Classs drug metabolism

THIS PRESENTATION IS FOR UNDER GRADUATE UNDERSTANDING AND EXAMINATION PURPOSE

1. DRUG METABOLISM Dr. RAGHU PRASADA M S MBBS,MD ASSOCIATE PROFESSOR DEPT. OF PHARMACOLOGY SSIMS & RC. 1

2. Biotransformation It is chemical alteration of drug in the body, wherein Non polar lipid soluble compounds are made polar lipid insoluble, so that they are easily excreted. Drugs which do not undergo biotransformation – Streptomycin, neostigmine ….(highly polar drugs) Primary site – Liver Others – Kidney, Intestine, Lungs, Plasma

3. Biotransformation Drug Biotransformation –convert lipophilic / hydrophobic drug ( to enter cells) to hydrophilic metabolites. Advantages Termination of drug action - (↓ toxicity) Reduced lipophilicity . Renal / biliary excretion ↑ - (↓renal reabs )

4. Biotransformation Metabolic changes by Enzymes ( Microsomal , Cytoplasmic , Mitochondrial) Spontaneous Molecular rearrangement – HOFMANN ELIMINATION Excreted unchanged (highly polar drugs) - Aminoglycosides , Methotrexate , Neostigmine

5. Biotransformation A) Drug inactivation - inactive or less active Propranolol , Pentobarbitone , Morphine , Chloramphenicol , Paracetamol , Ibuprofen , lignocaine B ) Active drug to Active metabolite- active metabolite Effect is due to parent drug and its active metabolite

6. Active drug –active metabolite Phenacetin - Paracetamol Phenyl butazone - Oxyphenbutazone Primidone - Phenobarbitone Diazepam - Oxazepam Digitoxin - Digoxin Amitriptyline - Nortriptyline Codeine - Morphine Spironolactone - Canrenone Allopurinol - Alloxanthine Cefotaxime - Des acetyl cefotaxime Morphine - Morphine 6 glucuronide

7. Prodrug =active drug C) Inactive drug ( Prodrug ) - Active drug Prodrugs are inactive drugs which need BT in the body to form active metabolites . ADV More stable Better BA Less toxicity Examples Levodopa - Dopamine Enalapril - Enalaprilat Dipivefrine - Epinephrine

8. Biotransformation -types TYPES BIOTRANSFORMATION REACTIONS - 2 TYPES Phase I / Non synthetic / Functionalization A functional group is generated Metabolite – active or inactive Phase II / Synthetic / Conjugation An endogenous radical is conjugated Metabolite is usually inactive

9. Biotransformation –phase I Phase I Reactions Oxidation Reduction Hydrolysis Cyclization Decyclization

10. Biotransformation-Phase II Reactions Phase II Reactions Glucuronide conjugation Acetylation Methylation Sulfate conjugation Glycine conjugation Glutathione conjugation Ribonucleotide / Ribonucleoside synthesis

11. PHASE I REACTIONS OXIDATION Addition of Oxygen / negatively charged radical or removal of Hydrogen / Positively charged radical Oxidation is the main process of metabolism Produces unstable intermediates - Epoxides , Superoxides , Quinones Oxidation – 9 types

12. PHASE I REACTIONS 1.OXIDATION AT NITROGEN ATOM Chlorpheniramine Dapsone Meperidine 2.OXIDATION AT SULPHUR ATOM Chlorpromazine Chloramphenicol

13. PHASE I REACTIONS ALIPHATIC HYDROXYLATION Hydroxyl group added to drug Salicylic acid to Gentisic acid Ibuprofen Tolbutamide , Chlorpropamide , AROMATIC HYDROXYLATION Phenytoin Phenobarbitone Propranolol

14. PHASE I REACTIONS DEALKYLATON AT OXYGEN ATOM Phenacetin to Paracetamol 6.DEALKYLATON AT NITROGEN ATOM Amitriptyline to Nortriptyline DEALKYLATON AT SULPHUR ATOM 6Methyl thiopurine to Mercaptopurine 8.OXIDATIVE DEAMINATION Amphetamine 9.DESULFURATION Parathion to Paraoxon

