Biotransformation

1. Biotransformation Siva Nageswararao Mekala Assistant Professor Dept. of clinical Pharmacology

2. Biotransformation It is the chemical transformation of the drug within the living organism

3. What happens to metabolized drug ? Lipid soluble and Unionized compounds Converted Water soluble or more ionized compounds reducing renal tubular reabsorption facilitating their excretion • Drug metabolism does not always result in detoxification and inactivation of drugs

4. Foreign chemicals or xenobiotics (substances foreign to the body) • Environmental contaminants as well as in our diets. • Plants (phytoallexins) eg, poisonous mushrooms • Drugs are considered xenobiotics Most are extensively metabolized in humans.

5. Humans exposed Environmental pollution, food additives, cosmetic products, agrochemicals, processed foods, and drugs. ( lipophilic chemicals) If there is absence of metabolism accumulate in the body, Resulting in toxicity

6. Biotransformation reactions may have four different consequences with respect to pharmacologic activity: • ACTIVATION: An inactive precursor is converted into pharmacologically active drug • L-dopa (inactive) to Dopamine(active) • prednisone(inactive) to prednisolone(active) Such inactive drugs are known as PRODRUGS

7. Prodrugs have following advantages: • Active form is more stable • Better bioavailability • Desired pharmacokinetics • Less side effects and toxicity e.g., Enalapril – Enalaprilat Bacampicillin – Ampicillin Acyclovir- Acyclovir triphosphate

8. ii. MAINTENANCE OF ACTIVITY: An active substance is converted into active metabolite e.g: Diazepam to Oxazepam Codeine to Morphine • INACTIVATION: An active drug is converted to inactive products e.g: Pentobarbital to Hydroxypentobarbital iv. ACTIVE DRUG TO HIGHLY TOXIC METABOLITE : eg: Paracetamole converts N-acetyl-p-benzoquinoneimine

9. Sites • Metabolizing enzymes are found in most tissues in the body with the highest levels located in the tissues of the gastrointestinal tract [liver, small and large intestines] • For orally administered drug---- LIVER Lower gut • In addition, drugs may be metabolized by gastric acid (eg, penicillin) • digestive enzymes (eg, polypeptides such as insulin) • enzymes in the wall of the intestine (eg, sympathomimetic catecholamines

10. Other organs that contain significant xenobiotic- metabolizing enzymes include the tissues of the nasal mucosa and lung

11. Biotransformation reactions can be assigned to two major categories: • PHASE - I (NON-SYNTHETIC REACTIONS): Convertion of the parent drug to a more polar metabolite by introducing or unmasking a functional group (–OH, –NH2, –SH). • PHASE - II(SYNTHETIC REACTIONS): Enzymes form conjugate of the substrate

12. NOTE: Drugs originally containing reactive groups NH2 , OH or COOH - can undergo direct phase 2 reactions

13. PHASE I In this enzymes carry out • Oxidation: Addition of Oxygen and/or removal of Hydrogen . It is the most important and common metabolic reaction. • Reduction: Removal of oxygen or addition of hydrogen • Hydrolytic reactions: Breakdown of the compound by addition of water • Cyclization: This is formation of ring structure from a straight chain compound • Decyclization : This is opening up of ring structure of the cyclic drug molecule.

14. OXIDATIVE REACTIONS: Eg: Phenytoin ,Phenobarbitone ,Propranolol, pentobarbitone • REDUCTION REACTIONS: Eg: Chloramphenicol , methadone , Halothane ,Warfarin • HYDROLYSIS REACTION: Eg: Esters: Procaine , Succinylcholine Amides: lignocaine ,Procainamide • CYCLIZATION: Proguanil • DECYCLIZATION: Barbiturates ,phynytoin

15. Classes of enzymes: • Cytochrome P450s (P450 or CYP), ii Flavin-containing monooxygenases (FMO) iii Epoxide hydrolases - Hydrolysis of epoxides (mES, sES)

16. CYTOCHROME P(CYP) • The CYPs are a superfamily of enzymes, all of which contain a molecule of heme that is noncovalently bound to the polypeptide chain • CYPs use O2, plus H+ derived from the NADPH to carry out the oxidation of substrates

17. CYPs metabolism of dietary and xenobiotic synthesis of endogenous compounds such as steroids, estrogens CYPs (> 50) metabolism of bile acids Specificity

18. CYP 19 Testosterone Aromatase (member of the cytochrome P450 superfamily) Estrogen(estradiol)

19. subfamily A CYP3A4 gene number 4 CYP 3A4 family 3

20. In humans the 12 CYPs that are important are: CYP1A1, 1A2, 1B1, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, 3A4, and 3A5 CYP1A2, 15% CYP2A6, 4% CYP2C9, 20% CYP2D6,5% CYP2E1, 10% CYP3A4 30%

21. PHASE II In Phase II consists of Conjugation reaction. Phase 2 "transferases“ • Sulfotransferases Addition of sulfate (SULT) • UDP-glucuronosyl Addition of glucuronic acid transferases (UGT) • Glutathione-S- Addition of glutathione transferases (GST) • N-acetyltransferases Addition of acetyl group (NAT) • Methyltransferases Addition of methyl group (MT)

22. PHASE II REACTIONS • Parent drugs or their phase I metabolites undergo coupling or conjugation reactions with an endogenous substance to yield drug conjugate • Conjugates are polar molecules that are readily excreted and often inactive.

