Drug Metabolism - Drug Design, Safety, and Development Principles PDF

Drug Metabolism Basic principles and importance to the pharmaceutical industry Importance of Drug Metabolism in a Pharmaceutical Industry It is all about EFFICACY AND SAFETY in a Pharmaceutical Industry! The goal of drug companies: Develop a safe and efficacious drug...

Drug Metabolism Basic principles and importance to the pharmaceutical industry Importance of Drug Metabolism in a Pharmaceutical Industry It is all about EFFICACY AND SAFETY in a Pharmaceutical Industry! The goal of drug companies: Develop a safe and efficacious drug Metabolism Indirectly Impacts Efficacy and Safety  Impact of Drug Metabolism on Efficacy, Safety and Pharmacokinetics Drug Metabolism Reactive Clearance Metabolite Pharmacologically Active Metabolite  In Exposure of the Drug Toxicity Activity  Why Address Metabolism?  Drug Metabolism issues: Can put a drug candidate on hold Leads to withdrawal of a drug  Failure of a drug candidate in development; Is costly Could lead to attrition of very promising candidates  Drug metabolism an asset to drug discovery Helps to address metabolism liabilities early on Metabolism a Primary Route of Clearance for Most Drugs  Role of Metabolism in Structure Based Drug Design Designing a new candidate Permeability entails optimization of Potency Selectivity Permeability Potency Selectivity Clearance Clearance Optimized Drug Candidate with Great Properties Early metabolism data can play an important role in modulating clearance  Drug Metabolism and Lead Optimization  Addressing undesirable ADME characteristics Fixing metabolic liability Identifying soft spots in a compound  Identifying factors that may impact drug safety Identifying toxicophores that undergo ‘metabolic activation’ Identifying metabolites with undesirable pharmacological & toxicological profile  Importance of Drug Metabolism in Development  Heightened awareness of the role of metabolites Metabolites can be mediators of pharmacological responses Both - off target and on target Identification of active metabolites  May lead to a discovery of new class of agents  Provide IP protection  Understand clearance pathways of a development candidate Helps in the management of potential toxicity due to a drug ❑ Plan for possible drug-drug interactions ❑ Design appropriate clinical studies  Pharmacogenomics Assessment Evaluate polymorphic enzymes early ❑ Avoid increased exposure of drugs in poor & extensive metabolizers Addressing undesirable ADME characteristics Fixing rapid metabolism of a compound  What are the Common Metabolic Reactions?  Phase I (Functionalization Reactions)  Oxidation - Most important!  Reduction  Hydrolysis  Phase II (Conjugation Reactions)  Glucuronidation - Most commonly encountered  Sulfation  Acetylation  Methylation  Amino Acid Conjugation  Glutathione Conjugation - Important if bioactivation occurs  Phase I Reactions (Functionalization Reactions)  Phase I (Functionalization Reactions) Oxidation - Most important! Reduction Hydrolysis Incorporation of ‘O’ R-H R-OH into the molecule Oxidation R-XR R-XH (X = N or O) Dealkylation R-R R=R Unsaturation Reduction R=O ROH Hydrolysis RCO2R RCO2H  Cytochrome P450 – A Major Oxidizing Enzyme Cytochrome P450 – represents a family of isozymes Broad substrate diversity – account for 75% of the known pharmaceuticals Predominantly found