Metabolism II

Phase II Reactions in Biotransformation

1. Phase II Reactions in Biotransformation Mr. K. K. Mali Assistant Professor, YSPM’s Yashoda Technical Campus, Faculty of Pharmacy, Satara

2. Content Introduction of biotransformation Phase II Reactions Conjugation with glucuronic acid Conjugation with sulphate moieties Conjugation with alpha amino acids Conjugation with glutathione and mercapturic acid formation Acetylation reaction Methylation reaction Miscellaneous conjugation reactions April 13, 2013 2 Phase II Reactions In Biotransformation

3. Content Factors affecting biotransformation of drugs Physicochemical properties of drug Chemical factors Induction of drug metabolising enzymes Inhibition of drug metabolising enzymes Environmental factors chemicals Biological factors Species differences Strain differences Sex differences Age Diet April 13, 2013 3 Phase II Reactions In Biotransformation

4. Content Factors affecting biotransformation of drugs Altered physiological factors Pregnancy Hormonal imbalance Diseased states Temporal factors Circadian rhythm April 13, 2013 4 Phase II Reactions In Biotransformation

5. Introduction of biotransformation April 13, 2013 5 Phase II Reactions In Biotransformation

6. Introduction of biotransformation April 13, 2013 6 Phase II Reactions In Biotransformation True detoxification reaction Phase II metabolites are Highly polar Free from pharmacological activity Non-carcinogenic due to conjugation

7. Phase II reactions Phase II metabolites are Attachment of small polar endogenous molecules such as glucuronic acid, sulfate and amino acids to Phase I metabolites or parent drugs Products are more water-soluble and easily excretable Trapping highly electrophilic molecules with endogenous nucleophiles such as glutathione prevent damage to important macromolecules (DNA, RNA, proteins) Regarded as true detoxifying pathway (with few exceptions) In general, appropriate transferase enzymes activate the transferring group ( glucuronate , sulphate , methyl, acetyl) in a coenzyme form April 13, 2013 7 Phase II Reactions In Biotransformation

8. Phase II reactions Characteristics of conjugation reactions Initial activation step Drug is activated or Conjugating reagent activated Capacity limited reaction a. Limited amount of conjugating reagent b. Limited amount of enzyme Order of capacity limited reaction Glucuronidation > Amino acid conjugation > Sulphation and Glutathione conjugation April 13, 2013 8 Phase II Reactions In Biotransformation

9. Phase II reactions Phase II reactions and their characteristics April 13, 2013 9 Phase II Reactions In Biotransformation Conjugation reaction Conjugating Agent Enzyme involved Activated intermediate Functional groups Glucuronidation Glucuronic Acid UDP- glucuronyl transferase UDPGA -OH, -COOH, -NH 2 , -SH Sulphation Sulphate Sulpho transferase PAPS -OH, -NH 2 Amino Acid Glycine Acyl transferase Acyl CoA -COOH, NH 2 Glutathione Glutathione Glutathione-S transferase - Alkyl halide, alkyl nitrate, alkyl epoxide , etc Acetylation Acetyl CoA N-Acetyl Transferase Acetyl CoA Hydrazines , -NH 2 , -SO 2 NH 2 Methylation L- Methionine Methyl Transferase S- adenosyl Methionine -OH, -NH 2 , -SH

10. Glucuronidation is the most common conjugation pathway The coenzyme, UDP glucuronic acid is synthesized from the corresponding phosphate UDP- glucuronic acid contains D- glucuronic acid in the a-configuration at the anomeric center, but glucuronate conjugates are b-glycoside, meaning inversion of stereochemistry is involved in the glucuronidation Glucuronides are highly hydrophilic and water soluble UDP glucuronosyl transferase is closely associated with Cyp450 so that Phase I products of drugs are efficiently conjugated Four general classes of glucuronides : O-, N-, S-, and C- Neonates have undeveloped liver UDP- glucuronosyl transferase activity, and may exhibit metabolic problem. For example, chloramphenicol ( Chloroptic ) leads neonates to “gray baby syndrome” Neonatal jaundice may be attributable to their inability to conjugate bilirubin with glucuronic acid Conjugation with glucuronic acid

