This document provides an overview of biotransformation (metabolism) through phases 1 and 2. Phase 1 reactions like oxidation, reduction, hydrolysis, cyclization and decyclization make drugs more polar through functional group changes. Phase 2 then involves conjugating reactions like glucuronidation, acetylation, methylation, sulfation and glutathione conjugation to make drugs more hydrophilic for excretion. The liver is a major site of biotransformation through microsomal and non-microsomal enzymes. First pass metabolism reduces bioavailability of orally administered drugs. Biotransformation is important for drug clearance, detoxification and activation of certain drugs for their actions.

1. BIOTRANSFORMATION
BY DHINESH
I M.PHARM (PHARMACOLOGY)
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2. CONTENTS
1. Biotransformation
2. Microsomal and non-microsomal enzymes
3. Phases of biotransformation
4. Comparison of two phases
5. First pass metabolism
6. Importance of biotransformation
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3. Pharmacokinetics
Drug at site of administration
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Drug in plasma
Drug in organs
Drug metabolite Elimination
Metabolism
Distribution
Absorption

4. Biotransformation (Metabolism)
 It is a process of making a drug/xenobiotics to more polar.
Lipid drugs - more absorbed , less excreted
Polar drugs- less absorbed, more excreted
Hydrophilic drugs are mostly excreted unchanged eg, neostigmine,
pancuronium, streptomycin etc.
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5. Which is responsible for
biotransformation?
“Enzymes”
These are proteins which are made up of amino acids.
Each has its unique function eg, oxidation, hydrolysis etc.
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6. Classification of Enzymes
Enzymes are classified based on the location where it is present
Enzymes mainly present in these three cell organelles
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i. Smooth endoplasmic reticulum
ii. Mitochondria
iii. Cytoplasm

7. Classification of enzymes
Enzymes present in smooth endoplasmic reticulum are called as microsomal
enzymes
Enzymes which are present in mitochondria and cytoplasm are called as Non-
microsomal enzymes
Main organs involved in the metabolism process- liver, kidney, intestinal
mucosa, lungs, plasma (non-microsomal enzymes)
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link

8. Classification of enzymes
Non - microsomal enzymes
Present in cytoplasm, mitochondria,
plasma
Some oxidations and reductions,
many hydrolytic reactions and all
conjugations except glucuronidation
Eg: The flavoprotein oxidases,
esterases, amidases and conjugases
(except glucuronide conjugase)
Microsomal enzymes
Located on smooth ER of liver ,
lungs, kidney, intestinal mucosa
Catalyze most of the oxidations ,
reductions, hydrolysis and
glucuronide conjugation
Eg: monooxygenases,
cytochrome P 450, glucuronyl
transferase.
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9. How metabolism is happening?
Through two chemical pathways
Phase 1 – functional group is exposed into the drug to make it more polar and
also susceptible for the conjugation in next phase
Phase 2 – It involves conjugation of the drug or the phase I metabolite and
make it easy to get excreted in urine or bile. Conjugation reactions have high
energy requirement
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i. Phase 1 (non synthetic pathway)
ii. Phase 2 (synthetic pathway)

10. Metabolism overview
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11. Phase 1 Reactions
1. Oxidation
2. Reduction
3. Hydrolysis
4. Cyclization
5. Decyclization
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12. Phase 1 Reactions
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1. Oxidation
 Oxidations are most important drug metabolising reactions.
 Loss of electron is defined as oxidation.
 Enzymes involved in this oxidation process include – CYP450 enzymes,
flavin monooxygenases and non microsomal enzymes include MAO.
Phase 1 Reactions

13. Phase 1 oxidation process by CYP450
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 Various oxidation process include - hydroxylation; oxygenation at C, N or S atoms; N or O-dealkylation,
oxidative deamination by CYP450 enzymes
 Eg: Barbiturates, phenothiazines, imipramine, ibuprofen, paracetamol etc , are metabolized by CYP450
enzymes
Barbiturate oxidation by CYP450
Pentobarbital

14. Phase 1 oxidation by Flavin
monooxygenases
Clozapine metabolism by flavin monooxygenase
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N
N
N
Cl
H
N CH3
N
+
N
N
Cl
H
N CH3
OH
Flavin monooxygenase

15. Phase 1 oxidation by non-microsomal
enzymes
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Acetaldehyde Acetic acidEthanol
CYTOPLASM
MITOCHONDRIA
ADH- Alcohol dehydrogenase
ALDH- Aldehyde dehydrogenase