in liver  In endoplasmic reticulum Loves lipophilic compounds Generally catalyzes compounds with high electron density Glucuronidation Elimination Atorvastatin Oxidative Metabolites Cytochrome P450 – A Major Oxidizing Enzyme Peter Guengerich CRT 2007 Human →57 genes 5 isoforms account for 95% of the P450 metabolism are 1A2, 2C9, 2C19, 2D6, 3A4. Next 3 isoforms are: 2C8, 2B6, 2J2  War-Head of P450 and Oxidation Cycle Fifth ligand X Reduction HO N O N Fe N N HO O Y Sixth Ligand Heme Moiety Incorporation THE WAR HEAD of oxygen OF P450 Incorporation of One more electron O O + O O Fe+3 Fe+4 Fe+5 Fe+4 S S S S Cytochrome P450 Compound I:Capture, Characterization, and C-H Bond Activation Kinetics Oxene Oxo-radical Oxo Jonathan Rittle and Michael T. Green Science 330 page 933 2010  Examples of Oxygen Insertion Reactions Epoxidation Aromatic Hydroxylation Carbamazepine Atorvastatin Flutamide Celebrex Alcohol Phencyclidine Hydroxylation of Aliphatic and Alicylic Compounds Acid Aldehyde  Examples of N- and S- Oxidations Nicotine Tamoxifen Sulfur Oxidation Thioridazine Tienilic acid Reactive Metabolite 15  Examples of Desaturation Reactions Abacavir Aldehyde (A Reactive Intermediate) Responsible for Idiosyncratic Acetaminophen Toxicity of the compound NAPQI Can form macromolecular adduct Can result in hepatotoxicity Amodiaquine Apriprazole Active Metabolite Desaturation to a reactive metabolite  Examples of Dealkylation Reactions Imipramine Desipramine N-Dealkylation Venlafaxine Desvenlafaxine Encainide Paroxetine O-Dealkylation Inactivation of Enzyme  Examples of Special Cases Gefitinib Oxidative Dehalogenation Mociprazine Furosemide Zetidoline De-phenylation Reaction Ring Opening  Metabolic insights to improve ADME properties Decreasing Clearance Tolbutamide Half life in plasma=5 hr Replacement of CH3 with NH2 Results in Toxic a Compound Cl replacement decreases clearance Chlorpropamide Half life in plasma 35 hr Metabolic insights to improve ADME properties Increasing Clearance Celecoxib Half life in rat Replace fluorine plasma 220 hr With methyl Half life in rats 3.5 hr Ahlstrom et al J Med Chem 2007  Reducing P450 Mediated Metabolic Liability The Magic of Fluorine Benzylic Hydroxylation Demethylation SCH48461 Ezetimibe (zetia) Aromatic Hydroxylation Fluorine blocks Amodiaquine bioactivation Reactive! Reference: Kevin Park et al. METABOLISM OF FLUORINE-CONTAINING DRUGS Annu. Rev. Pharmacol. Toxicol. 2001. 41:443–70  Reducing P450 Mediated Metabolic Liability Identifying Soft Spots A Pfizer Lead with High Metabolic Clearance Prevented Major site Lead Compound X = N, O of metabolism CYP metabolism High Metabolic Cl  Aldehyde Oxidase A Non-P450 Enzyme that is Gaining Importance  Molybdenum containing enzyme Belongs to molybdenum hydroxylase family of enzymes A redox enzyme Catalyzes both oxidations and reductions  Present in cytosol  Present in all tissues and organs  Broad Substrate activity Substrates are:  Aldehydes  Heteroaromatic rings Oxidation  Electrophilic metabolites of P450  N-oxides, sulfoxides, some heterocyclic rings – Reduction  War-Head of Aldehyde Oxidase and its Mechanism Heme Moiety Molybdenum The War-Head RH + H2O + O2 ROH + H2O2 Fifth ligand Hydroxylase X of P450 Largely inferred from Xanthine Oxidase HO N (Another molybdenum hydroxylase) O N Fe N N Aldehyde Cytochrome HO Oxidase P450 O Y Sixth Ligand Reduction Incorporation