11. Conjugation with glucuronic acid Catalysed by microsomal enzymes. Dominates at high substrate concentration Most common phase II reaction. Why? April 13, 2013 11 Phase II Reactions In Biotransformation

12. Conjugation with glucuronic acid Steps in glucuronidation formation Synthesis of activated coenzyme UDPGA Transfer of glucuronyl moiety from UDPGA to substrate RXH April 13, 2013 12 Phase II Reactions In Biotransformation pyrophosphorylase -D-Glucose-1-phosphate + UTP UDPG + PPi UDPG- dehydrogenase UDPG + 2NAD + + H 2 O UDPGA + 2NADH +2H + UDP- glucuronyl transferase UDPGA + RXH RX- glucuronic acid + UDP Where, X = O, COO, NH or S

13. Conjugation with glucuronic acid April 13, 2013 Phase II Reactions In Biotransformation 13

14. Conjugation with glucuronic acid 1. O- glucuronide conjugation a. OH compounds Ex. Alcohol:- Chloramphenicol Phenol:- Morphine, paracetamol b. COOH compounds Ex. Salicylic acid 2. N- glucuronide conjugation Ex. Desimipramine , Meprobamate 3. S- glucuronide conjugation Ex. Thiophenol 4. C- glucuronide conjugation Ex. Phenylbutazone April 13, 2013 14 Phase II Reactions In Biotransformation

15. Conjugation with glucuronic acid O- Glucuronide : Phenols Acetaminophen Morphine April 13, 2013 Phase II Reactions In Biotransformation 15

16. Conjugation with glucuronic acid O- Glucuronide : Alcoholes Chloramphenicol Propranolol April 13, 2013 Phase II Reactions In Biotransformation 16

17. Conjugation with glucuronic acid O- Glucuronide : Enols Hydroxycoumarine April 13, 2013 Phase II Reactions In Biotransformation 17

18. Conjugation with glucuronic acid O- Glucuronide : N- hydroxyamines /amides N- hydroxydapsone N-Hydroxy-2- acetylaminoflourene April 13, 2013 Phase II Reactions In Biotransformation 18

19. Conjugation with glucuronic acid O- Glucuronide : April 13, 2013 Phase II Reactions In Biotransformation 19 Aryl acids Salicylic Acid Arylalkyl Acids Fenoprofen

20. Conjugation with glucuronic acid N- Glucuronide : April 13, 2013 Phase II Reactions In Biotransformation 20 Arylamine 7-Amino-5-nitroindazole Alkylamines Desipramine

21. Conjugation with glucuronic acid N- Glucuronide : April 13, 2013 Phase II Reactions In Biotransformation 21 Sulphonamides Sulfisoxazole Amides Meprobamate

22. Conjugation with glucuronic acid N- Glucuronide : April 13, 2013 Phase II Reactions In Biotransformation 22 Amitryptiline 3 o Amines Cyproheptadine 3 o Amines

23. Conjugation with glucuronic acid N- Glucuronide : April 13, 2013 Phase II Reactions In Biotransformation 23 Cotinine Aromatic Amine

24. Conjugation with glucuronic acid S- Glucuronide : April 13, 2013 Phase II Reactions In Biotransformation 24 Sulfhydryl Methimazole Carbodithioic acid Disulfirum (reduced form)

25. Conjugation with glucuronic acid C- Glucuronide : April 13, 2013 Phase II Reactions In Biotransformation 25 Phenylbutazone

26. Conjugation with glucuronic acid Example: Benzoic acid April 13, 2013 26 Phase II Reactions In Biotransformation Water Soluble, Excreted

27. Conjugation with sulphate moiety Occurs less frequently than does glucuronidation presumably due to fewer number of inorganic sulfates in mammals and fewer number of functional groups (phenols, alcohols, arylamines and N- hydroxy compounds ) Catalysed by nonmicrosomal enzymes Less common Dominates at low substrate concentration April 13, 2013 27 Phase II Reactions In Biotransformation