15. PHASE I REACTIONS Main enzymes are the Oxygenases – MICROSOMAL MONOOXYGENASES in liver ( Cytochrome p450/CYP )- drugs CYP( 450)s require NADPH & Oxygen Drug Metabolizing Enzymes – 2 types Microsomal – CYP 450, UDPGT Non microsomal – Flavoprotein oxidases,esterases …

16. PHASE I REACTIONS NONMICROSOMAL OXIDATION Mitochondrial enzymes - MAO—Oxidative deamination of Adrenaline,5HT,Tyramine Cytoplasmic enzymes – Dehydrogenases Alcohol Oxidation to Acetaldehyde & Acetic Acid Plasma oxidative enzymes- Histaminase , Xanthine oxidas

17. PHASE I REACTIONS b) REDUCTION Addition of Hydrogen / positively charged radical or removal of Oxygen / negatively charged radical MICROSOMAL REDUCTION by Monooxygenases need NADPH & cytochrome c reductase . A. NITRO Reduction- Chloramphenicol to aryl amine metabolite B. KETO Reduction – Cortisone to Hydrocortisone ,

18. PHASE I REACTIONS c) HYDROLYSIS Drug is split combining with water Ester + water Esterases Alcohol & Acid Microsomal hydrolysis Pethidine to meperidinic acid Non microsomal hydrolysis – Esterases , Amidases & Peptidases Atropine to Tropic acid

19. PHASE I REACTIONS AZO Reduction Prontosil to Sulfanilamide NON MICROSOMAL REDUCTION Chloral hydrate to Trichloro ethanol

20. PHASE I REACTIONS d) CYCLIZATION Formation of ring structure from a straight chain compound . Eg : Proguanil e) DE CYCLIZATION Ring structure opened Phenytoin , Barbiturates

21. PHASE II REACTIONS CONJUGATION / TRANSFER Drug / phase I metabolite combines with endogenous substance derived from carbohydrates/ proteins. covalent bond formation between functional group of drug & endogenous substrate Endogenous- Glucuronic acid , Amino acids, Sulfates , Acetates, Glutathione Represent terminal inactivation – True detoxification reactions

22. PHASE II REACTIONS Conjugates-hydrophilic, ionized , ↑ mol.weight , Inactive Excreted in urine/ bile/ faeces . Phase II- need energy 7 types of reactions

23. PHASE II REACTIONS 1.CONJUGATION WITH GLUCURONIC ACID UDP glucuronyl transferases Conjugates with OH & COOH are conjugated with glucuronic acid derived from glucose Drug + UDPGA Microsomal Glucuronyl transferase Drug glucuronide + UDP Drugs - Aspirin , Paracetamol , PABA , Metronidazole , Morphine , Diazepam

24. PHASE II REACTIONS ↑ Mol.weight – favours biliary excretion Drug glucuronides excreted in bile are hydrolyzed by intestinal microfloral enzymes - parent drug released - reabsorbed into systemic circulation- ↓excretion ↑duration of action Oral contraceptives, Phenolphthalein Endogenous substrates - Steroid , Thyroxine , Bilirubin

25. PHASE II REACTIONS ACETYLATION Drugs with Amino or Hydrazine groups - INH, PAS, Hydralazine , Sulfonamides Procainamide , Dapsone . ( Code - SHIP) N Acetyl transferase Acetyl CoA Genetic polymorphism Acetylation - Rapid / Slow

26. PHASE II REACTIONS 3. CONJUGATION WITH SULFATE Drug groups-Amino, Hydroxyl Cytoplasmic Enzymes - Sulfotransferases / Sulfokinases . Methyl dopa , Steroids, Chloramphenicol , Warfarin

27. PHASE II REACTIONS CONJUGATION WITH GLYCINE Drug group – Carboxylic acid Salicylic acid , Benzoic acid 5. CONJUGATION WITH GLUTATHIONE Drug groups- Epoxide , Quinone Toxic metabolites of Paracetamol, Ethacrynic acid Cytoplasmic Enzyme - Glutathione S- Transferase

28. PHASE II REACTIONS 6. METHYLATION Drugs with Amino & Phenol groups Histamine , Adrenaline, Nicotinic acid, Dopamine, Methyl dopa , Captopril Enzyme- Methyl transferase Endogenous substance- Cysteine , Methionine

