biotransformation - Metabolism of drugs

1. By,
ABITH BABURAJ

2. The onset of pharmacological response
depends upon two pharmacokinetics
processes:
. Drug absorption
. Drug distribution
The duration and intensity of action depend
upon:
. Tissue redistribution of drug
. The rate of drug removal from the body

3. Elimination is defined as the irreversible loss of
drug from the body.
Elimination occurs by two processes
biotransformation and excretion.
. Biotransformation of drug is defined as the
chemical conversion of one form to another.

4. Drug biotransformation research, divided the
pathways of drug metabolism reaction into
two general categories-
. Phase I reactions
. Phase II reactions

5. Phase I reactions
These reactions generally precede phase II
reactions and include oxidative, reductive and
hydrolytic reactions.
The primary objective of phase I reactions are
. Increase in hydrophilicity
. Reduction in stability
. Facilitation of conjugation

6. Oxidative reactions
Oxidative reactions are the most important and
most common metabolic reactions.
. Almost all drugs that undergo phase I
biotransformation undergo oxidation at some
stage.
. A simple reason for oxidation being a
predominant reaction is that energy in
animals is primarily derived by oxidative
combustion of organic molecules containing
carbon and hydrogen atoms.

7. 1. Oxidation of aromatic carbon atoms
(aromatic hydroxylation)
This reaction proceeds via formation of a
reactive intermediate arene oxide (epoxide)
which in most cases undergoes
rearrangement to yield arenols. The arene
oxide intermediate is highly reactive and
know to be carcinogenic in some instance.

9. e.g conversion of acetanilide to paracetamol
Monosubstituted benzene derivative can be
hydroxylated at ortho, meta or para positions
but para- hydroxylated product is most
common
2. Oxidation of olefins
Oxidation of nonaromatic carbon- carbon
double bonds in analogous to aromatic
hydroxylation

11. e.g olefinic oxidation is conversion of
carbamazepine to carbamazepine-10,11-
epoxide the latter is coverted to
corresponding trans- 10,11-dihydrodiol.
3. Oxidation of benzylic carbon atoms
Carbon atoms attached directly to the aromatic
rings (benzylic carbon atoms) are
hydroxylated to corresponding carbinols.

14. 4. Oxidation of allylic carbon atoms
Carbon atoms adjacent to olefinic double
bonds also undergo hydroxylation in a
manner similar to benzylic carbons.
5. Oxidation of carbon atoms alpha to carbonyl
and imines
Several benzodiazepines contain a carbon atom
alpha to both carbonyl and imino functions
which readily undergoes hydroxylation

17. 6. Oxidation of aliphatic carbon atoms
Alkyl or aliphatic carbon atoms can be
hydroxylated at two positions at the terminal
methyl group and the penultimate carbon
atom of which the latter accounts for the
major product. Eg valporic acid

20. 8. Oxidation of carbon heteroatom systems
Biotransformation of C-N, C-S systems
proceeds in one of the two way:
. Hydroxylation of carbon atom attached to the
heteroatom and subsequent cleavage at
carbon –heteroatom bond
. Oxidation of the heteroatom itself

21. 9. Oxidation of carbon nitrogen system
a) N-Dealkylation
b) Oxidative deamination
c) N-Oxide formation
d) N-hydroxylation
10. Oxidation of carbon sulphur system
a) S- Dealkylation
b) Desulphuration
c) S-oxidation

22. 10. Oxidation of carbon oxygen systems
a)O- Dealkylation
11. Oxidation of alcohol, carbonyl and
carboxylic acid
12. Miscellaneous oxidative reactions
a) Oxidative aromatisation
b) Oxidative dehalogenation

23. .Bioreductions are also capable of generating
polar functional groups such as hydroxy and
amino which can undergo further
biotransformation or conjugation.
. A number of reductive reactions are exact
opposite of oxidation. For example:
Alcohol dehydrogenation- carbonyl reduction

24. 1. Reduction of carbonyls( aldehydes and
ketones)
Depending on their reactivity towards
bioreduction, carbonyls can be divided into 3
categories
. The aliphatic aldehydes and ketones
. The aromatic aldehydes and ketones
. The esters, acids and amides

26. 2. Reduction of alcohols and carbon- carbon
double bonds
These two reductions are considered together
because the groups are interconvertible by
simple addition or loss of a water molecule.
Before an alcohol is reduced, it is dehydrated
to C=C bond eg: bencyclane