of oxygen Where all the Incorporation of action takes place one more electron O O + O O Fe+3 Fe+4 Fe+5 Fe+4 The electrophilic S S S S Oxene intermediate A Molybdenum Pyranopterin Oxene Oxo-radical Oxo Co-factor (MoCo)  Introducing Nitrogens in a Molecule Increases its AO Susceptibility P450 AO Adding “N” reduces electron density in the ring e – Deficient carbons are susceptible to AO attack Napthalene Quinoline Quinoxaline P450 Only AO and P450 AO Oxygen source H2O  Drugs with Heteroaromatic Rings Metabolized by Aldehyde Oxidase Methotrexate Brimonidine Quinine Quinidine Deepak Dalvie  Why is it Important to Understand AO Catalyzed Metabolism?  Prediction of human PK parameters is a challenge Major species differences  Dog – very weak activity, Rat – moderate to high, Mouse – high  Inter-individual variation also observed  Activity is generally high in humans and is variable Leads to undesirable PK properties ( Rapid clearance, low Exposure) AO Substrates withdrawn from Development RO1 SGX523 P38 Inhibitor PF-945863 Carbezaran Ketolide Antibiotic FK3453 BIBX1382  Flavin Dependent Monooxygenases  Primary Function Oxygenation (insertion of an oxygen atom) of heteroatom- containing chemicals.  Oxidizes amines and sulfides to their respective N-oxides and sulfoxides.  Also responsible for oxidizing phosphorus.  FMO Has FAD (flavin adenosine dinucleotide) as a prosthetic group Is a NADPH and O2- dependent enzyme Present in the endoplasmic reticulum hence a microsomal enzyme. Thermally labile than CYPs War-Head of FMO :X  Other Functionalization Reactions?  Hydrolysis Hydrolysis of esters, carbamates , hydroxamates and amides Main Enzymes – esterases and amidases  Esterases are mainly responsible for metabolism of xenobiotics  Amidases – may metabolize peptides Hydrolysis of epoxides  Epoxide Hydrolases Hydrolysis of conjugates  Glucuronide and sulfate conjugates  Enzymes involved – -glucuronidases and sulfatases Cl NH2 NH2 H2O + HO N Clofibrate Esters N O O O O OH H3C COOCH2CH3 Procaine CH3  Metabolism of Clopidogrel and Prasugrel S S O OH O Prasugrel S S Cl hCE1 N Cl N O OH hCE2 F F N O N O SH Clopidogrel Plavix CO2H [O] O OH Cl O O N S O S OH SH S O CO2H Cl Active Moiety N Cl F N N F O N OH O O O O N N O O HO O N N O hCE1 N N Irinotecan O + O N OH O OH O NH CO2  Detoxication of Epoxides – Hydration by Epoxide Hydrolase Enzyme  Epoxides formed from oxidation of alkenes or arenes are generally hydrolyzed to diols by epoxide hydrolase or captured by other nucleophiles such as glutathione. O Epoxide Hydrolase OH (microsomal/cytosol) H2O OH HO H2O O OH Formation of the diol indicates that OH the epoxide is stable and is liberated O H from the active site. H2O OH H Toxicity can result if arene oxide is liberated from the active site  Examples of Epoxide Hydrolysis to Form Diols Benzopyrene A Classic Example Gluthetimide Phenytoin Triflubazam A Benzodiazepine Zolimidine  Alcohol and Aldehyde Dehydrogenases  Dehydrogenases are NAD-dependent and NADP-dependent enzymes Alcohol Dehydrogenases (ADH) – NAD dependent Aldehyde Dehydrogenases (ALDH) – NADP dependent  Alcohol Dehydrogenases Catalyze reversible dehydrogenation (oxidation) of alcohols to aldehydes H NAD+ H HO R OH O O H NADH R R Aldehyde Acid  Aldehyde Dehydrogenases Catalyze the conversion of aldehyde to carboxylic acid Rapid oxidation of aldehydes prevents the build up of