Pharmacology 306 Lecture 10: Biotransformation Phase I - PDF

Pharmacology 306 (PMCOL 306): Drug Disposition & Metabolism Lecture 10: Biotransformation Phase I Fatima Mraiche, PhD Associate Professor of Pharmacology, 2023 Vargo Teaching Chair Department of Pharmacology, FoMD...

Pharmacology 306 (PMCOL 306): Drug Disposition & Metabolism Lecture 10: Biotransformation Phase I Fatima Mraiche, PhD Associate Professor of Pharmacology, 2023 Vargo Teaching Chair Department of Pharmacology, FoMD October 2, 2024 Module 4, Lecture 10: Biotransformation Phase I and II Recommended reading: Goodman & Gilman’s: The Pharmacological Basis of Therapeutics, 14 Edition, Chapter 5: Drug Metabolism. (Access Pharmacy, UAlberta Library Databases) Shargel and Yu. Applied Biopharmaceutics and Parmacokinetics 8e, Chapter 6: Physiology of Drug Elimination Module 4 - Lecture 10: Biotransformation Phase I Learning Objectives By the end of the lecture, the student will: Describe the importance of drug metabolism (biotransformation) and its impact on the drug pharmacokinetics Describe the drug metabolism pathways, phases I and II (October 4, 2024) Identify the major enzymes involved in drug metabolism Describe the relevance of CYP enzymes and their genetic variability (post midterm) to pharmacokinetics and dosing Distinguish the effects of modifiers (enzyme inducers/competitive inhibitors) of drug metabolism on the pharmacokinetics of cleared drugs Terminology Drug Metabolism Biotransformation Parent compound * Active drug prodrug us. Metabolite Phase I Phase 2 Cytochrome P450 (CYP or P450) UDP glucuronosyltransferases (UGT) Enzyme induction Competitive inhibitor JModi tierot dingmetabousm s Where are we at? Drug Elimination Irreversible removal of drug from the body by all routes of elimination. Two major components: üExcretion: removal of the intact drug Kidney → urine üNon-volatile drugs are excreted mainly by renal excretion, a process in which the drug passes through the kidney to the bladder and ultimately into the urine. ü Other pathways: excretion of drug into bile, sweat, saliva, milk (via lactation), or other body fluids chcangen " üBiotransformation: drug is chemically converted in the body to a into active or inactive metabolites metabolite, enzymatic process Lecture Overview also convert from active to inactive produce its effect in lecture 11 change expression the of enzyme https://www.osmosis.org/library/oh/foundational-sciences/anatomy Biotransformation Process is also called drug metabolism, represents the M in ADME I Where? primarily in the liver active orin active Chemical transformation of a parent drug to another chemical, metabolite through enzyme ü Metabolites may undergo ( further metabolism, renal active inactive excretion, and/or biliary on excretion ü Metabolites are equally, more or less active than parent compound https://pharmacologycanada.org/Metabolism Why Is Drug Biotransformation Necessary Renal excretion and metabolism major processes of drug elimination ü> 90% of drugs Routes of elimination of the top 200 drugs Renal excretion: terminate the biologic activity of small molecular volumes or possess polar characteristics Metabolism: lipophilic xenobiotics are transformed to more polar and hence more readily excreted products Examples of Biotransformation: Into active drug: ü Tamoxifen is metabolized to 4- hydroxytamoxifen (30 -100-fold more active) than parent drug used for breast cancer Into inactive drug: create ü Felodipine is extensively metabolized reactive Toxic into metabolites with no therapeutic intermediate effects lead to the accumulation of inactive metabolites Into toxic metabolites: ü Acetaminophen is metabolized into a highly reactive intermediate metabolite that is hepatotoxic and nephrotoxic. NAPQI: should be neutralized by glutathione in normal condition (detoxifying) Normal: this metabolite is cleared via bile Overdose: the detoxification from glutathione pathways become overwhelmed, leading to accumulation of NAPQI damage by the toxic metabolite CMAJ September 18, 2012 184 (13) 1492-1496; DOI: https://doi.org/10.1503/cmaj.111338 Question V Why is it important to Identify and characterize a drug's metabolites? insight of Volume distribution How are lipophilic compounds converted to fast-soluble more hydrophilic products? water-soluble *easier to eliminate from the body via urine or bile Broad substrate specificity: single enzyme may metabolize very rare a large variety of chemically diverse compounds Enzyme multiplicity: many different enzymes may be involved in the biotransformation of a single drug diversity of enzymes allow for flexibility in metabolism and ensures the drugs can be processed effectively Product multiplicity: occurs when a substrate (drug) is converted enzymatically into several different metabolites into active, inactive or even toxic Polyfunctionality: drug may undergo several