Metabolic Changes of Drugs
Books: 1. Wilson and Gisvold’s Textbook of Organic Medicinal and
Pharmaceutical Chemistry 11th ed. Lippincott, Williams & Wilkins ed.
2. Foye’s Principles of Medicinal Chemistry
Introductory Concepts
■
Biochemically speaking: Metabolism means Catabolism
(breaking down of substances) + Anabolism (building up or
synthesis of substances)
■
But when we speak about drug metabolism, it is only catabolism
■
That is drug metabolism is the break down of drug molecules
■
So what is building the drug molecules? We use the word
“synthesis”, then
■
Drugs are synthesized in laboratory and thus is not an
endogenous event
■
Lipid soluble drugs require more metabolisms to become polar,
ionizable and easily excretable which involve both phase I and
phase II mechanisms.
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
■
Excretion physically removes drugs from the body
The major excretory organ is the kidney. The kidney is very good at excreting
polar and ionized drugs without any major metabolism. The kidney is unable to
excrete drugs with high LWPC
■
In general, by metabolism drugs become more polar, ionizable and thus
more water soluble to enhance elimination
■
It also effect deactivation and thus detoxication or detoxification
■
Many drugs are metabolically activated (Prodrugs)
■
Sometimes drugs become more toxic and carcinogenic
Metabolite Examples and notes
activity
Routes that result in the formation of inactive metabolites are often referred to as detoxification.
Inactive
OH
O
O
(detoxification)
Phenol sulphokinase
S
O
3'-Phosphoadenosine-5'phosphosulfate (PAPS)
Phenol
Similar activity
to the drug
OH
Phenyl hydrogen sulfate
The metabolite may exhibit either a different potency or duration of action or both to the
original drug.
CH3
CH3
O
O
O
N
Hydroxylation
N
Cl
H
N
N
OH
N-Demethylation
Ph
Diazepam
(Sustained anxiolytic action)
N
Cl
N
Cl
OH
Ph
Oxazepam
(short duration)
Ph
Temazepam
(Short duration)
CH3
CONHNHCH
CONHNH2
CH3
Different
activity
N-Dealkylation
N
Ipronazid
(Antidepressant)
N
Isoniazid
(Antituberculosis)
HO
Toxic
metabolites
NCOCH3
NHCOCH3
NH2
Other substances
responsible for
hepatotoxicity
Substances responsible
for methemoglobinamia
OC2H5
N-Hydroxyphenacetin
(Hepatotoxic)
OC2H5
Phenacetin
(Analgesic)
OC2H5
Phenetidine
Stereochemistry of Drug Metabolism
OH
OH
CH2COCH3
H
Ph
Ph
H
O
O
O
R-(+)-Warfarin
O
S-(-)-Warfarin
OH Major route
OH
CH2COCH3
HO
OH H2C
Ph
O
S-6-Hydroxywarf arin
H
CH3
OH
Minor route
O
O
R,S-(+)-alcohol derivative
CH3
H
Ph
O
O
R,R-(+)-alcohol derivative
COOH
Metabolism
H
COOH
R-(-)-Ibuprofen
(inactive)
H
CH3
OH H2 C
H
Ph
H
O
CH2COCH3
H
CH3
S-(+)-Ibuprofen
(active)
Sites of Drug Metabolism
 Liver: Major site, well organized with all enzyme systems
The first-pass effect
Following drugs are metabolized extensively by first-pass effect: Isoproterenol,
Lidocaine Meperidine, Morphine, Pentazocine, Propoxyphene, Propranolol,
Nitroglycerin, Salicylamide
 Intestinal Mucosa: The extra-hepatic metabolism, contains CYP3A4 isozyme
 Isoproterenol exhibit considerable sulphate conjugation in GI tract
 Levodopa, chlorpromazine and diethylstilbestrol are also reportedly metabolized
in GI tract
 Esterases and lipases present in the intestine may be particularly important
carrying out hydrolysis of many ester prodrugs
 Bacterial flora present in the intestine and colon reduce many azo and nitro
drugs (e.g., sulfasalazine)
 Intestinal b-glucuronidase can 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 Metabolism
CYP450, Hepatic microsomal flavin containing monooxygenases (MFMO
or FMO) Monoamine Oxidase (MAO) and Hydrolases
 Cytochrome P450 system: localized in the
smooth endoplasmic reticulum.
 Cytochrome P450 is a Pigment that, with CO
bound to the reduced form, absorbs maximally at
450nm
 Cytochromes are hemoproteins (heme-thiolate)
that function to pass electrons by reversibly
changing the oxidation state of the Fe in heme
between the 2+ and 3+ state and serves as an
electron acceptor–donor
 P450 is not a singular hemoprotein but rather a
family of related hemoproteins. Over 1000 have
been identified in nature with ~50 functionally
active in humans with broad substrate specificity
Simplified apoprotein portion
HOOC
CH3
L
N
N
Fe+3
N
N
CH3
CH2
CH3
HOOC
CH3
CH2
O
H R
Substrate binding site
Heme portion with
activated Oxygen
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
■
Currently, all P450’s are named by starting with “CYP” (CYtochrome P450,
N1, L, N2 - the first number is the family (>40% homology), the letter is the
subfamily (> 55% homology), and the second number is the isoform. The
majority of drug metabolism is by ~10 isoforms of the CYP1, CYP2 and
CYP3 families in humans
■
Major human forms of P450: Quantitatively, in the liver the percentages of
total P450 protein are: CYP3A4 – 28%, CYP2Cx – 20%, CYP1A2 – 12%,
CYP2E1 – 6%, CYP2A6 – 4%, CYP2D6 – 4%
■
By number of drugs metabolized the percentages are: CYP3A4 – 35%,
CYP2D6 – 20%, CYP2C8 and CYP2C9 – 17%, CYP2C18 and CYP2C19
- 8% CYP 1A1 and CYP1A2 -10%, CYP2E1 – 4%, CYP2B6 – 3%
Few Important CYP450 Isozymes
CYP
family
Main functions
CYP1
Xenobiotic metabolism
CYP2
Xenobiotic metabolism, Arachidonic acid metabolism
CYP3
Xenobiotic and steroid metabolism
CYP7
Cholesterol 7α-hydroxylation
CYP11
Cholesterol side-chain cleavage, Steroid 11β –
hydroxylation, Aldosterone synthesis
CYP17
Steroid 17α-hydroxylation
CYP19
Androgen aromatization
CYP21
Steroid 21-hydroxylation
CYP24
Steroid 24-hydroxylation
CYP27
Steroid 27-hydroxylation
EC
Recommended name
Family/gene
1.3.3.9 *
secologanin synthase
CYP72A1
1.14.13.11 *
trans-cinnamate 4-monooxygenase
CYP73
1.14.13.12 *
benzoate 4-monooxygenase
CYP53
1.14.13.13 *
calcidiol 1-monooxygenase
CYP27
1.14.13.15 *
cholestanetriol 26-monooxygenase
CYP27
1.14.13.17 *
-monooxygenase
CYP7
1.14.13.21 *
flavonoid 3'-monooxygenase
CYP75
1.14.13.28 *
3,9-dihydroxypterocarpan 6a-monooxygenase
CYP93A1
1.14.13.30 *
leukotriene-B4 20-monooxygenase
CYP4F
1.14.13.37 *
methyltetrahydroprotoberberine 14-monooxygenase
CYP93A1
1.14.13.41 *
tyrosine N-monooxygenase
CYP79
Drug Interactions & Metabolism
The drug interactions depend upon:
a) the isoform(s) required by the drug in question,
b) the isoforms altered by concomitant therapy,
c) the type of enzyme alteration (induction or
inhibition).
General Metabolic Pathways
Hydrolytic Reactions
 Esters and amides
 Epoxides and arene oxides
by epoxide hydrase
Phase II Conjugation
Phase I Functionalization
Drug
Metabolism






