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Drug Metabolism
By Dr. fatmah alomary
[email protected]
Drug Metabolism
Drug metabolism is the transformation of foreign
compounds ( xenobiotics) into a water soluble
derivatives which can be easily eliminated in
the urine.
Example
In General , the metabolism of xenobiotics
takes place in two steps known as
phase I &
phase II reactions
Phase I ( functionalization
reaction )
Is the process of increasing of the hydrophilicity of
lipophilic drug by introducing a polar functional
group eg; OH,COOH,NH2,SH to the molecule
through oxidative, reductive & hydrolytic
biotransformations.
Phase II ( conjugation reactions )



Is Linking of an endogenous solubilizing moiety
either to the original drug (if polar function is
already present) or to the phase I metabolite.
Common solubilizing groups are glucuronic acid,
various amino acids or sulphate groups.
The conjugate molecule, being more polar and
water-soluble, is usually excreted via the renal
route
Effect of metabolism on the
therapeutic activity of drugs
Factors affecting drug
metabolism





Genetic factors
Physiological factors
Pharmaceutical factors
Pharmacodynamic factors.
Enviromental factors.
Genetic factors
Biological half –life (t1/2) of various drug
Genetic Polymorphism:
Different expression of metabolizing enzymes according to the Race (ethnicity)
Physiological factors
Age ,Gender,maternity status,liver function & Nutritional status.
eg: Age which is the ability of the body to metabolize the drug
lower in v. young & elderly.
Pharmacodynamic factors
The dose, the route and the frequency of
administration of drugs & Drug interaction can affect
their metabolic profiles.
 Drugs given too frequently may overload the
metabolic system available to it, leading to elevated
blood and tissue levels of the drugs. The effect of
protein binding also influences the metabolism.
 Drug interactions for example:Phenobarbital stimulate the metabolism of
Diphenylhydantoin.
Plasma Concentration of anticoagulants such as Warfarin
are reduced by simultaneous application of barbiturate

Enviromental Factors
Inhaled gases,toxins eg:Nicotine (cigarette – 8 to 10 mg )
-Acute nicotine exposure

(From – insecticide sprays or tobacco)

Nausea, vomiting, salivation, diarrhea, dizziness, mental
confusion, weakness
-Fatal exposure (60 mg fatal for adult) 
Decreased blood pressure, irregular pulse, convulsions,
respiratory failure and death
-Cotinine - Major metabolite

-Lung – First site of metabolism
-Liver – Major site

-Half-life – about 2 hours





Phase I (Functionalization (
reactions

Oxidations (electron removal, dehydrogenation and hydroxylation)

Reduction ( electron donation, hydrogenation
and removal of oxygen )

Hydrolytic reactions of amides & esters.
-Two general types of enzyme systems take part in these
reactions:
-a) Microsomal Mixed Function Oxidases (MFOs)
Flavoprotein, NADPH-monooxygenase
Cytochrome P450
-b) Non-cytochrome oxidizing enzymes.
Xanthine oxidase
Alcohol/aldehyde dehydrogenase
I) Oxidation Reactions
The main enzymes involved in the oxidation of xenobiotics called
mixed – function oxidases (MFO) or monooxygenases, found
mainly in the liver but also occur to less extent in other tissues.
Cytochrom P450 ( CYP450 ) catalyze the majority of Drug
metabolism oxidation reactions.