28. Conjugation with sulphate moiety Steps in Sulphate Conjugation Synthesis of activated coenzyme PAPS Transfer of sulphate moiety from PAPS to substrate RXH April 13, 2013 28 Phase II Reactions In Biotransformation ATP- sulfurylase /Mg ++ ATP + SO 4 2- APS + PPi APS- phosphokinase /Mg ++ APS + ATP PAPS + ADP Sulfotransferase PAPS + RXH RX-SO 3 + PAP Where, X = O, NH

29. Conjugation with sulphate moiety Three enzyme-catalyzed reactions are involved in sulfate conjugation April 13, 2013 29 Phase II Reactions In Biotransformation

30. Conjugation with sulphate moiety Phenolic sulfation predominates Phenolic O- glucuonidation competes favorably with sulfation due to limited sulfate availability Sulfate conjugates can be hydrolyzed back to the parent compound by various sulfatases Sulfoconjugation plays an important role in the hepatotoxicity and carcinogenecity of N- hydroxyarylamides In infants and young children where glucuronyltransferase activity is not well developed, have predominating O-sulfate conjugation Examples include: a-methyldopa, albuterol , terbutaline , acetaminophen, phenacetin , p henol ( salbutamol , paracetamol), amines (aniline) April 13, 2013 30 Phase II Reactions In Biotransformation

31. Conjugation with sulphate moiety Examples April 13, 2013 31 Phase II Reactions In Biotransformation a -Methyldopa Albuterol Terbutaline

32. Conjugation with sulphate moiety Conversion of Minoxidil to Minoxidil Sulphate April 13, 2013 32 Phase II Reactions In Biotransformation Minoxidil Monoxidil Sulphate

33. Conjugation with alpha amino acids The first mammalian drug metabolite isolated, hippuric acid, was the product of glycine conjugation of benzoic acid Amino acid conjugation of a variety of caroxylic acids, such as aromatic, arylacetic , and heterocyclic carboxylic acids leads to amide bond formation Glycine conjugates are the most common Taurine , arginine , asparagine , histidine , lysine, glutamate, aspartate , alanine , and serine conjugates have also been found April 13, 2013 33 Phase II Reactions In Biotransformation

34. Conjugation with alpha amino acids Less common due to limited number of amino acids Steps: 1. Activation of carboxylic acid drug substrate 2. Acetylation of amino acid April 13, 2013 34 Phase II Reactions In Biotransformation Acyl synthetase RCOOH + ATP RCOAMP + H 2 O + PPi Acyl CoA transferase RCOAMP + CoASH RCOSCoA + AMP N- acyl transferase RCOSCoA + H 2 N-R’ -COOH RXH RCONH-R’-COOH + CoASH Where, R’ = -CH 2 If glycine , CH-CH 2 -CH 2 -NH 2 if glutamine

35. Conjugation with alpha amino acids Mechanism of Amino Acid conjugation Examples: 1. Aryl acid: Salicylic acid 2. Heterocyclic aryl acid: Nicotinic acid April 13, 2013 35 Phase II Reactions In Biotransformation An Acyl-CoA Intermediate Glycine Conjugate R = H Glutamine Conjugate R = CH 2 CH 2 CONH 2 Drug-COOH Drug-COOH

36. Conjugation with alpha amino acids Brompheniramine Metabolism April 13, 2013 36 Phase II Reactions In Biotransformation

37. Conjugation with glutathione April 13, 2013 Phase II Reactions In Biotransformation 37 Structure of glutathione Glutathione is a tripeptide ( Glu-Cys-Gly ) – found virtually in all mammalian tissues Its thiol functions as scavenger of harmful electrophilic parent drugs or their metabolites Examples include S N 2 reaction, S N Ar reaction, and Michael addition

38. Conjugation with glutathione Mechanisms of GSH conjugation 1. Nucleophilic substitution RX + GSH R-S-G + H + + x - Ex. Sulphates , Organophosphates 2. Nucleophilic addition R-CH=CH-R’ + GSH RCH-S-G-CH 2-R’ Ex. Paracetamol April 13, 2013 38 Phase II Reactions In Biotransformation