28. 3. Reduction of N- compounds ( nitro, azo and
N-oxide)
The N- containing functional groups that
commonly undergo bioreduction are nitro,
azo and n-oxide.
Eg: reduction of nitrazepam

30. 4. Miscellaneous reductive reactions
. Reductive dehalogenation: This reaction
involves replacement of halogen attached to
the carbon with the H-atom, eg: halothane
. Reduction of sulphur containing functional
group:
An example of S-S reductive cleavage is
disulphiram

31. . The reaction does not involve change in the
state of oxidation of the substrate
. The reaction results in a large chemical
change in the substrate brought about by
loss of relatively large fragments of the
molecule.
. The hydrolytic enzymes that metabolise
xenobiotics are the ones that also act on
endogenous substrates. Moreover their
activity is not confined to liver as they are
found in many other organs like kidney.

32. . Hydrolysis of esters and ethers
. Organic acid ( carboxylic acid) esters
. Inorganic acid esters
. Hydrolysis of amides( C-N bond cleavage)
. Hydrolytic cleavage of non- aromatic
heterocycles
. Hydrolytic dehalogenation
. Miscellaneous hydrolytic reactions

33.  Phase II reactions involve transfer of a suitable
endogenous moiety such as glucuronic acid,
sulphate, glycine etc.
 Phase II reactions are the real drug detoxication
pathway because
 The conjugate/products of phase II reactions are
absolutely free of pharmacological activity
 The conjugates/products of phase II reactions are
highly polar and thus easily excretable either in
bile or urine.

34. 1. Conjugation with glucuronic acid
Also called as glucuronidation, it is most
common and most important phase II
reaction
Glucuronide formation occurs in 2 steps-
. Synthesis of an activated coenzyme uridine-
5-diphospho-α-D-glucuronic acid from
UDP-glucose.

35. . Transfer of the glucuronyl moiety from
UDPGA to the substrate RXH in presence of
enzyme UDP-glucuronyl transferase to form
the conjugate.
. In this step, the α-configuration of
glucoronic acid undergoes inversion and thus
the resulting product is -D- glucuronide.

37. . O-Glucuronides
. N-glucuronides
. S-glucuronides
. C- glucuronides
2. Conjugation with sulphate moieties
Sulphation is similar to glucuronidation but is
catalysed by nonmicrosomal enzymes and
occurs less commonly as the moiety that
transfer sulphate to the subtrate is easily
depleted

38.  Sulphation also occurs in 2 steps
 synthesis of an activated coenzyme 3-
phosphoadenosine-5 phosphosulphate which
acts as a donor of sulphate to the substrate.
a) An intial interaction between the sulphate
and the adenosine triphosphate (ATP) to
yield adenosine-5-phosphosulphate(APS)
b) Activation of APS to PAPS

40.  Transfer of sulphate group from PAPS to the
substrate RXH in presence of enzyme
sulphotransferase and subsequent liberation
of 3-phosphoadenosine-5-phosphate.
3. Conjugation with alpha amino acids
This reaction also occurs to a limited extent
because of limited availability of amino acids

41. The reaction occurs in two steps
. Activation of carboxylic acid drug substrate
with ATP and coenzyme A to form acyl CoA
intermediate. Thus the reaction is a contrast
of glucuronidation and sulphation where the
donar enzyme is activated and not the
substrate
. Acylation of the -amino acid by the acyl CoA
in presence of enzyme N-acyl transferase

42. 4. Conjugation with glutathione and
mercapturic acid formation
Glutathione or GSH is a tripeptide with a
strongly nucleophilic character due to the
presence of a –SH (thiol) group in its structure
. The interaction between the substrate and the
GSH is catalysed by enzyme glutathione-S-
transferase to form S- substituted
glutathione conjugate

44. 5. Acetylation
This reaction is basically an acylation reaction
and thus similar to conjugation with -amino
acids.
. The general sequence of reaction is similar to
that for -amino acid conjugation the
enzyme involved is the nonmicrosomal N-
acetyl transferase

46. 6. Methylation
This reaction differs from general
characteristics of phase II reactions in several
ways:
. The metabolities formed are not polar or
water soluble
. The metabilites, in a number of instances,
have equal or greater pharmacological activity
than the parent drug