INTRODUCTION TO BIOTRANSFORMATION OF DRUG (METABOLISM OF PHENYTOIN AND CODEINE)
BIOTRANSFORMATION OF DRUG
(EG METABOLISM OF PHENYTOIN AND
ADAM SHAHUL HAMEED
M.TECH COMPUTATIONAL BIOLOGY
Biotransformation means chemical alteration of the drug in the
Biotransformation is a major mechanism for drug elimination.
Enzymatic processes in liver and other tissues that
modify the chemical structure of drugs render them more
water-soluble, increase their elimination, decrease their
Biotransformed metabolites are chemically different from the
Metabolism makes the compound less lipid soluble and more
polar and thus hydrophilic.
TYPES OF METABOLIC TRANSFORMATION
There are two phases in the metabolism of drugs:-
Phase 1 reaction. (Non synthetic phase).
This involves a change in drug molecule. It involves oxidation,
reduction or hydrolysis. This may result in activation, change
or inactivation of drug.
Phase II reaction. (Synthetic phase)
It involves formation of conjugates with drug or its metabolites
formed in phase 1 reaction. The conjugate is formed with an
endogenous substance such as carbohydrates and amino
Loss of electrons M = M+ + e-
Gain of oxygen R + O = RO
Hydroxylation; Oxygenation at C,N and S atoms;N or O
dealkylation, oxidative deamination
CYTOCHROME P-450 MONOOXYGENASE
(MIXED FUNCTION OXIDASE)
A large number of families (at least 18 in mammals) of
cytochrome P-450 (abbreviated “CYP”) enzymes exists
Each member of which catalyzes the biotransformation of a
unique spectrum of drugs. some overlap in the substrate
This enzyme system is the one most frequently involved in
phase I reactions.
CYTOCHROME P-450 MONOOXYGENASE
(MIXED FUNCTION OXIDASE)
Terminal oxidase of the mixed-function oxidase (MFO)
Located in the smooth endoplasmic reticulum of all
major organs and tissues
Uses NADPH as a source of reducing equivalents
THE P450 GENE SUPERFAMILY
Format of nomenclature:
The cytochrome P-450 families are referred to using an
arabic numeral, e.g., CYP1, CYP2, etc.
Each family has a number of subfamilies denoted by an
upper case letter, e.g., CYP2A, CYP2B, etc.
The individual enzymes within each subfamily are denoted
by another arabic numeral, e.g., CYP3A1, CYP3A2, etc.
The primary location of cytochrome P-450 is the liver,
Liver is the major organ of biotransformation
Kidney, Lungs, testes, skin intestines are the secondary organ of
Other tissues, including:
ovaries and testis
tissues involved in steroidogenesis and steroid metabolism.
The enzyme's subcellular location is the endoplasmic reticulum.
Mechanism of reaction
In the overall reaction:
the drug is oxidized
oxygen is reduced to water.
Reducing equivalents are provided by nicotinamide
adenine dinucleotide phosphate (NADPH), and
generation of this cofactor is coupled to cytochrome
Most of the reactions of cytochromeP450 involve
the addition of a hydroxyl group to a compound
which may be represented as
Gain of electrons M+ + e- = M
Loss of oxygen RO = R + O
Gain of hydrogen R + H = RH
Nitro to amino group
NO2 NO HNOH HNH
Chromium VI to Chromium III
Cr6+ + 3 e- Cr3+
CYCLIZATION AND DECYCLIZATION
This is formation of ring structure from a straight
chain compound, e.g. proguanil.
This is opening up of ring structure of the cyclic
drug molecule, e.g. barbiturates, phenytoin.
SYNTHETIC REACTIONS (PHASE 2 METABOLISM)
These involve conjugation of the drug or its phase I
metabolite with an endogenous substrate, generally derived
from carbohydrate or amino acid, to form a polar highly
ionized organic acid, which is easily excreted in urine or bile
Conjugation reactions have high energy requirement
For conjugation to take place, a compound should have an
appropriate group or centre eg – COOH, -OH, -NH, or –SH.
Conjugated metabolites are in variably less lipid soluble than their
This is the 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 which is derived from glucose.
Examples are chloramphenicol, aspum, paracetamol, lorazepam,
Not only drugs but endogenous substrates like bilirubin, steroidal
hormones and thyroxine utilize this pathway.