Glucuronic acid conjugation
Sulfate Conjugation
Glycine and other AA
Glutathion or mercapturic acid
Acetylation
Methylation
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
Reduction
 Aldehydes and ketones
 Nitro and azo
 Miscellaneous
Tetrahydrocannabinol (D1-THC) Metabolism
7
CH3
6
5
1
4
2
CH2OH
OH
OH
3
H3C
O
CH3
D1-THC
COOH
C5H11
OH
H3C
H3C
O
CH3
C5H11
O
CH3
7-Hydroxy-D1-THC
C5H11
D1-THC-7-oic Acid
COOR
OR
COO-
Where R =
O
H3C
O
CH3
C5H11
Glucuronide conjugate at either
COOH or phenolic OH group
OH
OH
HO
H
b-Glucuronyl
moiety
The metabolite is polar, ionisable and hydrophilic
Oxidative Reactions
Arenols
OH
Arene Oxides
O
Epoxides
O
C C
C
C
Benzylic, allylic
aliphatic C
Hydroxylation
C OH
C H
R N H
"Activated Oxigen"
[FeO]3+
Miscellaneous
Oxidations
S C
S P
O C
O P
Desulfuration
R N OH
R N CH2R
R O CH3
R N
R NH + O CHR
R N
O
S CH3
R OH
O
O-Dealkylation
SH,
S CH3
S-Dealkylation
and S-Oxidation
N-Hydroxylation
N-Dealkyaltion and
Oxidative Deamination
N-Oxide Formation
■
Hydroxylation is the primary reaction mediated by CYP450
■
Hydroxylation can be followed by non-CYP450 reactions including
conjugation or oxidation to ketones or aldehydes, with aldehydes
getting further oxidized to acids
■
Hydroxylation of the carbon α to heteroatoms often lead to cleavage of
the carbon – heteroatom bond; seen especially with N, O and S,
results in N–, S– or O–dealkylation.
■
Must have an available hydrogen on atom that gets hydroxylated, this
is important!!!
Aromatic Hydroxylation
R1
R1
R1
Spontaneous
CYP450
■
■
O
Mixed function oxidation of arenes to
arenols via an epoxide intermediate
arene oxide
Occurs primarily at para position
■
Substituents attached to aromatic ring
influence the hydroxylation
R1
R1
Epoxide
hydrolase
Epoxide
Hydrase
Major route of metabolism for drugs with
phenyl ring
■
■
OH
Aromatase
OH
OH
R1
Glutathione
OH
S
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
OH
OH
Glutathione
R1
Macromolecule
OH
Macromolecule
H
H
H
CYP2C19
N
O
O
N
H
Phenytoin
H
CH3
O
O
H
O
N
HO
N
N
Amphetamine
p-hydroxyphenytoin
O
OH
OH
O
C CH
H
N
CH3
ONa
CH3
O
HO
Warfarin sodium
17-a-Ethinylestradiol
O
HO
C
Propranolol
Ca+2
O
OH
O
CH3
N
F
N
CH3
N
H 3C
HN
CH3
O
C
O
Phenylbutazone
2
Atorvastatin
O
Cl
H
N
H3C
N
O
N S
OH
O
Cl
HN
H3C
Antihypertensive drug clonidine undergo little
aromatic hydroxylation and the uricosuric
agent probenecid has not been reported to
undergo any aromatic hydroxylation
Probenecid
Clonidine
CH3 O
N
Cl
N
N
N
S
CH3
CH3
Preferentially the more electron
rich ring is hydroxylated
Cl
Diazepam
Chlorpromazine
NIH Shift: Novel Intramolecular Hydride shift named after National Institute of Health where
the process was discovered. This is most important detoxification reaction for arene oxides
R
R
Spontaneous
Rearrangement
NIH Shift
+
O
Arene Oxide
R
R
-
OH
Arenol
H
H
O
H
OH
Oxidation of olefinic bonds (also called alkenes)
O
Epoxide hydrolase
Epoxide
Alkene
OHOH
trans dihydrodiol derivative
■
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 trans1,2-dihydrodiols (also called 1,2-diols or 1,2-dihydroxy compounds)
■
Terminal alkenes may form alkylating agents following this pathway
HO
O
Epoxide hydrolase
CYP3A4
N
O
N
NH 2
Carbamazepine
(Active)
OH
O
N
NH 2
Carbamazepine 10,11 epoxide
(Active & Toxic)
O
NH2
Carbamazepine trans 10,11 diol
(Inactive)
Q. Any similarities or dissimilarities with aromatic – NIH Shift, Conjugation
with macromolecules?
Benzylic Carbon Hydroxylation
R2
R2
R1
C
R1
H
O
C
O
S
N
H
■
Hydroxylate a carbon attached to a phenol group (aromatic ring)
■
R1 and R2 can 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
aldehyde and carboxylic acids and secondary alcohols are
converted to ketones by soluble alcohol and aldehyde
dehydrogenase
OH
O
O
N
H
CH3
O
S
CYP2C9
H
H3C
HO
Tolbutamide Metabolism
C
H
O
ONa
N
CH3
O
H3C
Tolmetin sodium
Dicarboxylic acid is the
major metabolite
N
H
O
N
H
CH3
Oxidation at Allylic Carbon Atoms
H
R1
C
C
H
H
C
C
R2 H
7
5
1
4
2
R3
R1
R4
C
OH
C
C
R3
R4
CH3
CH3
HO
HO
OH
OH
C
H
R2 H
7CH2OH
CH3
6
H
OH
OH
3
+
H3C
H3C
O
CH3
O
CH3
C5H11
D1-THC
+
H3C
C5H11
O
CH3
H3C
O
CH3
C5H11
6b-Hydroxy-D1-THC
6a-Hydroxy-D1-THC
7-Hydroxy-D1-THC
H2C
H2C
3
2
HO
OH
H
HO
N
N
1
H3CO
H3CO
N
Quinine
N
3-Hydroxyquinine
C5H11
O-Glucuronide Cojugate
O
O
O
CH3
O
2'
CH3
Hexabarbital
3'
O
O
CH3
O
OH
CH3
3'-Hydroxyhexabarbital
Pentazocine
O
CH3
O
O
CH3
3'-Oxohexabarbital
Hydroxylation at C a to C=O and C=N
CH 3
N
R
O
H
C
C
H
R'
R
O
H
C
C
CH 3
O
N
OH N-demethylation
3
N
R' Cl
H
N
O
N
Cl
O
OH
Cl
N
OH
The
benzodiazepines
are classic examples
with both functionalities
(3S) N-Methyloxazepam
or 3-Hydroxydiazepam
Diazepam
(CH3 CH 2 )2 NCH2 CH 2
CH 3
O
N
N
3
N
Cl
Oxazepam
O
3
N
O2 N
F
Flurazepam
The sedative hypnotic
glutethimide possesses
C a to carbonyl function
4
CH2 CH 3
C6 H5
3
Nimetazepam
HO
4
CH2 CH 3
C6 H5
1
O
N
O
H
Glutethemide
O
N
O
H
4-Hydroxyglutethemide
Aliphatic hydroxylation
R1
R1
H
H
H
C
C
C
H
H
H

