Drug Metabolism-Phase-I Reaction (Oxidation, Reduction, Hydrolysis), Phase-II Reaction (glucuronic acid, glycine, glutamine, glutathione, acetic acid), Factors affecting drug metabolism

1. Drug Metabolism
Dr. Gopal Krishna Padhy

2. Introduction
Metabolism refers to series of biochemical
h l h b dchemicals reactions occurring in the body to convert
drug substance to more easily extractable and
notoxic forms.
Metabolism plays an important role in eliminationp y p
of drugs and foreign substance from the body.
Liver is the primary site of metabolism for almostLiver is the primary site of metabolism for almost
every drug.
 S l f CYP450 (CYP) Several isoforms CYP450 (CYP) enzymes are
involved in drug metabolism; they are the most
important group of enzymes in phase I metabolism.

3. Phases of drug metabolismPhases of drug metabolism
Metabolism of drugs are divided into two phases
Phase-I
Phase-II
Phase-I
Phase-I converts a parent drug to a more polar
metabolites by inserting polar functional groups suchmetabolites by inserting polar functional groups such
as –OH, -COOH, -SH, -NH2 etc.
h f h l dThe reactions of phase-I involves oxidation,
reduction and hydrolysis.y y

4. Oxidation
A large number of drug substances or xenobiotics
undergoes metabolism by oxidation. These includesundergoes metabolism by oxidation. These includes
alcohol, aldehydes, olefins, amines, aromatic
compound etccompound etc.
Oxidation of alcohol
Aliphatic and aromatic alcohols undergo oxidation to
form the corresponding acids It is catalysed byform the corresponding acids. It is catalysed by
alcohol dehydrogenase.

5. Oxidation of aldehyde
Ald h d d id ti t f th diAldehydes undergo oxidation to form the corresponding
acids.
CHO COOH
O
Benzaldehyde Bezoic acid
O id ti f l fiOxidation of olefins
Olefins (compound containing double bond) undergo( p g ) g
oxidation to form the corresponding epoxide. The
epoxide my further undergoes enzymatic hydration toepoxide my further undergoes enzymatic hydration to
form trans 1,2-dixydroxides.

6. Example: Carbamazepine undergoes oxidation top p g
form carbamzepine-10,11-epoxide, which on
enzymatic hydration forms trans 10 11 dihydroxyenzymatic hydration forms trans-10,11-dihydroxy
carbamazepine.

7. Oxidation of aromatic ring
Aromatic compound undergoes oxidation to form
epoxide intermediate, which rearrange to form arenol.epoxide intermediate, which rearrange to form arenol.
R R
[O]
R
O OH
Arene Arene oxide ArenolArene Arene oxide Arenol
For example phenobarbital undergoes metabolism by
d f h d h b b loxidation to form hydroxyphenobarbital.
2 5 2 5

8. Oxidation of carbon atom α to the carbonyl group
Carbon atom adjacent to carbonyl function undergoes
oxidation to form hydroxy derivatives.y y .
e. g. Diazepam undergoes oxidation at 3rd position to form 3-
hydroxydiazepam.y y p
3 3

9. Reduction
A large drug substances containing aldehyde, nitro
and azo groups undergo metabolism by reduction.g p g y
Reduction of aldehyde
Aldehydes and ketones undergoes reduction by
enzymes called aldo-keto reductase.y
e.g. Bioreduction of chloral a sedative hypnotic drug
undergoes reduction to form trichloroethanolundergoes reduction to form trichloroethanol.

10. Reduction of nitro compound
Variuos aromatic nitro drugs undergo enzymatic
reduction to the corresponding aromatic aminesreduction to the corresponding aromatic amines.
e.g. Nitrazepam extensively metabolise to its 7-amino
b limetabolite.

11. Hydrolysis
Drugs containing ester and amide functional group
undergoes metabolism by hydrolysis.undergoes metabolism by hydrolysis.
e.g. Aspirin undergoes metabolism by hydrolysis of the
ester functional group to form salicylic acid and aceticester functional group to form salicylic acid and acetic
acid.
33
3

12. Phase-II
In Phase-II the metabolites formed in Phase-I are
converted to more polar and water soluble product byconverted to more polar and water soluble product by
attaching polar and ionisable moiety such as glucuronic
acid sulfate glycine and glutamic acidacid, sulfate, glycine and glutamic acid.
The resulting conjugated products are relatively water
soluble and readily excretable.
Glucuronic acidGlucuronic acid
Conjugation with glucuronic acid is the most commong g
reaction. The active form of glucuronic acid is UDP-
glucuronic acid.glucuronic acid.

13. Phenolic and alcoholic hydroxyls are the mosty y
common functional group undergoing
Glucuronidation in drug metabolism
O
Glucuronidation in drug metabolism.
R OH R O glucuronide
D lik hi t i h hl h i lDrugs like morphine, acetaminophen, chloramphenical
and propranolol undergo metabolism by
glucuronidation.

