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Metabolic Changes of
Drugs and RelatedOrganic Compounds
Kristine Mae F. Gante, RPhPharmacy Department
School of Health Sciences
Saint Paul University Philippines
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Topic Outline
General Pathways of Drug Metabolism
Sites of Drug MetabolismRoles of Cytochrome P-450
Monooxygenase in Oxidative
BiotransformationPhase 1
Phase 2
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Introductory Concepts
Biochemically speaking:
Metabolism means Catabolism(breaking down of substances) +
Anabolism(building up or synthesis
of substances)
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Introductory Concepts
But when we speak about drug
metabolism, it is only catabolism.That is drug metabolism is the
break down of drug molecules
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Introductory Concepts
So what is the term used to
describe building the drugmolecules?
We use the word synthesis, then
Drugs are synthesized in laboratory
and thus is not an endogenous
event
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What Roles are Played by
Drug Metabolism?
One of four pharmacokinetic
parameters, i.e., absorption,distribution, metabolism and
excretion (ADME)
Elimination of Drugs:
Metabolism and excretion together
are elimination.
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What Roles are Played by
Drug Metabolism?
In general, by metabolism drugs
become more polar, ionizable and
thus more water soluble to enhance
elimination.
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What Roles are Played by
Drug Metabolism?
It also affect deactivation and thus
detoxication or detoxification.
Many drugs are metabolically
activated (Prodrugs)
Sometimes drugs become more toxicand carcinogenic
G f
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General Pathways of Drug
Metabolism
2 Categories
Phase I FunctionalizationPhase II Conjugation
G h f
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General Pathways of Drug
Metabolism
2 Categories
Phase I Functionalization Oxidation
Reduction
Hydrolysis
G l P h f D
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General Pathways of Drug
Metabolism
2 Categories
Phase I Functionalization The purpose of these reactions is to
introduce a functional polar group(s)
[e.g. OH, COOH, NH2, SH] into thexenobiotic molecule to produce a more
water-soluble compounds.
G l P h f D
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General Pathways of Drug
Metabolism
2 Categories
Phase I metabolism can be achievedby:
Direct introduction of the functional
group (e.g. aromatic and aliphatichydroxylation)
G l P h f D
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General Pathways of Drug
Metabolism
Phase I metabolism can be achieved
by:
Modifying or unmasking existing
functionalities
e.g.: reduction of ketones and aldehydesto alcohols, oxidation of alcohols to
carboxylic acids, hydrolysis of esters and
amides
G l P h f D
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General Pathways of Drug
Metabolism
Phase I metabolism
Phase 1 reactions may not produce
sufficiently hydrophilic or inactive
metabolites, but they generally tend to
provide a functional group or handle the
molecule that can undergo subsequent
Phase II reactions.
G l P th f D
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General Pathways of Drug
Metabolism
Phase II metabolism
Its purpose is to attach small, polar,
ionizable endogenous compounds to the
functional handles of phase I
metabolites or parent compounds that
already have suitable existing functional
groups to form water-soluble conjugated
products.
G l P th f D
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General Pathways of Drug
Metabolism
Phase I and Phase II reactions
complement each other in
detoxifying, and facilitating the
elimination of drugs and xenobiotics.
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Biotransformation is a major
mechanism for drug elimination
Results of biotransformation:
Production of metabolites that aremore polar than the parent drug
usually terminates the pharmacologic
action of the parent drug After phase I reactions, similar or
different pharmacologic activity, or
toxicological activity.
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Results of Biotransformation
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Tetrahydrocannabinol (D1-THC) Metabolism
The metabolite is polar, ionizable and
hydrophilic
O C5H11
OH
CH3
H3C
CH3O C5H11
OH
CH2OH
H3C
CH3O C5H11
OH
COOH
H3C
CH3
O C5H11
OR
COOR
H3C
CH3
OCOO-
OHOH
HOH
1
7
2
345
6
D1-THC 7-Hydroxy-D
1-THC
D
1
-THC-7-oic Acid
Glucuronide conjugate at eitherCOOH or phenolic OH group
Where R =
-Glucuronyl
moiety
Phase I
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Possible consequences of
biotransformation:
Inactive metabolites (most common)
Metabolites with increased or decreased
potencies
Metabolites with qualitatively different
pharmacologic actionsToxic metabolites
Active metabolites from inactive
prodrugs.
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Possible consequences of
biotransformation:
Metabolites are often more polar than
the parent compounds.
This increased polarity may lead to:
A more rapid rate of clearance because of
possible secretion by acid or base carriers inthe kidney
It may lead to decreased tubular
reabsorption.
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Drug Metabolism Reactions
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Enzymes catalyzing phase I
biotransformation reactions
cytochrome P-450
aldehyde and alcohol dehydrogenase
deaminases
esterases
amidasesepoxide hydrase
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Enzymes catalyzing phase II
biotransformation reactions
glucuronyl transferase (glucuronide
conjugation)
sulfotransferase (sulfate conjugation)
transacylases (amino acid conjugation)
acetylases
ethylases
methylases
glutathione transferase
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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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Sites of Drug Metabolism
Liver
Major site
Well organized with all enzyme systems
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Hepatic microsomal enzymes
(oxidation, conjugation)
Extrahepatic microsomal enzymes
(oxidation, conjugation)
Hepatic non-microsomal enzymes
(acetylation, sulfation,GSH,
alcohol/aldehyde dehydrogenase,
hydrolysis, ox/red)
Drug Metabolism
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Sites of Drug Metabolism
The first-pass effect
Following drugs are metabolizedextensively by first-pass effect:
Isoproterenol, Lidocaine Meperidine,
Morphine, Pentazocine, Propoxyphene,Propranolol, Nitroglycerin, Salicylamide
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Sites of Drug Metabolism
Intestinal Mucosa:
The extra-hepatic metabolism, contains
CYP3A4 isozyme
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Sites of Drug Metabolism
Intestinal Mucosa:
Isoproterenolexhibit considerable
sulfate conjugation in GI tract
Levodopa, chlorpromazine and
diethylstilbestrol are also reportedlymetabolized in GI tract
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Sites of Drug Metabolism
Intestinal Mucosa:
Esterases and lipases present in the
intestine may be particularly important
carrying out hydrolysis of many ester
prodrugs
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Sites of Drug Metabolism
Intestinal Mucosa:
Bacterial flora present in the intestine
and colon reducemany azo and nitro
drugs (e.g., sulfasalazine)
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Sites of Drug Metabolism
Intestinal Mucosa:
Intestinal b-glucuronidasecan
hydrolyze glucuronide conjugates
excreted in the bile, thereby liberating
the free drug or its metabolite for
possible reabsorption (enterohepatic
circulation or recycling)
Enzymes Involved in Drug
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Enzymes Involved in Drug
Metabolism
CYP450
Hepatic microsomal flavin containingmonooxygenases (MFMO or FMO)
Monoamine Oxidase (MAO)
Hydrolases
Enzymes Involved in Drug
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Enzymes Involved in Drug
Metabolism
Cytochrome P450 system:
Localized in the smooth endoplasmicreticulum.