different reaction types  Pathways for Drug Metabolism With few exceptions, all xenobiotics are subjected to one or easier to excrete multiple enzymatic pathways that constitute: Phase 1 Phase 2 – October 4, 2024 As a general paradigm, metabolism serves to convert hydrophobic chemicals into more hydrophilic derivatives that can easily be eliminated from the body through urine or bile. https://www.pharmaguideline.com/2022/03/drug-metabolism-and-drug-metabolism-principles.html Phase 1 Reactions: small Phase 2 Reactions: the parent drug or the product of phase I metabolism is conjugated chemical modifications of the combined with a polar function, such as a glucuronide, drug molecule sulfate, or glutathione molecule Oxidations: the addition of a Conjugation Reactions hydroxyl group or the removal of Glucuronidation a methyl group Sulfonation CYTOCHROME P450 Glutathione \ Glycine I ~ oxidation chemical hydroxyl ⇒ Make more Acetylation occurring group polar Reductions UDP- Other Enzyme Families glucuronosyltransferases Flavin monooxygenases (FMO) (UGTs) are the most Monoamine oxidases (MAO) important of the phase II Esterases Aldehyde oxidase (AO) enzymes Aldehyde dehydrogenase (ALDH1A1) Aldo-keto reductases (AKR) Metabolism of phenytoin Phase 1: CYP facilitates 4-hydroxylation of phenytoin to yield HPPH. forming 4-Hydroxyphenytoin Phase 2: the hydroxy group serves as a because of the substrate for UGT, which conjugates a -OH group molecule of glucuronic acid using UDP- glucuronic acid is conjugated to GA as a cofactor. the -OH group ( substrate) ccofactor) Phase 1 and phase 2 reactions convert a very hydrophobic molecule to a larger hydrophilic derivative that is eliminated via the bile. HPPH, 5-(-4-hydroxyphenyl)-5- phenylhydantoin. Cytochrome P450 Enzyme System Superfamily of heme-containing proteins for enzyme’s catalytic activity- allows for oxidative reaction Primary phase I oxidative enzymes ü Metabolism of xenobiotics and endogenous (steroids, fatty acids, fat- soluble vitamins, prostaglandins, heme containing leukotrienes, and thromboxanes) biochemical processes ü Elimination of around 55% of drugs used clinically Function is carried out primarily by nine isoform specific enzymes or isoforms Classification system: based on the similarity of amino acid sequencing Characteristics of the Major Hepatic CYP Enzymes Major drug-metabolizing CYP families: CYP1, CYP2, and CYP3 80% of the oxidative phase I Most abundance reactions are thought to be - mediated by: CYP3A4, CYP2D6, CYP2C9, and CYP2C19 Relative importance of an individual enzyme in metabolizing drugs does not parallel its relative abundance. ü CYP2D6 20% of the drugs metabolized by the CYP system ü CYP2E1 is relatively abundant but plays little role in the metabolism of drugs CYP2D6 Though less than 5% of total, if has a major role in drug metabolism and highly polymorphic Metabolism of as much as 25% of commonly prescribed drugs üantidepressants, antiarrhythmics, beta-adrenergic antagonists, and opioids These genetic variants ==> affect an individual’s drug metabolism Most highly polymorphic CYP enzyme, with > 70 allelic variants individuals respond differently to drugs metabolized by CYP2D6 10% of the Caucasian population, 1% of the Asian population, and between 0% and 19% of the African population have a poor metabolizing phenotype of CYP2D6 (McGraw and Waller, 2012) üIncreased plasma concentration of the parent drug due to decreased metabolic clearance. Reduced ability to metabolize drug Higher risk of accumulation of the parent drug Prodrugs (drugs that require activation by metabolism that rely on CYP2D6 for conversion into their active forms may be ineffective; cannot be metabolized into its active form CYP3A4 Most abundant CYP in the liver, metabolizes over 50% of the clinically used drugs Liver expression of CYP3A4 is variable between individuals ü>30 allelic variants of CYP3A4 Substrates for CYP3A4 range from relatively small drugs like dapsone (leprosy) to relatively large compounds such as cyclosporine (immunosuppressive) reduce the metabolism of drugs Grapefruit (all sources) is a potent inhibitor of intestinal CYP3A4  lead to stronger pain relief, but active metabolites also higher risk of morphine- related toxicity inactive metabolites Crews et al. Clinical Pharmacology and Therapeutics. 2014; 95:376 - 382 Questions CYP2A6 is responsible for the inactivation of nicotine. CYP2A6 is a highly polymorphic enzyme whose activity varies between the sexes. 1. What information could be obtained from the nicotine metabolite ratio? ‘How efficiently an individual metabolizes nicotine’ High rat: o ⇒ active metabolite O Merabolites ( 1001 nicotine) ⇒ enzy me is not. working 2. Does reduced CYP2A6 result in higher or lower likelihood of smoking cessation? 