16. Phase I reactions - oxidations
Microsomal Oxidation
Aromatic hydroxylation
◦ Phenobarbitone – P-hydroxy phenobarbitone
Aliphatic hydroxylation
◦ Pentobarbitone – Hydroxy pentobarbitone
N – dealkylation
◦ Morphine – normorphine
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17. Phase I reactions
Microsomal Oxidation
O -dealkylation
◦ Codeine - morphine
S -dealkylation
◦ 6-methyl thiopurine - mercaptopurine
S – Oxidation
◦ Chlorpromazine - Chlorpromazine sulfoxide
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18. Phase I reactions
Microsomal Oxidation
N – oxidation
◦Trimethylamine – Trimethylamine N - oxide
Deamination
◦Amphetamine – phenylacetone derivative
Desulfarisation
◦Parathion - paraoxon
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19. Phase I reactions
Non - microsomal Oxidation
Mitochondrial oxidation
◦Adrenaline by MAO to VMA
Cytoplasmic oxidation
◦Alcohol by alcohol dehydrogenase to acetaldehyde
Plasma oxidation
◦Histamine by histaminase to IAA
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20. Phase I
ENZYME PERCENTAGE
CYP3A4 50%
CYP2D6 20%
CYP2C8 10%
OTHERS 20%
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21. Phase 1 Reactions
2. Reduction
This reaction involves gain of electrons (just opposite to oxidation) by
cytochrome P-450 enzymes.
Drugs primarily reduced are chloralhydrate, chloramphenicol, halothane and
warfarin
Eg: Halothene metabolism
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22. Phase I reactions - Reductions
Microsomal reductions
Nitro reduction
◦Chloramphenicol to its arylamine metabolite
Azo reduction
◦Protonsil to sulfanilamide
Keto reduction
◦Cortisone to hydrocortisone
Non – microsomal reductions
Chloral hydrate to trichloro ethanol
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23. Phase 1 Reactions
3. Hydrolysis:
This is defined as breakdown of drug molecule with the help a water molecule.
Esters, amides, polypeptides are hydrolyzed by esterases, amidases and
peptidases respectively.
 In addition, there are epoxide hydrolases which detoxify epoxide metabolites
of some drugs (eg: carbamazepine-epoxide) generated by CYP oxygenases.
Eg: procaine, lidocaine, choline esters, oxytocin etc.
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24. Phase 1 reactions
Hydrolysis example
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25. Phase I reactions - Hydrolysis
Microsomal hydrolysis
Pethidine to pethidinic acid by hepatic esterase
Non microsomal hydrolysis
Esterases, amidases, peptidase, protease,
phosphatase
Procaine – PABA
atropine – Tropic acid
Hydrolysis of beta lactam ring of penicillin
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26. Phase 1 reactions
4. Cyclization
This is formation of ring structure from a straight chain compound, e.g.
proguanil.
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27. Phase 1 Reactions
5. Decyclization:
This is opening up of ring structure of the cyclic drug molecule, Eg phenytoin.
This is generally a minor pathway.
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28. Phase 2 Reactions
1 ) Glucuronide conjugation:
It is most important synthetic reaction carried out by a group of UDP-
glucuronosyl transferases (UGTs)
Compounds with a hydroxyl or carboxylic acid group are easily conjugated with
glucuronic acid.
 Examples are— chloramphenicol, aspirin, paracetamol, lorazepam, morphine,
metronidazole
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29. Phase 2 glucuronide conjugation
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30. Phase 2 – Glucuronide conjugation
 Glucuronidation increases the molecular weight of the drug which favours its
excretion in bile.
Drug glucuronides excreted in bile can be hydrolysed by bacteria in the gut, the
liberated drug is reabsorbed and undergoes the same fate.
This enterohepatic cycling of the drug prolongs its action, e.g. phenolphthalein,
oral contraceptives.
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31. Phase 2 reactions
2) Acetylation
 Compounds having amino or hydrazine residues are conjugated with the help
of acetyl coenzyme-A,
Eg. sulfonamides, isoniazid, PAS, hydralazine, clonazepam, procainamide.
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32. Phase 2 reactions
3) Methylation
The amines and phenols can be methylated; methionine and cysteine acting as
methyl donors,
Eg. adrenaline, histamine, nicotinic acid, methyldopa, captopril,
mecarptopurine
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33. Phase 2 Reactions
4) sulphate conjugation
 The phenolic compounds and steroids are sulfated by sulfotransferases
(SULTs), e.g. chloramphenicol, methyldopa, and sex steroids.
5) Glycine conjugation
 Salicylates and other drugs having carboxylic acid group are conjugated with
glycine, but this is not a major pathway of metabolism.
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34. Phase 2 reactions
6) Glutathione conjugation
Forming a mercapturate is normally a minor pathway.
However, it has an important role in inactivating highly reactive quinone or
epoxide intermediates formed during metabolism of certain drugs, e.g.
paracetamol.
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35. Phase 2 reactions
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36. Phase 2 reactions
7) Ribonucleoside/nucleotide synthesis
 This pathway is important for the activation of many purine and pyrimidine
antimetabolites used in cancer chemotherapy.
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39. First pass / pre-systemic metabolism
Defined as the metabolism of a drug before reaching the systemic circulation
All orally administered drugs are exposed to drug metabolizing enzymes in the
intestinal wall and liver (where they first reach through the portal vein).
Presystemic metabolism can also occur in the skin (transdermally administered
drug) and in lungs (for drug reaching venous blood through any route) but
limitedly
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40. What is the importance of this
biotransformation ?
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41. References
Tripathi, K. (2016). Essentials of medical pharmacology. 6th ed.
new Delhi: Jaypee publisher, pp.23-29.
Ritter, J., Flower, R., Henderson, G., MacEwan, D., Loke, Y. and
Rang, H. (2018). Rang and Dale's pharmacology. 8th ed.
ELSEVIER, pp.133-137.
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