Reduction Reactions

- Have an important role in drugs contain carbonyl, nitro and azo

functional groups, which usually followed by conjugation reactions.

- Less common reduction reactions such as N-oxides, sulphoxides, S-S

and C-C cleavage reactions.

- Aldehydes and ketones:

- Source:
- From drugs.
- Oxidative deamination reactions.

- Aldehydes to primary alcohol (but most of its reaction is oxidation to

COOH), rare case is the conversion of chloral hydrate to
trichloroethanol (then phase II).
 Reduction Reactions
- Ketones to secondary alcohol then to conjugation reactions.

- These reactions are normally performed by aldo-keto reductase

(NADPH) or oxidoreductase enzyme (alcohol dehydrogenase).
 Reduction Reactions
- Propranolol as major metabolite after deamination is COOH
derivative and as minor is propranolol glycol.

O
OH
O N O H
H O N
OH Oxidative
H OH
OH
Deamination
+ H2N

Major
Minor

O
O OH
O OH
OH
OH
 Reduction Reactions

- Chlorphenramine first dealkylated, deaminated then it could undergo

oxidation or reduction.

Cl

Chlorphenramine
 Reduction Reactions

- Examples of compounds undergo oxidative deamination to ketone and

then reduction to alcohol (amphetamine, ephedrine).

Amphetamine Ephedrine
 Reduction Reactions

- Ketone reduction reactions are stereo-selective, which involve H

transfer to the carbonyl group and then one steroisomer will be
preferred over the other (Acetophenone, Warfarine).

Acetophenone Warfarine
 Reduction reactions
- Nitro compounds:

- The end product is primary amines.

- Aromatic nitro to nitroso to hydroxylamine to amine.

- Nitro reductase and NADPH are needed for nitro reduction (7-
nitrobenzodiazepine [clonazepam, nitrazepam]).
 Reduction reactions
- Nitro compounds:

Clonazepam

Nitrazepam
 Reduction reactions
- Azo compounds:

- Aromatic azo to hydrazo to cleavage to two aromatic amines.

- Prontosil (azo) to the active metabolite sulfanilamide.

H2 N

O PRONTOSIL
N NH2
H 2N S N
O
 Reduction reactions
- Azo compounds:

- Sulphasalazine is hydrolyzed to sulphapyridine and 5-aminosalicylic acid.

O H
N N
S

HOOC N
O SULPHASALAZINE
N

HO
 Other Reduction Reactions

A. (N-Oxide to tertiary amines).

- reduces the polarity of the tertiary amines, so reduce excretion.
 Other Reduction Reactions

B. (Disulphide reduction).
- Disulfiram is converted to N,N-dithylthiocarbamic acid and
sulindac (sulphoxide to sulfide).

N N
S N
S S S SH
S

HO

O
S
O
 Hydrolytic Reactions
- Esters and amides:
- Occur in various tissues and plasma.

- The products are (COOH, alcohols, phenols, and amines). The result is
more polar and easier to be conjugated.

- Enzymes involved for esters are esterases in liver, kidney and plasma and
for amides amidases, esterases and deacylases.

- For drugs contain esters, hydrolysis is the major route because it is

easily cleaved.

- Examples (Aspirin, cocaine, ritaline), normally esters are prodrugs that

are activated inside the body such as clofibrate, diphenoxylate.

O
O

N
 Hydrolytic reactions

- Amides hydrolyzed slowly compared to esters (procaine, procainamide).

- Other examples: indomethacin and prazocin.

 Hydrolytic reactions
- Other Hydrolysis reactions:

- Hydrolysis of proteins and hormones at their terminal amino acid groups

by aminopeptidases; insulin, GH, prolactin, and PTH.

- Hydrolysis of epoxides, and areneoxides.

- Hydrolysis of phosphate esters, carbamate esters, and cardiac

glycosides.
 Phase II: conjugation reactions
- Adds polar, small, endogenous and ionizable group to
phase I metabolite or parent xenobiotics such as
(glucuronic acid, sulfate, glycine, glutamine).

- Generally, these metabolites are nontoxic and not active.

- Other phase-II reactions such as acetylation and

methylation are not increasing water solubility, rather
they act to terminate or attenuate pharmacological
activity.

- The conjugated residues are first activated as coenzyme

before transfer and attachment by transferase enzymes.
 Phase II: Glucuronic acid conjugation

- The most common conjugation pathway due to:

- Readily available D-glucuronic acid (from D-glucose).
- Many functional groups can be united with glucuronic
acid.
- Its ionized carboxylic acid and the polar OHs increase
water solubility to high extent.
 Phase II: Glucuronic acid conjugation

- β-glucuronides formation involves two steps:

- Synthesis of activated coenzyme (UDPGA)
- Transfer of glucuronyl moiety to the xenbiotic by
UDP-glucuronyltransferases.
 Phase II: Glucuronic acid conjugation

- One step of glucuronation is sufficient to excrete a

compound, so di- process is not common.

- Common features of the binding are :

- occur at C1 of the glucuronic acid.
- The acceptor has the formula HXR, that the OH
of glucuronic acid will leave.
 Phase II: Glucuronic acid conjugation
- There are many functional groups that can be
glucuronated:
- O- glucuronation: hydroxy and carboxy.
- Hydroxy: alcoholic or phenolic are the most
common FGs that undergo glucuronation.
- Less common hydroxy groups undergo glucuronation
are enols, N-hydroxyl amines, and N-hydroxyl
amides.
- Carboxy: Aryl acids prefer conjugation with glycine
but could be glucuronated.
 Phase II: Glucuronic acid conjugation
- N- glucuronation:
- Occur occasionally with aromatic amines, amides
and sulphonamide. Considered minor pathway
compared to N-acetylation, or oxidative process.

