3 Metabolism

8/12/2019 3 Metabolism

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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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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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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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Drug Metabolism?

In general, by metabolism drugs

become more polar, ionizable and

thus more water soluble to enhance

elimination.


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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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Metabolism

2 Categories

Phase I Functionalization Oxidation

Reduction

Hydrolysis

G l P h f D


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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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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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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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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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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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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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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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Intestinal Mucosa:

The extra-hepatic metabolism, contains

CYP3A4 isozyme


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Intestinal Mucosa:

Isoproterenolexhibit considerable

sulfate conjugation in GI tract

Levodopa, chlorpromazine and

diethylstilbestrol are also reportedlymetabolized in GI tract


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Intestinal Mucosa:

Esterases and lipases present in the

intestine may be particularly important

carrying out hydrolysis of many ester

prodrugs


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Intestinal Mucosa:

Bacterial flora present in the intestine

and colon reducemany azo and nitro

drugs (e.g., sulfasalazine)


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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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Metabolism

CYP450

Hepatic microsomal flavin containingmonooxygenases (MFMO or FMO)

Monoamine Oxidase (MAO)

Hydrolases



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Metabolism

Cytochrome P450 system:

Localized in the smooth endoplasmicreticulum.

It is a Pigment that, with CO bound

to the reduced form, absorbsmaximally at 450nm



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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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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






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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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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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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



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Sulfate Conjugation



Acetylation

Methylation

Reduction


Nitro and azo

Miscellaneous

Oxidation

Aromatic moieties

Olefins

Benzylic & allylic C atoms



C-Heteroatom system


formation, N-hydroxylation)





aldehydes

Miscellaneous

Phase II -Conjugation

Phase I -Functionalization

Drug

Metabolism


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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Aflatoxin B1

Carcinogenic agent

Contains olefinic (C2-C3) double

bond adjacent to a cyclic ether

oxygen


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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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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



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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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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



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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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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



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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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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

+



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Valproic Acid (Depakene)

Undergoes both and - 1

Oxidation to 5- hydroxy and 4-

hydroxy metabolites respectively.



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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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Atoms

Glipizide

Oxidized to trans-4 and cis-3-

hydroxylcyclohexyl metabolite (6:1

ratio)



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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


85/164

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


86/164

Alicyclic Amines Often Generate Lactams


87/164

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


88/164

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.


89/164

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


90/164

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


91/164

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


92/164

C-N bond cleavage viaa-C hydroxylation(formation of carbinolamide) and N-

hydroxylation reactions

Oxidation involving C-O System (O-Dealkylation)


93/164

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)


94/164

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


95/164

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-


96/164

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


97/164

S-Dealkylation

C S R3

R1 C SR1 C OR1

HS R3+

R2R2

OHH

R2R3

CYP450 Spontaneous



98/164

Desulfuration

R1 C R2

S

R1 C R2

O



99/164

S-Oxidation

R1 S R2 R1 S R2

O

R1 S R2

O

O

Sulfoxide Sulfone



100/164

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


101/164

S-Dealkylation


102/164

Desulfuration


103/164

Desulfuration


104/164

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


105/164

Oxidative Dehalogenation

Hepatic Microsomal Flavin Containing

Monooxygenases (MFMO or FMO)

Non-Microsomal Oxidation Reactions



106/164

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


107/164


Halothane

Metabolized to trifluoroacetic acid

Halothanetrifluoroacetyl chloride

trifluoroacetic acid

The metabolite covalently binds in livermicrosomal proteins

O d D h l


108/164


Chloroform

It yields the chemically reactive

PHOSGENE (causes hepato- and

nephrotoxicity).



109/164

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



110/164

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


111/164

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


112/164

H3C

H2C

OH H3C

HC

O H3CC

O

OHAlcohol

DehydrogenaseAldehyde

Dehydrogenase

Ethanol Metabolism

Purine oxidation


113/164

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



114/164


Sulfate Conjugation



Acetylation

Methylation

Reduction


Nitro and azo

Miscellaneous

Oxidation

Aromatic moieties

Olefins Benzylic & allylic C atoms



C-Heteroatom system


formation, N-

hydroxylation)





aldehydes

Miscellaneous

Phase II -

Conjugation

Phase I -

Functionalization

Drug

Metabolism


Esters and amides

Epoxides and arene oxides

by epoxide hydrase

Reductive Reactions


115/164

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


116/164

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


117/164

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


118/164

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.



119/164

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)



120/164

and Ketones

Bioreduction of ketones leads to the

creation of asymmetric center , thereby

there are two possible sterioisomericalcohol.



121/164

and Ketones

Product Sterioselectivity

The preferential formation of one

isomer over the other


122/164

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


123/164

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


124/164

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


125/164

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


126/164

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


127/164

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


128/164

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


129/164

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



130/164


Sulfate Conjugation



Acetylation

Methylation

Reduction


Nitro and azo

Miscellaneous

Oxidation

Aromatic moieties

Olefins Benzylic & allylic C atoms



C-Heteroatom system


formation, N-

hydroxylation)C-O (O-dealkylation)




aldehydes

Miscellaneous

Phase II -

Conjugation

Phase I -

Functionalization

Drug

Metabolism


Esters and amides

Epoxides and arene oxides

by epoxide hydrase



131/164

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


132/164

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


133/164

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


134/164

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


135/164

Etomidate (Amidate, hypnotic): R-(+)-isomer is more rapidly hydrolyzed,

but S-(-)-isomer is more rapidly hydroxylated.


136/164

Phase 2 Reactions

Synthetic Conjugation

Phase I I


137/164

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


138/164

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


139/164

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


140/164

Glucuronic Acid Conjugation


141/164

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:-


142/164

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


143/164

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


144/164

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


145/164

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


146/164

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


147/164

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)


148/164

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-


149/164

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


150/164

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


151/164

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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