Metabolism of drugs

1. Kailas K Mali
HOD, Pharmaceutics
Yashoda Technical Campus,
Faculty of Pharmacy,
Satara

2. Contents
 Introduction
 Drugs metabolizing enzymes
 Chemical pathways of drug biotransformation
 Phase I reaction
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3. How Drug go out from body ?
Drug Elimination
Metabolism:
conversion of one
chemical
entity to another.
Excretion:
Loss of drug or its
metabolites

4. The main routes by which drugs and
their metabolites leave the body are:
-Kidney
-Hepatobiliary system
-Lungs
• Most drugs leave the body in the urine,
either unchanged or as polar metabolites
• Some drugs are secreted into bile via the
liver
• But most are reabsorbed from the intestine .

5. Introduction
 Elimination
 Irreversible loss of drug from the body
 Generally it is occurs by two processes
 Biotransformation
 Excretion
 Biotransformation
 Chemical conversion of one form to another
 Example:
Conversion of penicillin to penicilloic acid by the bacterial
penicillinase
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6. Introduction
 Biotransformation
 Drug metabolism (biotransformation or detoxication) is
the biochemical changes of the drugs and other foreign
substances in the body.
 This is leading to the formation of different metabolites with
different effects.
 Some of the compounds are excreted partially unchanged and
some are known to be converted to products, which may be
more active or more toxic than the parent compounds.
 The liver is the major site of drug metabolism, but specific
drugs may undergo biotransformation in other tissues.
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7. Introduction
 Biotransformation: Importance
 Drug metabolism is needed to convert non-polar
lipophilic compounds (lipid soluble) which the body
cannot excrete into more polar hydrophilic compounds
(water soluble) which the body can excrete them in
short period of time.
 Because if the lipid soluble non-polar compounds are
not metabolized to the polar water soluble compounds,
they will remain in the blood and tissues and maintain
their pharmacological effects for an indefinite time.
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8. Introduction
 Biotransformation Process
Highly lipophilic Lipophilic Polar Hydrophilic
Accumulation
Phase I metabolism
Phase II
metabolism
Hydrophilic
Excretion
Polar
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9. Classification of Metabolism
 Inactive metabolites
 Metabolites retain similar activity
 Metabolites with different activity
 Bioactivated metabolites (prodrug technique)
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10. Classification of Metabolism
 Inactive metabolites
 Conversion of active compound to pharmacologically
inactive
 Oxidation of 6-mercaptopurine to 6-mercapturic acid results in
loss of anticancer activity of this compound.
6-Mercaptopurine 6-Mercapturic acid (inactive)
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11. Classification of Metabolism
 Metabolites retain similar activity
 Some metabolite retain the pharmacological activity of their parent
compounds to a greater or lesser degree.
 Codeine is demethylated to the more active analgesic morphine
 Phenacetin is metabolized to more active paracetamol
 Imipramine is demethylated to the equiactive antidepressant
desipramine.
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12. Classification of Metabolism
 Metabolites with different activity
 Some metabolites develop activity different from that of their
parent drugs.
 Iproniazid (antidepressant) is dealkylated to isoniazid
(antitubercular)
 Retinoic acid (vitamin A) is isomerized to isoretinoic acid (anti-
acne agent).
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13. Classification of Metabolism
 Bioactivated metabolites (prodrug technique)
 Some inactive compounds are converted to active drugs within the
body.
 These compounds are called prodrugs.
 Prodrugs may have advantages over the active form (active
metabolite) as more stable, having better bioavailability or less side
effects and toxicity.
 Levodopa (antiparkinson disease) is decarboxylated in the neuron
to active dopamine.
 The prodrug sulindac a new non steroidal antiinflammatory drug
(sulfoxide) is reduced to the active sulfide
 Benorylate to aspirin and paracetamol
 The prodrug enalapril is hydrolysed to enalaprilat (potent
antihypertension).
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14. Biotransformation