39. Conjugation with glutathione S N 2 : Example April 13, 2013 39 Phase II Reactions In Biotransformation

40. Conjugation with glutathione S N Ar : Example April 13, 2013 40 Phase II Reactions In Biotransformation

41. Conjugation with glutathione Michael Addition: Example April 13, 2013 41 Phase II Reactions In Biotransformation

42. Mercapturic acid Conjugate April 13, 2013 42 Phase II Reactions In Biotransformation

43. Acetylation reaction Metabolism for drugs containing a primary amino group, (aliphatic and aromatic amines), amino acids, sulfonamides, hydrazines , and hydrazides The function of acetylation is to deactivate the drug, although N - acetylprocainamide is as potent as the parent antiarrhythmic drug procainamide ( Procanbid ) or more toxic than the parent drug, e.g., N - acetylisoniazid Examples: Aliphatic amines: Histamine Aromatic amines: PAS, PABA, Dapsone Sulphonamide : Sulphanilamide , Sulphapyridine Hydrazides : Isoniazide April 13, 2013 43 Phase II Reactions In Biotransformation

44. Acetylation reaction Acetylation is two-step, covalent catalytic process involving N-acetyl transferase April 13, 2013 44 Phase II Reactions In Biotransformation

45. Acetylation reaction Acetylation of P amino salicylic acid April 13, 2013 45 Phase II Reactions In Biotransformation

46. Acetylation reaction Acetylation of Procainamide April 13, 2013 46 Phase II Reactions In Biotransformation Procainamide NAPA Urine 85%

47. Methyl conjugation Characteristics: Metabolites formed are not polar Drug and metabolite having equal pharmacological activity Less important for xenobiotics Important in biosynthesis of endogenous amines, ex. Adrenaline, while as inactivation of endogenous amines, ex. NA, 5-HT, Histamine April 13, 2013 47 Phase II Reactions In Biotransformation

48. Methyl conjugation Minor conjugation pathway, important in biosynthesis of epinephrine and melatonin; in the catabolism of norepinephrine , dopamine, serotonin, and histamine; and in modulating the activities of macromolecules (proteins and nucleic acids) Except for the formation of quarternary ammonium salts, methylation of an amine reduces the polarity and hydrophilicity of the substrates A variety of methyl transferase , such as COMT ( catechol O-methyl transferase ), phenol-O- methyltransferase , N-methyl transferase , S- methyltransferase etc are responsible for catalyzing the transfer of methyl group from SAM to RXH April 13, 2013 48 Phase II Reactions In Biotransformation

49. Methyl conjugation Steps in methyl conjugation 1. Synthesis of an activated coenzyme SAM 2. Transfer of methyl group from SAM to substrate April 13, 2013 49 Phase II Reactions In Biotransformation

50. Methyl conjugation Examples 1. O- methylation : Morphine, L-DOPA 2. N- methylation : Normorphine , Nicotine 3. S- methylation : Propylthiouracil , 6-mercaptopurine April 13, 2013 50 Phase II Reactions In Biotransformation

51. Fatty Acid and Cholesterol Conjugation Hydroxyl-containing drugs can undergo conjugation with a wide range of endogenous fatty acids such as saturated acids from C10 to C16 and unsaturated acids such as oleic and linoleic acids Cholesterol ester metabolites have been detected for drugs containing either an ester or a carboxylic acid April 13, 2013 51 Phase II Reactions In Biotransformation

52. Miscellaneous conjugation reactions 1. Conjugation of cyanide 2. Conjugation with ribose Purine / Pyrimidine + Ribose Nucleotides 3. Conjugation with Taurine Bile acid + Taurine Bile April 13, 2013 52 Phase II Reactions In Biotransformation

53. Factors affecting biotransformation of drug 1. Physicochemical properties of drug 2. Chemical factors a. Enzyme Induction b. Enzyme Inhibition c. Environmental factors 3. Biological factors April 13, 2013 53 Phase II Reactions In Biotransformation

