1. BIOTRANSFORMATION OF
DRUGS
Moderator
Dr.B.L Kudagi prof & H.O.D

2. Introduction
• XENOBIOTICS  Foreign substances to the body.
• Xenobiotic sources – environmental pollution,food
additives,agrochemicals,drugs , others
• Large no.of diverse enzymes metabolize xenobiotics been called
xenobiotic metabolizing enzymes.
• Lipophilic chemicals in absence of metabolism would not be
efficiently eliminated , resulting in toxicity.
• Metabolism convert these hydrophobic chemicals into hydrophilic
derivatives that can be easily eliminated – diminishing biological
activity.

4. Biotransformation ( Metabolism )
It is chemical alteration of drugs in the body.
AIM : To convert lipid soluble compounds to
lipid insoluble so that they are not
reabsorbed .
Site : Liver, GIT, Lungs, kidney, plasma,
skin,others

5. Significance of biotransformation:
♦ defensive mechanism
♦ increases polarity of drug molecules
 restricts penetration through cellular
membrane
 reduces distribution
 promotes elimination

6. Biotransformation may lead to
1. Bioinactivation : Phenobarbitone to hydroxy
phenobarbitone
2. Bioactivation
a. Activation of inactive drug ( prodrug )
b. Active metabolite from an active drug
c. Toxic metabolites from relatively non-toxic drugs.

7. Activation of inactive drug (prodrug)
Prodrug : An inactive form of drug which gets metabolized in the
body to an active drug.
Eg : 1. Levodopa – Dopamaine.
2. Enalapril – Enalaprilat.
3. Valacyclovir – Acyclovir
4. Sulfasalazine – Mesalazine(5-ASA)

8. Active metabolite from an active drug
•
•
•
•
•
•
Diazepam – Oxazepam.
Amitriptyline – Nortriptyline.
Imipramine – Desimipramine.
Codeine – Morphine
Digitoxin – Digoxin
Procainamide – N-acetyl procainamide

9. Drug
Paracetamol
Toxic metabolite
N-acetyl -P-
Toxic effect
Hepatoxicity
benzoquinonimine
Isoniazid
Acetylhydrazine
Hepatoxicity
Halothane
Alkylating metabolites
Hepatoxicity
Cyclophosphamide
Acrolein
Sodium nitroprusside
Thiocyanate
Haemoohagic
cytitis
Acute toxic
psychosis

10. * Hofmann elimination : Atracurium undergoes spontaneous
molecular rearrangement in the plasma
( Non enzymatic Biotransformation )
* Most hydrophilic drugs are little biotransformed and are
largely excreted unchanged
Ex: streptomycin , neostigmine

11. Patterns of biotransformation

12. Phases of drug metabolism
Phase I : Nonsynthetic / Functionalization.
Phase II : Synthetic /Conjugation.
Phase I
Enzyme carry out Oxidation, Reduction ,Hydrolytic reactions.
Introduction of functional groups resulting in a modification of
the drug.
To increase the water solubility of the drug usually leads to
inactivation of the active drug.

13. The fate of Phase I(P450) Metabolites
 Inactive
Eg : Many
 Equally active
Eg : Fluoxetine Nor Fluoxetine
 More active
Eg : Losartan Active Metabolite (E -3174)
 Toxic
Eg : Acetominophen  N acetyl- p – benzoquinoneimine
(NABQI)
 Activation of ‘prodrug’
Eg : levo dopa  Dopamine.

14. Phase I
•
•
•
•
•
Oxidation reactions are carried by
– Cytochrome P450
– Flavin containing monooxygenases.
– Epoxide hydrolases (EH)
Reduction reaction
-NADPH cytochrome – C – reductase
-Nitro reductase.
-Azo reductase.
-Keto reductase
Hydrolysis reaction
-Epoxide hydrolase
-carboxylesterases
Cyclization.
Decyclization.
• Enzyme either adds or exposes a functional group – serve as
substrates for the phase II

15. Phase II
• Enzyme form a conjugate of the substrate
facilitate inactivation of toxic metabolites &
elimination of drugs.
• Produce metabolite with improved water
solubility & increased molecular weight.
• Several conjugating enzymes are glutathione S
transferase (GST),UGT,SULT,NAT,MT.
• Require substrate to have O2 , N2 ,S atoms that
serve as acceptor site.

16. Eg : Phenytoin

17. Sites of drug metabolism
• Highest level in tissues of GIT (liver ,SI ,LI).
• Liver – major “metabolite clearing house” both for endogenous
chemicals & xenobiotics.
• Xenobiotic metabolising enzymes located in epithelial cells of GIT
– initial metabolic processing of most oral medication.(1st pass
metabolism)
• Drug poorly metabolized remain longer period of time & has
longer elimination half lives.
• Other organs : tissues of nasal mucosa , lung , blood , CNS,
kidney.