 Pharmacological inactivation of drug
 Formation of metabolite with little pharmacological action
Phenytoin p- Hydroxy phenytoin
 Yield metabolite with equal activity
Phenylbutazone Oxyphenbutazone
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15. Biotransformation
 Rarely leads to toxicological activation of drugs
Paracetamol N- hydroxylated metabolite
(causes hepatic necrosis)
 Inactive drugs (prodrugs) also depend upon
biotransformation for activation process called as
pharmacological activation.
Chloramphenicol Palmitate Chloramphenicol
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16. Metabolite
activity
Examples and notes
Inactive
(detoxification)
Similar activity
to the drug
Different
activity
Toxic
metabolites
N
N
O
Ph
Cl
CH3
N
N
O
Ph
Cl
CH3
OH
N
H
N
O
Ph
Cl
Diazepam
(Sustained anxiolytic action)
Hydroxylation
Temazepam
(Short duration)
Oxazepam
(short duration)
N-Demethylation OH
N
CONHNHCH
CH3
CH3
N
CONHNH2
Ipronazid
(Antidepressant)
N-Dealkylation
Isoniazid
(Antituberculosis)
NCOCH3
HO
OC2H5
NHCOCH3
OC2H5
NH2
OC2H5
N-Hydroxyphenacetin
(Hepatotoxic)
Phenacetin
(Analgesic)
Phenetidine
Substances responsible
for methemoglobinamia
Other substances
responsible for
hepatotoxicity
OH
Phenol
Phenol sulphokinase
3'-Phosphoadenosine-5'-
phosphosulfate (PAPS)
O
S
O
O OH
Phenyl hydrogen sulfate
Routes that result in the formation of inactive metabolites are often referred to as detoxification.
The metabolite may exhibit either a different potency or duration of action or both to the
original drug.
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17. Stereochemistry of Drug Metabolism
O
CH2COCH3
Ph
H
OH
O
O
CH2COCH3
Ph
H
OH
O O
H2C
H
Ph
OH
O
OH
CH3
H
O
CH2COCH3
H
Ph
OH
O
O
H2C
H
Ph
OH
O
OH
H
CH3
S-(-)-Warfarin
S-6-Hydroxywarfarin
R-(+)-Warfarin
Major route
Minor route
R,S-(+)-alcohol derivative R,R-(+)-alcohol derivative
HO
CH3
H
COOH
COOH
H
CH3
Metabolism
R-(-)-Ibuprofen
(inactive)
S-(+)-Ibuprofen
(active)
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18. Stereochemistry of Drug Metabolism
N
P
O
O
NHCH2CH2Cl
CH2CH2Cl
N
P
O
O
NH2
CH2CH2Cl
NH
P
O
O
NHCH2CH2Cl
R-Ifosfamide
R-2-Dechloroethylifosfamide
S-3-Dechloroethylifosfamide
+
ClCH2CHO
Chloroacetaldehyde
(Neurotoxic)
N
P
O
O
H2N
ClH2CH2C
HN
P
O
O
ClH2CH2CHN
S-2-Dechloroethylifosfamide
R-3-Dechloroethylifosfamide
+
ClCH2CHO
Chloroacetaldehyde
(Neurotoxic)
N
P
O
O
ClH2CH2CHN
ClH2CH2C
S-Ifosfamide
CYP2B6
CYP2B6
CYP3A4
CYP3A4
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19. Drug metabolizing enzymes
 Enzymes
 Differs from those that metabolize food materials
 Two types
 Microsomal enzymes
 Non-Microsomal enzymes
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20. Drug metabolizing enzymes
 Microsomal
 Catalyze majority of drug biotransformation reactions, are derived
from rough endoplasmic reticulum.
 Catalyzes oxidative, reductive, hydrolytic and glucuronidation
reactions
 Lipoidal nature leads to selectivity towards lipid soluble substrate
 Do not interact with natural endogenous substances, which are
water soluble
Lipid soluble substrate Water soluble
metabolite
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21. Drug metabolizing enzymes
 Non Microsomal enzymes
 Non specific enzymes present in soluble form in cytoplasm
attached to mitochondria,
 Act on water soluble xenobiotics
 Oxidases, peroxidases, esterase etc.
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22. Biotransformation Pathways
 Drug metabolism reactions have been divided into two
classes:
 Phase I reaction (functionalization ) and
 Phase II reaction (conjugation)
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23. General Metabolic Pathways
 Glucuronic acid conjugation
 Sulfate Conjugation
 Glycine and other AA
 Glutathion or mercapturic acid
 Acetylation
 Methylation
Reduction
 Aldehydes and ketones
 Nitro and azo
 Miscellaneous
Oxidation
 Aromatic moieties
 Olefins
 Benzylic & allylic C atoms and
a-C of C=O and C=N
 At aliphatic and alicyclic C
 C-Heteroatom system
C-N (N-dealkylation, N-oxide
formation, N-hydroxylation)
C-O (O-dealkylation)
C-S (S-dealkylation, S-oxidation,
desulfuration)
 Oxidation of alcohols and
aldehydes
 Miscellaneous
Phase II -
Conjugation
Phase I -
Functionalization
Drug
Metabolism
Hydrolytic Reactions
 Esters and amides
 Epoxides and arene oxides
by epoxide hydrase
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24. Phase I Reaction
 Polar functional groups are either