It is a Pigment that, with CO bound
to the reduced form, absorbsmaximally at 450nm
Enzymes Involved in Drug
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Enzymes Involved in Drug
Metabolism
Cytochrome P450 system:
Cytochromes are hemoproteins(heme-thiolate) that function to pass
electrons by reversibly changing the
oxidation state of the Fein hemebetween the 2+ and 3+ state and
serves as an electron acceptordonor
Cytochrome P450:
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Cytochrome P450:
Naming
Before we had a thorough
understanding of this enzyme system,
the CYP450 enzymes were named
based on their catalytic activity
toward a specific substrate, e.g.,aminopyrine N-demethylase now
known as CYP2E1.
Cytochrome P450:
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Cytochrome P450:
Naming
CYP N1L N2
N1 - Family (>40% homology)L - subfamily (> 55% homology)
N2- isoform (specific enzyme
responsible for a particular reaction)
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Drug Metabolism
Nomenclature
CYP2D6
Family
Sub-Family Individual Gene
Isoform
Cytochrome P450:
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Cytochrome P450:
Naming
Major human forms
of P450:
Quantitatively, in
the liver the
percentages oftotal P450 protein
are:
CYP3A4 28%
CYP2Cx 20%
CYP1A2 12%
CYP2E1 6%
CYP2A6 4%
CYP2D6 4%
Cytochrome P450:
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Cytochrome P450:
Naming
By number of drugs metabolized:
CYP3A4 35%
CYP2D6 20%
CYP2C8 and CYP2C9 17%
CYP2C18 and CYP2C19 - 8% CYP 1A1 and CYP1A2 -10%
CYP2E1 4%
CYP2B6 3%
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OTHER
36%
CYP2D6
2%
CYP2E1
7%
CYP 2C
17%
CYP 1A212%
CYP 3A4-5
26%
RELATIVE HEPATIC CONTENT
OF CYP ENZYMES
% DRUGS METABOLIZED
BY CYP ENZYMES
ROLE OF CYP ENZYMES IN HEPATIC
DRUG METABOLISM
CYP 1A2
14%
CYP 2C9
14%
CYP 2C19
11%
CYP2D6
23%
CYP2E
5%CYP 3A4-5
33%
Few Important CYP450
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Few Important CYP450
Isozymes
CYP
family
Main functions
CYP1 Xenobiotic metabolism
CYP2 Xenobiotic metabolism, Arachidonic acid metabolismCYP3 Xenobiotic and steroid metabolism
CYP7 Cholesterol 7-hydroxylation
CYP11 Cholesterol side-chain cleavage, Steroid 11 hydroxylation,
Aldosterone synthesis
CYP17 Steroid 17-hydroxylation
CYP19 Androgen aromatization
CYP21 Steroid 21-hydroxylation
CYP24 Steroid 24-hydroxylation
CYP27 Steroid 27-hydroxylation
Few Important CYP450
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Few Important CYP450
Isozymes
Recommended name Family/gene
secologanin synthase CYP72A1
trans-cinnamate 4-monooxygenase CYP73
benzoate 4-monooxygenase CYP53
calcidiol 1-monooxygenase CYP27
cholestanetriol 26-monooxygenase CYP27
-monooxygenase CYP7
flavonoid 3'-monooxygenase CYP75
3,9-dihydroxypterocarpan 6a-monooxygenase CYP93A1
leukotriene-B420-monooxygenase CYP4F
methyltetrahydroprotoberberine 14-monooxygenase CYP93A1
tyrosine N-monooxygenase CYP79
R l f CYP M i
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Role of CYP Monooxygenases in
Oxidative Biotransformation
Oxidation of Xenobiotics
RH + NADPH + O2+ H+
ROH + NADP++ H2O
Mixed function oxidases or
monooxygenases
DrugNADP
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Electron flow in microsomal drug oxidizing system
CO
hCYP-Fe+2
Drug
CO
O2
e-
e-
2H+
H2O
Drug
CYPR-Ase
NADPH
NADP+
OH
Drug
CYP Fe+3
PCDrug
CYP Fe+2
Drug
CYP Fe+2
Drug
O2
CYP Fe+3
OH
Drug
R l f CYP M i
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Role of CYP Monooxygenases in
Oxidative Biotransformation
Oxidation of Xenobiotics
It requires molecular oxygen and thereducing agent NADPH.
One atom of Oxygen is introduced
into the substrate (RH) and the otheratom is incorporated in water.
R l f CYP M i
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Role of CYP Monooxygenases in
Oxidative Biotransformation
1. 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 ofthis cofactor is coupled to cytochrome
General Metabolic Pathways
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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
O idati e Reactions
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Oxidative Reactions
OH O
C C
O
C C
C H
C OH
O CO P
S C
S PS CH3
SH, S CH3
O
R O CH3
R OH
R N H
R N
R N C H2R
R N
R N OH
R NH
O
CHRO"Activated Oxigen"
[FeO]3+
Arene OxidesArenols
Epoxides
Benzylic, allylic
aliphatic CHydroxylation
MiscellaneousOxidations +
DesulfurationS-Dealkylation
and S-Oxidation
O-Dealkylation N-HydroxylationN-Dealkyaltion and
Oxidative DeaminationN-Oxide Formation
Aromatic Hydroxylation
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Aromatic Hydroxylation
Mixed function oxidation of arenesto arenols via an epoxideintermediate arene oxide
Major route of metabolism fordrugs with phenyl ring
Occurs primarily atparaposition
Substituents attached to aromaticring influence the hydroxylation
Activated rings (with electron-richsubstituents) are more susceptiblewhile deactivated (with electronwithdrawing groups, e.g., Cl, N+R3,COOH, SO2NHR)are generally slowor resistant to hydroxylation
R1 R1
OH
R1
O
R1
OH
OH
R1
SGlutathione
R1
Macromolecule
Spontaneous
Epoxide hydrolase
Glutathione
Macromolecule
R1
OH
OH
Aromatase
CYP450
OH
OH
Epoxide Hydrase
H
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N
N
O
H
H
O N
N
O
H
H
O
CYP2C19
HO
H
CH3
CH3
OH
NO
CH3
H
H
N
C CH
OH
HO
Phenytoin p-hydroxyphenytoinAmphetamine
Propranolol17-a-Ethinylestradiol
O
CH3
O O
ONa
Ca+2
HN
O
H3C
CH3 F
C
N
CO
OH
HO O
2
Warfarin sodium
Atorvastatin
CH3
O
O
N
N
Phenylbutazone
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Cl
Cl
HN
HN
N H3C
O
O
H3C
N S
OH
O
ClonidineProbenecid
Antihypertensive drug clonidineundergo little aromatichydroxylation and the uricosuricagent probenecid has not been
reported to undergo any aromatichydroxylation
Diazepam Chlorpromazine
CH3
Cl
O
N
N
Cl
CH3
CH3NN
S
Preferentially the moreelectron rich ring ishydroxylated
NIH Shift: Novel Intramolecular Hydrideshift named after National Instituteof Health where the process was discovered. This is most importantdetoxification reaction for arene oxides
R
O
SpontaneousRearrangement
R
-
O H
H+
NIH Shift
R
O
H
H
R
OH
Arenol
Arene Oxide
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Oxidation of Aromatic Moieties
PCB & TCDD
Resistant to aromatic oxidationMetabolic stability coupled to the
lipophilicity explains their long
persistence in the body onceabsorbed.
