PHAR435 Drug Metabolism Lecture Notes Fall 2024 PDF

PHAR435: PHARMACOKINETICS DRUG METABOLISM Dejan Nikolic, PhD. 1  Drug Metabolism Overview § Part of Elimination: Metabolism and Excretion. § Definition: Chemical alteration of the drug by the human body. § Goal: to convert lipophilic drug molecule...

PHAR435: PHARMACOKINETICS DRUG METABOLISM Dejan Nikolic, PhD. 1  Drug Metabolism Overview § Part of Elimination: Metabolism and Excretion. § Definition: Chemical alteration of the drug by the human body. § Goal: to convert lipophilic drug molecules into polar species suitable for excretion. § Sites: Nearly every tissue has some metabolic capacity. However, the major site of metabolism is the liver. Other organs with significant capacity are GI tract (including microbiome), kidneys, lungs and skin. § Drugs are metabolized by specialized enzymatic systems 2  Package Insert for AMRIX® (Cyclobenzaprine) Metabolism and Excretion Cyclobenzaprine is extensively metabolized and is excreted primarily as glucuronides via the kidney. Cytochromes P-450 3A4, 1A2, and, to a lesser extent, 2D6, mediate N- demethylation, one of the oxidative pathways for cyclobenzaprine. Cyclobenzaprine has an elimination half- life of 32 hours (range 8-37 hours; n=18); plasma clearance is 0.7 L/min following single-dose administration of AMRIX. 3  THE JOURNEY OF A DRUG liver diet Foreign substance? Control! drugs Conjugation Systemic circulation Renal elimination + OH/COOH Absorption intestine 2 NH C O HO Portal vein O O Biliary O H C elimination HO O HO HO Drugs/compounds will be metabolized in the liver for better elimination: 4 First-pass effect. Enterohepatic recirculation of some drugs, for example chloramphenicol.  PHARMACOLOGICAL CONSEQUENCES OF METABOLISM § Activity is lost (pharmacological deactivation). Metabolic transformation results in termination of pharmacological activity. Majority of drugs are inactivated by metabolism. § Activity remains (active Phenytoin metabolites). Metabolic transformation does not cause loss of activity. Activity of the metabolite may be stronger or weaker than that of the parent drug 5 Buspirone 6-hydroxybusprione 5-HT1A ~ 24nM 5-HT1A~ 72nM  Metabolic activation (therapeutic). Parent drug is not active, but is activated by metabolic transformation. These molecules are called prodrugs. Enalapril Enalaprilate O O N N OH Tamoxifen 4-Hydroxytamoxifen 6  Metabolic activation (toxicological): Metabolic transformation converts drugs into toxic species. This is a highly undesirable phenomenon. O HN O O High doses O HN HN P4502E1 N 40% GSH O-glucuronide SG OH O OH 20-40% O acetaminophen Glutathione conjugate HN Detoxification Oxidation/arylation of proteins/DNA O O S O O Hepatic necrosis 7 7  METABOLIC REACTIONS Phase I Phase II § Oxidation § Conjugation Ø Hydroxylation Ø Glucuronic acid Ø Deamination Ø Sulfuric acid Ø Dealkylation Ø Glutathione Ø Sulfoxidation Ø Amino acid § Reduction Ø Acetylation § Hydrolysis Ø Methylation 8  METABOLIC FATE OF DRUG MOLECULES: A SUMMARY Liver Systemic Circulation Eliminated unchanged by kidneys Phase I Phase II Phase II Portal Vein Kidneys 9  Metabolism can be complex: Propranolol - at least 11 phase I and 12 phase II metabolites 10  Enzymes involved in drug metabolism? (partial list) Cytochrome P450 MAJOR PHASE I UDP-glucuronosyl transferase MAJOR PHASE II Epoxide hydrolase glutathione S-transferase N-acetyltransferase flavin-containing monooxygenase prostaglandin synthase quinone reductase alcohol dehydrogenase aldehyde and ketone dehydrogenase N-methyl and O-methyl transferase sulfotransferase monoamine oxidase 11  Cytochrome P450s: The engine of drug metabolism Membrane bound proteins located in the smooth ER. Most abundant in the liver. Hemoproteins OOC OOC N N Fe N N RH + O2 + NADPH ---> ROH + NADP+ + H2O Monooxygenase reaction 12 Examples of Phase I reactions: oxidations Aromatic hydroxylation Aliphatic hydroxylation N-dealkylation N-dealkylation N-oxidation 13  THE CYTOCHROME P450 SUPERFAMILY § We call it superfamily because there are many different isoforms of the enzyme. § Different isoforms are necessary to handle large number of diverse chemical structures. § P450s are classified in families and subfamilies. § Family: >40% sequence homology. Labeled with numbers § Subfamily: >55% sequence homology. Labeled with letters § Individual isoforms receive an extra number after subfamily letter. § Thus, CYP2E1 means that this isoform belongs to family 2, subfamily E and is the first identified enzyme from the family. § 18 families; 41 subfamilies; 57 different isoforms § Majority involved in metabolism of endogenous compounds, i.e. steroids, bile acids. 