electrophilic intermediates  Metabolism of alcohols is so rapid that carboxylic acids are most commonly detected  Substrates of Alcohol Dehydrogenases N O H Felbamate NH 2 HO N N O N N N N Abacavir O O Hydroxyzine NH 2 NH 2 OH Hydrolysis O ADH ADH OH H N N N O O O NH 2 ADH O O ALDH ALDH O O N N O N O OH NH 2 HO Spontaneous O Cetrizine O O  ADH and ALDH work in synergy Reactive Intermediate  Substrates of Aldehyde Dehydrogenase Synergy with CYP450 HO NH O NH O CYP P P O N Cl O N Cl Alkylates DNA Cl Cl Cyclophosphamide Mustard O H2N O H2N Acrolein H2N O O O P P P O HO O N Cl O N Cl + HO N Cl Deactivation Activation under by ALDH anaerobic conditions Cl Cl in tumor cells Cl  Cyclophosphamide is a prodrug that is activated by P450 but metabolized by ALDH to an acid  Other substrates include aldehyde products from oxidation of alcohol  Conjugation Reactions UGT  Main Enzymes B-  Glucuronyltransferases H R O O nucleophile  Sulfotransferases CO2H HN O  Glutathione transferases OH d+ O O O N OH O P O P O O OH O O- H H B H H H OH H UGT Mechanism of Glucuronidation Nucleophilic NH2 functional group N N reacts with a high O O N N energy co-substrate -O S O P O O O OH H H O H H H H O OH B- B O P O- O- Mechanism of Sulfation 1.36 36 Common Glucuronidation & Sulfation Reactions OH OGlu Can also undergo Sulfation Catalyzed by Sulfotransferases R OGlu R OH Hydroxyphenyl Alcohols Uridine Glucuronyl Transferases Carboxylic Acids Hydroxamates R O O O OH OGlu HO R N R N H H O Acids do not undergo sulfation R Sulfation results in bioactivation OGlu  Examples of Glucuronidation and Sulfation OH HO H CH3 N O Cl2 O NH O NH O2N HO HN O Chloramphenicol Acetaminophen OH Hydroxyphenytoin CH3 N Dapsone O2 Morphine S Has the potential to undergo OH O OH glucuronidation at 2 sites H2N N H OH Results in a active metabolite O F CO2H CP-544439 O H N N N O2 S F F N H2N H H HO N O O H Trovan F  Acyl Glucuronides  Many carboxylic acid drugs tend to be metabolically more stable, due to the intrinsic higher polarity.  But many carboxylic acids can be glucuronidated to form acyl glucuronides.  Acyl glucuronides are potentially toxic and immunogenic due to chemical reactivity after acyl migration. O HOOC HOOC HOOC HOOC O O O R O HO HO HO O HO O R HO OH O OH HO OH O OH O OH OH O O R R HOOC HOOC OH OH H2 N Protein HO HO HO N Protein HO O O O H H O O R R Bioactivation of Amines via Oxidation & Sulfation Sulfate conjugates play an important role in the metabolic activation of N-hydroxylamines and N-hydroxamates to reactive intermediates OH OSO3H N CH3 + N N CH3 CH3 N N N N N N DNA ADDUCTS Hydroxylamines H OSO3H + N N N O O O H3C H3C H3C oxidation and sulfation  Glutathione Conjugation NH 2 NH 2 H H HS N CO2H ES N CO2H E + O O O NH O NH Electrophile CO2H CO2H Glutathione GSH Adduct  GSH - An electrophile scavenger  GSH is also a radical scavenger  GSH adduct formation - an indicator of reactive metabolites  Commonly used as a reactive metabolite assay in early discovery  Can be catalyzed by glutathione S-transferase or can be a chemical reaction  Concentration of GSH in the liver is ~10 mM  Categories of Substrates Undergoing GSH Conjugation At Saturated Carbon atoms GSH RCH 2 X RCH 2 SG X = leaving gp Cl, Br, I, sulfate etc.. At Carbon atoms of Aromatic or Heteroaromatic Rings X SG GSH ,  - Unsaturated Systems (Michael Acceptors) X = NO2, Cl, Br etc... GSH GS Z Z At Carbons of