Oxidation of olefinic bonds (also called alkenes)
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Oxidation of olefinic bonds (also called alkenes)
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 spontaneousand result in alkylation of endogenous molecules
Susceptable to enzymatic hydration by epoxide hydrase to form trans-
1,2-dihydrodiols (also called 1,2-diolsor 1,2-dihydroxy compounds)
Terminalalkenes 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)
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Oxidation of Olefins
Epoxidation of the olefinic 10,11 double
bondFurther conversion to 1,2 diols
Protriptyline (Vivactil)
Antipsychotic
Cyproheptadine (Periactin) H1Antagonist
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Oxidation of Olefins
Aflatoxin B1
Carcinogenic agent
Contains olefinic (C2-C3) double
bond adjacent to a cyclic ether
oxygen
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Oxidation of Olefins
Aflatoxin B1
It is oxidized to the corresponding
2,3-oxide (extremely reactive)
The oxide binds covalently to DNA,
RNA and proteins2,3-dihydro-2-(N 7-guanyl)-3-
hydroxyaflatoxin B1
Benzylic Carbon Hydroxylation
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Benzylic Carbon Hydroxylation
Hydroxylate a carbon attached to a phenyl group (aromatic
ring)
R1and R2can produce steric hindrance as they get larger
and more branched
So a methyl group is most likely to hydroxylate
Primary alcohol metabolites are often oxidized further to
aldehydeand carboxylic acidsand secondary alcohols are
converted to ketones by soluble alcohol and aldehyde
dehydrogenase
CR1
R2
H CR1
R2
OH
Benzylic Carbon Hydroxylation
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Benzylic Carbon Hydroxylation
Dicarboxylicacid is themajor
metabolite
ONa
O
CH3
H3C
O
N
Tolmetin sodium
Tolbutamide Metabolism
OOO
CH3NHNH
S
H3C
OOO
CH3NHNH
S
C
CYP2C9
HOH
H
Oxidation at Benzylic Carbon
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Oxidation at Benzylic Carbon
Atom
Celecoxib
Undergoes benzylic oxidation at its
C-5 methyl group to give
hydroxycelecoxib as a major
metabolite.
Oxidation at Allylic Carbon
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Oxidation at Allylic Carbon
Atom
1-THC
It contains 3 allylic carbon centers
(C7, C6, C3).
Allylic oxidation occurs at C-7 to yield
7-hydroxy-1-THCIt is as active or even more active
than the parent compound.
Oxidation at Allylic Carbon Atoms
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Oxidation at Allylic Carbon Atoms
C C CC R3R1
R2 R4
C C CC R3R1
R2 R4
OHHHH H H
HH
O C5H11
OH
CH3
H3C
CH3O C5H11
OH
CH2OH
H3C
CH3O C5H11
OH
CH3
H3C
CH3
HO
O C5H11
OH
CH3
H3C
CH3
HO
D1-THC
12
345
6
77
7-Hydroxy-D1-THC 6a-Hydroxy-D
1-THC 6-Hydroxy-D
1-THC
+ +
N
NHO
H3CO
H2C
H
N
NHO
H3CO
H2C
OH
Quinine
1
2 3
3-Hydroxyquinine
Oxidation at Allylic Carbon
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Oxidation at Allylic Carbon
Atom
Examples:
Hexobarbital
Pentazocin
Safrole
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O
O
O
CH3
CH3
2' 3' O
O
O
CH3
CH3
O
O
O
CH3
CH3
OH O
O-Glucuronide Cojugate
Hexabarbital 3'-Hydroxyhexabarbital 3'-Oxohexabarbital
Pentazocine
Oxidation at Carbon Atoms to
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Oxidation at Carbon Atoms to
Carbonyls and Imines
Alpha carbon
Carbon adjacent to the carbonyl (C=O)
and imino (C=N) functionalities
Hydroxylation at C to C=O and C=N
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Hydroxylation at C to C O and C N
The benzodiazepinesare classic exampleswith both
functionalities
The sedativehypnoticglutethimidepossesses C a tocarbonyl function
R C C R'
O H
H
R C C R'
O H
OH
N
N
CH3 O
Cl
3
N
N
CH3 O
Cl
OHN
HN
O
Cl
OHN-demethylation
N
N
(CH3CH2)2NCH2CH2 O
Cl N
N
CH3O
O2N
3 3
Diazepam (3S) N-Methyloxazepam
or 3-HydroxydiazepamOxazepam
F
Flurazepam Nimetazepam
NH
C6H5
CH2CH3
OO NH
C6H5
CH2CH3
OO
HO
1
344
Glutethemide 4-Hydroxyglutethemide
Oxidized to their3-hydroxymetabolites
Oxidation at Carbon Atoms to
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Oxidation at Carbon Atoms to
Carbonyls and Imines
Hydroxylation of the carbon alpha to
the carbonyl moieties generally occur
at a limited extent in drug
metabolism.
Oxidation at Aliphatic or
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Oxidation at Aliphatic or
Alicyclic Carbon Atoms
Oxidation
Metabolic oxidation at the terminal
methyl group
- 1Oxidation
Oxidation of the penultimate carbonatom (next-to-the-last carbon)
Aliphatic hydroxylation
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p y y
Catalyzes hydroxylation of the and
-1carbons in aliphatic chains Generally need three or more
unbranched carbons
C C CR1
C C CR1
OH
C C CR1 OH
H
H H
H
H
H
H
H
H H
H
H
H
H
H
H
H
H
H
N
N
H
H
O
O
O
N
N
H
H
O
O
OOH
CYP450
OH
O
CH3
CH3H3C
OH
O
CH3
CH3H3C
OH
CYP450
Pentobarbital Metabolism
Ibuprofen MetabolismOH
O
CH3
CH3HOOC
+
Oxidation at Aliphatic or
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Oxidation at Aliphatic or
Alicyclic Carbon Atoms
Valproic Acid (Depakene)
Undergoes both and - 1
Oxidation to 5- hydroxy and 4-
hydroxy metabolites respectively.