 introduced into the molecule
 or modified by oxidation, reduction or hydrolysis.
 or convert lipophilic molecules into more polar molecules by
introducing or exposing polar functional groups.
 Aromatic and aliphatic hydroxylation or reduction of ketones and
aldehydes to alcohols.
 Phase I reactions may increase or decrease or leave unaltered the
pharmacological activity of the drugs
 Objectives
 Increase in hydrophilicity
 Reduction in stability
 Facilitation of conjugation
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25. Phase I Reaction
 Reactions in Phase I Metabolism
 Oxidation reaction
 Reduction
 Hydrolysis
 Hydration
 Isomerisation
 Miscellaneous
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26. Phase I Reaction
 Enzymes catalyzing phase I reactions include
 cytochrome P-450
 aldehyde and alcohol dehydrogenase
 deaminases
 esterases
 amidases
 epoxide hydratases
 Location of these enzymes
 numerous tissues
 some are present in plasma.
 Subcellular locations include
 cytosol
 mitochondria
 endoplasmic reticulum
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27. Cytochrome P 450 Monooxygenase
 General features
 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 specificities.
 This enzyme system is the one most frequently involved in phase I
reactions.
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28. Cytochrome P 450 Monooxygenase
 General features
 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.
 Cytochrome P-450 catalyzes numerous reactions, including:
 aromatic and aliphatic hydroxylations
 dealkylations at nitrogen, sulfur, and oxygen atoms
 heteroatom oxidations at nitrogen and sulfur atoms
 reductions at nitrogen atoms
 ester and amide hydrolysis
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29. Cytochrome P 450 Monooxygenase
 General features
 The CYP3A subfamily is:
 responsible for up to half of the total cytochrome P-450 in the
liver
 accounts for approximately 50% of the metabolism of
clinically important drugs.
 CYP3A4 is a particularly abundant enzyme.
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30. Cytochrome P 450 Monooxygenase
 Representative P450 isozymes
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31. Cytochrome P 450 Monooxygenase
 Cytochrome P450
 The primary location of cytochrome P-450 is the liver,
 Other tissues, including:
 the adrenals
 ovaries and testis
 tissues involved in steroidogenesis and steroid metabolism.
 The enzyme's subcellular location is the endoplasmic reticulum.
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32. Cytochrome P 450 Monooxygenase
 Cytochrome P450
 Name based on its light absorption at 450 nm when complexed with
carbon monoxide
 Hemoprotein (heme-thiolate) containing an iron atom which can
alternate between the ferrous (Fe++) and ferric (Fe+++) states
 Electron acceptor
 Serves as terminal oxidase
N N
NN
CH3
HOOC
HOOC
CH3 CH3
CH2
CH3
CH2
Fe+3
L
O
H R
Substrate binding site
Heme portion
with activated
Oxygen
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33. Cytochrome P 450 Monooxygenase
 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 P-450 reductase.
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34. Oxidation Reaction
 Addition of oxygen or removal of hydrogen.
 Normally the first and most common step involved in the drug
metabolism
 Majority of oxidation occurs in the liver and it is possible to occur in
intestinal mucosa, lungs and kidney.
 Most important enzyme involved in this type of oxidation is
cytochrome P450
 Increased polarity of the oxidized products (metabolites) increases
their water solubility and reduces their tubular reabsorption, leading
to their excretion in urine.
 These metabolites are more polar than their parent compounds and
might undergo further metabolism by phase II pathways
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35. Oxidation Reaction
 Catalysed by microsomal enzymes
 Require both molecular oxygen and reducing agent
NADPH, they are referred to as mixed function oxidases
(MFO).
 MFO consists of 3 components
 Cytochrome P450- heme protein, transfer of oxygen atom
 Flavoprotein- NADPH dependent, acts as electron carrier, catalyze
the reduction of Cytochrome P450
 Phosphatidylcholine- facilitate electron transfer
RH + O2 + NADPH + H+ ROH + H2O + NADP+
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36. CYP450 oxidation- reduction cycle
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37. Electron flow in microsomal drug oxidizing system
