Drug Metabolism PDF

D R U G M E TA B O L I S M Dr. Ahmed Elkerdawy Senior Lecturer in Pharmaceutical Chemistry [email protected] JBL2W25 Learning Objectives 1. Demonstrate understanding of knowledge about different drug metabolism phases. 2. Integrate knowledge from Phase I and II reactions to predict the po...

D R U G M E TA B O L I S M Dr. Ahmed Elkerdawy Senior Lecturer in Pharmaceutical Chemistry [email protected] JBL2W25 Learning Objectives 1. Demonstrate understanding of knowledge about different drug metabolism phases. 2. Integrate knowledge from Phase I and II reactions to predict the possible metabolic pathways of different drug chemical structures. Drug Metabolism Metabolism is the process by which the body converts xenobiotics (as drugs) usually into a less toxic, more polar form that can be readily excreted. Sites of Drug Metabolism: The liver is the primary organ of drug metabolism. The lung, kidney, intestine, skin and placenta can also carry out drug metabolizing reactions. Sites of Drug Metabolism The largest proportion of Drug Metabolism takes place in liver due to: 1. Liver is rich in almost all drug metabolizing enzymes. 2. The liver is a well perfused organ, therefore, drugs can access to the metabolizing enzymes. 3. Orally administered drugs pass through the liver before reaching the systemic circulation and thus become subject to metabolism (First pass effect). Drug Metabolism Possibilities 1. Active drug → Inactive metabolite(s) (Most common pathway) CH3 COOH Oxidation O O O O S S O NH N NH N O H H Tolbutamide carboxylic acid metabolite [oral hypoglycemic] [inactive metabolite] Drug Metabolism Possibilities 2. Active drug → Active metabolite(s) (Co-activation) H3C H3C N N O O H3COCO OCOCH3 HO OH Heroin Morphine [4 times more active] [active ] Drug Metabolism Possibilities 3. Active drug → Toxic metabolite(s) (metabolic toxicity) OH OH O HO HO Diethyl stilbsterol [DES] [anti-cancer] [carcinogenic epoxide metabolite] Drug Metabolism Possibilities 3. Active drug → Toxic metabolite(s) (metabolic toxicity) Drug Metabolism Possibilities 4. Inactive drug → Active metabolite(s) NH2 N N Reduction NH2 SO2NH2 Prontosil red [inactive] Drug Metabolism Possibilities 4. Inactive drug → Active metabolite(s) NH2 NH2 N N SO2NH2Sulphanilamide Reduction [anti-microbial] NH2 + NH2 SO2NH2 Prontosil red H2N [inactive] NH2 Triamino benzene Drug Metabolism Possibilities 4. Inactive drug → Active metabolite(s) (Prodrugs) NH2 NH2 N N SO2NH2Sulphanilamide Reduction [anti-microbial] NH2 + NH2 SO2NH2 Prontosil red H2N [inactive] NH2 Triamino benzene Drug Metabolism Possibilities 5. Active parent drug → Active metabolite having a different pharmacological action CO NH NH CO NH NH2 Dealkylation N N Iproniazide Isoniazide [anti-depressant] [anti-T.B.] Drug Metabolism This is carried through Phase I and/or Phase II Phase I (Functionalization) Add, modify or expose a polar functional group. Phase II (Conjugation) Attachment of hydrophilic substituent. Cytochrome P450 family are examples of enzymes involved in Phase I, which account for about 75% of the total metabolism. CYP3A4, CYP2D6, CYP2C9 and CYP2C19 are important for the metabolism of drugs in humans. Some drugs may pass directly into Phase II. Drug Metabolism Drug Metabolism Drug Metabolism Phase I Phase II Oxidation Reduction Hydrolysis Drug Metabolism 1- Aromatic Oxidation 2- Alkene (Olefinic Oxidation) 3- Benzylic Oxidation 4- Allylic Oxidation 5- Aliphatic Oxidation  Aromatic [= Aryl = Arene] Oxidation Epoxide Arene Arene Oxide M Arene oxide is highly reactive → binds