Oxidation at Aliphatic or
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Oxidation at Aliphatic or
Alicyclic Carbon Atoms
Amobarbital
Pentobarbital
Thiamylal
Secobarbital
Chlorpropramide
Meprobamate
Glutethimide
Ethosuximide
Phenylbutazone
Alicyclic (nonaromatic
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ring) Hydroxylation
Acetohexamide (Dymelor)Metabolism
Cyclohexyl group is commonly present in many drugmolecules
The mixed function oxydase tend to hydroxylate atthe 3 or 4 positionof the ring
Due to stericfactors if position 4 is substituted it isharder to hydroxylate the molecules
H3C
O
OOO
NH
NH
S
H3C
O
OOO
NH
NH
SCYP450
OH
Oxidation at Alicyclic Carbon
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Oxidation at Alicyclic Carbon
Atoms
Glipizide
Oxidized to trans-4 and cis-3-
hydroxylcyclohexyl metabolite (6:1
ratio)
Oxidation at Alicyclic Carbon
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Oxidation at Alicyclic Carbon
Atoms
Phencyclidine (PCP)
4-hydroxypiperidyl (Aliphatic)
4-hydroxycyclohexyl derivatives
(Alicyclic)
Oxidation Involving Carbon-
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g
Heteroatom Systems
C-N, C-O and occasionally C-S Two basic types of biotransformation processes:
1. Hydroxylation of alpha-C attacheddirectly to the
heteroatom (N,O,S). The resulting intermediate is
often unstable and decomposes with thecleavage of the C-X bond:
Oxidative N-, O-, and S-dealkylation as well as
oxidative deamination reaction fall under this
category
R X C
H
R X C
O
H
a a R XH
O
+
Usually Unstable
Oxidation Involving Carbon-
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g
Heteroatom Systems
Two basic types of biotransformation processes:2. Hydroxylation or oxidation of heteroatom(N, S
only, e.g., N-hydroxylation, N-oxide formation,
sulfoxide and sulfone formation)
Metabolism of some N containing compounds
are complicated by the fact that C or N
hydroxylated products may undergo secondary
reactions to form other, more complex metabolicproducts (e.g., oxime, nitrone, nitroso, imino)
N-Dealkylation (Deamination)
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C N
H
R2
R1 C
R2
R1R3
R4
O + HN R3
R4
C N
OH
R2
R1 R3
R4
CYP450 Spontaneous
NCH3
CH3
N NCH2
CH3
N N
CH3
NCYP2C19
Spontaneous
OH
H
Deamination and N-dealkylation differ only in the point of reference; If the drug is R1or R2
then it is a deamination reaction and If the drug is R3or R4then it is an N-dealkylation
In general, least sterically hindered carbon (a) will be hydroxylated first, then the next, etc.
Thus the more substituent on this C, the slower it proceeds; branchingon the adjacent
carbon slows it down, i.e. R1, R2= H is fastest.
Any group containing an a-H may be removed, e.g., allyl, benzyl. Quaternary carboncannot be removed as contain no a-H
The more substituents placed on the nitrogen the slower it proceeds (steric hindrance)
The larger the substituents are the slower it proceeds (e.g. methyl vs. ethyl). In general,
small alkyl groups like Me, Et and iPro are rapidly removed; branching on these
substituents slows it down even more
Imipramine N-Dealkylation
C-N systems
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1. Aliphatic (1o, 2o,3o,) and alicyclic (2oand 3o)
amines;2. Aromatic and heterocyclic nitrogen
compounds;
3. Amides Enzymes:
1. CYP mixed-function oxidases: a-C
hydroxylation and N-oxidation
2. Amine oxidases or N-oxidases (non-CYP,NADPH dependent flavoprotein and require
O): N-oxidation
C-N systems
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R1 N C
H
a R1 N C
O
a
H
R1 NH +
O
Carbinolamine
R2 R2 R2
3oor 2oamine 2oor 1oamine
C
H
NH2
aC
O
NH2
a
H
NH3+
O
Carbinolamine1oamine Carbonyl Ammonia
3o Aliphatic and alicyclic amines are metabolized by
oxidative N-dealkylation(CYP) Aliphatic 1o, 2o amines are susceptible to oxidative
deamination, N-dealkylation and N-oxidation reactions
Aromatic amines undergoes similar group of reactions
as aliphatic amines, i.e., both N-dealkylation and N-oxidation
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Tertiary Aliphatic and Alicyclic Amines
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Imipramine is monodemethylated todesmethylimipramine(major metabolite).
Very little of the bisdemethylated metabolite isdetected.
NCH3
CH3
N NCH2
CH3
N N
CH3
NCYP2C19
Spontaneous
OH
H
Imipramine N-Dealkylation
H3C
CH3
CH33oAmine drugs
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CH3
CH3
CH3
CH3
O
N
HN
N
CH3
N
O
NH2C
ON
CH3
H3C
CH3
CH3
CH3N
O
CH3
CH3
N
Br
NN
CH3
CH3
DisopyramideLidocaine Tamoxifen
Diphenhydramine
Cl
CH3
CH3NN
S
Chlorpromazine Benzphetamine Brompheniramine
O
N
CH3
CH3O
HO OH
N
O
CH3
H
CH3
O
CH3
NAlicyclic Amine drugs
Meperidine Morphine Dextromethorphan
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Alicyclic Amines Often Generate Lactams
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Alicyclic Amines Often Generate Lactams
CH3
N
CH3
N O
Cyproheptadine Lactum metabolite
NH
O
H3C NH
O
OHH3C
C6H52
1
Phenmetrazine Carbinolamine
intermediate
3
C6H5
NH
O
OH3C
C6H5
3-Oxophenmetrazine
COOCH3
HN
Hydrolysis COOH
HN
COOH
HN
OMethylphenidate Ritalinic Acid 6-O xoritalinic Acid
2o& 1oAmines
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Generally, dealkylation of secondary amines occurs beforedeamination. The rate of deamination is easily influenced by stericfactors both on the a-C and on the N; so it is easier to deaminate aprimary amine but much harder for a tertiary amine.