CO
hu
CYP-Fe+2
Drug
CO
O2
e-
e-
2H+
H2O
Drug
CYP
R-Ase
NADPH
NADP+
OHDrug
CYP Fe+3
PC
Drug
CYP Fe+2
Drug
CYP Fe+2
Drug
O2
CYP Fe+3
OHDrug
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38. Oxidation Reaction
 Oxidation of aromatic carbon atoms
 Oxidation of olefins (C=C)
 Oxidation of Allylic carbon atom
 Oxidation of Alicyclic carbon atom
 Oxidation of Carbon- hetero atom
 Oxidation of Carbon – Nitrogen system
 N- dealkylation
 Oxidative deamination
 N- oxide formation
 N- hydroxylation
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39. Oxidative Reactions
OH O
C C
O
C C
C H
C OH
O C
O P
S C
S P S CH3
SH, S CH3
O
R O CH3
R OH
R N H
R N
R N CH2R
R N
R N OH
R NH
O
CHRO
"Activated Oxigen"
[FeO]
3+
Arene OxidesArenols
Epoxides
Benzylic, allylic
aliphatic C
Hydroxylation
Miscellaneous
Oxidations +
Desulfuration
S-Dealkylation
and S-Oxidation
O-Dealkylation N-Hydroxylation
N-Dealkyaltion and
Oxidative Deamination
N-Oxide Formation
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40. Oxidation of aromatic carbon atom
 Aromatic Hydroxylation
 Mixed function oxidation of arenes to arenols via an epoxide
intermediate arene oxide
 Major route of metabolism for drugs with phenyl ring occurs
primarily at para position
 Substituents attached to aromatic ring influence the
hydroxylation
 Activated rings (with electron-rich substituents) are more
susceptible while deactivated (with electron withdrawing
groups, e.g., Cl, N+R3, COOH, SO2NHR) are generally slow
or resistant to hydroxylation
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41. Oxidation of aromatic carbon atom
 Aromatic Hydroxylation
N
N
O
H
H
O N
N
O
H
H
O
CYP2C19
HO
Phenytoin to p-hydroxyphenytoin
R
O
Spontaneous
Rearrangement
R
-
O H
H+
NIH Shift
R
O
H
H
R
OH
Arenol
Arene Oxide
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42. Oxidation of aromatic carbon atom
Para- hydroxylated product is most common
Acetanilide Paracetamol
Oxidation
Aromatic Hydroxylation
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43. EpoxideAlkene trans dihydrodiol derivative
Epoxide hydrolaseO OHOH
■ The second step may not occur if the epoxide is stable, usually it is more
stable than arene oxide
■ May be spontaneous and result in alkylation of endogenous molecules
■ Susceptable to enzymatic hydration by epoxide hydrase to form trans-1,2-
dihydrodiols (also called 1,2-diols or 1,2-dihydroxy compounds)
■ Terminal alkenes may form alkylating agents following this pathway
NH2O
N
NH2O
N
NH2O
N
Epoxide hydrolaseCYP3A4
O HO OH
Carbamazepine Carbamazepine 10,11 epoxide Carbamazepine trans 10,11 diol
(Active) (Active & Toxic) (Inactive)
Oxidation of Olefins (Alkenes)
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44. Oxidation of Allylic carbon atom
Hexobarbital
3’
2’
OH
Hydroxylation
3’- Hydroxy hexobarbital
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45. Oxidation of carbon atoms alpha to carbonyls and Imines
Diazepam to 3 hydroxy desmethyldiazepam
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46. Oxidation of Alicyclic Carbon Atom
■ Cyclohexyl group is commonly present in many drug molecules
■ The mixed function oxydase tend to hydroxylate at the 3 or 4 position
of the ring
■ Due to steric factors if position 4 is substituted it is harder to
hydroxylate the molecules
H3C
O
OOO
N
H
N
H
S
H3C
O
OOO
N
H
N
H
S
CYP450
OH
Acetohexamide Metabolism
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47. Oxidation of Alicyclic Carbon Atom
Minoxidil 4’- Hydroxy Minoxidil
Hydroxylation
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48. Oxidation of Carbon - Hetero Atoms
 Biotransformation of C-N, C-O and C-S system proceeds in
two ways:
 Hydroxylation of carbon atom attached to heteroatom e. g. N-, O-
and S- dealkylation, oxidative deamination and desulphuration
 Oxidation of heteroatom itself
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49. Oxidation of carbon- nitrogen system
 N- Dealkylation
 Oxidation of alpha- carbon atom to generate an intermediate
carbinolamine rearranges by cleavage of C-N bond to yield N-
Dealkylated product and carbonyl of alkyl group.
 Tertiary nitrogen>sec. nitrogen favors dealkylation due to high lipid
solubility
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50. N- Dealkylation
Diazepam Desmethyldiazepam + Formaldehyde
Oxidation of carbon- nitrogen system
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51. Oxidative Deamination