Strained Rings Z = COR, CO2R, CN, NO2 OH O GSH Other Activated Double Bonds SG Isocyanates, Thioisocyanates O O GSH OH R N C O R N C S O GS Example of Deactivation by Glutathione Adduct Formation A Classic Example - Acetaminophen Detoxification H N O UGTs, STs R CH3 Major O Pathway H N O CH3 H HO Minor N O Pathway N O CH3 CH3 HO Bioactivation O S G CYP2E1 (EtOH Inducible) Glutathione Adduct Quinone Imine 1.43 Conversion to Active Metabolites One important reason why metabolism studies are important  Conversion to Active Metabolites Drug Detoxification Metabolism Bioactivation Pharmacologically Active Metabolites  All metabolism reactions can lead to active metabolites  Oxidation Reactions Resulting in Active Metabolites that are Marketed as Drugs H H N N N N O O HO O O Phenacetin Acetaminophen HO N S N S O OH S S OH Thioridazine Mesoridazine N N OH OH N Terfenadine Fexofenadine N N N N H N Imipramine Desipramine O NH N O Cl Cl H N N Loratadine Desloratadine Amitriptyline Nortriptyline N N O HO HO HO Venlafaxine Desvenlafaxine  Drugs Undergoing Oxidation and Resulting in Generation of Active Metabolites In Vivo HO O OH HN O O HN HN N + O N N HO HO HO F F F Atorvastatin O Cl Cl CH3 CO2H N N N N N N H2N O H2N O N N Carbamazepine N N N N Losartan N N O O H H HN HN Carboxylosartan OH  All these act on the same target CF3 CF3 NO2 NO2  Can impact the efficacy of the Flutamide 2-Hydroxyflutamide parent drug  Bioactivation via Conjugation Formation of Active Conjugative Metabolites F F O O N N F F HO HO GluO HO Ezetimibe Phase II conjugates can Active metabolite also be active HO HO GluO O N O O N + N HO GluO HO Morphine Active Metabolite  Impact of Polymorphism on Efficacy - Tamoxifen  Tamoxifen – a standard of endocrine therapy for treatment of breast cancer. CYP2D6  Therapeutic effect is mainly due to metabolites 4-Hydroxytamoxifen 4-hydroxytamoxifen & Tamoxifen Endoxifen CYP3A4/3A5  Alteration of a genetically CYP3A4/3A5 polymorphic enzyme will alter concentrations of the metabolites Will impact outcome of CYP2D6 receiving this therapy  Knowledge of genetic variation Endoxifen will help to individualize therapy Goetz et. al. (2008) Clin. Pharmacol. Ther. 83, 160-165  Genetic Polymorphism Its Impact on Drug Development  Genetic Polymorphisms arise from: Differences found in genes that encode enzymes  Major Polymorphic CYP Enzymes include 2D6, 2C9, 2C19, 3A5, 1A, 2A6, 2E1 Metabolize majority of drugs  Phase II enzymes are also polymorphic UGT1A1, GSTs, NAT – have shown polymorphism  Polymorphisms are one of the primary causes of variation in PK Therapeutic failure, adverse effects, and toxicity in selected subpopulations undergoing treatment are observed  Polymorphic Metabolism of Clopodigrel and Inadequate Platelet Inhibition CYP2C19 CYP2B6 2-Oxoclopidogrel Clopidogrel CYP2C19 CYP2B6 Inhibition of Platelet Aggregation Factor  Clopidogrel – current standard of care for coronary artery disease  CYP2C19 variant allele lowers metabolism significantly  Patients with genetic alteration are at risk of cardiovascular disease  FDA has recently changed the prescription information for this drug to highlight the importance of CYP2C19 Ellis et. al. Pharmacogenomics (2009) 10:1799-1817  Effect of CYP2D6 Polymorphism on Efficacy of Drugs CYP2D6  Poor metabolizers result in reduced formation of active metabolite and therefore Codeine Morphine ineffective