CH3
HNCH3
CH3
NH2
CH3
O
CH2
O
NH3
Methampetamine Ampetamine Phenylacetone
Cl
NHCH3
O
Cl
NH2
O
Ketamine Norketamine
Exceptions: Some 2o and 3oamines can undergo deamination directlywithout dealkylation.
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Propranolol
O
HN CH3
CH3
OH
Direct OxidativeDeamination
O
HN CH3
CH3
OHO
OH
O
HN CH3
CH3
OHO
NH2
CH3H3C
OH
O H OH2N
Carbinolamine
O
H
O
NH3
Oxidative DeaminationThrough Primary Amine
AldehydeMetabolite
Primary Amine Metabolite(Desisopropyl Propranolol)
N-Oxidation
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N N
H H H OH
N O
1aromatic amine Hydroxylamine Nitroso
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
1amine Hydroxylamine Nitroso Nitro
O
R C N
H
H
R C N
H
H
CH3
H
CH3
OH
R C N
H
H
2amine Hydroxylamine Nitrone
CH2
O
R C N
H
H
R C N
H
H
CH3
CH3
CH3
CH3
3amine N-Oxide
O
Aromatic amines
1 amines
2 amines
3 amines
The attack is on the unbonded electrons so 3o amines can be oxidized
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Cl
H3C
CH3H
H
N
Cl
H3C
CH3OH
H
NCYP450
The attack is on the unbondedelectrons so 3 amines can be oxidized
Generally, only occurs if nothing else can happen, so it is a rare reaction
Performed by both amine oxidasesand hepatic MFOs
Good examples would include amines attached to quaternary carbonssince
they cannot be deaminated
H3C
CH3
H
H
NNH2
PhentermineAmantadine
Chlorphentermine N-HydroxylationHydroxylamine
Nitroso
Nitro
Amides
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C-N bond cleavage viaa-C hydroxylation(formation of carbinolamide) and N-
hydroxylation reactions
Oxidation involving C-O System (O-Dealkylation)
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C O R3 HO R3+
H
R1
R2
C O R3
OH
R1
R2
CYP450 SpontaneousR1 C
R2
O
Converts an ether to an alcohol plus a ketone or aldehyde
Oxidation involving C-O System (O-Dealkylation)
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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
NH3C
H3C
OH
Spontaneo
usCY
P450
Trimethoprim O-Dealkylation
O
H
OH3C
ON
CH3
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CH3O
CH3H
N OH
CH3
Cl
O
N
O
O
N
O
ON
NH2
N
NH3C
H3C
H3CO
H
CH3
CH3
OH
NO
OO OH
CH3
CodeinePhenacetin Indomethacin
Prazosin
Metoprolol
One exception that appears to be a form of O-
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H3C C OH H3C C OH
OH
H3C C O
H H
H
H
CYP450 Spontaneous
One exception that appears to be a form of O-dealkylation is the oxidation of ethanol by
CYP2E1 In this case R3 is hydrogen instead of carbon
to form the terminal alcohol rather than anether
The enzyme involved is CYP2E1 and has beenhistorically referred to as the MicrosomalEthanol Oxidizing System (MEOS)
Oxidation involving C-S System
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S-Dealkylation
C S R3
R1 C SR1 C OR1
HS R3+
R2R2
OHH
R2R3
CYP450 Spontaneous
Oxidation involving C-S System
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Desulfuration
R1 C R2
S
R1 C R2
O
Oxidation involving C-S System
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S-Oxidation
R1 S R2 R1 S R2
O
R1 S R2
O
O
Sulfoxide Sulfone
Oxidation involving C-S System
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N
N
SCH3
NH
N
6-(Methylthio)-purine
N
N
SCH2
NH
N
OH
N
N
SH
NH
N
CH2
O
6-Mercaptopurine
S-Dealkylation
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S-Dealkylation
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Desulfuration
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Desulfuration
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CH3S
CH3
NN
S
CH3S
CH3
NN
S
CH3S
CH3
NN
S
CH3S
CH3NN
S
CH3S
CH3NN
S
OO
O
O O O
Thioridazine
Ring Sulfoxide Ring Sulfone
Mesoridazine
Sulforidazine
S-Oxidation
Other Oxidative Biotransformation Pathway
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Oxidative Dehalogenation
Hepatic Microsomal Flavin Containing
Monooxygenases (MFMO or FMO)
Non-Microsomal Oxidation Reactions
Oxidative Dehalogenation
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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 wont proceed
The intermediate acyl halide is very reactive
O2N
OH
OH
NHCOCCl
O
HCl
O2N
OH
OH
NHCOC
O
OH
O2N
OH
OH
NHCOCCl2
OHO2N
OH
OH
NHCOCHCl2
Chloramphenicol
Oxamyl ChlorideDerivative
Oxamic Acid
Derivative
TissueNucleophiles
Covalent Binding(Toxicity)
QWhat is Gray Baby Syndrome?
O d D h l
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Oxidative Dehalogenation
Halothane
Metabolized to trifluoroacetic acid
Halothanetrifluoroacetyl chloride
trifluoroacetic acid
The metabolite covalently binds in livermicrosomal proteins
O d D h l
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Oxidative Dehalogenation
Chloroform
It yields the chemically reactive
PHOSGENE (causes hepato- and
nephrotoxicity).