MFO is an old terminology,the enzyme are most frequently known
as CYP450 Superfamily
-The enzyme systems carrying out this biotransformation are referred to as
monooxygenases or microsomal (non specific enzymes in liver).
-The reaction requires both molecular oxygen and the reducing
agent (Activation of O2  1 atom goes to organic molecule, the other
reduced to H2O0.
-NADPH (nicotinamide adenosine dinucleotide phosphate).
-Monooxygenases are made up of several
components:1) Cytochrome P-450 which is the most important component and is
responsible for transferring an oxygen atom to the substrate R-H.
2) Cofactors supply the reducing equivalents
(electrons) needed in the overall metabolic oxidation
a) NADPH. Dependent cytochrome P-450
reductase.
b) NADH. Linked cytochrome P-450.
*Cytochrome P-450 is found in high concentration in the
liver, also present in other tissues like lung, kidney, intestine,
skin, placenta and adrenal cortex.
C) FMO is also a member of the mono-oxygenase system
*It is characterized by the substrate nonspecificity, this
versatility may be attributed to the multiple forms of the
enzyme.
- Consequently, the biotransformation of a parent xenobiotic
to several oxidized metabolites is carried out not just by one
form of P-450 by several different forms.
-It is now actually proven that the metabolism of drug is
carried out by different isoforms,members of the CYP450
superfamily,eg:CYP2A1,CYP2D6,CYP3A4…etc
CLASSIFICATION
-A large number of families (at least 18 in mammals) of
cytochrome P-450 (abbreviated “CYP”) enzymes exists
as well as many subfamilies.
each member catalyzes the biotransformation of a unique
group of drugs
-CYP450 SUPERFAMILY: classified according to sequence
homology.
-High homology: > 90%, intermediate: ~ > 60%; Low: ~ >
40%
-FAMILY: members have > 40% homology (low). E.g.:
CYP1 vs. CYP2
-SUBFAMILY: members have > 60% homology
(intermediate). E.g.: CYP2A vs. CYP2B
ISOFORM: CYP2A1, CYP2A2. (High)
Major reactions of oxygenation catalyzed
by CYP450:
1-Carbone oxidation reaction
2-N-Oxygenation reactions.
3-S-oxidation .
1-Carbone oxidation reactions:
a)Hydroxylation of Saturated aliphatic C atom.
b)Hydroxylation of aromatic ring
c)Oxidation of unsaturated aliphatic
2-N-Oxygenation reactions
Major reactions of oxygenation
catalyzed by CYP450
3-S-oxidation
.
Oxidation reactions
1.
Carbon oxidation reaction
A) –Aliphatic hydroxylation
B) -Aromatic hydroxylation
6.
N-Dealkylation
N-oxide formation
Oxidative Deamination
O-Dealkylation reactions
S-Dealkylation .
7.
S-oxidation reactions
2.
3.
4.
5.
A) –Aliphatic hydroxylation
i)saturated aliphatic carbon atoms