18. Xenobiotic Metabolizing Enzymes

20. Structure of Cytochrome
• Name – CYP reduced form combine with CO giving a product
whose absorption peak is at 450 nm
• CYP are superfamily of enzymes containing molecule of heme that
is non covalently bound to polypeptide chain.
• CYP uses O2, CYP 450, CYP450 reductase, NADPH for oxidation
of substrates.

21. CYP
• CYP Responsible for metabolizing vast majority of therapeutic drugs.
• Existence of 57 functional genes and 58 Pseudo genes in humans.
• Genes grouped based on AA sequence into a large number of families
and sub families.
• Cytochrome P450 Nomenclature,
e.g. for CYP2D6
» CYP = cytochrome P450
» 2 = genetic family
» D = genetic sub-family
» 6 = specific gene
• In Humans 12 CYP’s involved in xenobiotic metabolism fall into
families 1 -3.

22. Cytochrome P450 3A4/5
• CYP3A4 is responsible for metabolism of 50% of all
drugs
• Expressed in liver,intestine,kidney
• Inducers Barbiturates,Carbamazepaine,phenytoin,rifampicin
• Inhibitors
-Erythromcin,clarithromycin,ketoconazole,flucanazole

23. Cytochrome P450 2D6
• This is next most important CYP isoform which
metabolizes nearly 20-30% drugs including
– Antiarrhythmics, β- blockers
– Trcyclic antidepressant,SSRIs
• Inhibition of the enzyme by quinidine results in
failure of conversion of codine to morphine,
analgesic effect of codeine is lost.

25. Flavin containing Monooxygenases (Ziegler’s enzyme)
• Super family of phase I enzymes expressed at high levels in the liver
bound to ER.
• Six families of FMOs, FMO3 – most abundant in the liver.
• Metabolize nicotine, H2 receptor blocker, antipsychotics [clozapine],
antiemetics [itopride].
• Genetic deficiency  Fish odor syndrome due to lack of metabolism
of TMAO [ trimethylamine N oxide ]  TMA.
• Minor contributors to drug metabolism - produce benign metobolites.
• Not involved in drug-drug interactions.
• Eg : Itopride metabolized by FMO3
: Cisapride metabolized by CYP3A4

26. Hydrolytic Enzymes
Epoxide Hydrolase

Two Forms: sEH – Expressed in cytosol
mEH – Membrane of ER

Participates in the deactivation of potentially toxic derivates
generated by CYP eg: Carbamazepine to carbamazepaine 10 and 11
epoxide by a CYP.
* valproic acid inhibit mEH- significant DI with Carbamazepine.
This leads to efforts to devolp new drugs such as gabapentin that
is metabolized by CYPs and not by EHs
*

27. Conjugation Enzymes
• Phase 2 conjugation enzymes are synthetic in nature – result in
formation of metabolite with increase in molecular mass.
• Terminate biological activity of the drug.
• Characteristic feature – dependency on the catalytic reaction for
cofactors such as UDP-GT,PAPS,GST,NAT,SULT.
• All reactions are carried out in cytosol of the cell, exception of
glucuronidation.
• Catalytic rates of phase 2 reaction are significantly faster than rates
of CYP’s.
• So rate of elimination depends on Phase 1.

28. Glucuronidation
• Most important phase 2 reaction catalyzed by UDPGlucuronosyltransferases (UGTs).
• UGT2 – Greater specificity for glucuronidation of steroids.
• UGT1A1 – Glucuronidation of bilirubin.
Crigler Najjars syndrome type 1 & type 2
•
Most common genetic polymorphism – Gilberts syndrome (10%)
(mutation in UGT1A1 gene).
• Drug toxic are simvastatin,atorvastatin,ibuprofen,ketoprofen

29. Sulfation
• SULT located in cytosol metabolise various substrates.
• 11 SULT isoforms identified – role in human homeostasis.
SULT1B1  catalysis of cholesterol .
SULT1A3  catecholamine
SULT1E1  estrogens
SULT2A1  DHEA
SULT1 Sulfation of phenolic molecules.
• SULT1A1  Most abundant in human tissue.