Non-Microsomal Oxidation Reactions
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Monoamine oxidase (outer membrane of
mitochondria, flavin containing enzyme )
Dehydrogenases (cytoplasm)
Purine oxidation (Xanthene oxidase)
C N HR1
R2
H
R3
CR1
R2
O + H N H
R3
Monoamine oxidase
Non-Microsomal Oxidation Reactions
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Two MAOs have been identified: MAOAand MAOB.Equal amounts are found in the liver, but the brain
contains primarily MAOB; MAOA is found in theadrenergic nerve endings
MAOA shows preference for serotonin,catecholamines, and other monoamines with phenolic
aromatic rings and MAOB prefers nonphenolicamines
Metabolizes 1 and 2 amines; N must be attached to-carbon; both C & N must have at least one
replaceable H atom. 2 amines are metabolized byMAO if the substituent is a methyl group
Phenylisopropylamines such as amphetamine andephedrine are not metabolized by MAOs but arepotent inhibitorsof MAOs
R2R2 R1 C OR1 C O
Alcohol dehydrogenase Aldehyde dehydrogenase
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C OHR1
HCR1
2
O
R1 C O
OH
R1 C O
H
Metabolizes 1 and 2 alcohols and aldehydescontaining at least oneHattached to a-C;
1 alcohols typically go to the aldehyde then acid; 2 alcohols are converted to ketone, which cannot be
further converted to the acid. The aldehyde is converted back to an alcohol byalcohol (keto) reductases (reversible), however, itgoes forward as the aldehyde is converted tocarboxylic acid;
3 alcohols and phenolic alcohols cannot be oxidizedby this enzyme; NoHattached to adjacent carbon
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H3C
H2C
OH H3C
HC
O H3CC
O
OHAlcohol
DehydrogenaseAldehyde
Dehydrogenase
Ethanol Metabolism
Purine oxidation
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O
HN N
NH
NH
O
O
HN N
N NH
O
HN N
NH
NH
O
OH
O
HN
H
N
NH
NH
O
O
Hypoxanthine Xanthine Uric acid(hydroxy tautomer)
Uric acid(keto tautomer)
Xanthine
oxidase
Xanthine
oxidase
General Metabolic Pathways
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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
Reductive Reactions
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Bioreduction of C=O (aldehyde and keton) generates alcohol
(aldehyde 1oalcohol; ketone 2oalcohol)
Nitro and azo reductions lead to amino derivatives
Less Common Reactions:
Reduction of N-oxides to their corresponding 3o
amines andreduction of sulfoxides to sulfides are less frequent
Reductive cleavage of disulfide (-S-S-) linkages and reduction
of C=C are minorpathways in drug metabolism
Reductive dehalogenation is a minor reaction primarily differfrom 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
Reduction of Aldehydes & Ketones
H H
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C=O moiety, esp. the ketone, is
frequently encountered in drugs andadditionally, ketones and aldehydes arisefrom deamination
Ketonestend to be converted to alcohols
which can then be glucuronidated.
Aldehydes can also be converted toalcohols, but have the additional pathway
of oxidation to carboxylic acids.
R C O
H
R C
H
OH
H
Aldehyde 1alcohol
R C O
R2
R1 C
R2
OH
H
Ketone 2alcohol
Reduction of Aldehydes & Ketones
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Reduction of ketonesoften leads to the creation
of an asymmetric center and thus twostereoisomeric alcohols are possible
Reduction of a, unsaturated ketones found insteroidal drugs results not only in the reduction of
the ketonebut also of the C=C Aldoketo oxidoreductases carry out
bioreductions of aldehydes and ketones.
Alcohol dehydrogenase is a NAD+ dependent
oxidoreductase that oxidizes alcohols but in thepresence of NADH or NADPH, the same enzymecan reduce carbonyl compounds to alcohols
Reduction of Aldehydesd K t
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and Ketones
Chloral Hydrate
Can undergo enzymatic reduction to
form trichloroethanolas a major
metabolite (pharmacologically active)
Further glucoronidation lead to an
inactive conjugated product that is
easily excreted in the urine.
Reduction of Aldehydesd K t
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and Ketones
Propranolol
It is converted to an intermediate
aldehyde by N-dealkylation or
oxidative deamination.
The aldehyde is oxidized to carboxylic
acid (naphthoxylactic acid) but a small
fraction is reduced to the alcohol
derivative (propranolol glycol)
Reduction of Aldehydesd K t
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and Ketones
Bioreduction of ketones leads to the
creation of asymmetric center , thereby
there are two possible sterioisomericalcohol.
Reduction of Aldehydesd K t
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and Ketones
Product Sterioselectivity
The preferential formation of one
isomer over the other
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R1C
R2
O
N
R
HH
H2N
OH+
R1C
R2
HO H+
N+
R
H2N
O
Ketone Chiral AlcoholRed Nicotinamide moiety
of NADPH or NADH
Ox Nicotinamide moiety
of NADP+or NAD+
Ketone reduction involves a hydride transfer from the
reduced Nicotinamide moiety of the cofactor NADPH or
NADH to the carbonyl carbon atom of the ketone.
Reduction of Aldehydesand Ketones
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and Ketones
Acetohexamide
Is the metabolized in the liver to give
primarily (S)(-)-hydroxyhexamide.
The metabolite is as active as the
parent compound.
It is eliminated through the kidneys.
Reduction of Aldehydesand Ketones
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and Ketones
Acetohexamide
It is not recommended to diabetic
patients with kidney failure because of
the possible accumulation of
hydroxyhexamide.
OH H2C
O
CH3
C H
OH H2C CH3
C6H5
HO H
OH H2C CH3
C H
H OH
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O O
H
C6H5
R(+)-Warfarin
O O
H
C6H5
O O
H
C6H5
R,S(+)-Warfarin R,R(+)-Warfarin
+
Warfarin
Undergoes extensive reduction of its side chain keto group.
R,S (+) is the major metabolite
Naltrexone
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Reduction of the 6-keto functionality can lead to either 6-or
6-hydroxy metabolites depending on the species.
Humans and rabbits highly sterioselective (6-hydroxy
metabolites )
Chickens- 6-hydroxy metabolite
Monkeys and guinea pigsboth but predominantly CH2
HO
OH
O
N
O
Naltrexone
Reduction of Nitro & Azo Compounds
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N NR
Azido
NH2R
Amine
NH + N N
N2
N N R2R1 R1 NH2 H2N R2+
Azo Two 1amines
HNR1
Hydrazo
HN R2
R C N
H
H
R C N
H
H
H
H
H
OHR C N
H
H
R C N
H
H
O
O
1amineHydroxylamineNitrosoNitro
O
R1and R2are almost always aromatic
Usually only seen when the NO2functional group is attached directlyto an aromatic ring and are rare
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to an aromaticring 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 reductasesin intestine can reduce both nitro and azo
Cl
HO
O2N N
NO
O
NNaNN
O2NOS
NH
O O
N
N
O
HO
OHN
SNH2
O O
N
NH2
N
H2NS
NH2
O O
H2N NH2
NH2
H2N
+
Prontosil Sulfanilamide 1,2,3-Triaminobenzene
Clonazepam SulfasalazineDantrolene
Reduction of Sulfur Containing Compound
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XSulfoxide reduction
(Cannot reduce a sulfone)
R1 S R2
O
R1 S R2 R1 S R2
O
O
R1 S S R1 SHR2 HS R2+Disulfide reduction
H3C
H3C CH3
S
S
CH3NS
SN H3C
H3CS
SHN
Disulfiram N,N-Diethylthiocarbamic
Acid
O
H3C
OH
OF
CH3
S
H
Sulindac
SulfoneSulfoxide
General Metabolic Pathways
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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
Hydrolytic Reactions
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Enzymes: Non-microsomal
hydrolases; however, amide hydrolysis
appears to be mediated by liver
microsomal amidases, esterases, and
deacylases Electrophilicityof the carbonyl carbon,
Nature of the heteroatom, substituents
on the carbonyl carbon, and
substituents on the heteroatom
influence 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
Table: Naming carbonyl - heteroatom groups
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
R1 C R2
O
+
The Reactions
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R1 C
O
O R2 R1 C
O
OH HO R2
R1 C
OHN 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
The Reactions
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O C NR1
OHO C NR1
OOH HO C OH
OHN 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 CHN N
OR2
R3R1 C OH
O
H2N NR2
R3+
Hydrazide Hydrazine
Carbamate hydrolysis
Urea hydrolysis
Hydrazide hydrolysis
Drug Examples
OH OH
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H3COO
O
N
CH3
O
Cocaine
OHO
O
N
CH3
O
H3COO
N
CH3
HO+
Benzoylecgonine Methylecgonine
H3C O
O
O
H3C OOH
O OH
+
Aspirin Salicylic Acid
CH3
CH3N
H2N
O
O
CH3
CH3N
H2N
O
HN
Procainamide
Procaine
H2N
O
OH
Slow Hydrolysis
Rapid Hydrolysis
OH
OH3C
O
CH3
Cl
O
N
Indomethacin
CH3
CH3
CH3
CH3
O
N
HN
Lidocaine
O
O
N
O
ON
NH2
N
NH3C
H3C
Prazosin
Stereoselectivity of Hydrolysis
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Etomidate (Amidate, hypnotic): R-(+)-isomer is more rapidly hydrolyzed,
but S-(-)-isomer is more rapidly hydroxylated.