- Some compounds form quaternary ammonium

glucuronide metabolite.
 Phase II: Glucuronic acid conjugation
- S-glucuronation:
- With the thiol group.
- C- glucuronation:
- Novel form of conjugation occur in little examples
(Phenylbutazone).
 Phase II: Sulphate conjugation

- Occur mainly with phenols, and to less extent alcohols,

aromatic amines, and N-hydroxy compounds.

- There is limited amount of sulphate, so it is limited

reaction.

- used extensively by the body to excrete endogenous

compounds such as steroids, heparin, catecholamines.

- The process involve formation of the coenzyme and then

transfer process by sulfotransferase.
 Phase II: Sulphate conjugation
NH2
N N
O O
N N HXR PAP
O S O P O O O
O OH O S XR
H2O3PO OH O

3'-Phosphoadenine-5'-phosphosulfate

- Lead to water soluble and inactive metabolite, but some O-

sulfate and N-hydroxy compounds give toxic metabolites.
 Phase II: Sulphate conjugation

- Low level of glucuronyltransferase or undeveloped enzyme

may cause acetaminophen to be mainly excreted as sulfate
conjugate in neonates rather than glucuronyl derivative as
both processes are competitive.
 Phase II: Sulphate conjugation
- O-sulfate ester conjugates and N-hydroxy compounds
are important as they could cause reactive toxic
intermediates.

- Carcinogenic species such as 2-acetylaminofluorene

mediate toxicity thorough O-sulfate esters that
generate electrophilic niternium species.
 Phase II: Sulphate conjugation
- Phenacetin: is metabolized by N-hydroxylphenacetin and
then conjugated with sulfate which in turn binds covalently
to microsomal protein causing hepato and nephrotoxcity.
 Phase II: Aminoacids conjugation
- Glycine and glutamine used to conjugate (COOH) aromatic
acids and arylalkyl acids.
- This process is limited due to competition with glucuronic
acid and limited supply of amino acids.

Glycine Glutamine
 Phase II: Aminoacids conjugation

- The process of conjugation is as follows:

- Activation of the carboxylic acid containing compound
to form Acyl-CoA.
- Acylation of glycine or glutamate by N-acyltransferase
which occur in mitochondria of liver and kidney.

COOH
O O C O COOH
H2N R
H C R
OH SCoA N
H H
 Phase II: Aminoacids conjugation
- Glycine conjugation
 Phase II: Aminoacids conjugation
- Glutamine conjugation occur mainly for arylacetic acids
such as phenylacetic acid and 3-indolylacetic acid.
Phase II: Aminoacids conjugation
Phase II: Aminoacids conjugation
 Phase II: GSH (mercapturic acid conjugates)
- Important pathway for detoxifying chemically reactive
electrophilic compounds.

- GSH is a tripeptide (γ-glutamyl-cysteinyl glycine) that

found in many tissues. After conjugation GSH-
conjugates will be degraded to mercapturic acid
derivatives.

NH2
H
N OH
HS
O O
O NH
OH

O
 Phase II: GSH (mercapturic acid conjugates)

- Sulfhydryl group is considered the important group

that interact with electrophilic positions in the toxic
compounds.

- The enzyme involved in the GSH conjugation is called

glutathione S-transferase, and the degradation step is
performed by renal and hepatic microsomal enzymes.

- No need to form activated coenzyme or substrate.

 Phase II: Acetylation
- Important pathway for drugs containing primary amino groups
(ArNH2, H2NC6H4SO2NHR, hydrazines (-NHNH2), hydrazides
(CONHNH2), and primary aliphatic amines).

- The end product will be amides that are expected to be nontoxic

and inactive.

- No enhancement of water solubility, so it is expected to terminate

activity or detoxification process (not always).

- The source of acetyl group is acetyl-CoA, and then transferred by

the enzyme N-acetyltransferase.

- Examples of aromatic primary amines; procainamide, dapsone, Nitro

derivatives; clonazepam, sulphonamides; sulphamethoxazole,
sulfanilamide (crystallurea), hydrazine; hydralazine, hydrazide;
INH, aliphatic amines (minor compared to oxidative deamination);
histamine and mescaline
Phase II: Acetylation
Phase II: Acetylation
Phase II: Acetylation
 Phase II: Methylation
- Used for biosynthesis of endogenous compounds (ephedrine and
melatonine).

- Used also for inactivation of endogenous compounds (dopamine,

seratonine).

- Minor pathway for xenobiotic compounds.

- Reduces water solubility except the case of creation quaternary

ammonium compound.

- S-adenosylmethionine (SAM) is the coenzyme and then there is

transferase enzymes that deliver this group.

- Methyltransferase enzymes are important such as catechol-O-

methyltransferase (COMT), phenol-O-methyltransferase.

- COMT is important in performing O-methylation for

neurotransmitters norepinephrine and dopamine.
 Phase II: Methylation
- Catecholes are metabolized by COMT, methyldopa and
isoproterenol to mono methylated (only C3 OH), terbutaline is not
O-methylated.

- Little case where phenols were O-methylated (minor); Morphine to

codeine.

- N-methylation of xenobiotic compounds is very low such as

amantadine.

- N-methylation present at heteroatom such as nicotine and nicotinic

acid to give quaternary ammonium products.

- S-methylation for drugs contain thiols (6-mercaprtopurine,

propylthiouracil)