Bio Transformation

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By :-

A. Ashwan Kumar

Pharmaceutical Chemistry

09171s0201



Definition

y Biotransformation of drugs is defined as theconversion of the drug molecule from one chemicalform to another.

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Biotransformation may result in:

a. Pharmacological inactivation of drugs, i.e. it r esults information of metabolites with liitle or no pharmacologic activity; e.g. conver sion of Phenytoin to p- hydr oxy

phenytoin.

b. Yield metabolites with equal activity; e.g. conver sion of

phenyl butazone to oxyphen butazone.c. Rar ely lead to toxicologic activation of drugs, i.e. it r esults in

formation of metabolites with high tissue r eactivity; e.g. conver sion of paracetamol to r eactive metabolites that causehepatic necr osis.

d. Pharmacological activation; e.g. conver sion of enala pr il toenala pr ilat

e. Change in Pharmacologic activity; e.g.conver sion of diazepam(tranquilizer) to oxazepam(anticonvulsant).

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Metabolic transformation of drugs occur betweenabsorption of drugs and its renal elimination.

The reactions fall into 2 categories:y Phase I reactions

y Phase II reactions

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Phase I reactions Phase I Reactions usually precede phase II reactions and these can be

classified into three types:

1) OXIDATIVE REACTIONS.

2) REDUCTIVE REACTIONS.

3) HYDROLYTIC RECTIONS.

A Polar group fucntionsal group is either introduced or unmasked if already present on the otherwise lipid soluble substrate, e.g. OH, -COOH,-NH2 and SH.

Thus , these reactions are also called as functionalization reaction orasynthetic reactions.



Ox idative Reactions Oxidative reactions are the most important and most common metabolic

reactions.

Almost all the drugs that undergo phase I biotransformations undergooxidation at some stage or the other.

Oxidation of xenobiotics in nonspecifically catalyzed by a number of enzymes

located in the microsomes. Such enzymes require both molecular oxygen and the reducing agent NADPH

to effect reaction.

They are therfore referred to as the mixed function oxidases.

The overall reaction involving the substrate RH which yields the product ROH,is given by the following equation:

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Since only one oxygen atom from the molecular oxygen is incorporated in theproduct formed, the mixed function oxidases are also called as

monooxygenases.

The multienzyme mixed function oxidase system located in the endoplasmicreticulum of hepatic cells, is composed of an electron transfer chain consistingof 3 Components.

A heme protein known as cytochrome P-450, which is actually a family of enzymes. Itis a terminal oxidase and plays the important role of transferring an oxygen atom to

the substrate RH and convert it to ROH

A second enzyme, the f lavoprotein known as cytochrome P-450 reductase which isNADPH dependent. It functions as an electron carrier, catalyzing the reduction of cytochrome P-450 to the ferrous form by transferring an electron from NADPH.

A heat stable lipid component known as phosphatidylcholine.its function is tofacilitate electron transfer from NADPH to cytochrome

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1.Oxidation of aromatic carbon atoms:

This reaction proceeds via formation of a reactive inremediate arene oxide(epoxide) which in most cases undergoes rearrangement to yiels arenols and in some casescatechols and glutathione conjugates.

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2.Oxidation of Olefins: Oxidation of non aromatic carbon-carbon double bonds is analogous to

aromatic hydroxylation i.e. it proceeds via formation of an epoxide to yield 1,2-dihydrodiols.

E.g. Conversion of carbamazepine to carbamazepine-10,11-epoxide; the latter isconverted to corresponding trans-10,11-dihydrodiol.

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Monosubstituted benzene derivatives can be hyroxylated at ortho-, meta- or para-postions but para-hydroxylated product is most common

e.g. Conversion of acetanilide to paracetamol and phenylbutazone tooxyphenbutazone

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3.Oxidation of Benzylic Carbons:

Carbon atoms attached directly to the aromatic rings(benzylic carbons atoms)are hydroxylated to corresponding carbinols.If the product is a primary carbinol, it is further oxidized to aldehydes and then to carboxlic acids, e.g.tolbutamide. A secondary carbinol is converted to a ketone.