54. Factors affecting biotransformation of drug April 13, 2013 54 Phase II Reactions In Biotransformation Factor Reaction type Age (newborns, children, elderly) Decreasing of metabolism speed Pregnancy Increasing of metabolism speed Genetic factor Various reactions Liver pathology Decreasing of excreation speed of drugs, depending on their kinetics, type and stage of liver disease, increasing of bioavailability and decreasing of excretion speed of orally administered drugs with high hepatic clearence GI pathology Changes in metabolism in GI epithelium Nutrition character Increasing of metabolism speed of certain drugs in case of diet with dominance of proteins and carbohydrates Decreasing of metabolism speed in case of heavy digestive disorders linked with starvation (total or protein)

55. Factors affecting biotransformation of drug April 13, 2013 55 Phase II Reactions In Biotransformation Environment Increasing of metabolism speed if in contact with chlorine insecticides Alcohol — one time consumption Depressing of enzymes that metabolise drugs — chronic consumption Induction of enzyme system Smoking Increasing of metabolism of certain drugs (i.e. theophyllin) Way of excretion Metabolism in liver before entering system circulation (first going-through effect) after peroral administration of drugs Time of introduction of drugs Circade changes in drugs metabolism Interaction of drugs Stimulation and depression of enzyme reaction

56. Factors affecting biotransformation of drug Physicochemical properties of drug Molecular size and shape pKa Acidity/ Basicity Liphophilicity Steric and electronic characteristics The chemical structure (the absence or presence of certain functional groups) of the drug determines its metabolic pathways. April 13, 2013 56 Phase II Reactions In Biotransformation

57. Factors affecting biotransformation of drug Physicochemical properties of drug Steric and electronic characteristics April 13, 2013 57 Phase II Reactions In Biotransformation

58. Factors affecting biotransformation of drugs Enzyme induction Increased drug metabolising ability of the enzymes by drug Properties of enzyme inducers 1. Lipophilic 2. Long half life Mechanism of enzyme induction Increased liver size and blood flow Increased total and microsomal protein content Increased stability of enzymes Increased synthesis of cytochrome P450 April 13, 2013 58 Phase II Reactions In Biotransformation

59. Factors affecting biotransformation of drugs Enzyme induction Classification of enzyme inducers 1. Phenobarbital type inducers Ex. Phenytoin , warfarin etc. 2. Polycyclic hydrocarbon type inducers Ex. Cigarette smoking can cause increased metabolism and elimination of theophylline . Autoinduction Ex. Carbamazepine , Meprobamate , Rifampicin April 13, 2013 59 Phase II Reactions In Biotransformation

60. Factors affecting biotransformation of drugs Enzyme induction Consequences of enzyme induction Increased rate of metabolism Decrease in drug plasma concentration Enhanced oral first pass metabolism Reduced bioavailability If metabolite is active or reactive, increased drug effects or toxicity April 13, 2013 60 Phase II Reactions In Biotransformation

61. Factors affecting biotransformation of drugs Enzyme induction April 13, 2013 61 Phase II Reactions In Biotransformation

62. Factors affecting biotransformation of drugs Enzyme inhibition Decrease in the drug metabolising ability Classification of enzyme inhibition Direct inhibition Competitive inhibition Non-Competitive inhibition Product inhibition Indirect Inhibition Repression Altered physiology April 13, 2013 62 Phase II Reactions In Biotransformation

63. Factors affecting biotransformation of drugs Enzyme inhibition Direct inhibition Competitive inhibition Ex. Methacholine inhibit metabolism of Ach for choline esterase Non-Competitive inhibition Ex. INH inhibit metabolism of Phenytoin Product inhibition Ex. Xanthine oxidase inhibitor such as allopurionol Indirect Inhibition Repression Ex. Disulphiram Altered physiology Due to nutritional deficiency or hormonal imbalance April 13, 2013 63 Phase II Reactions In Biotransformation

64. Factors affecting biotransformation of drugs Enzyme inhibition Consequences of Enzyme Inhibition Increase in the plasma concentration of parent drug Reduction in metabolite concentration Exaggerated and prolonged pharmacological effects Increased liklihood of drug-induced toxicity April 13, 2013 64 Phase II Reactions In Biotransformation