treatment CYP2D6 Encanide O-Desmethylencanide  Poor metabolizers display (Active metabolite) greater toxicity CYP2D6 Phenformin Hydroxy-Metabolite  Effect of Polymorphic Enzymes and Toxcity Camptosar Anti-tumor Agent Effect of Polymorphic Enzyme on toxicity Lack of UGT1A1 in some patients leads to increase in concentration of SN-38 - results Hydrolysis in diarrhea and neutropenia SN38 Active Metabolite UGT1A1  Effect on Toxicity by Polymorphic N-Acetyltransferase-2 Isoniazid Hydralazine Dapsone N-Acetyltransferase-2 N-Acetylation results in reduced oxidative metabolism and reduced toxicity Variable N-acetylation Results in variably toxic N-acetylated metabolite Increasing adverse effects and complicates dosing Amonafide N-Acetylamonafide (Increased toxicity)  Drug Safety - One of the leading causes for candidate attrition  Circumstantial evidence links metabolic activation to toxicity Drug Stable Excretion Metabolite Metabolism METABOLITE (Reactive/Chemically Unstable Metabolites) FORMATION Reaction with macromolecules Altered Cellular Adverse Drug Function Reaction 1.55  Drugs That Have Been Discontinued Due to Hepatotoxicity Withdrawn from the market Temporarily withdrawn or withdrawn from certain countries Troglitazone Zileuton Bromfenac Pemoline Tienilic acid Trovafloxacin Temafloxacin Tolcapone Nomifensin Nimesulide Perhexilin Felbamate Ibufenac Benoxaprofen And the list keeps growing ……… 1.56  Drugs Associated With IADRs Drugs Withdrawn Temp. Withdrawn Marketed Drugs Aclcofenac (antiinflammatory) or Withdrawn in Abacavir (antiretroviral) Isoniazid (antibacterial) Cutaneous ADRs Hepatitis (can be fatal) Hepatitis, rash Alpidem (anxiolytic) other Countries Acetaminophen (analgesic) Phenytoin (anticonvulsant) Hepatitis (fatal) Agranulocytosis, cutaneous ADRs Hepatitis (fatal) Captopril (antihypertensive) Procainamide (antiarrhythmic) Amodiaquine (antimalarial) Aminopyrine (analgesic) Cutaneous ADRs, agranulocytosis Hepatitis, agranulocytosis Hepatitis, agranulocytosis Agranulocytosis Carbamazepine (anticonvulsant) Sulfamethoxazole (antibacterial) Amineptine (antidepressant) Nefazodone (antidepressant) Hepatitis, agranulocytosis Agranulocytosis, aplastic anaemia Hepatitis, cutaneous ADRs Hepatitis (> 200 deaths) Clozapine (antipsychotic) Terbinafine (antifungal) Benoxaprofen (antiinflammatory) Trovan (antibacterial) Agranulocytosis Hepatitis, cutaneous ADRs Hepatitis, cutaneous ADRs Hepatitis Cyclophosphamide (anticancer) Ticlopidine (antithrombotic) Bromfenac (antiinflammatory) Zileuton (antiasthma) Agranulocytosis, cutaneous ADRs Agranulocytosis, aplastic anaemia Hepatitis (fatal) Hepatitis Dapsone (antibacterial) Tolcapone (antiparkinsons) Carbutamide (antidiabetic) Agranulocytosis, cutaneous ADRs, Hepatitis (fatal) Bone marrow toxicity aplastic anaemia Trazodone (antidepressant) Ibufenac (antiinflammatory) Diclofenac (antiinflammatory) Hepatitis Hepatitis (fatal) Hepatitis Trimethoprim (antibacterial) Iproniazid (antidepressant) Felbamate (anticonvulsant) Agranulocytosis, aplastic anaemia, Hepatitis (fatal) Hepatitis (fatal), aplastic anaemia cutaneous ADRs Metiamide (antiulcer) (fatal), severe restriction in use Thalidomide (immunomodulator) Bone marrow toxicity Furosemide (diurectic) Teratogenicity Nomifensine (antidepressant) Agranulocytosis, cutaneous ADRs, Valproic acid (anticonvulsant) Hepatitis (fatal), anaemia