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Drug Metabolism: Phase I NEPHAR 305 Pharmaceutical Chemistry I Drug Metabolism: Phase I Assist.Prof.Dr. Banu Keşanlı Drug Metabolism ¾ Drug’s biochemical modification or degradation, usually through specialized enzymatic systems ¾ Xenobiotic: a chemical which is found in an organism but which is not normally produced or expected to be present in it ¾ Drug metabolism often converts lipophilic chemical compounds into more readily excreted polar products ¾ Duration and intensity of the pharmacological action of drugs is important ¾ Drug metabolism can result in toxication if the metabolite of a compound is more toxic than the parent drug or chemical ¾ or detoxication (process of preventing toxic entities from entering the body in the first place) by the activation or deactivation of the chemical A prodrug is a pharmacological substance (drug) that is administered in an inactive (or significantly less active) form. Once administered, the prodrug is metabolised in vivo into an active metabolite. Importance of Drug Metabolism Basic premise: Lipophilic Drugs Hydrophilic Metabolites (Not excreted) (Excreted) Water soluble increased renal excretion and Decreased tubular re-absorption of lipophilics Importance of Drug Metabolism Metbolism Termination of Drug - Bioinactivation - Detoxification - Elimination Metabolism Bioactivation - Active Metabolites - Prodrugs - Toxification Phase I or Functionalization Reactions Oxidative Reactions • Oxidation of aromatic moieties • Oxidation of olefins • Oxidation at benzylic, allylic carbon, carbon atoms α to carbonyl and imines • Oxidation at aliphatic and alicyclic carbon • Oxidation involving carbon-heteroatom systems: Carbon-nitrogen systems (aliphatic and aromatic amines; N-dealkylation, oxidative deamination, N-oxide formation, N-hydroxylation) Carbon-oxygen systems (O-dealkylation) Carbon-sulfur systems (S-dealkylation, S-oxidation, and desulfuration) • Oxidation of alcohols and aldehydes • Other miscellaneous oxidative reactions Reductive Reactions • Reduction of aldehydes and ketones • Reduction of nitro and azo compounds • Miscellaneous reductive reactions Hydrolytic Reactions • Hydrolysis of esters and amides • Hydration of epoxides and arene oxides by epoxide hydrase Phase II or Conjugation Reactions • Glucuronic acid conjugation • Sulfate conjugation • Conjugation with glycine, glutamine, and other amino acids • Glutathione or mercapturic acid conjugation • Acetylation • Methylation Transformation of Xenobiotics by Biological Systems ¾ Phase I and Phase II reactions are biotransformations of chemicals that occur during drug metabolism ¾ Oxidative biotransformations require both molecular oxygen and the reducing agent NADPH (reduced form of nicotinamide adenine dinucleotide phosphate) ¾ One atom of molecular oxygen (O2) is introduced into the substrate R-H to form R-OH and the other oxygen atom is incorporated into H2O OH O2 NADPH Aromatic Hydroxylation ¾ Major route of metabolism for many drugs containing a phenyl group (and aromatic) ¾ Proceeds initially with an “arene oxide” intermediate ¾ Hydroxylation occurs at para position Activated rings – electron donating substituents Deactivated rings don’t get oxidized – toxic ¾ Dihydrol metabolite formation is possible ¾ Undergoes further conversions to polar water soluble glucuronide or sulfate conjugates, which are readily excreted in the urine (a) TethedralTetrahedral ara ürün H H intermediate O (b) H H O OH Oxidation Involving Carbon-Heteroatom Systems ¾ Hydroxylation of α-carbon atom attached to the heteroatom (N, O, S) results in unstable intermediate which decomposes via cleavage of carbon – heteroatom bond ¾ Hydroxylation or oxidation of heteroatom (N, O, S) could fall under these mechanisms: N-hydoxylation, N-oxide formation, sulfoxide, sulfone formation ¾ Structural factors determine the metabolic pathway – complicated ¾ Nitrogen functionalities such as amines, amides are found in natural products (morphine, nicotine etc) and in numerous drugs (antihistamines, phenothiazine etc) Tertiary aliphatic and Alicyclic amines Oxidative N-dealkylation: Oxidative removal of alkyl groups Small groups such as methyl, ethyl, isopropyl removed rapidly, t-butyl is impossible H H O O RN C RN C R NH + C 1 α 1 α 1 R2 R2 R2 Secondary Carbonyl Tertiary amine Carbinolamine amine (aldehyde or ketone) Antiestrogenic agent Tamoxifen (Nolvadex) H3C H3C NCH2CH2O O HNH2CH2CO H3C + C CH2CH3 CH2CH3 H H Tamoxifen Secondary and Primary Amines • Oxidative N-dealkylation • Oxidative deamination • N-oxidation 9 Carbinolamine pathway to give corresponding primary amine metabolite through N-dealkylation 9 Which then is susceptible to oxidative deamination • initial α-C hydroxylation followed by C-N bond cleavage to give carbonyl metabolite and ammonia ¾ Metabolism of