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.
3.
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
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Diazepam – Oxazepam.
Amitriptyline – Nortriptyline.
Imipramine – Desimipramine.
Codeine – Morphine
Digitoxin – Digoxin
Procainamide – N-acetyl procainamide
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
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
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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.
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.
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.
24.
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
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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
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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
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Cytosolic N-acetyl transferases (NAT) – most polymorphic of all
human xenobiotic drug metabolizing enzymes.
Eg: Isoniazid – (5-15%), experience toxicities.
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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.
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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 )
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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
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
43. Clinical relavence of enzyme inhibition
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• 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