 Cleavage occurs at bond between amino group link to
larger portion of drug molecule. Formed amines are
simple.
 Primary aliphatic amines undergo deamination
Phenyl acetone
+ NH3
AmmoniaAmphetamine
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52. N Oxidation
 Aliphatic amine
Imipramine Imipramine N- oxide
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53. N Oxidation
 Alicyclic amine
Nicotine Nicotine 1’ N- oxide
O
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54. N- Hydroxylation
Lidocaine
N- Hydroxy lidocaine
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55. Oxidation of Carbon- sulphur system
■ S-Dealkylation
■ Desulfuration
■ S-Oxidation
C S R3R1 C SR1 C OR1 HS R3+
R2R2
OHH
R2
R3
CYP450 Spontaneous
Steric hindrance discussion similar to N-dealkylation
R1 C R2
S
R1 C R2
O
R1 S R2 R1 S R2
O
R1 S R2
O
O
Sulfoxide Sulfone
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56. Oxidation of Carbon- sulphur system
 S- Dealkylation
N
N
S
CH3
N
H
N
6-(Methylthio)-purine
N
N
S
CH2
N
H
N
OH
N
N
SH
N
H
N
CH2
O
6-Mercaptopurine
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57. Oxidation of Carbon- sulphur system
 Desulphuration
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58. Oxidation of Carbon- sulphur system
 S Oxidation
CH3
S
CH3
NN
S
CH3
S
CH3
NN
S
CH3
S
CH3
NN
S
CH3S
CH3
NN
S
CH3
S
CH3
NN
S
O
O
O
O O O
Thioridazine
Ring Sulfoxide Ring Sulfone
Mesoridazine Sulforidazine
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59. Oxidation of Carbon- oxygen system
 O- Dealkylation
C O R3 HO R3+
H
R1
R2
C O R3
OH
R1
R2
CYP450 Spontaneous
R1 C
R2
O
■ Converts an ether to an alcohol plus a ketone or aldehyde
■ Steric hindrance discussion similar to N-dealkylation
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60. Oxidation of Carbon- oxygen system
 O- Dealkylation
O
O
O
NH2
NH2
N
N
CH3
H3C
H3C
O
O
O
NH2
NH2
N
N
CH2
H3C
H3C
OH
O
O
NH2
NH2
N
N
H3C
H3C
OH
Spontaneous
CYP450
Trimethoprim O-Dealkylation
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61. Oxidation of Carbon- oxygen system
 O- Dealkylation
Codeine Morphine
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62. Oxidative Dehalogenation
 O- Dehalogenation
R C
H
Cl
Cl
R C
OH
Cl
Cl
R C
O
Cl
R C
O
OH
+
H Cl
+H2O
CYP450
H Cl
+
Spontaneous
• Requires two halogens on carbon
• With three there is no hydrogen available to replace
• With one, the reaction generally won’t proceed
• The intermediate acyl halide is very reactive
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63. Oxidative Dehalogenation
 O- Dehalogenation
O2N
OH
OH
NHCOCCl
O
HCl
O2N
OH
OH
NHCOC
O
OH
O2N
OH
OH
NHCOCCl2
OHO2N
OH
OH
NHCOCHCl2
Chloramphenicol
Oxamyl Chloride
Derivative
Oxamic Acid
Derivative
Tissue
Nucleophiles
Covalent Binding
(Toxicity)
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64. Hepatic Microsomal Flavin Containing Monooxygenases
C
H3C
NH N
S
H
N
H
N
CH3
N
N
C
H3C
NH N
S
H
N
H
N
CH3
N
NO
MFMO
• Oxidize S and N functional groups
• Mechanism is different but end products are similar to those
produced by S and N oxidation by CYP450
• FMO’s do not work on primary amines
• FMO’s will not oxidize substrates with more than a single charge
• FMO’s will not oxidize polyvalent substrates
Cimetidine MFMO S-Oxidation
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65. Oxidation catalysed by other enzymes
 Alcohol dehydrogenase
 A soluble enzyme, found almost exclusively in the
parenchymal cells of the liver
 Converts ethanol to acetaldehyde
 Converts methanol to formaldehyde
 Converts ethylene glycol to its respective aldehyde
metabolites
 Is inhibited by pyrazole
CH3CH2OH + NAD+  CH3CHO + NADH + H+
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66. Oxidation catalysed by other enzymes
 Aldehyde dehydrogenase
 Found primarily in the liver. Oxidizes free aldehydes and has
broad substrate specificity
 Is inhibited by disulfiram
 Mitochondrial enzyme
 involved in the metabolism of acetaldehyde
 Cytosolic enzyme
 oxidizes xenobiotic aldehydes
 Microsomal enzyme
 oxidizes xenobiotic aldehydes
CH3CHO + NAD+  CH3COOH + NADH + H+
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67. Oxidation catalysed by other enzymes
 Xanthine oxidase
 This enzyme specifically oxidizes xanthine containing drugs
 Theophylline
O
HN
N
N
H
N
H
O
O
HN
N
N N
H
O
HN
N
N
H
N
H
O
OH
O
HN
H
N
N
H
N
H
O
O
Hypoxanthine Xanthine Uric acid
(hydroxy tautomer)
Uric acid
(keto tautomer)
Xanthine
oxidase
Xanthine
oxidase
4/13/2018 67