14. Glycine
Many aromatic carboxylic acids form conjugate with
glycine. benzoic acid conjugates with glycine to formglycine. benzoic acid conjugates with glycine to form
hippuric acid, which is a well known metabolic reaction.
COOH CONHCH COOHCOOH
+ H2N CH2 COOH
CONHCH2 COOH
+ H2O
Benzoic acid Glycine Hippuric acid
COOH CONHCH2 COOH
OH OH
+ H2N CH2 COOH
Benzoic acid Glycine Salicyluric acid
+ H2O
Benzoic acid Glycine Salicyluric acid

15. Glutamine
Glutamine conjugation occurs mainly with phenylacetic
acid to form phenylacetyl glutamineacid to form phenylacetyl glutamine.
O COOH O
H
COOH
CH2 C
O
OH + H2N C
COOH
CH2CH2CONH2
H
CH2 C
H
N C CH2CH2CONH2
H
Glutamine conjugateGlutamic acidPhenyl acetic acid Glutamine conjugateGlutamic acidPhenyl acetic acid

16. Glutathione
A wide range of organic compounds such as alkyl or
aryl halides nitro compounds alkanes get conjugatedaryl halides, nitro compounds, alkanes get conjugated
with cysteine of glutathione.
Acetic acid
A t l C A i th ti f f ti id th t t kAcetyl-CoA is the active form of acetic acid that takes
part in conjugation reaction.
Drugs containing primary amino group such as
sulfonamide, hydrazides and hydrazine undergoesy y g
metabolism by acylation.

17. O O CH3
C NHNH2
O
N-Acetylation
C NHNH
O
C
O
CH3
N N
Isoniazid N-Acetylisoniazid

18. Factors affecting drug metabolism
Age differences
Age related differences in drug metabolism is quite
apparent in the newborn.apparent in the newborn.
In new born babies deficiency of oxidative and
conjugative enzymes are responsible for reducedconjugative enzymes are responsible for reduced
metabolic capability.
e.g. Oxidative (Cytochrome P450 ) metabolism of
tolbutamide appears to be low in newborn.pp
The ability of infants to conjugate chloramphenicol
with glucuronic acid appears to be responsible forwith glucuronic acid appears to be responsible for
gray baby syndrome.

19. Species and strain differences
The metabolism of many drugs and xenobiotics are often
species dependant.p p .
Metabolism of amphetamine occurs by two different pathway
i e Oxidative deamination or aromatic hydroxylationi.e. Oxidative deamination or aromatic hydroxylation.
In human, guinea pig, and rabbit deamination appears to be
the predominate pathwaythe predominate pathway.
However in in rat, aromatic hydroxylation appears to be more
importantimportant.
CH CH
NH2
CH CH CH
NH2
CHHO
Aromatic
Hydroxylation
Oxidative
deaminationO
CH2 CH CH3
CH2 CH CH3HO
CH2 C CH3
Phenylacetone Amphitamine p-Hydroxyamphetamine
RatHuman, Rabbit

20. Hereditary or genetic differences
Marked difference in metabolism of several drugs in
human occurs due to genetic factorshuman occurs due to genetic factors.
Genetic factors influence the rate of oxidation of
h i h lb i liphenytoin, phenylbutazone, nortriptyline etc.
The rate of oxidation of these drugs varies widely
among different individual.

21. Gender differences
The rate of metabolism of drugs also varies due to
gender difference in animals.
Adult male rat metabolize several drugs at muchg
faster rat than female rat.
e g N-demethylation of aminopyrin oxidation ofe.g. N demethylation of aminopyrin, oxidation of
hexobarbital, glucuronidation of o-aminophenol.

22. Enzyme induction
The activity of hepatic enzymes such as cytochrome
P 450 can be increased by exposure to diverse drugsP-450 can be increased by exposure to diverse drugs ,
pesticides etc.
Th b hi h h i i f h dThe process by which the activity of these drug
metabolising enzymes is increased is termed enzyme
induction.
e.g. Phenobarbital can increase the metabolism ofg
cortisol,Vitamin D in human.

23. Ph b bit l i d th l lt fPhenobarbital induces the glucuronyltransferase
enzyme, thereby enhances the conjugation with
glucuronic acid.
Other chemicals such as polycyclic aromaticp y y
hydrocarbons and pesticide may induce certain oxidative
pathway and thereby alter drug responsepathway and thereby alter drug response.

24. Enzyme inhibition
Several drugs and food material can inhibit drug
metabolism. With decreased drug metabolism, a drug
often accumulated leading to prolonged duration ofg p g
action and serious adverse effect.
Enzyme inhibition can occur by diverse mechanismEnzyme inhibition can occur by diverse mechanism
such as inactivation of drug metabolising enzymes,
i t f ith t i th i d h t t i itinterference with protein synthesis and hepatotoxicity
leading to impairment of enzyme activity.

25.  h lb h b h b l fe.g. Phenylbutazone inhibits the metabolism of
warfarin. This can lead to increased hemorrhage ing
patient on both warfarin and phenylbutazone
therapy.therapy.
Similarly the metabolism of phenytoin is inhibited
by chloramphenicol and isoniazidby chloramphenicol and isoniazid.

26. Steriochemical aspects
Many drugs are often administered as racemic
mixture in humans. The two enantiomer present in the
racemic mixture may differ in pharmacological activity.y p g y
The individual enantiomers of racemic mixture also
may be metabolised by different pathwaymay be metabolised by different pathway.
e.g. In dogs the (+) enantiomer of sedative hypnotic
l t thi id i h d l t d i il t α t thglutethimide is hydroxylated primarily at α to the
carbonyl to yield 5-hydroxyglutethimide, whereas the
(-) enantiomer undergoes aliphatic hydroxylation of its
C-2 ethyl group.y g p