65. Factors affecting biotransformation of drugs Enzyme inhibition “Enzyme inhibition is more important clinically than enzyme induction for drugs having narrow therapeutic index.” April 13, 2013 65 Phase II Reactions In Biotransformation

66. Factors affecting biotransformation of drugs Biological Factors Species differences Strain differences Sex differences Age Diet Altered physiological factors Pregnancy Hormonal imbalance Disease states Temporal factor Circadian rhythm April 13, 2013 66 Phase II Reactions In Biotransformation

67. Factors affecting biotransformation of drugs Biological Factors Species differences April 13, 2013 67 Phase II Reactions In Biotransformation

68. Factors affecting biotransformation of drugs Biological Factors Species differences Phenylbutazone half-life is 3 h in rabbit, ~6 h in rat, guinea pig, and dog and 3 days in humans. Strain differences Isoniazid is known to be acetylated by N- acetyltransferase into inactive metabolite. The rate of acetylation in asian people is higher or faster than that in eurpoean or north american people. Fast acetylators are more prone to hepatoxicity than slow acetylator . Isoniazide (fast (Whites) and slow (Eskimos) acetylators ) April 13, 2013 68 Phase II Reactions In Biotransformation

69. Factors affecting biotransformation of drugs Biological Factors Sex differences Responsiveness to certain drugs is different for men and women Males shows greater metabolism than female Hormonal changes during development have a profound effect on drug metabolism Metabolism of Diazepam, caffiene , and paracetamol is faster in females than in males while oxidative metabolism of lidocaine , chordiazepoxide are faster in men than in females April 13, 2013 69 Phase II Reactions In Biotransformation

70. Factors affecting biotransformation of drugs Biological Factors Age Neonates: (0-2 months) Slow biotransformation Drug eliminated as unchanged Caffene - Half life 4 d Chloramphenicol - Gray baby syndrome or cyanosis Sulphonamides - Renal toxicity Paracetamol - Hepatotoxicity Infants: (2 months to one year) Same as neonates with improved enzyme activity. April 13, 2013 70 Phase II Reactions In Biotransformation

71. Factors affecting biotransformation of drugs Biological Factors Age Children : (1-12 years) Fast biotransformation Require large dose of drug Elderly person: Liver size reduced Blood flow decreased Enzyme activity decreased Drug conjugation not affected April 13, 2013 71 Phase II Reactions In Biotransformation

72. Factors affecting biotransformation of drugs Biological Factors Diet Low protein diet decreases drug metabolising ability. High protein diet increases drug metabolising ability. High protein-carbohydrate ratio increases MFO activity. Fat free diet decreases CYP 450 levels. Vitamin deficiency of A,C,E, and B can result in a decrease of oxidative pathway in case of vitamin C deficiency , while vitamin E deficiency decreases dealkylation and hydroxylation. Ca, Mg, Zn deficiencies decreases drug metabolism capacity whereas Fe deficiency increases it. April 13, 2013 72 Phase II Reactions In Biotransformation

73. Factors affecting biotransformation of drugs Biological Factors Diet Essential fatty acid (esp. Linoleic acid) deficiency reduce the metabolism of ethyl morphine and hexobarbital by decreasing certain drug-metabolizing enzymes. Grapefruit juice decrease metabolism of many drugs. Starvation- decreases amount of glucuronide formation. Malnutrition in women- increases metabolism of sex hormone. Alcohol ingestion results in short term decrease followed by increase in enzyme activity. April 13, 2013 73 Phase II Reactions In Biotransformation

74. Factors affecting biotransformation of drugs Altered physiological factors Pregnancy Metabolising ability decreased- high levels of steroid hormons Metabolism of promazine and pethidine reduced. Metabolism of anticonvulsants increased- induction by circulating progesterone. Hormonal imbalance Enzyme induction or inhibition April 13, 2013 74 Phase II Reactions In Biotransformation