H
H
H
C
C
C
H
H
H
■
OH
Catalyzes hydroxylation of the ω and ω-1
carbons in aliphatic chains
Generally need three or more unbranched
carbons
■
H
R1
H
OH H
C
C
C
H
H
H
H
O
H
O
N
H
N
CYP450
Pentobarbital Metabolism
O
O
N
O
N
OH
H
CH3
OH
CH3
CH3
O
O
CYP450
OH
H3C
H 3C
H
CH3
O
Ibuprofen Metabolism
O
OH
+
CH3
HOOC
OH
CH3
Alicyclic (nonaromatic ring)
Hydroxylation
■
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
OH
O
O
S
H3C
N
H
O
O
N
H
O
S
CYP450
H3C
O
O
Acetohexamide Metabolism
N
H
O
N
H
Oxidation Involving CarbonHeteroatom Systems
■
C-N, C-O and occasionally C-S
■
Two basic types of biotransformation processes:
1.
Hydroxylation of a-C attached directly to the heteroatom (N,O,S).
The resulting intermediate is often unstable and decomposes with
the cleavage of the C-X bond:
H
O
H
R
X
Ca
R
X
Ca
O
R
XH
+
Usually Unstable
Oxidative N-, O-, and S-dealkylation as well as oxidative deamination
reaction fall under this category
2.
■
Hydroxylation or oxidation of heteroatom (N, S only, e.g., Nhydroxylation, 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 metabolic products (e.g.,
oxime, nitrone, nitroso, imino)
C-N systems
■
■
■
■
Aliphatic (1o, 2o, 3o,) and alicyclic (2o and 3o) amines; Aromatic and heterocyclic
nitrogen compounds; 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
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 Ndealkylation and N-oxidation
H
O
H
R1
N
Ca
R1
N
Ca
O
R1
R2
R2
3o or 2o amine
NH
+
R2
Carbinolamine
2o or 1o amine
H
o
H
O
Ca
Ca
NH2
NH2
1 amine
Carbinolamine
O
+
Carbonyl
NH3
Ammonia
N-Dealkylation (Deamination)
H
R1
C
N
R3
OH
CYP450
R1
R2 R4
C
Spontaneous
N
R1
R3
C
R2
R2 R4
O
+
HN
R3
R4
■
Deamination and N-dealkylation differ only in the point of reference; If the drug is R1 or R2
then it is a deamination reaction and If the drug is R3 or R4 then 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; branching on 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 carbon
cannot 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 i–Pro are rapidly removed; branching on these
substituents slows it down even more
OH
N
N
CH3
CYP2C19
CH3
Imipramine N-Dealkylation
N
N
CH2
CH3
Spontaneous
N
N
H
CH3
Alicyclic Amines Often Generate Lactams
N
OH
N
N
CH3
N
CH3
N
CH3
N
Cotinine
Carbinolamine
Nicotine
1
C6 H 5
O
2
3
H 3C
C6 H 5
N
H
Phenmetrazine
COOCH 3
HN
Methylphenidate
O
C6 H 5
H 3C
Hydrolysis
N
OH
H
Carbinolamine
intermediate
O
N
N
CH3
CH3
Cyproheptadine
H 3C
N
O
H
3-Oxophenmetrazine
COOH
COOH
HN
HN
Ritalinic Acid
O
O
6-Oxoritalinic Acid
O
Lactum metabolite
CH3
3oAmine drugs
H3C
CH3
N
N
CH3
CH3
H
N
N
C
CH3
O
CH3
H3C
CH3
N
O
O
CH3
Lidocaine
NH2
CH3
Tamoxifen
Disopyramide
CH3
O
N
CH3
CH3
N
N
S
N
CH3
N
CH3
CH3
N
CH3
CH3
Cl
Br
Diphenhydramine
Alicyclic Amine drugs
Chlorpromazine
Benzphetamine
Brompheniramine
CH3
CH3
CH3
N
N
N
H
O
O
Meperidine
CH3
HO
O
Morphine
OH
O
CH3
Dextromethorphan
2o & 1o Amines
O
CH3
HN
CH2
CH3
NH3
NH 2
CH3
O
Phenylacetone
Ampetamine
Methampetamine
Cl
NHCH 3
O
Ketamine
CH3
Cl
NH 2
O
Norketamine
Generally, dealkylation of secondary amines occurs before deamination. The rate of
deamination is easily influenced by steric factors both on the a-C and on the N; so it is
easier to deaminate a primary amine but much harder for a tertiary amine.
Exceptions: Some 2o and 3o amines can undergo deamination directly without dealkylation.
OH
O
HN
OH
OH
O
O
O H
Direct Oxidative
CH 3 Deamination
HN
CH 3
H2 N
CH 3
CH 3
Propranolol
OH
OH
O
Oxidative Deamination
Through Primary Amine
O
O
H3C
HN
O
H
CH 3
CH 3
Aldehyde
Metabolite
NH3
Carbinolamine
CH3
NH 2
Primary Amine Metabolite
(Desisopropyl Propranolol)
O
N-Oxidation
H
H
H
OH
N
N
Hydroxylamine
Nitroso
N
O
Aromatic amines
1 aromatic amine
H
H
1° amines
R
C
H
N
R
H
H
C
N
CH3
R
C
H
H
2 amine
H
C
N
H
3 amine
CH3
CH3
R
C
H
H
C
N
N
CH3
N O
CH3
N-Oxide
R
C
H
Nitroso
Nitro
H
R
OH
O
H
CH3
Hydroxylamine
H
R
OH
H
H
3° amines
R
Hydroxylamine
H
R
N
H
1 amine
2° amines
C
H
H
C
N
H
Nitrone
CH2
O
O
N
O
■
The attack is on the unbonded electrons so 3o amines can be oxidized
■
Generally, only occurs if nothing else can happen, so it is a rare reaction
■
Performed by both amine oxidases and hepatic MFO’s
■
Good examples would include amines attached to quaternary carbons since
they cannot be deaminated
H
H3C
N
H
Cl
Chlorphentermine N-Hydroxylation
H
H3 C
N
NH2
CH3
Hydroxylamine
Nitroso
H
CH3
Phentermine
N
CYP450
CH3
Cl
H
H3C
Nitro
Amantadine
OH
Amides
C-N bond cleavage via a-C
hydroxylation (formation of
carbinolamide) and Nhydroxylation reactions
Oxidation involving C-O System (O-Dealkylation)
H
R1
C
OH
CYP450
O
R3
R1
R2
C
Spontaneous
O
R3
R1
C
R2
O
+
HO
R3
R2
■
Converts an ether to an alcohol plus a ketone or aldehyde
■
Steric hindrance discussion similar to N-dealkylation
OH
O
H3C
H 3C
H3C
O
O
NH2
N
NH2
CY
P4
50
O
O
N
us
eo
tan
on
Sp
H 3C
O
CH2