23. Type of Endogenous Examples Conjugation Reactant • GlucuronidationUDP glucuronic acid morphine (Hydroxyl or carboxyl acid group) diazepam • Acetylation Acetyl-CoA isoniazid, dapsone (Amino or hydrazine residues) • Glutathione Glutathione Acetaminophen Conjugation (GSH), (Quinone or epoxide reactive intermediates)

24. Type of Endogenous Examples Conjugation Reactant • Glycine GlycineAcyl-CoA Salicylic acid Conjugation nicotinic acid (Having carboxylic acid group)-minor pathway • SulfationPhosphoadenosylEstrone (Phenolic or steroids) phosphosulfate methyldopa • Methylation S-Adenosyl Dopamine methionine, epinephrine, histamine (Amines and phenols) • Water WaterBenzopyrene Conjugation Leukotriene A4

25. (highly lipophilic) CYP (PHASE I) Phenytoin 4-hydroxyphenytoin (Slightly water soluble) UGT+UDP-GA (PHASE II) 4-hydroxyphenytoin beta-d-glucuronide (highly water soluble)

27. Drug metabolizing enzymes Microsomal Non microsomal Cytoplasm and mitochondria Located in smooth ER Catalyzes all conjugation reactions except glucuronidation Catalyzes phaseI and Phase II glucuronidation INDUCIBLE BY DRUGS NON- INDUCIBLE BY DRUGS

28. Hofmann Elimination • Inactivation of a drug in the body fluids by spontaneous molecular rearrangement without the agency of enzyme (Non enzymatic degradation in the plasma) E.g., ATRACURIUM - cleavage of the N-alkyl portion in the benzylisoquinoline

29. ENZYME INDUCTION Repeated administration of certain drugs increases the synthesis of microsomal enzymes -Chemically induce P450 enhancing the rate of its synthesis reducing its rate of degradation

30. Induction involves microsomal enzymes in liver and other organs • It increases rate of metabolism by 2-4 fold

31. Various substrates inducing P450 are • Drugs: inducing CYP3A4 are Barbiturates, carbamazepine, phenytoin, rifampin • Environmental pollutants: induce CYP1A1/2 enzymes- activation of procarcinogens Eg: tobacco smoke, charcoal-broiled meat, polycyclic hydrocarbons

32. Clinical relevance of enzyme induction • Drug drug interactions: • Oral contraceptive and rifampicin : Rifampicin induces the drug metabolizing enzyme of oral contraceptives, thus enhancing the metabolism and leading to contraceptive failure. • Phenytoin accelerates metabolism of vit.D3 and lead to osteomalacia

33. Therapeutic benefits: To treat neonatal jaundice - phenobarbitone can be given to infant soon after birth to induce hepatic glucuronyltransferase (Bilirubin is conjugated and excreted)

34. - In Cushing’s syndrome (Abnormally high levels of cortisol) phenytoin may reduce the manifestations by enhancing degradation of steroids • Chronic poisoning : faster metabolism of accumulated poisonous substance • Vegetables like cabbage ,spinach etc can induce microsomal enzymes and promotes the rapid elimination of the drugs

35. ENZYME INHIBITION • Certain drug substrates inhibit cytochrome P450 enzyme activity cimetidine and ketoconazole bind tightly to the P450 heme iron reduce the metabolism of endogenous substances competitive inhibition.

36. Factors affecting drug biotransformation • AGE: • Increased susceptibility to the pharmacologic or toxic activity of drugs is seen in young and very old patients compared with young adults .In neonates mainly due to diminished activity of hepatic microsomal enzymes. • SEX: • dependent differences in drug metabolism exist in humans for ethanol, propranolol, some benzodiazepines, estrogens, and salicylates. • N-Demethylation of erythromycin F>M. • Propranolol oxidation M>F.This is most probably due to hormonal differences

37. DIET AND ENVIRONMENTAL FACTORS • Charcoal-broiled foods and cruciferous vegetables induce CYP1A enzymes • Grapefruit juice inhibits the CYP3A metabolism of coadministered drug substrates • Cigarette smokers metabolize some drugs more rapidly than nonsmokers because of enzyme induction

38. DRUG DRUG INTERACTIONS DURING METABOLISM: • Enzyme-inducing drugs like sedative-hypnotics, antipsychotics, anticonvulsants, the antitubercular drug rifampin, and insecticides may increase the requirement dose of drugs like warfarin

39. DISEASES AND METABOLISM • Disease like alcoholic hepatitis, cirrhosis impair hepatic drug metabolising enzymes E.g. half-life of diazepam in patients with liver abnormality are greatly increased, with a corresponding prolongation of their effects

40. GENETIC VARIATION (Pharmacogenetics) • Genetic factors that affect enzyme level leads to difference in metabolism

41. Defect Enzyme Drug Clinical Involved Consequences • N-Acetylation N-acetyl Isoniazid Peripheral transferase neuropathy • Ester Plasma Sch Prolonged hydrolysis cholinesterase apnea • G6PD Deficiency –Hemolysis when exposed to certain drugs like Sulphonamides , Salicylates

42. THANK YOU…!