to cellular macromolecules [M] as DNA and proteins → toxicity Toxic Aromatic [= Aryl = Arene] Oxidation Major Detoxification of NIH Shift Arene oxide Phenolic [Arenol] Metabolite Epoxide Hydrolase Enzyme Trans Dihydrodiol Metabolite Arene Oxide Glutathione-S-Transferase Enzyme Glutathione = GSH Glutathione Adduct Aromatic [= Aryl = Arene] Oxidation Rules: Monosubstituted →Hydroxylation in: 1. Para-position 2. Electron-Rich ring EDG EWG E-Rich E-Deficient  → CH2-CH-NH2 rapid P-hydroxylation HO CH2-CH-NH2 CH3 CH3 electron donating gp. Amphetamine Fast metabolism & have short t1/2  → Cl O Cl Cl O Cl TCDD Clonidine These compounds resist metabolism & have long t1/2  → O N H OH Chlorpromazine Propranolol Diazepam  → OH or HN OH O N O H Phenytoin Phenytoin Phenylbutazone  OH OH C CH 2 17 alpha-ethinyl estradiol HO 3  Hydroxylation occurs in electron Rich ring and in the p-position If p-position If two rings occupied Different Identical o-hydroxylation The electron rich ring One ring only only  Alkene [Olefinic] Oxidation: Epoxide Hydrolase Enzyme Trans Alkene Epoxide Dihydrodiol Cellular Toxicity Alkene [Olefinic] Oxidation: Examples Alclofenac Epoxide (minor) Dihydroxy (major) Modify carbamazepine to decrease possible toxicity Carbamazepine Carbamazepine- Trans-10,11- (Antipsychotic 10,11-epoxide dihydroxyderivative and antiepileptic) (Anticonvulsant)  Aromatic Olefinic Intermediate Epoxide Product 1- Phenol (Major) Trans-dihydrodiol 2- DNA Adduct 3- Trans-dihydrodiol 4- Glutathione adduct  Allylic C Oxidation:  Benzylic C Oxidation: Primary Alcohol Aldehyde Carboxylic Acid Phase II  𝛚 Carbon Aliphatic Oxidation: 𝛚-1 Carbon Ibuprofen 𝛚 oxidation 𝛚-1 oxidation  [ultimate or terminal C] & -1 [penultimate C] oxidation. This oxidation commonly takes place in drug molecules with straight or branched alkyl chains. Alcohol metabolites formed  further oxidation to aldehydes & ketones. [Or directly conjugated with glucuronic acid].  Aliphatic Oxidation: Valproic acid Oxidation Reactions Alcohols and Aldehydes Oxidation: Alcohol Dehydrogenase Aldehyde Dehydrogenase Primary Alcohol Aldehyde Carboxylic Acid Alcohol Dehydrogenase Tertiary Alcohol NOT Secondary Alcohol Ketone oxidized This reaction may be reversed readily by alcohol dehydrogenase enzyme [2ry alcohol are more polar, so more likely to be conjugated than ketone] Oxidation Reactions Direct conjugation Aldehydes [1ry alc.] ketones [2ry alc.] Oxidation Reactions Oxidation of heteroatom-containing compounds (1) Hydroxylation of C directly attached to a (2) Direct attack on the heteroatom heteroatom [N or S but not O] [N, S or O] ( to it)  There must be one H atom at least on Oxidation Reactions this carbon. N & C-N Oxidation: C-N Oxidation is Major Oxidative Oxidative N- Oxidative N- Deamination Dealkylation Dealkylation Tertiary Amine Secondary Amine Primary Amine Aldehyde Chlorpromazine Oxidation Reactions Catalyzed by amine oxidase enzyme. It’s called oxidative N-dealkylation due to removal of an alkyl group [especially Me]. Removal of first alkyl gp from 3ry amine is faster than removal of second alkyl gp. Small alkyl group rapidly removed (Me, Et, Pro} than large alkyl groups. What about t-Butyl group?? Oxidation Reactions N & C-N Oxidation: N Oxidation is Minor Tertiary Amine N-Oxide Metabolite Heterocycle N-Oxide Metabolite Imipramine N-Oxide Metabolite Quinoline N-Oxide Metabolite Oxidation Reactions S & C-S Oxidation: 1) S-Oxidation 2) S-Dealkylation Sulfide Thiol Sulfide Sulfoxide Sulfone Chlorpromazine Oxidation Reactions S & C-S Oxidation: 3) Desulphuration Oxidative