CH3
OH
N
NH2
N
NH2
H 3C
H 3C
Trimethoprim O-Dealkylation
O
O
N
NH2
N
NH2
CH3
N
H3C
O
H
N
O
O
OH
CH3
CH3
N
O
O
O
OH
O
CH3
Codeine
H3C
H3 C
O
CH3
Cl
Phenacetin
N
NH2
OH
O
N
N
O
Indomethacin
N
O
O
H3C
Prazosin
O
H
N
CH3
Metoprolol
■
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 an ether
■
The enzyme involved is CYP2E1 and has been historically referred to
as the Microsomal Ethanol Oxidizing System (MEOS)
H
H3C
C
H
OH
CYP450
OH
H3C
C
H
Spontaneous
OH
H3C
C
H
CH3
O
Oxidation involving C-S System
H
■
S-Dealkylation
R1
C
OH
CYP450
S
R3
R1
R2
C
S
R3
Spontaneous
R1
C
R2
R2
Steric hindrance discussion similar to N-dealkylation
O
S
■
Desulfuration
R1
C
R1
R2
C
R2
O
■
R1
S-Oxidation
S
R1
R2
S
O
R2
R1
S
N
N
N
H
6-(Methylthio)-purine
N
S
CH2 OH
N
N
N
N
H
R2
O
Sulfone
Sulfoxide
CH3
S
O
CH2
SH
N
N
N
H
6-Mercaptopurine
N
O
+
HS
R3
O
H3C S
COOH
H
N
O
N
O
H
Methitural
CF3
2-Benzylthio-4trifluoromethyl benzoic acid
H3C
H3C
O
S
P
O
O
Parathione
O
S
N
O
H
Pentobarbital
N
O
H
Thiopental
NO2
H3C
H3C
O
H
N
N
S CH2C6H5
S
O
H
O
P
O
O
Paraoxone
NO2
N
O
N
S
S
CH3
S
Ring Sulfoxide
N
CH3
CH3
S
Thioridazine
N
CH3
N
S
N
CH3
S CH3
O
Mesoridazine
N
O S
O
N
CH3
CH3
S
Ring Sulfone
N
N
CH3
S
S
CH3
O O
Sulforidazine
Oxidative Dehalogenation
H
R
C
OH
CYP450
Cl
R
Cl
■
C
O Spontaneous
O
R C
R C
+H2O
Cl
OH
+
+
Cl
Cl
Requires two halogens on carbon
H
■
With three there is no hydrogen available to
replace
■
With one, the reaction generally won’t proceed
■
The intermediate acyl halide is very reactive
OH
OH
O2N
OH
NHCOCHCl2
O2N
Chloramphenicol
O2N
OH
O2N
Q. What is Gray Baby Syndrome?
Cl
OH
HCl
OH
NHCOCCl2
OH
H
Cl
OH
NHCOC OH
O
Oxamic Acid
Derivative
OH
NHCOCCl
O
Oxamyl Chloride
Derivative
Tissue
Nucleophiles
Covalent Binding
(Toxicity)
Hepatic Microsomal Flavin Containing
Monooxygenases (MFMO or FMO)
■
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
H3C
S
NH
N
H
N
H
N
N
C
MFMO
H3C
CH3
S
NH
N
Cimetidine MFMO S-Oxidation
Q. What is the difference with MFO?
N
O
H
N
H
N
N
C
CH3
N
Non-Microsomal Oxidation Reactions
■
Monoamine oxidase (outer membrane of mitochondria, flavin containing enzyme )
■
Dehydrogenases (cytoplasm)
■
Purine oxidation (Xanthene oxidase)
Monoamine oxidase
H
R1
C
N
R2 R3
H
R1
C
R2
O
+
H
N
H
R3
■
Two MAOs have been identified: MAO–A and MAO–B. Equal amounts are found in
the liver, but the brain contains primarily MAO–B; MAO–A is found in the adrenergic
nerve endings
■
MAO–A shows preference for serotonin, catecholamines, and other monoamines
with phenolic aromatic rings and MAO–B prefers non–phenolic amines
■
Metabolizes 1° and 2° amines; N must be attached to α-carbon; both C & N must
have at least one replaceable H atom. 2° amines are metabolized by MAO if the
substituent is a methyl group
■
b–Phenylisopropylamines such as amphetamine and ephedrine are not metabolized
by MAOs but are potent inhibitors of MAOs
Alcohol dehydrogenase
R2
R1
C
Aldehyde dehydrogenase
R2
OH
R1
H
C
R1
C O
O
R1
C
H
O
OH
Metabolizes 1° and 2° alcohols and aldehydes containing at least one “H” attached 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 by
alcohol (keto) reductases (reversible), however, it goes forward as the aldehyde is converted to
carboxylic acid; 3° alcohols and phenolic alcohols cannot be oxidized by this enzyme; No “H”
attached to adjacent carbon
H2
C
Ethanol Metabolism
H3C
Alcohol
Dehydrogenase
OH
H3C
Aldehyde
Dehydrogenase
H
C
OH
H3C
O
C
O
Purine oxidation
O
O
N
HN
N
N
H
Hypoxanthine
Xanthine
oxidase
N
HN
O
N
H
Xanthine
Molybdenum Containing
O
O
N
H
Xanthine
oxidase
N
HN
O
H
N
HN
O
OH
N
H
N
H
Uric acid
(hydroxy tautomer)
O
N
H
N
H
Uric acid
(keto tautomer)
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
Reduction of Aldehydes & Ketones
H
R
C
O
H
Aldehyde
R
C
H
OH
H
1 alcohol
R
C
O
R2
Ketone
R1
C
OH
R2
2 alcohol
■
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
■
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
O
H
O
+
C
R1
O
H
H
HO
H2N
R2
Chiral Alcohol
O
OH H2C
HO
OH H 2C
CH3
H
O
O
CH2
O
O
OH
OH H2C
CH3
H
O
R,R (+)-Warfarin
O
OH
C6H5
CH3
N
OH
OH
H3 C
Naloxone
H
O
N
O
Ox Nicotinamide moiety
of NADP+ or NAD+
+
R,S (+)-Warfarin
R (+)-Warfarin
O
N+
C 6H5
H
HO
+
H
CH3
C6H 5
O
R2
H2N
R
R
Red Nicotinamide moiety
of NADPH or NADH
Ketone
C
R1
N
H
O
O
H3 C
OH
O
OH
O
H2N
OH
Daunomycin
HO
O
O
Naltrexone
CH3
CH3
OH
C
O
OH
H
C
CH
CH
H
HO
Norethindrone
H2
C
CH3
CH
H
H2
C
C
NH2
O
Amphetamine
Phenylacetone
OH
OH
H
H
C
C CH3
NHCH3
(-)-Ephedrine
C
3b,5b-Tetrahydronorethindrone
CH3
H2
C
CH3
CH
OH
1-Phenyl-2-propanol
OH
H
C
CH3
O
1-Hydroxy-1-phenylpropane-2-one
C
H
CH
CH3
OH
1-Phenyl-1,2-propandiol
Reduction of Nitro & Azo Compounds
H
R
C
N
R
O
H
N
C
N
R
O
H
R2
R1
H
H
N
R
H
N
H
N
R
N
N
Azido
NH
H
NH2
+
H 2N
Two 1 amines
R
NH2 + N
Amine
H
1 amine
Hydrazo
Azo
N
H
R1
R2
C
OH
Hydroxylamine
Nitroso
N
C
H
Nitro
R1
H
H
O
N
N2
R2