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SO2NHCONHC4H9

CH3

SO2NHCONHC4H9

CH2OH

SO2NHCONHC4H9

CHO

SO2NHCONHC4H9

COOH

Alcohol dehydrogenase

Tolut bamide 1ºCarbinol Corr esponding aldehyde Corr esponding -COOH



4.Oxidation of Allylic Carbons:Carbon atoms adjacent to olefinic double bonds(are allylic carbon atoms) also

undergo hydroxylation in a manner similar to benzylic carbons,

e.g. hydroxylation of hexobarbital to 3` hydroxy hexobarbital

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NH N

OO

O

NH N

OO

O

OH

CH3

CH3

CH3

CH3

Hexobarbital3'-Hydroxy barbital



5.Oxidation of Aliphatic Carbons: Alkyl or aliphatic carbon atoms can be hydroxylated at two positions- at the

terminal methy group(called as -oxiadtion) and the pentultimate carbonatom(called as -1 oxidation).

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CH COOH

CH COOH

CH COOH

H5C2

H5C2

CH2

CH2

H5C2

HOCH2

CH2

CH2

H5C2

H5C2

CH2

CH

OH

Valproic acid

Valproic acid

Valproic acid

5-hydroxy

4-hydroxy

(major product)



6.Oxidation of C-N Systems:

a).N-Dealkylation: Alkyl groups attached directly to nitrogen atom in nitrogen bearing

compounds are capable of undergoing N-dealkylation reaction, e.g. secondary and tertiary aliphatic and aromatic amines, tertiary alicyclic amines and N-substituted amides and hydrazines.

M

echanism involves oxidation of -carbon to generate an intermediatecarbinolamine which rearranges by cleavage of C-N bond to yield the N-dealkylated product and the corresponding carbonyl of the alkyl group.

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y A representative example of each class of compounds undergoing N-dealkylation is given below



b).Oxidative Deamination This reaction proceeds via the carbinolamine pathway but

here the C-N bond cleavage occurs at the bond that linksamino group to the larger portionof the drug molecule.

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Primary aliphatic amines readily undergo deamination, e.g.

amphetamine, while secondary and tertiaty amines are deaminated only when bilky groups are attached to nitrogen, e.g. propanolol.Primary amine metabolites formed by N-dealkylation or decarboxylationalso undergo deamination.

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CH2 CH NHCH2

CH3

Amphetamine

CH2 CH

CH3

Phenyl Acetone

O + NH3



c).N-Oxide Formation

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y N-oxides are formed only by nitrogen atoms having basic properties.y Thus, amines can form N-oxides but amides cannot.

y Four groups of tertiary amines that form N-oxides are

y Aliphatic amines-e.g. imipramine

y Alicyclic amines-e.g. nicotine

y

Nitrogen atoms of aromatic heterocycles-e.g. trimethoprimy Amines attached to aromatic rings-e.g. N,N- dimethy aniline.

y The N-Oxide products are highly water soluble and excreted in urine.



d). N-Hydroxylation:

This is usually displayed by nonbasic nitrogen atomssuch as amide nitrogen, e.g. Lidocaine.

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N-Hydroylation of amides often leads to generation of chemically reactive intermeduates capable of binding covalently with

macromolecules, e.g. paracetamol. Paracetamol is safe in therapeutic doses since its reactive metabolite

imidoquinone is neutralized by glutathione; however, in high doses,the glutathione level becomes insufficent abd significant covalenttissue binding, thus occurs resulting in hepatotoxacity.

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7.Oxidation of C-S Systems:

a).S-Dealkylation:The mechanism of S-dealkylation is analogous to N-dealkylation i.e. it proceeds

via -carbon hydroxylation.

e.g. 6-methyl mercaptopirine

b).S-Oxidation: Apart from S-dealkylation, thioethers can also undergo S-oxidation reactions to

yield sulfoxides which may be further oxidized to sulfones.e.g. Phenothiazines

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c). Desulfuration:

The reaction also involves cleavage of carbon-sulfurbond. The product is the one with c=o bond.