15 are involved in metabolism of drugs. § Only families 1,2 and 3 appear to be responsible for metabolism of drugs 14  CYP P450 FAMILY TREE Superfamily CYP Family CYP1 CYP2 CYP3 CYP1A CYP1B CYP2C CYP2D Subfamily CYP2C19 CYP2D6 Member 15 CYP2C19*2 CYP2D6*9 Allele 15 16  Glucuronosyl Transferases: the Terminators Full name is Uridine Diphosphate Glucuronosyl Transferases (UGTs) Located in ER; also a family of enzymes Catalyze transfer of activated glucuronic acid moiety (UDP-glucuronic acid) onto a nucleophilic site on the drug molecule (-OH, -SH, -NH, -COOH) Glucuronides are hydrophilic and are excreted into the bile or urine. Those excreted in bile may undergo enterohepatic recirculation Glucuronides are for the most part pharmacologically inactive with few exceptions. 17  Glucuronidation Reaction - - CO2 CO2 UDP-GLUCURONOSYL O XR RXH TRANSFERASE O OH OH O UDP HO HO UDP OH OH URIDINE DIPHOSPHATE β-GLUCURONIDE CONJUGATE α-GLUCURONIC ACID Examples of Glucuronidation Reactions 19 What are clinically important aspects of drug metabolism? Induction Inhibition Genetic polymorphism 20  What are metabolic drug-drug interactions? Drug-drug interaction is a phenomenon when one drug affects the activity of another drug when both are administered together. Many interactions at the metabolism level are known Perpetrators: drugs that cause the interaction. Usually inhibitors or inducers of drug metabolism Victims: drugs that exhibit side effects or rendered inactive by the perpetrator drugs. Those are typically drugs that have only 1 or 2 significant elimination pathways 21 Perpetrators and victims in the context of DDI 22  St. John's Wort induces P450 3A4 Herbal supplement used by millions to improve mood One of components (hyperforin) induces CYP3A4 Drugs affected: Cyclosporin Antidepressants Birth control pills Indinavir (HIV) Irinotecan (Cancer) Warfarin (anticoagulant) Hypericum perforatum P450 3A4 induction reduces bioavailability of these drugs Wanwimolruk, S., and Prachayasittikul, V. (2014) Cytochrome P450 Enzyme Mediated Herbal Drug Interactions (Part 1). Excli Journal 13, 347-391. 23  Genetic polymorphism Not everyone is created equal re: P450 Loss-of-function variants lead to reduced clearance and increased plasma concentration Gain-of-function variants lead to increased clearance and lower plasm concentration Most important polymorphic isoforms:CYP2D6, CYP2C9 and CYP2C19 24  Classic Example: CYP2D6 phenotypes Distribution of sparteine metabolism phenotypes in a Caucasian population. Phenotypes: UM, ultrarapid metabolizer; EM, extensive- ; IM, intermediate-; PM, poor metabolizer. 25  Package Insert for Imipramine “The biochemical activity of the drug metabolizing isozyme cytochrome P450 2D6 (debrisoquin hydroxylase) is reduced in a subset of the Caucasian population (about 7% to 10% of Caucasians are so-called "poor metabolizers"); reliable estimates of the prevalence of reduced P450 2D6 isozyme activity among Asian, African, and other populations are not yet available. Poor metabolizers have higher than expected plasma concentrations of tricyclic antidepressants (TCAs) when given usual doses. Depending on the fraction of drug metabolized by P450 2D6, the increase in plasma concentration may be small, or quite large (8-fold increase in plasma AUC of the TCA).” 26  Summary of metabolic reactions GST P450 ST PHASE II UGT PHASE I Esterases Amidases Alcohol/Aldehyde Dehydrogenase Epoxide Hydrolase P450 Reductase Quinone Reductase The percentages shown refer to 6767 metabolites = 100%; Redox reactions blue; hydrolyses yellow; conjugations red. Testa, B., Pedretti, A., and Vistoli, G. (2012) Reactions and enzymes in the metabolism of drugs and other xenobiotics. Drug Discov. Today 17, 549-560. 27