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
O
H2N
O
S
O
N
H2
O
S
N N
NH2
H2N
Prontosil
N
H2
H2N
+
NH2
NH2
Sulfanilamide
1,2,3-Triaminobenzene
H
N
O
O
O2N
S
N
Cl
HO
N
O
N
N N
O
Clonazepam
N
H
O
O2N
O
NNa
O
OH
Sulfasalazine
N
Dantrolene
Reduction of Sulfur Containing Compounds
O
O
Sulfoxide reduction (Cannot reduce a sulfone)
R1
S
R1
R2
S
X
R2
Sulfoxide
R1
S
R2
O
Sulfone
Disulfide reduction
R1
S
S
R2
H3C
SH
+
N
S
S
N
HS
R2
H3C
CH3
S
H3C
R1
S
H3C
CH3
N
SH
S
N,N-Diethylthiocarbamic
Acid
Disulfiram
O
F
OH
CH3
H
H3 C
S
O
Sulindac
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
■
■
■
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
Table: Naming carbonyl - heteroatom groups
R1
R1
R2
Name
Susceptibility
to Hydrolysis
C
O
Ester
Highest

O
C
S
Thioester
C R2
+
O
O
Carbonate
C
N
Amide
O
N
Carbamate
N
N
Ureide
Lowest
The Reactions
O
Ester hydrolysis
R1
O
C O R2
R1
C OH
O
Amide hydrolysis (slower)
R1
C
HO R2
O
H
N R2
R1
C OH
H2N R2
Carbonate hydrolysis
O
O
R1
O
C
O
R2
R1
HO
+
OH
Carbonate
O
C
O
HO
R2
R2
+
HO
Carbonic acid derivative
C
H
OH
O
C
O
+
O
C
O
+
O
H
O
H
Carbonic acid
Carbamate hydrolysis
O
O
R1
O
C
R2
N
R1
OH
+
HO
C
HN
N
R3
+
HO
R3
R3
Carbamic acid derivative
Carbamate
O
R2
R2
C
H
OH
Carbonic acid
Urea hydrolysis
R1
R2
O
N
C
N
R3
R4
Urea derivative
O
R1
R2
NH
+
HO
C
HN
R1
C
+
R3
R4
Carbamic acid derivative
HO
C
H
O
OH
Carbonic acid
O
O
Hydrazide hydrolysis
N
O
R2
R3
H
N
N
Hydrazide
R2
R3
R1
C
OH
+
H2N
N
R2
R3
Hydrazine
C
O
+
O
H
Drug Examples
OH
O
O
H3C
OH
O
OH
O
+
O
OH
H3C
CH3
N
H3C
O
O
Salicylic Acid
HO
CH3
O
CH3
N
Cl
N
O
Indomethacin
N
H3C
O
H3 C
Slow Hydrolysis
O
HO
O
N
N
N
O
NH2
Prazosin
H 2N
CH3
H2N
OH
N
CH3
O
Rapid Hydrolysis
H
N
N
O
CH3
O
CH3
CH3
Procaine
CH3
Methylecgonine
N
Procainamide
O
O
N
Benzoylecgonine
CH3
CH3
O
O
Cocaine
H
N
H3C
+
O
O
H2 N
O
O
Aspirin
O
H3C
OH
Lidocaine
CH3
O
Stereoselectivity of Hydrolysis
 Etomidate (Amidate, hypnotic): R-(+)-isomer is more rapidly hydrolyzed,
but S-(-)-isomer is more rapidly hydroxylated.
The Concept of Prodrugs and Antedrugs
Prodrug
M
D
M
D activation M
D
M
D
Antedrug
D
M
D
M
inactivation
ID
M
= Barrier & ID = inactive drug, D = active drug, M = modifier
(I)
Prodrug: Need metabolic activation
(II)
Antedrug: Active drug that is quickly inactivated thereby minimizing
systemic effects
Prodrugs and Related Terms
■
Albert in 1958 coined the term prodrug to refer a pharmacologically inactive
compound that is metabolically activated in the mammalian system
■
Hard Drugs are not susceptible to metabolic or chemical transformation, have
high lipid solubility and thus accumulation or high water solubility
O
O
S
F3C
N
NH2
O
N
CH3
Celecoxib: t1/2 10-12 h in humans
■
S
F3C
N
NH2
O
N
Cl
t1/2 ca. 680 h (Liver toxicity)
Soft drugs are active compounds that after exerting its action undergo
inactivation to give a nontoxic product. Indeed soft drugs are a group of modified
compounds that are also designed to delivery the drugs in to the brain (the
chemical delivery system). Bodor coined the term.
Basic Concepts of Prodrugs
■
Carrier-linked prodrugs: a pro-moiety is attached, which is not necessary for
activity but may impart some desired property to the drug, such as increased
lipid or water solubility, or site-directed delivery
■
Advantages may include:
■
1.
increased absorption
2.
alleviation of pain at the site of injection if the agent is given parenterally
3.
elimination of an unpleasant taste associated with the drug
4.
decreased toxicity
5.
decreased metabolic inactivation
6.
increased chemical stability
7.
prolonged or shortened action
Bioprecursor prodrugs contain no pro-moiety but rather rely on metabolism to
introduce the functionality necessary to create an active species
O
OH
Cl
H
N
Cl
O
O2N
O
O
O
OH
HO
O
O-Na+
O
ONa
O
Prodrug: Chloramphenicol Hemisuccinate Na Salt
O
Prodrug: Prednisolon Hemisuccinate Sodium Salt
■
Inactive as it is and activated by hydrolysis by plasma esterases to chloramphenicol/
prednisolon
■
Increased water solubility for parenteral administration, which otherwise would
precipitate and cause pain by damaging surrounding tissues
CH
3
OH
N
Cl
H
N
H3C
Cl
O
CH3
Cl
O OH
O
O2N
H
N
CH3
O
HO
O
O
Prodrug: Chloramphenicol Palmitate
O
S
CH3
(CH2)14CH3
Prodrug: Clindamycin Palmitate
■
Inactive as it is; activated by hydrolysis by intestinal esterases to chloramphenicol/
clindamycin
■
Minimize their bitter taste and improve their palatability in pediatric liquid suspensions
O
S
N
H
O
O
CH3
N
CH3
O
COONa
Prodrug: Carbenicillin Indanyl Ester
■
Inactive as it is and activated by hydrolysis by plasma esterases to carbenicillin
■
Lipophilic indanyl ester furnish improved oral bioavailability
F
O
F
O
CH3
CH3
CH3
H
H
H
H3C
S
OH
OH
OH
H3C
O
F
H3C
S
O
S
O
O
Sulfide
(Active)
Sulindac
(Inactive)
Sulfone
(Inactive)
Prodrugs of Functional Groups