conversion of thio group to carbonyl group Oxidation Reactions C-O Oxidation: O-Dealkylation Ether Alcohol or Phenol CH3 CH3 Small alkyl gps rapidly O-dealkylated. E.g. Codeine. N N O O H3C O OH HO OH Codeine Morphine Oxidation Reactions If several non-equivalent methoxy groups  selective dealkylation of only one methoxy. E.g. Trimethoprim. Trimethoprim Drug Metabolism Phase I Phase II Oxidation Reduction Hydrolysis Drug Metabolism Aldehydes Ketones Reductase Reductase Primary Alcohol Secondary Alcohol Drug Metabolism Aldehydes Majority of aldehydes oxidized to carboxylic acids but may be reduced to alcohols especially if attached to e-withdrawing group. Drug Metabolism Ketones Ketones are generally resistant to oxidation and therefore undergo reduction to secondary alcohols Ketones: give 2ry alcohol [usually of asymmetric centers]  two possible isomers. O H H OH HO CH3 CH3 CH3 + acetophenone S(-) methyl phenyl carbinol R(+) methyl phenyl carbinol 75 % 25 % Drug Metabolism Examples Chloral Primary Alcohol Acetohexamide Secondary Alcohol Drug Metabolism Nitro Sulfone Reductase Sulfoxide Reductase Reductase Primary Amine Sulfide (Thioether) Drug Metabolism Examples Chloramphenicol Primary Amine Sulindac Sulfide Drug Metabolism Azo Disulfide Reductase Reductase Primary Amine Thiol Drug Metabolism Sulfasalazine Primary Amine Drug Metabolism N S S N N SH + HS N S S S S Disulfiram N,N-diethyldithiocarbamic acid Disulfiram Thiol Drug Metabolism Aldehydes Ketones Nitro Sulfone Azo Disulfide R-CH=O R-CO-R NO2 R-SO2-R RN=NR’ R-S-S-R’ Enzyme Reductase Products 1ry alcohol 2ry alcohol NH2 R-SO-R RNH2 + RSH + R’SH (Isomers) →R-S-R R’NH2 (Thiol) Major or Minor (oxidation Major minor is major) c.f. electron withdrawing group Drug Metabolism Phase I Phase II Oxidation Reduction Hydrolysis Drug Metabolism Phase I Hydrolysis Reactions Esters Amides Esterase Amidase Acid Alcohol Acid Amine Drug Metabolism Esters Catalyzed by Esterases which are non-specific & widely distributed in liver, kidney, intestine. Major pathway for esters  facile hydrolysis. Hydrolytic products are alcohol & acid  contain functional groups that can be conjugated Drug Metabolism Phase I Hydrolysis Reactions Examples Esterase Procaine Amides are slowly hydrolyzed than Esters and so, Amidase longer duration Procainamide Considered as a strategy of drug design Drug Metabolism Esterase Aspirin Amidase Lidocaine Drug Metabolism Phase I Hydrolysis Reactions Examples Lactone is a cyclic ester Esterase Lovastatin 𝛃-lactamase Lactam is a cyclic amide 𝛃-Lactam antibiotic Drug Metabolism Esters Amides Enzyme Esterase Amidase Distribution Liver, Kidney, Intestine Liver Products Acid and alcohol Acid and Amine Rate Rapid Slow ( Long duration) Major or minor Major pathway Substituent effect Electron withdrawing group increases the rate of hydrolysis Cyclised Lactone by esterase Lactam by lactamase Drug Metabolism Phase I Phase II Conjugation Oxidation Reactions Reduction Hydrolysis Phase II Reactions ❑ Most phase II reactions are conjugation reactions catalyzed by transferase enzymes. ❑ Phase II is capable of converting parent xenobiotics or its phase I metabolites to conjugated products which are: More polar Water soluble products Readily excretable Conjugation Reactions: Biologically inactive Glucuronic acid conjugation √ Non-toxic Glutathione conjugation √ Methylation Acetylation Sulfate conjugation √ Amino Acid conjugation Phase II Reactions Glucuronic Acid Conjugation: Most Common Pathway GA UDPGA ❑ Glucuronic acid is a sugar acid derived from glucose (Readily available), with