aplastic anaemia Hepatitis (fatal), teratogenicity Practolol (antiarrhythmic) Halothane (anesthetic) Severe cutaneous ADRs Hepatitis Remoxipride (antipsychotic) Imipramine (antidepressant) Aplastic anaemia Hepatitis Sudoxicam (antiinflammatory) Indomethacin (antiinflammatory) Hepatitis (fatal) Hepatitis Tienilic Acid (diuretic) Hepatitis (fatal) Tolrestat (antidiabetic) Hepatitis (fatal) For most of these drugs, bioactivation to reactive metabolites Troglitazone (antidiabetic) has been demonstrated in vitro or in vivo Hepatitis (fatal) Zomepirac (antiinflammatory) Kalgutkar AS and Soglia JR (2005). Hepatitis, cutaneous ADRs Exp. Opin. Drug Metab. & Toxicol. 1:91-141) 1.57  Some Examples of Reactive Metabolite Formation A Classic Example - Acetaminophen Detoxification H N O Detoxification UGTs, STs H R CH3 N O Major O Pathway CH3 H HO N O S G HO CH3 Quinone Imine Glutathione Adduct Minor Pathway N O CH3 Bioactivation O Glutathione Depletion CYP2E1 (EtOH Inducible) Reactions with cellular proteins  Dose-dependent hepatotoxin  Hepatotoxicity occurs at high doses (> 4 g/day)  About 5% of dose is metabolized to a reactive quinone-imine (CYP 2E1, 1A2, 3A4) 1.58  Mechanism Based Inhibitors Formation of Ultra-reactive Metabolite  Forms a covalent bond with nucleophilic residue in the enzyme  This kills the enzyme  Total active enzyme is decreased  The activity is regained following resynthesis of the enzyme  The adduct formed with can result in severe toxicity Furafylline Clopidogrel Mibefradil Tienilic Acid MBI of CYP2C8 Gemfibrozil Gemfibrozil Raloxifene Not an MBI Glucuronide  Irreversible Inhibition Highly Reactive Intermediate Drug + Enzyme Drug Enzyme Drug Enzyme D + E EI E-I [I] Drug Binds to Enzyme Simple Catalysis X Converts to Reactive Metabolite M Metabolite Reactive Metabolite Bites the Enzyme Forms Enzyme Adduct Toxicity  Example of MBI in Action Inactivation of CYP2C9 by Tienilic Acid  Forms reactive sulfoxide metabolite or an epoxide Tienilic Acid intermediate  Covalently binds to CYP2C9  Results in antibodies against CYP 2C9. Enzyme-SH Enzyme-SH  Withdrawn due to autoimmune H+ H+ hepatotoxicity Incidence of 0.01-0.07% (< 1 in 1000) Covalently binds to CYP 2C9  Inactivation of CYP3A4 By Raloxifene CYP3A4 HS-Enzyme Raloxifene Extended Quinone Methide Raloxifene inactivates CYP3A4 after metabolic activation Adduct formation  MI Complex and Inactivation of CYP2D6  Paroxetine: Undergoes metabolic activation 1,3-benzdioxole ring scission by P4502D6 is principal pathway of elimination in humans  P4502D6 Inactivation F Carbene O O P450 2D6 O O O O O HO O N O N Fe N Paroxetine N H N HO O P450 An Carbene – Iron MI Complex with Inactivation Heme Complex  Effect of Inactivation of CYP2D6 Results in increase of Plasma Concentrations of the Second Drug Time (hr) J Clin Pharmacol 2002;42:1219-1227 1.64 Withdrawals from US Market due to Drug-Drug Interactions Drug Inhibition Drugs Market Period Terfenadine 1985-1998 Mibefradil 1997-1998 Astemizole 1988-1999 Cisapride 1999-2000  Summary  Drug metabolism impacts drug discovery in a big way Both - Efficacy & Safety  Understanding drug metabolism of newly synthesized candidate has become a norm in most pharmaceutical industries  Knowledge of organic chemistry is clearly important Elucidating the mechanism of biotransformation reactions Helps in the med chemists in implementing it to drug design