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Phase 2 Reactions
Synthetic Conjugation
Phase I I
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Phase II - combines functional group of compoundwith endogenous substance
E.g.Glucuronic acid, Sulfuric acid, Amino Acid, Acetyl.
Products usually very hydrophilic
The final compounds have a larger molecular weight.
How We Get To Phase 2
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Most of the drugs do not become polar uponphase 1 reactions.
The Body is left with a plan to further
metabolize the Drugs
Goal of Phase 2 : Make substances more
soluble that couldnt be done in the Phase 1
reactions.
Synthetic Reactions / Phase I I
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These reactions usually involves covalent attachments
of small polar endogenous molecules such as
Glucoronic acid, Sulfate, Glycine to either unchanged
drugs or Phase I product having suitable functional
groups as COOH,-OH,-NH2,- SH.
Thus is called as Conjugation reactions.
Since the product formed is having high molecular
weight so called as synthetic reactions.
The product formed is hydrophilic in nature with totalloss of pharmacologic activity so called as a true
detoxification reaction
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Glucuronic Acid Conjugation
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Four general classes of glucuronides: O-,
N-, S-, and C-
Neonates have undeveloped liver UDP-
glucuronosyltransferase activity, and may
exhibit metabolic problem. For example,
chloramphenicol (Chloroptic) leads
neonates to gray baby syndrome
Neonatal jaundice may be attributable to
their inability to conjugate bilirubin with
glucuronic acid
Glucuronide formation occurs in 2 steps:-
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1. Synthesis of an activated coenzyme uridine-5- diphospho -alpha-D- Glucuronic acid (UDPGA) from UDP- glucose (UDPG).
-D-Glucose-1-phosphate + UDPG +UTP Ppi
UDPG +2NAD + H2O UDPGA
+2NADH + 2H+
Pyrophosphorylase
UDPG - Dehydrogenase
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2. Transfer of the glucuronyl moiety from UDPGA to
the substrate RXH in presence of enzyme UDP-glucuronyl transferase to form the conjugate.
UDPGA + RXH RXGlucuronic Acid +UDP
Where,
X = O, COO, NH or S
UDP-Glucuronyl transferase
03-12-2010 143KLECOP, Nipani
Formation of Glucuronide Conjugate
UTP PPi
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OHOHO
HOOPO 3
2-
HOOHO
HO
HO O
HOUTP PPi
Phosphorylase
a-D-Glucose-1-phosphate
P
O
O
O -P
O
O-O
ON
HO OH
NH
O
O
OHOOC
HOHO
HOO P
O
O
O -P
O
O-O
ON
HO OH
NH
O
O
2NAD+
2NADH
UDPG
dehydrogenase
RXH
UDPOHOHO
HOXR
HO
UDP-Glucuronyl-transferase(microsomal)
-D-Glucuronide
UDPG
Uridine-5'-diphospho-a-D-Glucose (UDPG)
O
Types of Compounds Forming Glucuronides
TYPE EXAMPLES
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TYPE EXAMPLES
O-Glucuronide
Phenols
Alcohols
Enols
N-hydroxyamines/amides
Acetaminophen morphine
Chloramphenicol Propranolol
Hydroxycoumarine
N-hydroxydapsone N-Hydroxy-2-acetylaminoflourene
OH
CH3
O
HN
HO OH
N
O
CH3
O2N
Cl
ClO
HN
OH
OH
H
CH3
CH3
OH
NO
O O
OH
SO2
H2N NHOH N
CH3
OH
Aryl acidsOH
COOH
CH3
Salicylic acid
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Arylalkyl acids
O
OH
O
N
H
N
NH2
O2N
Fenoprofen
N-Glucuronides
Arylamines7-Amino-5-
nitroindazole
AlkylaminesN
H
CH3
N
Desipramine
AmidesH3C
O
NH2
NH2
H3C O
O
O
Meprobamate
Sulfonamides
OO
H2N
NH
S
CH3
CH3
NO
Sulfisoxazole
3o
Amines
CH3N
Cyproheptadine
Sulfate Conjugation
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Occurs less frequently than does
glucuronidation presumably due tofewer number of inorganic sulfates inmammals and fewer number offunctional groups (phenols, alcohols,
arylamines and N-hydroxy compounds)
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Sulfotransferases are widely-distributed enzymes Cofactor is 3-phosphoadenosine-5-phosphosulfate
(PAPS)
Produce highly water-soluble sulfate esters,eliminated in urine, bile
R OH R O SO3
1 Synthesis of an activated coenzyme 3-phosphoadenosine-5-
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1. Synthesis of an activated coenzyme 3 -phosphoadenosine-5 -
phosphosulfate (PAPS) which acts as a donor of sulfate to the
substrate. This also occurs in two steps- an initial interaction between
the sulfate and the adenosine triphosphate (ATP) to yield
adenosine-5-phosphosulfate (APS) followed by activation of
latter to PAPS.
ATP + SO42- APS + Ppi
APS + ATP PAPS + ADP
ATP Sul furylase/Mg++
APS Phosphokinase/Mg++
2. Transfer of sulfate group from PAPS to the substrate RXH in
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g p
presence of enzyme Sulfotransferase and subsequent liberation
of 3- phosphoadenosine-5-phosphate(PAP).