75. Factors affecting biotransformation of drugs Altered physiological factors Disease states Liver- Primary site for metabolism, Diseases like hepatic carcinoma, hepatitis, cirrhosis, obstructive jaundice decreases metabolism. Kidney: Glycine conjugation of salicylates , oxidation of vitamin D, hydroysis of procaine impaired in renal disease. CCF and MI: decrease blood flow to liver- decrease metabolism of high extraction ratio drugs- Propranolol , lidocaine Diabetes: decreased glucuronidation April 13, 2013 75 Phase II Reactions In Biotransformation

76. Factors affecting biotransformation of drugs Temporal factor Circadian rhythm Diurnal variations or variations in enzyme activity with light cycle Enzyme activity maximum in early morning. Enzyme activity minimum in late afternoon. Ex: aminopyrine , hexobarbital , imipamine April 13, 2013 76 Phase II Reactions In Biotransformation

77. Factors affecting biotransformation of drugs Genetic Variation wide variability in the response to drugs between individuals consequences of such variation may be therapeutic failure or an adverse drug reaction genetic diversity is the rule rather than the exception with all proteins, including drug metabolizing enzymes allelic variants with different catalytic activities from that of the wild-type form have been identified inheritance leads to subpopulations (genetic polymorphisms) with different drug metabolizing abilities lack of activity reduction in catalytic ability enhanced activity April 13, 2013 77 Phase II Reactions In Biotransformation

78. Factors affecting biotransformation of drugs Genetic Variation frequency of the polymorphism often varies according to the ethnic ancestry of the individual CYP2D6 is extensively studied, the gene for CYP2D6 is highly polymorphic It’s expression leads to 3 phenotypes (phenotype is the expression of genetic make-up) Extensive metabolizers ( EMs ) have functional enzyme activity Intermediate metabolizers ( IMs ) have diminished enzyme activity Poor metabolizers ( PMs ) have little or no activity 5-10% of Caucasians and 1-2% of Asians exhibit the PM phenotype April 13, 2013 78 Phase II Reactions In Biotransformation

79. Factors affecting biotransformation of drugs Genetic Variation Debrisoquine , formerly used in the treatment of hypertension, is metabolized by CYP2D6 to 4-hydroxydebrisoquine Remarkable interindividual variation in pharmacological effect of the drug Urine of volunteers given debrisoquine was examined for presence of 4-hydroxydebrisoquine One subject had a very low conversion of parent drug to metabolite was very sensitive to the antihypertensive effects of debrisoquine April 13, 2013 79 Phase II Reactions In Biotransformation

80. Factors affecting biotransformation of drugs Drug dosing 1- An increase in drug dosage would increase drug concentration and may saturate certain metabolic enzymes. 2- when metabolic pathway becomes saturated, an alternative pathway may be pursued. April 13, 2013 80 Phase II Reactions In Biotransformation

81. Factors affecting biotransformation of drugs Route of administration Orally administered drugs are absorbed from the GIT and transported to the liver before entering the systemic circulation. Thus the drug is subjected to hepatic metabolism (first pass effect) before reaching the site of action. Sublingually and rectally administered drugs take longer time to be metabolized than orally taken drugs. Nitroglycerine is ineffective when taken orally due to hepatic metabolism. IV administration avoid first pass effect because the drug is delivered directly to the blood stream. April 13, 2013 81 Phase II Reactions In Biotransformation

82. References Brahmankar DM , Jaiswal SB. Biopharmaceutics and Pharmacokinetics A Treatise. 2 nd edition. Vallabh Prakashan;New Delhi, 2009 Page No. 139- 169. Gibson G. Gordon, Skett Paul. Introduction to drug Metabolism. 3 rd edition, Nelson Thorne Publishers; Chetenham , 2001 Page No. 1-13, 37- 62. Wilson and Gisvold’s Textbook of Organic Medicinal and Pharmaceutical Chemistry 11 th ed. Lippincott, Williams & Wilkins ed. Foye’s Principles of Medicinal Chemistry April 13, 2013 82 Phase II Reactions In Biotransformation

83. April 13, 2013 83 Phase II Reactions In Biotransformation