Pharmacokinetics Biotransformation/MetabolismLearning Objectives & Knowledge Points • Be able to list consequences of drug metabolism • Be able to list sources of variation in metabolism • Be able to discuss Phase I and Phase II reactions • Be able to explain the effects of drug-drug interactions and food on CYP450 activity and the resultant effect on plasma drug levels • Be able to explain the effects of pharmacogenetic variation on biotransformation activity and the resultant effect on plasma drug levels • Be able to discuss the effects of ethnic prevalence of genetic variations in enzyme activity on plasma drug levels • Be able to explain the role of alterations in drug inactivation in adverse drug reactions (ADRs)Drug Biotransformation • Drug biotransformation involves enzymes used for xenobiotics (i.e., foreign compounds such as drugs) & metabolism of endogenous compounds (e.g., steroid hormones, cholesterol, active Vitamin D congeners) • Typically, drugs converted to more polar (i.e., more hydrophilic/ water soluble) metabolite(s) • Variability in metabolism is the cause of a significant portion of the variability in biological activity of the drug – Pharmacogenetic variation as well as drug-drug & food drug interactions can affect drug biotransformation • Biological Consequences of Drug Metabolism – Reduction in plasma drug levels – Change biological activity of the drug – Possible generation of toxic metabolitesDrug Metabolism Phase I & Phase II reactions • Metabolism mainly in liver & intestines, but also lung, nasal mucosa, plasma, brain & kidney • Phase I – Introduction of functional groups (e.g., of –OH, -COOH, -NH2 or –SH) by reduction/oxidation (i.e., redox reaction) or by hydrolysis • Cytochrome P450 • Flavin-containing monooxygenases (NADPH-cytochrome P450 reductase) • Epoxide hydrolases – Enzymes in endoplasmic reticulum and mitochondria – Little effect on water solubility, but dramatically alters biological activity – Involved in Bioactivation of prodrug usually via hydrolysis of ester or amide linkageDrug Metabolism Phase I & Phase II reactions • Phase II – Covalent linkage of hydrophilic function group onto molecule to create highly polar metabolite. – Conjugation reactions usually require the substrate to have oxygen (hydroxyl or epoxide groups), nitrogen, or sulfur atoms that serve as acceptor sites for a hydrophilic moiety (e.g., glutathione, glucuronic acid, sulfate, or an acetyl group) • glutathione-S-transferase • UDP-glucuronosyltransferase • Sulfotransferase • N-acetyltransferase • methyltransferaseisoniazid N-acetylisoniazid Phase I & II are functions Not a sequenceRelative contributions of different Phase I enzymes to metabolism of drugs in clinical usePrevalent CYP Isoforms in the Human Liver other 3A 2D6 2C 2E1 2A6 1A2 2D6 2C8-10 Cytochrome P450 Located in mitochondria & smooth endoplasmic reticulum Encoded by 63 human genes in 18 families Nomenclature: CYP 1 A 2 Isoform family isoform subfamily individual gene product in subfamilyCytochrome P450 Drug-drug interactions & polymorphisms in different CYP isoforms  individual variability in pharmacokinetic properties especially plasma drug levels that may manifest as adverse drug reactions (ADRs) • Drug can be a substrate for multiple cytochrome P450s – Amitriptyline substrate for CYP2C19, 2C9, & 2D6 • Drugs be substrate, inhibitors, and/or inducers of the P450s • Food can also affect cytochrome P450 activity • Polymorphisms of different isoforms can result in dramatic differences in activity (i.e., P450s subject to pharmacogenetic variation)Function CYP1A2 CYP2C19 CYP2C9 CYP2D6 CYP2E1 CYP3A4 Substrate Caffeine Clozapine Cyclobenzaprine Fluvoxamine Imipramine Mexiletine Olanzapine Pimozide Propranolol Tacrine Theophylline Warfarin Amitriptyline Citalopram Clomipramine Cyclophosphamide Diazepam Imipramine Lansoprazole Nelfinavir Omeprazole Phenytoin Amitriptyline Celecoxib Diclofenac Flurbiprofen Ibuprofen Losartan Naproxen Phenytoin Piroxicam SMX Tolbutamide Warfarin Amitriptyline Clomipramine Codeine Desipramine Dextromethorphan Imipramine Metoprolol Nortriptyline Oxycodone Paroxetine Propafenone Risperidone Thioridazine Timolol Tramadol Venlafaxine Acetaminophen Chlorzoxazone Dapsone Enflurane Ethanol Halothane Isoflurane Isoniazid Aliskiren Alprazolam Astemizole Buspirone CCB Carbamazepine Cisapride Cyclosporine Doxorubicin Erythromycin Etoposide Fentanyl HIV PI Iphosphamide Lovastatin Midazolam Pimozide Quinidine Quinine Simvastatin Tacrolimus Terfenadine Triazolam Inhibitor Cimetidine Ciprofloxacin Citalopram Diltiazem Enoxacin Erythromycin Fluvoxamine Mexiletine Ofloxacin Tacrine Ticlopidine Cimetidine Felbamate Fluoxetine Fluvoxamine Ketoconazole Lansoprazole Omeprazole Paroxetine Ticlopidine Amiodarone Fluconazole Fluoxetine Fluvastatin Isoniazid Metronidazole Paroxetine Phenylbutazone SMX/TMP Sulfaphenazole Ticlopidine Amiodarone Chlorpheniramine Fluoxetine Haloperidol Indinavir Paroxetine Propafenone Quinidine Ritonavir Sertraline Thioridazine Ticlopidine Disulfiram Water cress Amiodarone Cimetidine Conivaptar Cyclosporine Danazol Diltiazem Fluconazole Grapefruit juice HIV PI Itraconazole Ketoconazole Macrolides Miconazole Nefazodone Omeprazole Quinidine Ritonavir Verapamil Inducer Carbamazepine Tobacco Carbamazepine Norethindrone Phenobarbital Rifampin Secobarbital Ethanol Isoniazid Carbamazepine Rifabutin Rifampin RitonavirDrug-drug interactions on P450s Inhibition  Drugs can inhibit different P450 isoforms • Inhibition due to: – Binding of parent drug (e.g., cimetidine, ketoconazole) or metabolite (e.g., erythromycin) to heme iron – Irreversible, covalent modification of apoprotein by reactive drug metabolite (e.g., chloramphenicol) • Drug can be substrate & inhibit same or different P450 isoform – Fluvoxamine is substrate for CYP1A2 & also inhibits this CYP – Citalopram is substrate for CYP2C19 but inhibits CYP1A2 • Effects of P450 inhibition? – Prolong drug action – Increase plasma drug levels – Increase bioavailability with PO admin – [Drug] may reach toxic levels (e.g., with ketoconazole inhibition of CYP2C19) – Lopinavir co-admin with sub-threshold dose Ritonavir to inhibit CYP3A4 & ↑ plasma levels lopinavirDrug-drug interactions on P450s Induction  Drugs can induce different P450 isoforms – Carbamazepine is