Compounds having amino or hydrazine residues are
conjugated with the help of acetyl coenzyme-A, e.g.
sulfonamides, isoniazid, PAS, hydralazine, clonazepam,
The amines and phenols can be methylated;
methionine and cysteine acting as methyl donors, e.g.
adrenaline, histamine, nicotinic acid, methyldopa,
SULFATE AND GLYCINE CONJUGATION
The phenolic compounds and steroids are
sulfated by sulfotransferases (SULTs).
e.g. chloramphenicol, methyldopa, adrenal and sex
Salicylates and other drugs having
carboxylic acid group are conjugated with glycine, but this
is not a major pathway of metabolism.
Forming a mercapturate is normally a minor pathway. However, it
serves to inactivate highly reactive quinone or epoxide intermediates
formed during metabolism of certain drugs, e.g. paracetamol.
When large amount of such intermediates are formed (in poisoning
or after enzyme induction), glutathione supply falls short-toxic
adducts are formed with tissue constituents to tissue damage.
This pathway is important for the activation of many purine and
pyrimidine antimetabolites used in cancer chemotherapy.
These are located on smooth endoplasmic reticulum
(a system or microtubules inside the cell), primarily in liver,
also in kidney, intestinal mucosa and lungs.
The monooxygenases, cytochrome P 450, glucuronyl
transferase, etc. are microsomal enzymes.
They catalyse most of the oxidations, reductions, hydrolysis
and glucuronide conjugation
Microsomal enzymes are inducible by drugs, diet and other
These are present in the cytoplasm and mitochondria of
hepatic cells as well as in other tissues including plasma
The flavoprotein oxidases, esterases, amidases and
conjugases are nonmicrosomal enzymes.
Reactions catalysed are:
Some oxidations and reductions, many hydrolytic
reactions and all conjugations except-glucuronidation.
This refers to inactivation of the drug in the body fluids
by spontaneous molecular rearrangement without the
agency of any enzyme, e.g. atracurium.
RESULTS OF BIOTRANSFORMATION
Production of metabolites that are more polar than the
Usually terminates the pharmacologic action of the
After phase I reactions, similar or different
pharmacologic activity, or toxicologic activity.
METABOLISM OF DRUG LEADS TO
Inactivation of drug: The active drug is converted in to inactive
metabolites & excreted. Ex-lidocaine, ibuprofen. Also conversion of
phenytoin to p-hydroxy phenytoin.
Active metabolites from equally active metabolites: The drug is
converted in to similar active metabolites. Ex- conversion of codeine in to
morphine, having similar activity.
Active metabolites from inactive drug (prodrug): Some drugs given in
the form of inactive form which are made active by metabolism.
Ex- enalapril which activated in the form of enalaprilat, which is prodrug.
METABOLISM OF DRUG LEADS TO
Conversion in to toxic substances: Xenobiotics metabolizing
enzymes are responsible for elimination of drug but may convert it in to
This occurs when the enzyme convert the drug in to unstable
intermediate which has affinity towards cellular component & causing
They have carcinogenic activity, when the electron deficient atom is
formed which reacted with DNA & RNA of the cell and causing
mutation of gene. Other ex- conversion of paracetamol to toxic
metabolite causing hepatic toxicity.
It is a Chronic medical condition produced by sudden
changes in the electrical function of the brain.
Pharmacokinetics of phenytoin
Well absorbed when given orally, however, it is also available as iv. (for
80-90% protein bound
Induces liver enzymes (Very Important)
Metabolized by the liver to inactive metabolite
Metabolism shows saturation kinetics and hence t ½ increases as the dose
Excreted in urine as glucuronide conjugate
Plasma t ½ approx. 20 hours
Therapeutic plasma concentration 10-20 μg/ml (narrow)
Dose 300-400 mg/day
Codeine or 3methylmorphine (a naturally occurring
methylated morphine) is an opiate used for its
analgesic, antidiarrheal, antihypertensive, anxiolytic,
antidepressant, sedative and hypnotic properties.
Codeine is used to treat mild to moderate pain and to
1.Essential of medical Pharmacology by K.D Tripathi and
Jaypee page no 23 to 30
2.Biochemistry by U.Sathya narayana and U.Chakrapani.
3.Modern Pharmacology with Clinical applications by
Lipinncott (Sixth Edition )
4.Pharmacy Tutor www.pharmatutor.com
5.Wikipedia Biotransformation of Drug.
My Sincere Thanks to SHRIKANTH (Comp Bio )