Propranolol both by direct deamination and deamination of its primary amine metabolite OH OH OH H O O CH O CH O O CH C CH2 C CH C C H H 2 H2 2 HN HN H CH3 CH3 C C H H CH CH3 Propranolol 3 Aldehyde metabolite NH Direct oxidative Carbinolamine 2 deamination OH NH3 OH oxidative O CH deamination through primary amine C CH2 O CH H2 C CH2 H HN CH3 2 C NH2 O H CH3 O Primary amine metabolite C (Desisopropyl Propranolol) Carbinolamine H3C CH3 N-Dealkylation Oxidative Deamination ¾ Metabolic N-oxidation of secondary amine leads to N-oxygenated products, N-hydroxylamines which are susceptible to further oxidation giving nitrone metabolites - OH O + NH N N CH 3 CH3 CH2 Secondary Hydroxylamine Nitrone amine CF3 CH3 CF3 CH3 CF3 CH3 CH + CH HN CH3 N 3 - N 3 OH O Hydroxylation Nitrone ¾ Secondary amines undergo oxidative dealkylation and deamination more than N-oxidation ¾ Primary aliphatic amines are biotransformed by oxidative deamination or by N-oxidation ¾ Monoamine oxidase (MAO) enzymes are responsible for oxidative deamination ¾ Structural features, e.g. α-substituents determine whether C or N oxidation will occur • If no hydrogen atom on α-C then α-C hydroxylation is impossible Aromatic Amines and Heterocyclic Nitrogen Compounds ¾ Tertiary aromatic amines undergo oxidative N-dealkylation and N-oxide formation ¾ Secondary aromatic amines undergo N-dealkylation and N-oxide formation ¾ Primary amines undergo N-oxidation generating N-hydroxylamine metabolite ¾ Oxidation of hydroxylamine derivative to nitroso derivative is possible ¾ N-oxidation is minor compared to other biotransformations such as N-acetylation aromatic hydroxylation ¾ Conjugation pathways are observed (e.g. sulfate conjugation) CH3 a Ar N Ar NHCH3 CH3 CH2OH Ar N CH3 b CH3 + Ar N O- CH3 Oxidation of Alcohols and Aldehydes ¾ Primary alcohols are oxidized to aldehydes which often undergo facile oxidation to generate polar carboxylic acid derivatives ¾ Secondary alcohols are oxidized to ketones – more likely to form conjugates or be reduced back to alcohol form ¾ Catalyzed by soluble alcohol dehydrogenase present in liver and other tissues + + RCH2OH + NAD RCHO + NADH + H RCHO + NAD+ RCOOH + NADH COOH NH H3C CH2OH R=COH Mefenamic acid R=COOH Reductive Reactions ¾ Plays an important role in the metabolism of many compounds containing carbonyl, nitro and azo groups ¾ Bioreduction of carbonyl compounds generates alcohol derivatives ¾ Nitro and azo reductions lead to amino derivatives ¾ Hydroxyl and amino moieties of the metabolites are more susceptible to conjugation than the functional groups of the parent compounds ¾ Facilitate drug elimination Reduction of Aldehydes and Ketones ¾ Carbonyl containing drugs and metabolites are common ¾ Aldehydes are more readily oxidized to carboxylic acids then reduced to alcohols ¾ Ketones are resistant to oxidation, mainly reduced to secondary alcohols ¾ Aldo-keto reductase enzymes are responsible for reduction ¾Bioreduction of ketones often leads to the creation of asymmetric center thus 2 possible stereoisomeric alcohols O CH3 CHCH2CH2 N CHCH2CH CH3 Cl CH2 Cl CH2 ChlorpheniramineKlorfeniramin AldehitAldehyde yapısımetabolitendaki metabolit oxidationOksidasyon reductionRedüksiyon CHCH2COOH CHCH2CH2OH Cl CH2 Cl CH2 O HO H H OH C C C CH CH CH3 3 3 + AcetophenoneAf S-(-)-Methyl R-(+)-Methyl Phenyl Carbinol Phenyl Carbinol O OH H H OH C C C CH CH CH OH CH2 3 OH CH2 3 OH CH2 3 C C6H5 C C6H5 C C6H5 + H H H O O O O O O R(+)-Warfarin R,S-(+)-Alcohol R,R-(+)-Alcohol Major diastereomer Minor diastereomer Hydrolytic Reactions Hydrolysis of Esters and Amides ¾Metabolism of ester and amide linkages in many drugs is catalyzed by hydrolytic enzymes in tissues and plasma ¾ Metabolic products formed (carboxylic acids, alcohols, phenols, amides) are polar ¾ Functionally more susceptible to conjugation and excretion than the parent ester or amide ¾ Hydrolysis is a major biotransformation pathway for ester functionality ¾ Many parent drugs have been chemically modified or derivatized to generate prodrugs to overcome some undesirable property (e.g. bitter taste, poor absorption, poor solubility, irritation at site of injection) ¾ Ester derivatives are ideal prodrug candidates ¾ Amides are hydrolyzed slower compared to esters COOH O COOH OCCH3 OH + CH3COOH Acetic acid Aspirin Salicylic acid (Acetylsalicylic acid) O O O C OCH3 C OH C OCH CH N H 3 3 H O CH3 N H CH3 N C O OH + H O C H O H Cocaine Benzoylecgonine Methylecgonine O Slow hydrolysis H2NCNHCH2CH2N(C2H5)2 Yavaş hidroliz Prokainamit O Procainamide H2NCOH O H2NCOCH2CH2N(C2H5)2 HFastızlı hidroliz hydrolysis Prokain Procaine NEPHAR 305 Pharmaceutical Chemistry I Drug Metabolism Phase II Prof. Dr. Hakkı Erdoğan Assist.Prof. Banu Keşanlı PHASE II REACTIONS 1. Glucuronidation 2. Sulfate Conjugation 3. Acetylation
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