68. Oxidation catalysed by other enzymes
 Amine oxidase (N oxidation)
 This enzyme specifically oxidizes xanthine containing drugs
 Imipramine (require NADPH and molecular oxygen)
4/13/2018 68

69. Reductive Reactions
 Bioreduction of C=O (aldehyde and keton) generates alcohol (aldehyde
→ 1o alcohol; ketone → 2o alcohol)
 Nitro and azo reductions lead to amino derivatives
 Reduction of N-oxides to their corresponding 3o amines and reduction
of sulfoxides to sulfides are less frequent
 Reductive cleavage of disulfide (-S-S-) linkages and reduction of C=C
are minor pathways in drug metabolism
 Reductive dehalogenation is a minor reaction primarily differ from
oxidative dehalogenation is that the adjacent carbon does not have to
have a replaceable hydrogen and generally removes one halogen from a
group of two or three
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70. Reductive Reactions
 Reduction of N- compounds
 Nitro- reduction
 Azo- reduction
 N- oxide
 Reduction of Aldehydes/ ketones
 Reduction of Alcohols and C=C bond
 Miscellaneous Reductive Reactions
 Reductive Dehalogenation
4/13/2018 70

71. Reductive Reactions
 Nitro and Azo Reduction
N NR
Azido
NH2R
Amine
NH + N N
N2
N N R2R1 R1 NH2 H2N R2+
Azo Two 1 amines
H
NR1
Hydrazo
H
N R2
R C N
H
H
R C N
H
H
H
H
H
OH
R C N
H
H
R C N
H
H
O
O
1 amineHydroxylamineNitrosoNitro
O
4/13/2018 71