금연 Reduced CYP2A6 activity leads to higher nicotine levels ==> higher likelihood of smoking cessation because nicotine stays in the body longer, reducing the need for. frequent intake CYP1A2 Metabolism of 5% of marketed drugs: clozapine, olanzapine, and theophylline Has genetic polymorphisms 15% of the Japanese, 5% of the Chinese, and 5% of the Australian non- smoker populations are classified as CYP1A2 poor metabolizers CYP1A activity is induced by smoking status ==> Smokers generally have higher levels of CYP1A compared to nonsmoker üSmokers have higher theophylline clearance than nonsmokers Therefore, smokers require higher doses of certain drugs to achieve the same therapeutic effect üEnhanced enzyme levels are thought to cause faster substrate clearance, which has been associated with treatment failures for clozapine in smokers need to adjust dosages for clozapine !! Factors Affecting CYP Enzymes Inhibitors reduce an enzyme's ability to metabolize other drugs ü Increased concentrations of these drugs and toxicity. inhibitor of CYP3A4 ü Consumption of grapefruit juice can inhibit CYP3A4, blocking the metabolism of numerous drugs. Inducers: increase the amount of an enzyme ü More rapid metabolism and decreased, possibly subtherapeutic plasma concentrations of its substrates. Decrease efficacy of the drug, as it may be metabolized too quickly to maintain therapeutic levels ü Smoking (CYP1A) e.g. theophylline  Example of Enzyme Induction Increase drug metabolism, increase in metabolites (decrease drug concentration) Enzyme induction Example: Chronic EtOH (enzyme alcohol- enzyme inducer- risk of toxic metabolite inducer) and metabolized into highly Acetominophen reactive intermediate metabolite Toxic Chronic alcohol use metabolite? increases the risk of acetaminophen-induced liver toxicity e.g. Toxic metabolite (NAPQI0) for Acetominophen https://www.youtube.com/watch?v=Dtbkc8F_ff0 Competitive Inhibition - Decrease drug metabolism, less metabolites, increase drug concentration since the drug metabolized more slowly metabolized by CYP3A4 - Statins (Atorvastatin, Rosuvastatin) and grapefruit juice CYP3A4 inhibitor (contain furanocoumarins) - CYP3A4 inhibited by furanocourmarins Competitive inhibition e.g. grapefruit juice https://www.youtube.com/watch?v=Lt1-mjMFniE Flavin-Containing Monooxygenases Expressed at high levels in the liver Six families of FMOs, with FMO3 the most abundant in liver FMO3 metabolize nicotine, H2 receptor antagonists (cimetidine), antipsychotics (clozapine), and antiemetics (itopride) compared to cytochrome P450 family (CYP enzymes) Minor contributors to drug metabolism, and they almost always produce benign nonelectrophilic metabolites the byproducts of their metabolic actions are harmless and nonreactive FMOs are not readily inhibited and are not induced by any of the xenobiotic receptors their activity does not significantly increase in response to external substances like drugs or environmental toxin Question In contrast to CYPs, FMOs would not be expected to be involved in drug-drug interactions, why? Because they are not easily induced or inhibited and play a smaller role in drug metabolism compared to CYP enzymes Reflections, Upcoming Lectures: How is drug metabolism impacted by the following: Drug-drug and drug-food interaction QUESTION: How have these factors made Individual variability development of drugs more time consuming and costly due? Age-related variability The effects of gender, race and/or ethnicity The impact of diseases The impact of genetic background Overview https://www.osmosis.org/library/oh/foundational-sciences/anatomy Homework Do Phase III reactions exist or is it just Phase I and Phase? Module 4 - Lecture 10: Biotransformation Phase I Learning Objectives By the end of the lecture, the student will: Describe the importance of drug metabolism (biotransformation) and its impact on the drug pharmacokinetics Describe the drug metabolism pathways, phases I and II (October 4, 2024) Identify the major enzymes involved in drug metabolism Describe the relevance of CYP enzymes and their genetic variability (post midterm) to pharmacokinetics and dosing Distinguish the effects of modifiers (enzyme inducers/competitive inhibitors) of drug metabolism on the pharmacokinetics of cleared drugs References: Goodman & Gilman’s: The Pharmacological Basis of Therapeutics, 14 Edition, Chapter 5: Drug Metabolism. (Access Pharmacy, UAlberta Library Databases) Rosenbaum SE. Basic Pharmacokinetics and Pharmacodynamics. 2nd Edition. Chapter 5.Drug Elimination and Clearance. https://www.osmosis.org/learn/Pharmacokinetics:_Drug_metabolism https://pharmacologycanada.org/Metabolism Animation CYP: http://jeremyhammond.com/acp/tutorial.htm Kearns GL and Ritschel WA. Handbook of Basic Pharmacokinetics, Including Clinical Applications, 7th. Chapter 13 Drug Biotransformation. https://doi.org/10.21019/9781582121260.ch13 Shargel and Yu. Applied Biopharmaceutics and Parmacokinetics 8e, Chapter 6: Physiology of Drug Elimination