PAPS + RxH Rx-SO3+ PAP
X= O,NH
Examples of compounds undergoing sulfation are:
Phenol Paracetamol , Salbutamol
Alcohols Aliphatics C-1 to C-5 Arylamines Aniline
Sulfotransferase
Sulfation of Drugs
Phenolic sulfation predominates
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COOHH3C
H
H
N
HO
HOHO CH3
HOCH3
CH3
HOH
N
Phenolic sulfation predominates
Phenolic O-glucuonidation competes favorably with sulfation due
to limited sulfate availability
Sulfate conjugates can be hydrolyzed back to the parentcompound by various sulfatases
Sulfoconjugation plays an important role in the hepatotoxicity andcarcinogenecity of N-hydroxyarylamides
In infants and young children where glucuronyltransferase activityis not well developed, have predominating O-sulfate conjugation
Examples include: a-methyldopa, albuterol, terbutaline,acetaminophen, phenacetin
a-Methyldopa
CH3CH3
CH3
OH
HOH
NHO
Albuterol Terbutaline
Sulfation of Drugs
Acetaminophen
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Acetaminophen
O-sulfate conjugate is the mainurinary metabolite in infants andyoung children.
Amino Acid Conjugation
The first mammalian drug metabolite isolated, hippuric
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acid, was the product of glycine conjugation of benzoic
acid
Amino acid conjugation of a variety of caroxylic acids, suchas aromatic, arylacetic, and heterocyclic carboxylic acids
leads to amide bond formation Glycine conjugates are the most common
Taurine, arginine, asparagine, histidine, lysine, glutamate,aspartate, alanine, and serine conjugates have also been
found
COH
R O
Benzoic Acid, R = HSalicylic Acid, R = OH
CONHCH2COH
R O O
Hippuric Acid, R = HSalicyluric Acid, R = OH
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Alternative to glucuronidation Two principle pathways
-COOH group of substrate conjugated with -NH2
of Glycine, Serine, Glutamine, requiring CoA
activation
E.g. conjugation of benzoic acid with Glycine
to form Hippuric acid
Aromatic -NH2or NHOH conjugated with -COOHof Serine, Proline, requiring ATP activation
1. Activation of carboxylic acid drug substrate with ATP and
coenzyme A (CoA) to form an acyl CoA intermediate. Thus, the
reaction is a contrast of glucuronidation and sulfation where the
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reaction is a contrast of glucuronidation and sulfation where the
donor coenzyme is activated and not the substrate.
RCOOH + ATP RCOAMP + H2O + Ppi
RCOAMP + CoA-SH RCSCoA + AMP
2. Acylation of the alpha- amino acid by the acyl CoA in presence
of enzyme N-acyl transferase.
RCSCoA RCONH-RCOOH
+ NH2-R-COOH + CoA- SH
Acetyl Synthetase
Acyl CoA Transferase
N-Acetyl transferase
Brompheniramine Metabolism
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Br
NN CH3
CH3
P450
Br
NNH
CH3
P450
Br
NNH2
P450
Br
N CHO
Br
NN
CH3
CH3
Br
N
O
HN COOH
Br
N COOH
Brompheniramine
Aldehydedehydrogenase
GlycineN-acyltransferase
Carboxylic Acid metabolite
Brompheniramine N-oxide Glycine conjugate
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Glutathione-S-transferase catalyzes conjugation with
glutathione
Glutathione is tripeptide of Glycine, Cysteine, Glutamic
acid
Glutathione Conjugation
HNH2 O
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Glutathione is a tripeptide (Glu-Cys-Gly) found
virtually in all mammalian tissues
Its thiol functions as scavenger of harmful
electrophilicparent drugs or their metabolites
NH
HN
NH2
O
HS
O
O
HO
O
OH
NH
HN
NH2
OS
O
O
OH
O
HO
S
HN
NH
O O
OH
O
HO
Glutathione reduced form (GSH) Glutathione oxidized form (GSSG)
Mercapturic Acid Conjugates
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NH
HN
NH2
OS
O
O
HO
O
OH
Drug
Amino Acid(AA)
-Glutamyltranspeptidase
-Glutamyl-AA
NH2
HN
S
O
O
HO
Drug
Glutathione Conjugate
Glycine
CysteinylGlycinase
NH2HO
S
O
Drug
S-substitutedCysteineDerivative
AcetylCoA CoASH
N
H
H2N
S
O
Drug
CH3
O
Mercapturicacid conjugate
Acetyl Conjugation
Metabolism for drugs containing a primary amino group, (aliphatic and aromatic
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amines), amino acids, sulfonamides, hydrazines, and hydrazides
The function of acetylation is to deactivate the drug, although N-acetylprocainamide is as potent as the parent antiarrhythmic drug procainamide
(Procanbid) or more toxic than the parent drug, e.g., N-acetylisoniazid
Acetylation is two-step, covalent catalytic process involving N-acetyl transferase
H3C SCoA
O CoASH
H3C X
O
H2N R
X-
H3C
O
NHR
X-
N-Acetylation of amines
Geneticpo lymorph ismin N-acetyltransferaseactivity
Multiple NAT2 alleles (NAT2*5, *6, *7, and *14) have substantially decreasedacetylation activity and are common in Caucasians and populations of African
descent. In these groups, most individuals carry at least one copy of a slow
acetylator allele, and less than 10% are homozygous for the wild type (fast
acetylator) trait. The ratio of NAT2 activity is 7 in Caucasians to 18 in the Chinese
population.
Example of Acetylated Drugs
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O
CH3
CH3
ONH NH2
O
OHSHO
Cilastatin
NHHN
SH3C
HO
COOHO
N
Imipenem
Methyl Conjugation Minorconjugation pathway, important in biosynthesis of epinephrine
and melatonin; in the catabolism of norepinephrine dopamine
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and melatonin; in the catabolism of norepinephrine, dopamine,
serotonin, and histamine; and in modulating the activities of
macromolecules (proteins and nucleic acids)
Except for the formation of quarternary ammonium salts, methylation
of an amine reduces the polarityand hydrophilicity of the substrates
A variety of methyl transferase, such as COMT(catechol O-methyl
transferase), phenol-O-methyltransferase, N-methyl transferase, S-
methyltransferase etc are responsible for catalyzing the transfer ofmethyl group from SAMto RXH
H3CS
H2N COOH
S+
H2N COOH
O
HO OH
AdCH3 HX-R
CH3-X-R
S
H2N COOH
O
HO OH
AdMethionine
adenosyltransferase
Methyltransferase+
Mthetionine
S-Adenosylmethionine
Mechanism of methyl conjugation
ATP PPi + Pi
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Age Differences
Species and Strain Differences
Hereditary or genetic factor
Sex differences
Enzyme induction/inhibition
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End of Presentation
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