E.g. Thioamides- thiopental

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8.Oxidation of C-O Systems:

a).O-Dealkylation:

This reaction is similar to N-dealkylation and proceeds by -carbonhydroxylation to from an unstable hemiacetal or hemiketalintermediate which spontaneously undergoes C-O bond cleavage toform alcohol and a carbonyl moiety.

E.g. Phenacetin to Paracetamol

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9.Oxidation of Alcohol, Carbonyl and Carboxylicacid:

These reactions are mainly catalyzed by nonmicrosomal enzymes,dehydrogenases.

Primary and secondary alcohols and aldehydes undergo oxidation relatively easily but tertiary alcohols,ketones and carboxylic acids are resistant as such a

reaction involves cleavage of C-C bonds. Primary alcohols are metabolized to aldehydes and further to carboxylic acids.

Secondary alcohols are metabolized to ketones.

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Reductive reactions They ar e ca pable of generating polar f unctional gr ou ps like hydr oxy and amino

which f ur ther under go biotransformation r eactions or conjugation.

A number of r eductive r eaction ar e exact opposite of oxidation. for exam ple:

Such r eactions may be catayzed by the same enzyme (true r ever si ble r eaction) or by

differ ent enzymes (a ppar ent r ever si ble r eaction).

Since r ever si ble r eactions usually lead to conver sion of inactive metabolites into

active drug, they may r esult in delay of drug r emoval f r om the body and hence

pr olongation of action

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1.Reduction of Carbonyls (Aldehydes and ketones):

Depending on their reactibity towards reduction, carbonyls can bedivided into 3 categories:

1. The aliphatic aldehydes and ketones.

2. The aromatic aldehydes and ketones.

3. The esters, acids and amides.

The order of reactivity of these categories of drugs inundergoingreduction is 1>2>3 i.e. aliphatic aldehydes and ketones undergoextensive reduction where as ester,acids and amides are least reactive.

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A representative example of compounds undergoing redutive reactions is givenbelow.

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2.Reduction of Alcohols and carbon carbon doublebonds:

These two reactions are considered together because the groups areinterconvertible by simple addition or loss of a water molecule.

Before an alcohol is reduced, it is dehydrated to c=c bond, e.g. bencyclane.

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3.Reduction of N-Compounds(Nitro, Azo and N-oxide):a. Reduction of nitro group proceeds via fromation of nitroso and hyroxylamine

intermediates to yield amines.

For example, reduction of nitrazepam.

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b. Reduction of azo compounds yield primary amines via formation of hydrazo intermediate which undergoes cleavage at N-N bond.

e.g. reduction of Prontosil

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Hy drol y sis The r eaction does not involve change in the oxidation

state of the substrate.

The r eaction r esults in the lar ge chemical change in

the substrate due to loss of lar ge f ragments in thesubstrate.

The hydr olytic enzymes that metabolize xeno biotics

ar e the ones that also act on endogenous substances.

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1.Hydrolysis of Esters :

Esters on hydrolysis yield alcohol and carboxylic acid.the

reaction is catalyzed by esterases.

E.g.

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2.Hydrolysis of Amides: Amides are hydrolyzed slowly in comparison to esters. The reaction is catalyzed

by amidases, involves C-N Cleavage to yield carboxylic acid.

E.g.

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Phase II Reactions This involve transfer of a suitable moiety such as glucronic acid, sulfate,

glycine etc., in presence of a enzyme transferase to drugs or metabolitesof phase I reactions having suitable functional groups to form highly polar, readily excretable and pharmacologically inert conjugates.

Phase II reactions are called as real drug detoxification pathways.

The moieties transferred to the substrates in a phase II reaction possess3 characteristics:

1. They are simple endogenous molecules.

2. They are of large molecular size.

3. They are strongly polar or ionic in nature in order to render the substrate

water soluble.