Carboxylic acids and alcohols: Most common

Amines and azo linkages: Not been used much

Carbonyl compounds: Not found to be used widely
Carboxylic Acids and Alcohols
Converted to ester prodrugs which are often hydrolyzed to active drug by different types of
esterase enzymes:
Ester hydrolase
Lipase
Cholesterol esterase
Acetylcholinesterase
O
Drug
O
Promoiety
Drug
OH +
O
Drug
HO Promoiety
Esterase
or
Carboxypeptidase
Cholinesterase
O
O
O
Promoiety
Drug
OH + HO
Promoiety
Microflora in the gut
Manipulation of steric and electronic properties of promoiety allows control of rate and
extent of hydrolysis
Advantage of Prodrug Formation I: Increased absorption of hydrophilic drugs by
making less hydrophilic or more lipophilic
H3C
CH3
CH3
OH
O
O
H3C
O
H
NH+
CH3
OH
NH+
CH3
HO
Esterase
O
CH3
CH3
Dipivefrin
HO
H
Epineprine
H3C
O
CH3
CH3
OH
Pivalic Acid
Prodrug of Epinephrine: Dipivefrin

More lipophilic, thus achieve higher intraocular concentration

Hydrolysis occur in cornea, conjunctiva, and aqueous humor after
ophthalmic application
Advantage of Prodrug Formation II: Masking unpleasant taste
Chloramphenicol palmitate and Clindamycin palmitate has already been shown.
Other drugs include
CH3
O
O
S
N
O
O
CH3
H3C
HO
N
CH3
O
H2N
H3C
HO
CH3
O
CH3 H C
O
3
H N CH3
O
O
OH
O
CH3
O
CH3
H3C
N-Acetyl sulfisoxazole
CH3
O
O
O
O
CH3
OH
CH3
O
CH3
CH3
Erythromycin estolate
O
O
H3C
O
H3C
O
N CH3
O
O
H3 C
H3C
H3C
H 3C
CH3
O
O
CH3
O
O
O
O
CH3
O
Troleandomycin
CH3
CH3
CH3
O
CH3
O
CH3
Not all carboxylic esters hydrolyzed in vivo where double ester approach is used
H
N
R1
O
S
N
O
CH3
Esterase
CH3
No Reaction
COOR2
(R2 = Ethyl, Propyl, Butyl, Phenyl)
Penicillin Esters
H
N
R1
S
Esterase
O
N
O
R3
COOR2
(R2 = Ethyl, Propyl, Butyl, Phenyl)
Cephalosporin Esters
No Reaction
N
H 2N
S
OCH3
N
H
N
O
S
N
OCH3
CH3 O
O
O
O
CH3
O
Cefpodoxime Proxetil
(Prodrug)
N
H2 N
S
O
S
N
OCH3
CH3
O
O H
CH3
O
O
N
H 2N
S
OCH3
N
H
N
O
CH3
Esterase
OCH3
N
H
N
O
CH3
+ CO2 + HO
S
N
OCH3
O
H3C
O
+
O
OH
H3C
CH3
CH3
Advantage of prodrug formation III: Increase hydrophilicity and thus water solubility
to apply parenterally or also orally when compounds are too lipophilic to formulate in
liquid dosage form
O
Drug
O
O
C
H2
O
O-Na+
C
H2
HO
Drug OH +
O
C
H2
C
H2
Succinates
O
Drug O
O
P O-Na+
OH
Drug OH
+
HO
P O-Na+
OH
Phosphates
O
-
O
O
Drug
Drug
OH +
O
O
O
Rapid and thus the prodrug is unstable
O
O-Na+
H
H3C
CH3 H
N
N
O OH
CH3
CH3
N
H2O H3PO4
Cl
Phosphatase
O
HO
O
S CH3
P
O
OH
OClindamycin Phosphate
H
N
H3C
O OH
CH3
Cl
O
HO
Clindamycin
OH
S
CH3
Chemical Delivery System
The site specific delivery of drugs is an important way of increasing
drug’s therapeutic index. The knowledge of prodrug and drug
metabolism is used to concentrate drugs at its target site thus
minimizing the systemic toxicity.
HO
CH2CH COOH
HO
NH2
BBB; Active transport to CNS
by L-Amino acid delivery system
HO
HO
HO
CH2CH COOH
HO
CH2CH2NH2
NH2
L-Dopa
Dopamine (Active)
Antedrugs (Soft Drugs)
I stopped taking medicine
as I prefer original disease
to side effects
!!
Because,
Vioxx’ll treat pain
but who’ll treat
vioxx
??
Safety-Based Drug Withdrawals
from U.S. Market (2006-2007)
Drugs
Vioxx
(Rofecoxib)
Ximelagatran
(Exanta)
Therapeutic activity
Antiinflammatory
(COX-2 inhibitor)
Date
approved
Date
withdrawn
05/99
09/04
Myocardial Infarction etc.
2006
Hepatotoxicity
Anticoagulant
Primary health risk
Tegaserod
(Zelnorm)
IBS, constipation
2002
2007
Cardiovascular ischemic
events
Aprotinin
(Trasylol)
Induce bleeding
during sergery)
1960s
2007
Ischemic colitis and Severe
constipation
Why the Adverse Drug Reactions Occur?
 Because of unintended systemic actions in most
therapeutic classes of drugs
To bring a drug from concept to market

It takes about 10-15 years

$897 millions to $1.7 billions

Overall attrition rate 10,000:1
What is Antedrug?
An active synthetic drug which is inactivated by a metabolic
process upon entry into the systemic circulation.
Therefore, a true antedrug acts only locally.
True Antedrug
Partial Antedrug
Inactive Metabolite
Less active metabolite
Lee HJ and Soliman MRI (1982). Science, 215, 989.
OCOR
OH
OH
O
O
OH
OH
IA
A
Prodrug
O
OH
Antedrug
A
O
OH
OH
OH
O
O
OH
OH
CO 2R
Hydrocortisone
(IA = Inactive Compound, A = Active Compound)
IA
CO 2-
Chemical Approaches
1) The Carboxylic Esters and Amides
2) 20-Thioester Derivatives
3) g-Butyrolactone Derivatives
O
CH3
HO
CH3
H
H
CO2R
O
OH
HO
CH3
hydrolysis in plasma
H
O
CH3
H
H
H
O
Stable locally and active
Inactive
Inactivation of Steroid 21-ate Esters in Bood Plasma.
COOH
OH
O
CH3
HO
CH3
H
F
H
O
OCOCH3
OH
CO2Me
t1/2 6.3 min
plasma
O
O
CH3
HO
CH3
F
O
O
HO
CH3
H
F
H
O
OH
OH
CO2Me
t1/2 90 min
plasma
CH3
HO
CH3
F
O
O
SCH2F
OCOEt
Me
H
CH3
CH3
HO
CH3
Liver
H
O
Inactivation of Fluticasone Propianate
OCOEt
Me
H
F
F
OH
F
Inactive
H
H
H
OH
OH
COOH
Advantages of Antedrugs

Localization of the drug effects

Elimination of toxic metabolites, increasing the therapeutic index

Avoidance of pharmacologically active metabolites that can lead
to long-term effects

Elimination of drug interactions resulting from metabolite
inhibition of enzymes

Simplification of PK problems caused by multiple active species
M.O.F.Khan*, K.K.Park, H.J.Lee. Antedrugs: An Approach to Safer Drugs. Curr. Med. Chem., 12(19), 22272239, 2005.
Phase II: Drug Conjugation