30. N-acetylation
•
Cytosolic N-acetyl transferases (NAT) – most polymorphic of all
human xenobiotic drug metabolizing enzymes.
Eg: Isoniazid – (5-15%), experience toxicities.
•
Two functional NAT gene in human, slow acetylation pattern
attributed mostly to polymorphism in NAT 2 gene.
Slow phenotype predisposed to toxicity
Hydralazine (NAT2) at therapeutic doses  extreme
hypotension and tachycardia in slow acetylators.
Sulfonamides  slow acetylators are predisposed to drug
induced hypersensitivity reactions.
•
•
•

31. Methylation
• Xenobiotics undergo O-, N-,S- methylation.
• N-methyltransferase are COMT, POMT, TPMT.
• TPMT – catalysis the S- methylation of aromatic and cyclic
sulfhydryl compounds.
• Genetic deficiency of TPMT – severe toxicities of thio purine
drugs.(Azathioprine , 6-mercaptopurine )

34. First pass metabolism (FPM)
Metabolic degradation before the drug reaches
systemic circulation.
Gut lumen : Benzyl Pn. by acid ,
Insulin by proteolytic enzyme
Liver
: Main site of metabolism
Skin
: Transdermally administered drugs
Lungs
: Drugs reaching venous blood through
any route.

35. Drugs with high FPM
a. Oral dose
b. Individual variation
c. In liver disease , Bioavailability
d. 2 drugs competing with each other’s FPM.
Ex :- CPZ & propranolol

37. Factors affecting (modifying) Drug metabolism.
1. Age.
2. Sex.
3. Species.
4. Nutritional factors.
5. Diseases.
6.Genetic variation
7.Drug-Drug interaction

39. Diseases :Pathological liver condition..
Hypothyroidism  reduces the rate of metabolism.
Hyperthyroidism  enhances the rate of metabolism.
Genetic factors :•Genetic variation in the rate of drug metabolism
 Fast / slow acetylators. Eg : Isoniazid.
 Atypical pseudocholinesterase. Eg : Succinylcholine.
 Faulty expression of CYP2D6 Eg: codeine

40. INDUCERS
ENZYMES
drug affected
[increased metabolism]
Phenobarbitone, phenytoin,
Carbamazepaine,
glucocarticoids
CYP3A4
Rifampicin, phenobarbitone
CYP3A4&2C9
Oral Contraceptives,
Warfarin
Smoking, omeprazole
CYP1A2
Warfarin,Theophylline
Chronic ethanol intake, Isoniazed
CYP2E1
Ethanol itself
,General anaesthetics
Barbiturates,CCB,
Theophylline

41. Clinical relavance of enzyme induction
•
•
•
•
OC pills – rifampicin or phenytoin
Warfarin – Barbiturates
Barbiturates – own metabolism
Ethanol drinkers – paracetamol ( N acetyl p
benzoquinoneimine )
• Neonatal jaundice – Phenobarbitone
• Cushings syndrome - phenytoin

42. INHIBITORS
ENZYME INHIBITED
Drugs affected
[decreased
metabolism]
Cimetidine
Hepatic microsomal mixed
function oxidase
Phenytoin, Warfarin, Anti
Depressants, Theophyline,
Diazepam, Quinidine,
Testosterone
Sodium valpaorate
Hepatic microsomal mixed
function oxidase
Phenytoin, phenobarbital,
primidon
Erythromycin
Hepatic microsomal mixed
function oxidase
Theophyline, Warfarin,
carbamazepine, cyclosporin
Disulfiran,tolbutamide,
metranidazole
Aldehyde dehydrogenase
Alcohol,Warfarin
Carbidopa
L-aromatic aminoacid
decorboxylase
L-dopa

43. Clinical relavence of enzyme inhibition
•
•
•
•
•
• Theophylline - Erythromycin or chloramphenicol
Phenytoin – chloramphenicol or dicumarol
Terfenadine – Ketaconazole or Chloramphenicol
L-dopa – Carbidopa
Alcohol – disulfiran or Metranidazole
d-tubocurarine - Neostigmine

44. Role in Drug development process
 Successful drug development - Efficacy & Safety.
 Compound subjected to analysis by human liver cells or extracts , to
predict the rate of metabolism.
 If a single CYP metabolizes , then decision can be made about drug
interaction especially for elderly people.
 Ideal drug candidate – metabolized by several CYP’s so that variability
in expression levels of one CYP not significantly impact its metabolism
and Pk

45. References
• Pharmacological basis of Therapeutics – Goodman & Gilman
12th Edition .
• Principles of pharmacology – HL Sharma & KK sharma 2nd
edition .
• Pharmacology – Rang & Dale 6th Edition.
• Text book of pharmacology – K. D. Tripathi.7th Edition.
• Basics & clinical pharmacology – Katzung 11th edition

46. Thank u