs normally inactivated by enzyme e.g. erythromycin affects cyclosporin metabolism Therapeutic effects – consequence of inhibition e.g. Disulfiram (aldehyde dehydrogenase inhib) – produces adverse reaction to ethanol, also inhibits metabolism of other drugs, including warfarin, which it potentiates Inhibition of the conversion of a prodrug to active metabolite can result in loss of activity. E.g. Proton pump inhibitors (such as omeprazole) and the antiplatelet drug clopidogrel widely co-prescribed BILIARY EXCRETION AND ENTEROHEPATIC CIRCULATION Liver cells transport substances from plasma to bile using transport systems e.g. organic cation transporters (OCTs), organic anion transporters (OATs) and P-glycoproteins (P-gps) Enterohepatic circulation – hydrophilic drug conjugates (esp glucuronides) concentrated in bile and delivered to the intestine glucuronide can be hydrolysed, regenerating active drug; free drug reabsorbed cycle repeated – reservoir of recirculating drug e.g. morphine RENAL EXCRETION Renal clearance - volume of plasma containing the amount of substance that is removed from the body by the kidneys in unit time Drugs differ greatly in the rate at which they are excreted by the kidney Three processes account for renal drug excretion: 1. glomerular filtration 2. active tubular secretion 3. passive reabsorption (diffusion from the concentrated tubular fluid back across tubular epithelium) Mechanisms by which one drug can affect the rate of renal excretion of another are by: altering protein binding (hence filtration) inhibiting tubular secretion altering urine flow and/or urine pH INDIVIDUAL VARIATION Ethnicity – ethanol, propranolol Age - hepatic microsomal enzyme activity declines Pregnancy - Cardiac output increases - increased renal blood flow and GFR, increased renal elimination of drugs Disease Drug interaction Pharmacokinetic interaction e.g. MAO, warfarin GENETIC VARIATION Plasma cholinesterase deficiency – suxamethonium Drug acetylation deficiency Drug targeting ? ANY QUESTIONS? We made it! This Photo by Unknown Author is licensed under CC BY-SA Please use the discussion board for general questions and queries: https://surreylearn.surrey.ac.uk/d2l/le/252546/discussions/List