Attachment of small polar endogenous molecules such as
glucuronic acid, sulfate and amino acids to Phase I metabolites or
parent drugs

Products are more water-soluble and easily excretable

Attenuate pharmacological activity and thus toxicity

Trapping highly electrophilic molecules with endogenous
nucleophiles such as glutathione prevent damage to important
macromolecules (DNA, RNA, proteins)

Regarded as true detoxifying pathway (with few exceptions)

In general, appropriate transferase enzymes activate the
transferring group (glucuronate, sulphate, methyl, acetyl) in a
coenzyme form
Glucuronic Acid Conjugation

Glucuronidation is the most common conjugation pathway

The coenzyme, UDP glucuronic acid is synthesized from the corresponding
phosphate

UDP-glucuronic acid contains D-glucuronic acid in the a-configuration at
the anomeric center, but glucuronate conjugates are b-glycoside, meaning
inversion of stereochemistry is involved in the glucuronidation

Glucuronides are highly hydrophilic and water soluble

UDP glucuronosyltransferase is closely associated with Cyp450 so that
Phase I products of drugs are efficiently conjugated

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
Formation of Glucuronide Conjugate
UTP
HO
HO
HO
PPi
O
HO
Phosphorylase
OPO 32a-D-Glucose-1phosphate
HO
HO
HO
O
O
HO
O
O
NH
O P O P O
O-
O-
O
HO
UDPG
N
O
2NAD
U
D
OH PG
+
2N
de
hy
dr
og
en
A
DH
as
e
HOOC
HO
O
HO
O
O
HO
O P O P O
OUDP-Glucuronyl-transferase
(microsomal)
HO
HO
HO
O
RXH
O
XR
HO
b-D-Glucuronide
UDP
O-
O
NH
O
HO
N
O
OH
Uridine-5'-diphosphoa-D-Glucose (UDPG)
Types of Compounds Forming Glucuronides
TYPE
EXAMPLES
CH3
O
H
N
N
CH3
O-Glucuronide
OH
Phenols
Acetaminophen
HO
O
morphine
OH
OH
Cl
H
N
CH3
CH3
O
OH
O2N
Alcohols
H
N
O
Cl
OH
Chloramphenicol
Propranolol
OH
O
Enols
O
Hydroxycoumarine
H2N
N-hydroxyamines/amides
S
O2
NHOH
CH3
N
OH
N-hydroxydapsone N-Hydroxy-2-acetylaminoflourene
COOH
OH
Aryl acids
Salicylic acid
CH3
O
OH
O
Fenoprofen
Arylalkyl acids
NH2
N-Glucuronides
Arylamines
O
H
N
S
N 7-Amino-5-
Sulfonamides
nitroindazole
O2N
N
Alkylamines
O
N
H2N
O
N
CH3
N
H
CH3
Sulfisoxazole
H
CH3
3o
Amines
N
CH3
Desipramine
O
NH2
H3C
Amides
O
O
H3C
Meprobamate
NH2
O
Cyproheptadine
N
S-Glucuronides
HS
N
CH3
Sulfhydryl
Methimazole
H3C
S
Carbodithioic acid
H3C
N
SH
Disulfirum (reduced form)
O
C-Glucuronides
N
CH3
N
O
Phenylbutazone
Sulfate Conjugation

Occurs less frequently than does glucuronidation presumably due to
fewer number of inorganic sulfates in mammals and fewer number of
functional groups (phenols, alcohols, arylamines and N-hydroxy
compounds)

Three enzyme-catalyzed reactions are involved in sulfate conjugation
O
-
ATP
O
-
-
O S O
O
Sulfate
PPi
Mg+2
ATP sulfurylase
O
O
O S O P O
O
-
O
O
Ad
ATP
ADP
-
O
O S O P O
O
-
O
+2
Mg
APS phosphokinase
HO
OH
Adenosine-5'phosphosulfate (APS)
O
Ad
RXH
PAP
O
Sulfotransferase
(soluble)
-2
O3 PO
OH
3'-phosphoadenosine-5'phosphosulfate (PAPS)
-
O S XR
O
Sulfate
conjugate
Sulfation of Drugs

Phenolic sulfation predominates

Phenolic O-glucuonidation competes favorably with sulfation due to limited
sulfate availability

Sulfate conjugates can be hydrolyzed back to the parent compound by
various sulfatases

Sulfoconjugation plays an important role in the hepatotoxicity and
carcinogenecity of N-hydroxyarylamides

In infants and young children where glucuronyltransferase activity is not
well developed, have predominating O-sulfate conjugation

Examples include: a-methyldopa, albuterol, terbutaline, acetaminophen,
phenacetin
OH
H
HO
N
H3C
HO
H
COOH
N
HO
HO
a-Methyldopa
Albuterol
OH
H
CH3
CH3
CH3
HO
H
N
OH
CH3
CH3
CH3
Terbutaline
Possible Mechanism of Phenacetin Toxicity
Electrophilic nitreneum
Amino Acid Conjugation

The first mammalian drug metabolite isolated, hippuric acid, was the
product of glycine conjugation of benzoic acid
R
O
COH
Benzoic Acid, R = H
Salicylic Acid, R = OH
R
O
O
CONHCH2COH
Hippuric Acid, R = H
Salicyluric Acid, R = OH

Amino acid conjugation of a variety of caroxylic acids, such as 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
Mechanism of Amino Acid conjugation
Drug-COOH
An Acyl-CoA Intermediate
Glycine Conjugate R = H
Glutamine Conjugate R = CH2CH2CONH2
Brompheniramine Metabolism
N
CH3
N
CH3
CH3
NH
N
P450
NH2
N
P450
Br
Brompheniramine
Br
N
CHO
N
P450
Br
H
N
N
O
Br
Brompheniramine N-oxide
COOH
Aldehyde
dehydrogenase
Br
CH3
N
CH3
N
Br
Glycine conjugate
Br
Carboxylic Acid metabolite
Glycine
N-acyltransferase
COOH
Glutathione Conjugation
NH 2
H
N
HO
O
HO
HS
H
N
O
O
N
H
O
OH
O
O
O
O
O
N
H
S
S
H
N
NH 2
HO
NH 2
Glutathione reduced form (GSH)
N
H
OH
O
O
OH
O
Glutathione oxidized form (GSSG)

Glutathione is a tripeptide (Glu-Cys-Gly) – found virtually in all
mammalian tissues

Its thiol functions as scavenger of harmful electrophilic parent drugs
or their metabolites