its sixth carbon atom oxidized to a carboxylic acid. ❑ Activation of glucuronic acid to Uridine Diphosphate (UDP)-Glucuronic acid [Active form]. Phase II Reactions Glucuronic Acid Conjugation: Most Common Pathway UDP-glucuronyltransferases R-X-H UDPGA -Glucuronide Transfer of glucuronyl group from UDPGA to an appropriate substrate catalyzed by microsomal enzymes called UDP-glucuronyltransferases. Phase II Reactions Glucuronic Acid Conjugation: Most Common Pathway O or N or S ↑ Solubility & Glucuronide Excretion Binds to all polar groups: ❖ OH: phenols, alcohols, enols, N-hydroxylamines, N- hydroxylamides ❖ COOH: aryl acids, arylalkyl acids ❖ NH: arylamines, alkylamines, amides, sulfonamides, tertiary amines ❖ SH: Thiols Phase II Reactions Glucuronic Acid Conjugation: Aspirin O-Glucuronide (Acyl) Paracetamol O-Glucuronide (Ether) O 2N N N N H CH3 NH2 N 7- amino-5- H nitrobenzimidazole N-Glucuronide 7-amino-5-nitronidazole N-Glucuronide desipramine  Phase II Reactions 6-Mercaptopurine S-Glucuronide SH N N N H 3C C4H9 SH methimazole S-Glucuronide  Phase II Reactions PAPS Sulfate Conjugation: Sulfotransferase Enzyme PAPS 1. Activation of inorganic sulfate to 3’-PhosphoAdenosine-5’-PhosphoSulfate [PAPS][Active form]. 2. Binding to the polar group of the compound: Phenols and alcohols, aromatic amines, and N- hydroxyl compounds (Mostly Phenolic OH). 3. N- sulfonation of aromatic amines is a minor pathway. 4. Amount of sulfate available is limited. 5. Utilized to conjugate endogenous compounds e.g. steroids, catecholamines, and thyroxin. Phase II Reactions Sulfate Conjugation: ❑ Neonates and young children have a decreased glucuronidating capacity because of undeveloped glucuronyl transferases or low levels of these enzymes. ❑ Sulfate conjugation is well developed and becomes the main route of paracetamol conjugation in this pediatric group. Paracetamol O-Glucuronide Sulfate Conjugate Main metabolite in adult Main metabolite in infant Phase II Reactions Glutathione Conjugation: Detoxification Glutamic Acid Tripeptide Glycine Cysteine γ-glutamylcysteinylglycine found in most tissues 1. Glutathione [GSH] binds to Electrophilic drugs [E+] by the nucleophilic sulphydryl group present in glutathione 2. The reaction is catalyzed by Glutathione-S-Transferase Enzyme. ❑ Electrophilic functional groups, such as epoxides, alkyl halides, sulphonates, disulphides, carbon-carbon double bonds and radical species ❑ Take place in most cells, especially those in the liver and kidney Phase II Reactions GSH = Glutamic a-Cysteine-Glycine Glutathione-S enzymatic cleavage Drug + GSH Drug -SG COOH transferase CH NH Drug S CH2 - glutamic a - Glycine 2 Drug-Cysteine N- acetylation excretion Mercapturic a derivative CoA acetylCoA 1. Enzymatic cleavage of glycine and glutamic acid forming S-substituted Cysteine 2. N-Acetylation of S-substituted Cysteine forming Mercapturic acid [the excretable form]. Phase II Reactions Glutathione Conjugation: Glutathione-S-Transferase E+ GSH Conjugate Aromatic Derivative Arene Oxide [toxic] 1. Cleavage of Glycine and Glutamic acid. 2. N-Acetylation Mercapturic Acid Excretable NON-toxic  PHAY0006 Timetabling Indicative Reading Patrick, GL. An Introduction to Medicinal Chemistry, 6th Edition, Oxford, Oxford University Press, 2017. ISBN 0-19-969739-7 Drug Metabolism and Pharmacokinetics Quick Guide, Siamak Cyrus Khojasteh , Harvey Wong , Cornelis E.C.A. Hop, 2011, Springer. ISBN 978-1-4419-5629-3 Handbook of Drug Metabolism, Third Edition, Paul G. Pearson, Larry C. Wienkers, 2019, Taylor and Francis. ISBN 9780429190315