29. PHASE II REACTIONS 7. RIBONUCLEOTIDE /RIBONUCLEOSIDE SYNTHESIS Action of Purine & Pyrimidine antimetabolites 6 Mercaptopurine

30. Enzyme inhibition INHIBITION OF DRUG METABOLISM One drug can inhibit the metabolism of another drug ↑ in circulating levels of slowly metabolised drug Prolongation or potentiation of its effects Consequences Precipitate toxicity of the object drug. can be therapeutically beneficial. Eg : Aversion of alcohol with disulfiram , Reversal of Skeletal Muscle paralysis of d- tc by N eostigmine

31. Enzyme inhibition Valproate Ketoconazole Cimetidine Ciprofloxacin Erythromycin INH

32. Enzyme induction MICROSOMAL ENZYME INDUCTION Drugs , insecticides, carcinogens will induce the synthesis of microsomal enzyme proteins Accelerated metabolism and reduced pharmacological response Consequences Drug- drug interactions Can lead to toxicity. Eg : Alcoholics more prone to hepatotoxicity of paracetamol due to↑ production of NABQI , Pptn of a/c intermittent porphyria by barbiturate

33. Enzyme induction Therapeutic benefit. Eg : To treat neonatal jaundice Decreased duration of action. Eg : OCP failure Griseofulvin Phenytoin , Primidone Rifampicin Smoking Carbamazepine Phenobarbitone

34. Elimination by the Liver Metabolism - major 1) Phase I and II reactions 2) Function: change a lipid soluble to more water soluble molecule to excrete in kidney 3) Possibility of active metabolites with same or different properties as parent molecule Biliary Secretion – active transport, 4 categories

35. The Enterohepatic Shunt Portal circulation Liver gall bladder Gut Bile duct Drug Biotransformation; glucuronide produced Bile formation Hydrolysis by beta glucuronidase

36. Liver P450 systems Liver enzymes inactivate some drug molecules First pass effect (induces enzyme activity) P450 activity is genetically determined: Some persons lack such activity  leads to higher drug plasma levels (adverse actions) Some persons have high levels  leads to lower plasma levels (and reduced drug action) Other drugs can interact with the P450 systems Either induce activity (apparent tolerance) Inactivate an enzyme system

37. Drug-CYP Interactions Enzyme (CYP) Substrate Inhibitor Inducer 1A2 Clozapine , haloperidol Cimetidine Tobacco smoke 2B6 Bupropion Thiotepa Phenobarbital 2C19 Citalopram Fluoxetine Prednisone 2C9 Fluoxetine Paroxetine Secobarbital 2D6 Most ADs, Aps CPZ , ranitidine Dexamethasone 2E1 Gas anesthetics Disulfiram Ethanol 3A4,5,7 Alprazolam Grapefruit juice Glucocorticoid

38. Liver P450 systems Multiple CYP gene families have been identified in humans, and the categories are based upon protein sequence homology Most of the drug metabolizing enzymes are in CYP 1, 2, & 3 families . CYPs have molecular weights of 45-60 kDa . Frequently, two or more enzymes can catalyze the same type of oxidation, indicating redundant and broad substrate specificity. CYP3A4 is very common to the metabolism of many drugs; its presence in the GI tract is responsible for poor oral availabilty of many drugs

39. Liver P450 systems Monoamine Oxidase (MAO), Diamine Oxidase (DAO) - MAO (mitochondrial) oxidatively deaminates endogenous substrates including neurotransmitters (dopamine, serotonin, norepinephrine , epinephrine); drugs designed to inhibit MAO used to affect balance of CNS neurotransmitters (L-DOPA); MPTP converted to toxin MPP+ through MAO-B. DAO substrates include histamine and polyamines. Alcohol & Aldehyde Dehydrogenase - non-specific enzymes found in soluble fraction of liver; ethanol metabolism Xanthine Oxidase - converts hypoxanthine to xanthine , and then to uric acid. Drug substrates include theophylline , 6-mercaptopurine. Allopurinol is substrate and inhibitor of xanthine oxidase ; delays metabolism of other substrates; effective for treatment of gout.

40. Acetominophen Metabolism ~60% ~35% CYP2E1* CYP1A2 CYP3A11 NAPQI N-acetyl-p-benzoquinone imine *induced by ethanol, isoniazid Protein adducts, Oxidative stress Toxicity

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