Examples include SN2 reaction, SNAr reaction, and Michael addition
SN2 Examples
GSH
R X SG + Y
GlutathioneS-Transferase
A.
SN2
R X Y
-
1. CH3O2SO
Busulfan
OSO2CH3
ONO2
2.
H
ONO2
SG
X = C, O, S; Y = leaving group or epoxide
CH3O2SO
ONO2
-
SG
O NO2
H
SG
S+ G
ONO2
ONO2
-
SG H
O SG
ONO2
+
OH
Nitroglycerine
CH 3
O
O
CH3
O
Naproxcinod
O
O
N
O
GSSG
SNAr Examples
X
SG
GSH
B.
SNRr
Z
Z
N
1.
N
O
+
N O
S
H3C
N
-
N
-
SG
N
N
N
H
Azathioprine
O
+
N OSG
S
N
H3C
N
N
N
N
N
H
N
SH
NO2
+
N
N
SG
H3C
N
N
1-Methyl-4-nitro-5H
(S-glutathionyl) 6-Mercaptopurine
imidazole
Michael Addition
H+
C.
-
Z
SG
Z
SG
Michael Addition
CH3
N
CH3
N
CH3
N
SG
-
SG
HO
O
OH
HO
O
O
CH3
SG N
O
O
O
HO
GS
OH HO
OH
CH3
N
O
OH
Mercapturic Acid Conjugates
Drug
O
HO
S
H
N
O
O
O
Amino Acid
g-Glutamyl-AA
(AA)
OH
N
H
NH 2
g -Glutamyl
transpeptidase
Drug
O
HO
S
H
N
NH 2
O
Glutathione Conjugate
Drug
Glycine
Cysteinyl
Glycinase
Acetyl
CoA
S
HO
NH 2
O
S-substituted
Cysteine
Derivative
CoASH
Drug
S
H 2N
O
N
H
O
Mercapturic
acid conjugate
CH 3
Acetyl Conjugation

Metabolism for drugs containing a primary amino group, (aliphatic and aromatic
amines), amino acids, sulfonamides, hydrazines, and hydrazides

The function of acetylation is to deactivate the drug, although Nacetylprocainamide 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
O
H3 C
X-
O
CoASH
SCoA
H3 C
O
X
H2N
R
H3 C
NHR
X-
N-Acetylation of amines
Genetic polymorphism in N-acetyltransferase activity
Multiple NAT2 alleles (NAT2*5, *6, *7, and *14) have substantially decreased
acetylation 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
O
O
HO
S
NH
O
OH
NH2
CH3
CH3
Cilastatin
HO
H3C
N
S
H
N
O
COOH
Imipenem
NH
Fatty Acid and Cholesterol Conjugation

Hydroxyl-containing drugs can undergo conjugation with a wide
range of endogenous fatty acids such as saturated acids from
C10 to C16 and unsaturated acids such as oleic and linoleic acids

Cholesterol ester metabolites have been detected for drugs
containing either an ester or a carboxylic acid
O
O
HO
O
Cl
N
Cl
OH
Cholesterol
Fatty Acid
Cl
O
Prednimustine
O
(CH2) 10
COOH
Methyl Conjugation

Minor conjugation pathway, important in biosynthesis of epinephrine
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 polarity and hydrophilicity of the substrates

A variety of methyl transferase, such as COMT (catechol O-methyl
transferase), phenol-O-methyltransferase, N-methyl transferase, Smethyltransferase etc are responsible for catalyzing the transfer of
methyl group from SAM to RXH
H2 N
COOH
H2N
ATP
H2 N
COOH
PPi + Pi
Methyltransferase
H3CS
Methionine
adenosyltransferase
+
S
O
Mthetionine
HO
Ad
CH3
HX-R
OH
S-Adenosylmethionine
Mechanism of methyl conjugation
CH3 -X-R
COOH
+
S
O
HO
Ad
OH
Case Study
Case 2. Imagine yourself as a drug information specialist at a poison control center.
A technician from the coroner’s office is investigating a case and requires assistance
in identifying the possible sources of benzodiazepines (BZDs) in the toxicology
profile of a particular corpse. The technician has identified four distinct BZDs in this
blood sample. She believes that the major component is diazepam (1) (72% of the
identified BZDs) and that the remaining three components are metabolites (NOTE:
the assay identifies only active compounds).
O
H
N
N
Cl
Q. What are the three structures of
potential ACTIVE metabolites for
diazepam?
CH3
O
O
H
N
N
2
OH
N
Cl
N
Cl
CH3
O
N
1
4
OH
N
Cl
3
Assignment: Due by this Friday
http://www-home.cr.duq.edu/~harrold/basic_concepts_index.html
Study Guide
1. What Roles are Played by Drug Metabolism? Know with structural
examples
2. Role of stereochemistry in metabolism of drugs with example of warfarin,
ibuprofen and itomidate
3. What is first pass effect; enterohepatic circulation? Why and how they
occur? Drug examples
4. Metabolisms in the intestinal mucosa
5. CYP450, Hepatic microsomal flavin containing monooxygenases (MFMO
or FMO) Monoamine Oxidase (MAO) and Hydrolases. Drugs metabolised
by these enzymes and the active sites of these enzymes. Types of
metabolic reaction catalyzed by these enzymes
6. Specific CYP enzymes with the number of drugs they metabolize
7. Few CYP family with their main functions
8. Drug interaction basics related to metabolic enzymes
Study Guide Cont.
9. Mechanism and routes of aromatic hydroxylation. The effects of electron
donating and withdrawing groups in aromatic hydroxylation. Drug examples.
What is NIH shift?
10. Oxidation of olefins. Role of epoxide hydrolase. Can olefenic epoxide be
converted to alcohol as in aromatic epoxide by NIH shift?
11. What type of C in a drug molecule can not be hydroxylated?
12. What is allylic and benzylic hydroxylation? Show drug examples.
13. Show the drug examples where hydroxylation occur on Cα to C=O and C=N
bonds
14. Show the drug examples where hydroxylation occur at aliphatic and alicyclic
carbon atoms. Which carbons are more easily hydroxylated?
15. What is N-oxidatin and N-dealkylation. What enzymes are involved? How do
you differentiate between N-dealkylation and deamination. Drug examples.
What types of drugs generates lactams instead of causing dealkylation?
16. What is the difference between mixed function oxidases and amine oxidases?
Study Guide Cont.
17. What is the difference between ethanol oxidation and O-dealkylation?
18. What is S-dealkylation, desulfuration and S-oxidation? Drug examples.
19. How does steric factors influence S- O- and N-dealkylations?
20. Oxidative dehalogenation with special example of chloramphenicol. Why
chloramphenicol cause toxicity to the babies?
21. What is MFMO and its active site? What types of functional groups are
metabolized by this enzyme? Drug examples.
22. MAO, dehydrogenases, xanthene oxidases and their functions with drug
examples. Difference between MAO-A and MAO-B.
23. Alcohol and aldehyde dehydrogenases, the coenzymes and the types of drugs
they work on.
24. Azo and nitro reductases, their coenzymes and the drugs they act on.
Study Guide Cont.
25. Different types of hydrolytic enzymes. Compare rate of hydrolysis of esters,
amides, carbonates and carbamates.
26. What are prodrugs and antedrugs? What are the advantages? Examples.
27. What are different types of conjugation reactions?
28. The enzymes and substrates involved in glucuronidation, and sulfate
conjugation.
29. Why acetaminophen is toxic to neonates? Mechanism of phenacetin and
acetaminophen toxicity.
30. What types of drugs or metabolites may form glycin conjugates?
31. What are different mechanisms involved in glutathione conjugation? What is
mercapturic acid conjugate? Mercapturic acid conjugate of acetaminophen is a
sign of its toxicity – why?
32. Mechanism of acetylation. What is slow and fast acetylator?
33. What is COMT? What coenzymes is involved in its action? What types of drugs
and/or neurotransmitters are metabolized by COMT?