72. Reductive Reactions
 Nitro and Azo Reduction
• R1 and R2 are almost always aromatic
• Usually only seen when the NO2 functional group is attached directly to an
aromatic ring and are rare.
• Nitro reduction is carried out by NADPH-dependent microsomal and soluble
nitroreductases (hepatic)
• NADPH dependent multicomponent hepatic microsomal reductase system
reduces the azo
• Bacterial reductases in intestine can reduce both nitro and azo
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73. Reductive Reactions
 Reduction of Nitro group
Nitrazepam 7- Amino metabolite
4/13/2018 73

74. Reductive Reactions
 Reduction of Carbonyls
 Depending on reactivity towards bioreduction,
carbonyls can be divided into 3-
 Aliphatic aldehydes and ketones
 Aromatic aldehydes and ketones
 Esters, acids and amides
 The order of reactivity is i > ii> iii
 Reduction of i and ii yields primary and sec. alcohols.
4/13/2018 74

75. Reductive Reactions
■ Reduction of Carbonyls
■ C=O moiety, esp. the ketone, is frequently encountered in
drugs and additionally, ketones and aldehydes arise from
deamination
■ Ketones tend to be converted to alcohols which can then be
glucuronidated. Aldehydes can also be converted to
alcohols, but have the additional pathway of oxidation to
carboxylic acids
■ Reduction of ketones often leads to the creation of an
asymmetric center and thus two stereoisomeric alcohols are
possible
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76. Reductive Reactions
• Reduction of Carbonyls
• Reduction of a, b –unsaturated ketones found in steroidal drugs
results not only in the reduction of the ketone but also of the C=C
• Aldo–keto oxidoreductases carry out bioreductions of aldehydes
and ketones. Alcohol dehydrogenase is a NAD+ dependent
oxidoreductase that oxidizes alcohols but in the presence of
NADH or NADPH, the same enzyme can reduce carbonyl
compounds to alcohols
R C O
H
R C
H
OH
H
Aldehyde 1 alcohol
R C O
R2
R1 C
R2
OH
H
Ketone 2 alcohol
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77. Reduction of Carbonyls
Reduction of Aliphatic Aldehydes
Cl3C-CHO.H2O Cl3C-CH2OH
Chloral Hydrate Trichloroethanol
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78. Reduction of Carbonyls
Reduction of Aliphatic Ketones
R1
C
R2
O
N
R
H
H
H2N
OH
+
R1
C
R2
HO H
+
N
+
R
H2N
O
Ketone Chiral AlcoholRed Nicotinamide moiety
of NADPH or NADH
Ox Nicotinamide moiety
of NADP
+
or NAD
+
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79. Reduction of Carbonyls
Reduction of Alicyclic Ketones
Naltrexone Isomorphine derivative
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80. Reduction of Carbonyls
Reduction of Aromatic Ketones
Acetophenone Methyl phenyl carbinol
4/13/2018 80

81. Miscellaneous Reductive Reactions
Reductive Dehalogenation
4/13/2018 81
Trifluoroacetic acidHalothane
CF3-C-H
Br
Cl
CF3CH3 CF3COOH
1,1,1- trifluoroethane

82. Hydrolytic Reactions
 Hydrolyzes (adds water to) esters and amides and their
isosteres; the OH from water ends up on the carboxylic acid
(or its isostere) and the H in the hydroxy or amine
 Differs from oxidative and reductive reactions in 3 respects
 Does not involve change in the state of oxidation of substrate
 Results in large chemical change in substrate, loss of large fragment
 Hydrolytic enzymes acts on endogenous substrate
 Hydrolysis of esters and ethers
 Hydrolysis of amides
 Hydrolytic Dehalogenation
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83. Hydrolytic Reactions
■ Enzymes: Non-microsomal hydrolases;
however, amide hydrolysis appears to be
mediated by liver microsomal amidases,
esterases, and deacylases
■ Electrophilicity of the carbonyl carbon,
Nature of the heteroatom, substituents
on the carbonyl carbon, and
substituents on the heteroatom
influnce the rate of hydrolysis
■ In addition, Nucleophilicity of
attacking species, Electronic charge,
and Nature of nucleophile and its steric
factors also influence the rate of
hydrolysis
R1 R2 Name Susceptibility
to Hydrolysis
C O Ester Highest
C S Thioester
O O Carbonate
C N Amide
O N Carbamate
N N Ureide Lowest
Naming carbonyl - heteroatom groups
R1 C R2
O