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Reactions involved 1. Glycoside conjugation - glucur onidation

2. Sulf ate - sulf ation

3. Glutathione (G-SH)4. Methylation

5. Acylation

i. Acetylation

ii. Amino acid conjugationiii. Deacetylation

6. Phosphate conjugation

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G lucuronidation:

TYPES OF GLUCURONIDES FORMED

y o-glucur onides:- xeno biotics with hydr oxyl and /or

carboxyl f unctions form o-glucur onides

(ether glucur onides , ester glucur onides)y N-glucur onides :- xeno biotics with amide , amine

and sulfonamide form N-glucur onides(

y S-glucur onides :-xeno biotics with thiols (SH) form

S-glucur onides (thioether glucur onides)

y C-glucur onides :- xeno biotics with nucleophillic

carbon atoms form C-glucur onides

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Formation of glucuronide:

a-D-glucose-1-phosphate + UTP UDPG+PPi

UDPG + 2NAD* + H2O UDPGA + 2NADH + 2H*

UDPGA+RXH RX-Glucuronic acid + UDP

Pyro phosphorylase

UDPG-dehydrogenase

UDP-Glucuronyl transferase

O

H O

O HO HO H

O H

Glucose

O

C O 2 H

O HOO H

O H

UDP-Glucuronate

P P O

H O O H

N

N

O

O

RX

H

O

C O 2 H

O HO H

O H

XR

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lucuronidation of phenol

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SULFAT I ON y It occurs in 2 steps

y X = O, NH; PAPS = 3-phospho adenosine- 5-phosphosulfate

y APS = adenosine- 5- phosho sulfate

y E.g. Phenols(Paracetamol), alochols, arylamines.

ATP + SO42- APS + PPi

APS + ATP APS-Phosphokinase/Mg+2

PAPS + ADP

PAPS + RXHRX-SO3 + PAP

sulphotransferase

ATP-sulfurylase/Mg+2

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Sulfation of phenol and toluene

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C onjugation with A lpha Am ino A cids

y It is a limited extent reaction because of limited availability of amino acids.

y R = CH2 (if glycine) or >CH-CH2-CH2-CONH2 (IFGLUTA MINE)

RCOOH + ATP RCOAMP + H2O

RCOAMP + CoASH RCOSCoA + AMP

RCOSCoA + H2N-R'-COOH RCONH-R'COOH + CoASH

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Examples of drugs forming glycine or glutamineconjugates

y Aliphatic acids iso propoxyacetic acid

y Alicyclic acids cholic acid

y Aryl acids salicylic acid

y Heterocyclic aryl acids nicotinic acid

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C onjugation with G lutathione and Mercapturic A cid

for m ation

H S NH

H N

O C O 2H

O

N H 2

C O 2HR -X + S NH

H N

O C O 2H

O

N H 2

C O 2H

R

Glutathione conjugate

S O H

H N

O

O

R

Mercaptur ic acid der.

SH

Strong nucleophile

Eg

paracetamol

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A cet y lation:y Acetate derived from acetyl coenzyme A conjugates with

several drugs including isoniazid, hydralazine andprocainamide. Acetylating activity resides in the cytosol

and is widely distributed, occurring in leukocytes andgastrointestinal cells as well as in liver, in which it ispresent in reticuloendothelial rather than parenchymalcells.

y Eg: histamine, procainamide, PABA, sulphanilamide,

isoniazid

NH2HOOC

OH

NHCOCH3HOOC

OH

+ CH3COSCoA

N-acetyl transferase

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Meth y lation:.Methylation of substrate proceeds in two steps

1) synthesis of activated coenzyme s-adenosylmethionine(SA M),

the donor of methyl group,from L-methionine and ATP2) transfer of methyl group from SA M to the substrates

containing a free amino , hydroxyl or thiol groups .

Eg;-phenols morphinepriamry aliphatic amines norephidrinethiols 6-mercaptopurine

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Miscellaneous O ther conjugation reactions include

y C onjugation with c y anide

yC onjugation with ribose

y C onjugation with taurine

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References:

y Richard B. Silverman, The Organic Chemistry of drugdesign and drug action, Pg.no: 405-479.

y

William & lemke ,Foyes Principles of M

edicinalChemistry 6th edition ,Pg.no:65-126.

y S.N.Pandeya Textbook of Medicianal Chemistry Vol-1,Pg.no:31-62.

y

Wilson and Gisvolds Textbook of Organic Medicinalpharmaceutical Chemistry.

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T hank y ou

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