Saturated aliphatic C-H bonds are metabolised
by hydroxylation on the penultimate carbon
atom (ω-1 )and on the ultimate carbon(ω)to
lesser extent.
ii)Enzymatic introduction of a hydroxyl group
into cyclohexane ring generally occurs at C-3 or
C-4
-In humans the trans-4-hydroxycyclohexyl
product has been reported as a major
metabolite of acetohexamide ( hypoglycemic
agent )
iii(Terodiline
Aromatic p-hydroxylation predominate with Risomer where as benzylic hydroxylation is
preferred with S-isomer.
iv)Tolbutamide
CYP450
Tolbutamide
CYP450
Pentobarbital
CYP450
CYP450
Ibuprofen
CYP450
Phenmetrazine
CYP450
Valproic Acid
v(
VI)Oxidation at Benzylic Carbon Atoms
Benzylic carbon atoms are susceptible to oxidation forming the corresponding
alcohol or carbinol which is further oxidized to or conjugated with glucuronic acid.
H3C
O
C
Tolmetin
N
CH3
CH2COOH
HOOC
O
C
N CH2COOH
CH3
Dicarboxylic Acid Metabolite
)Oxidation at Carbon Atoms Alpha to Carbonyl and
Imines
An important class of drugs undergoing this type of oxidation is the
benzodiazepines e.g. diazepam and flurazepam. The C-3 carbon atom is  to both a
lactam carbonyl and an immino functionality.
(CH3CH2)2NCH2CH2
H3C
N
O
N
3
Cl
H3C
N
O
O
3
3
N
Cl
N
N
O2N
F
Diazepam
Nimetazepam
Flurazepam
to carbonyl group generally occurs only to a limited Hydroxylation of the carbon atom
extent e.g. glutethimide
3
4
2
1
O
N
H
CH2CH3
C6H5
O
Glutethimide
HO
3
4
2
1
O
N
H
CH2CH3
C6H5
O
4-Hydroxyglutethimide
vi) Aliphatic hydroxylation (alkene
epoxidation).
B) Aromatic Hydroxylation (Oxidation of aromatic rings) :
Aromatic epoxidation:
It involves oxidation of aromatic compounds (arenes) to their phenolic
metabolites (arenols).
R
R
R
O
Arene
Arene oxide
OH
Arenol
It is a major route of metabolism for many drug containing phenyl
groups.
Rules for Aromatic Oxidation:
-In most of drugs containing aromatic moieties, microsomal aromatic
hdroxylation occurs at the para-position.
-Microsomal aromatic hydroxylation reactions proceed most readily in
activated (electron-rich) rings e.g. rings containing electron donating group as NH2
group.
-Deactivated aromatic rings (e.g., those containing electron-withdrawing groups as
Cl, N+R3, COOH, SO2NHR are generally slow or resistant to hydroxylation.
Cl
N
H
Cl
COOH
Cl
H
N
N
H
SO2N(CH2CH2CH3)2
Clonidine hydrochloride
Probenecid
Cl
8
O
2
Cl
Cl
7
O
3
Cl
2,3,7,8-Tetrachlorodibenzo-p-dioxin
(TCDD)
For compounds in which two aromatic rings are present, hydroxylation occurs
preferentially in the more electron-rich ring.
7
H
N
Cl
O
N
S
3 Cl
N
CH2CH2CH2N(CH3)2
Chlorpromazine
Cl
Diazepam
p-Chlorobiphenyl
When para- position of aromatic ring is occupied the oxidation occurs in
ortho- position.
CH3 OH
C
CH
2
HO
3
17-Ethinylestradiol
Estradiol
CYP450
CYP450
CYP450
2) N-dealkylation
Metabolic oxid. of C-N & C-S involve hydroxylation of alpha
carbone atom attached directly to heteroatom(N,O,S)
General Mechanism:
a) Hydroxylation of the -carbon atom attached
directly to the heteroatom.
R
H
X C
R
O
X C
H
Usually unstable
R =N
=O
=S
R
XH
+
O
C
Aldehyde or ketone
R - NH2 1 ry amine
R - OH alcohol
R - SH thioalcohol
b) Hydroxylation or Oxidation of the
Heteroatom (N, S only):
Hydroxylol
C N
C N OH
C N
O
This reaction is catalyzed by cytochrome P-450 and N-oxide
amine oxiases or N-oxidases.
C S
C S O
O
C S
O
Sulphoxide
Sulphone
N-dealkylation‫ؤ‬
cont…..
It involves oxidation of tertiary and secondary amines.
oxidative alpha-hydroxylation at alpha-C then
dealkylation.
i) Oxidation of Tertiary Aliphatic Amines:
It is characterized by oxidative removal of alkyl group (particularly –CH3
group) form tertiary aliphatic and alicylic amines. Removal of the first alkyl group
occurs more rapidly than the removal of the second alkyl group. Bisdealkylation
may occur but very slowly.
i)
O
HCH
O
HCH
CH3
CH2CH2CH2N
CH3
Imipramine
ii)
CH3 minor
CH2CH2CH2N
H
CH2CH2CH2NH2
Desmethylimipramine
(desipramine)
Bisdesmethylimipramine