+
4/13/2018 83

84. Hydrolytic Reactions
R1 C
O
H
N R2 R1 C
O
OH H2N R2
O C O R2R1
O
HO C O R2R1
O
OH HO C OHR2
O
HO O C O O H
H
+++
Carbonate Carbonic acid derivative Carbonic acid
Ester hydrolysis
Amide hydrolysis (slower)
Carbonate hydrolysis
R1 C
O
O R2 R1 C
O
OH HO R2
4/13/2018 84

85. Hydrolytic Reactions
O C NR1
O
HO C NR1
O
OH HO C OH
O
HN O C O O H
H
+++
Carbamate Carbamic acid derivative
Carbonic acid
R2
R3
R2
R3
R2
R3
N C N
O
HO C N
O
NH HO C OH
O
HN O C O O H
H
+++
Urea derivative Carbamic acid derivative
Carbonic acid
R3
R4
R3
R4
R2
R3
R1
R2
R1
R2
R1 C
H
N N
O
R2
R3
R1 C OH
O
H2N N
R2
R3
+
Hydrazide Hydrazine
Carbamate hydrolysis
Urea hydrolysis
Hydrazide hydrolysis
4/13/2018 85

86. Hydrolysis: Drug Examples
H3C
OO
O
N
CH3
O
Cocaine
OHO
O
N
CH3
O
H3C
OO
N
CH3
HO
+
Benzoylecgonine Methylecgonine
H3C O
O
O
OH
H3C O
OH
O
OH
OH
+
Aspirin Salicylic Acid
CH3
CH3N
H2N
O
O
CH3
CH3N
H2N
O
H
N
Procainamide
Procaine
H2N
O
OH
Slow Hydrolysis
Rapid Hydrolysis
OH
O
H3C O
CH3
Cl
O
N
Indomethacin
CH3
CH3
CH3
CH3
O
N
H
N
Lidocaine
O
O
N
O
O
N
NH2
N
N
H3C
H3C
Prazosin
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87. Hydrolysis of esters
 Organic acid esters
H3C O
O
O
OH
H3C O
OH
O
OH
OH
+
Aspirin Salicylic Acid
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88. Hydrolysis of esters
 Inorganic acid esters
4/13/2018 88
CH3
CH3 C O S CH3
H O
O CH3
CH3 C OH
H
HO S CH3
O
O
+
Isopropyl methane
sulphonate
Isopropanol Methanesulponic
acid

89. Hydrolysis of esters
 Hydrolysis of Amide (C-N bond cleavage)
 Reaction catalysed by amidases
 Cleavage yield carboxylic acid and amine
R1 C
H
N N
O
R2
R3
R1 C OH
O
H2N N
R2
R3
+
Hydrazide Hydrazine
4/13/2018 89

90. Hydrolysis of esters
 Hydrolysis of Amide (C-N bond cleavage)
 Secondary amide with aliphatic substitution
Procainamide
PABA
4/13/2018 90

91. Hydrolysis of esters
 Hydrolysis of Amide (C-N bond cleavage)
 Secondary amide with aromatic substitution
Lidocaine
2, 6 Xylidine
N, N-Diethyl
glycine
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92. Hydrolysis of esters
 Hydrolysis of Amide (C-N bond cleavage)
 Tertiary amide
Carbamazepine Iminostilbene
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93. Metabolism of Phenytoin
4/13/2018 93

94. References
 Brahmankar DM , Jaiswal SB. Biopharmaceutics and Pharmacokinetics
A Treatise. 2nd edition. Vallabh Prakashan;New Delhi, 2009 Page No.
139- 169.
 Gibson G. Gordon, Skett Paul. Introduction to drug Metabolism. 3rd
edition, Nelson Thorne Publishers; Chetenham, 2001 Page No. 1-13, 37-
62.
 Wilson and Gisvold’s Textbook of Organic Medicinal and
Pharmaceutical Chemistry 11th ed. Lippincott, Williams & Wilkins ed.
 Foye’s Principles of Medicinal Chemistry
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