iii
-Oxidation of Secondary Amines
Amines can undergo deamination. Amphetamine for example is deaminated to
phenyl acetone and ammonia
CH2
CH
HN
CH3
CH3
Methamphetamine
O
HCH
CH2
CH3
CH
NH2
Amphetamine
NH3
CH2
C
O
CH3
Phenylacetone
CYP450
Nicotine
Cotinine
CYP450
CYP450
Nornicotine
Norcotinine
3) N-Oxide
formation:
-The biotransformation of amines is the same as the carbon and nitrogen
oxidation reactions seen for aliphatic amines but tertiary and secondary aromatic
amines are rarely encountered in medicinal agents
.
Mephentermine
Mephentermine N-Oxide
4) Oxidative Deamination :
Oxidative deamination of most exogenous primary amines is carried out
by the mixed oxidases. However, endogenous primary amines, such
as dopamine, norepinephrine, tryptamine and serotonin, are
metabolized through oxidative deamination by monoamine oxidases
(MAO).
Amines can undergo deamination. Amphetamine for example is ..
. deaminated to phenyl acetone and ammonia
Mechanism:
CH2
.
CH3
CH
NH2
Amphetamine
-Carbon
Hydroxylation
CH2
O H
C CH3
NH2
Carbinolamine
NH3
CH2
C
O
CH3
Phenylacetone
This process is similar to N-dealkylation, in that it involves an initial -carbon
hydroxylation reaction to form a carbinolamine intermediate, followed by carbonnitrogen cleavage to the carbonyl metabolite and ammonia in primary amines
5) Oxidative Dealkylation
Oxygen alkyl groups are removed by liver microsomal preparation by a
mechanism involves α-hydroxylation of the alkyl groups
i)The metabolism of these systems occurs through oxidative
O-dealkylation by microsomal enzymes.
H3C
N H
H3C
N H
O-demethylation by
Cyt P-450 (O2)
O
H3C
N H
O-demethylation by
Cyt P-450 (O2)
HO
HO
O
OH
O
OH
Codeine
OH
Morphine
Normorpthine decrease
CH2CO2H
CH3O
N
C
O
CH3
O
HN
Cl
CH3
OC2H5
Phenacetin
CH3O
CH3O
N
N
N
NH2
Prazosin
N
C
O
O
.
6) S-dealkylation
S
CH3
N
N
N
N
H
S
CH2
OH
S
N
N
N
N
6-(Methylthio)purine
OH
N
N
N
H
CH2
N
H
6-Mercaptopurine
O
HN
S
N
H
CH2CH2S CH3
CHCH2CH2CH3
CH3
O
Methitural
Methitural
S-demethylated metabolite
+
O
HCH
i-Oxidation of Sulfur:
O
O
Sulfone
Sulfoxide S
Oxidation of S
S
O
Thioethers or sulfides, for example, Chlorpromazine and
Cimetidine are oxidized to their sulphoxides
H3C
HN
CH2
S
H
N
CH2
CH2
N
Cimetidine
X
N
Metiamide
X
S
C
C
X
H3C
NHCH3
CH2
HN
N
S
O
H
N
CH2
CH2
Sulfoxide Metabolite
N
S
N
CH2 CH2
N
CH3
Thioridazine
2
S
CH3
C
X
NHCH3
ii- Desulfuration
It is the conversion of thione (C = S) to the corresponding (C = O).
O
HN
S
N
H
CH2CH3
CHCH2CH2CH3
CH3
O
Thiopental
O
HN
O
N
H
CH2CH3
CHCH2CH2CH3
CH3
O
Pentobarbital
II.Reduction
-Play an important role in the metabolism of compunds containing
azo,nitro,carbonyl.
-Bioreduction of nitro & azo lead to amino derivatives ,where as carbonyl
compounds reductions lead to alcohol analogs…
1-Azo-reduction
2-Nitro reduction
3-Reduction of Carbonyl group
E.g.: The opioid receptor antagonist Naltrexone is reduced in
humans to it’s secondary alcohol metabolite
Bio reduction of sedative – hypnotic Chloral hydrate yields trichloroethanol.
This oxidation is non- microsomal is believed to take place by alcohol
dehydrogenase.
4-Reduction of Sulphur containing group
prodrug
inactive
. active
III-Hydrolytic Reactions:
Metabolism of ester & amide linkage in many drugs catalyzed
by hydrolytic enzyme(esterase and amidasea).
Procaine
Procaineamide
Example
Procaine
Short acting local anesthetic
T1/2 = 40-84 second
Procainamide
Long acting antiarrhythmic
T1/2 = 2.5-4.5 hr
Ester vs. Amide bond
The duration of actions of ester drugs are less than the amide
analogues.why?
Procaine (ester type) injection or topical is usually shorter
acting than its amide analogue procainamide administered
similarily
•

Ester bond is relatively weaker than amide bond, it will be rapidly
hydrolyzed by esterase enzyme
Nucleophilic attack of hydroxide
anion on ester and amide
Esterases and Amidases:
Esters are more prone to hydrolysis.
Converted to more W.soluble carboxylic acids.
E.g.: Meperidine, Succinylcholine.
Sterically hindered esters might be excreted
unchanged??? Why?
Amides:
More resistant to hydrolysis than esters why?,
Advantage: Procaine vs. Procainamide.
Procaine: ester, very short half life, destroyed
shortly after entering circulation
Procainamide:
longer half life than procaine.
more than 60% excreted unchanged