BIOTRANSFORMATION Prof. Dr. Basavaraj K. Nanjwade M. Pharm., Ph. D Department of Pharmaceutics KLE University’s College of Pharmacy BELGAUM – 590010, Karnataka, India Cell No:

Transcript BIOTRANSFORMATION Prof. Dr. Basavaraj K. Nanjwade M. Pharm., Ph. D Department of Pharmaceutics KLE University’s College of Pharmacy BELGAUM – 590010, Karnataka, India Cell No:

BIOTRANSFORMATION

03-12-2010

Prof. Dr. Basavaraj K. Nanjwade

Department of Pharmaceutics KLE University’s College of Pharmacy BELGAUM – 590010, Karnataka, India Cell No: 0091 9742431000 E-mail: [email protected]

M. Pharm., Ph. D KLECOP, Nipani 1



Contents:-



Introduction



Phase I reaction



Phase II reaction



Factors affecting Biotransformation



Reference

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

Introduction:

Important terms:



Detoxication (detoxification)



Prodrugs



Active Metabolite



Reactive Metabolite



Metabolism

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Xenobiotics

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

Definition:

 “Biotransformation of drug is defined as the

conversion from one chemical form to another”.

the term is used synonymously with

metabolism

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• • 

Biotransformation leads to:

Pharmacologic Inactivation of Drug

Ex:

Active Drug Inactive Drug

Salicylic Acid Salicyluric Acid • • •

Active Metabolite From An Inactive Drug

Ex.

Inactive(Prodrug)

Aspirin

Active

Salicylic Acid •

No Change in Pharmacologic Activity

•

Ex.

Active Active Drug

Codeine Morphine 03-12-2010 KLECOP, Nipani 5

Drug metabolizing organs

• Liver is the heart of metabolism • Because of its relative richness of enzymes in large amount.

• Schematic chart of metabolizing organ (decreasing order) • Liver > lungs > Kidney > Intestine > Placenta > Skin > Brain > Testes > Muscle > Spleen 03-12-2010 KLECOP, Nipani 6

Drug Metabolizing Enzymes

Microsomal Non Microsomal

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Microsomal Enzymes

• •

Found predominately in the smooth Endoplasmic Reticulum of liver Other areas:

– – –

Kidney Lungs Intestinal mucosa

Non-microsomal enzymes

•

Found in the cytoplasm and mitochondria of hepatic cells

•

Other tissues including plasma

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•

Microsomal Enzymes

Non-synthetic/ Phase I reactions

–

Most oxidation and reduction

–

Some hydrolysis

•

Synthetic/ Phase II reactions

–

ONLY Glucuronide conjugation

Non-microsomal enzymes

•

Non-synthetic/ Phase I reactions

–

Most hydrolysis

– • Synthetic/ Phase II reactions –

Some oxidation and reduction ALL except Glucuronide conjugation

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Microsomal Enzymes

•

Inducible

–

Drugs, diet, etc .

03-12-2010

Non-microsomal enzymes

•

Not inducible

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Why Biotransformation?

• Most drugs are excreted by the kidneys.

• For renal excretion drugs should: – have small molecular mass – be polar in nature – not be fully ionised at body pH • Most drugs are complex and do not have these properties and thus have to be broken down to simpler products.

• Drugs are lipophilic in nature.

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• Strongly bound to plasma proteins.

• This property also stops them from getting eliminated.

• They have to be converted to simpler hydrophilic compounds so that they are eliminated and their action is terminated.

When and How?

• Biotransformation occur between absorption and elimination from kidneys.

• Drugs administered orally can biotransform in the intestinal wall.

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

Metabolic reaction:



Phase I reaction



Phase II reaction



Oxidation



Reduction



Hydrolysis

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Conjugation

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

Phase I:

 A polar functional group is either introduced or unmasked if already present on the otherwise lipid soluble Substrate, 

E.g

–OH, -COOH, -NH2 and –SH.

 Thus, phase I reactions are called as

functionalization reactions

 Phase I reactions are Non-synthetic in nature.

 The majority of Phase I metabolites are generated by a common hydroxylating enzyme system known as cytochrome P450.

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

Oxidative reaction:

1) Oxidation of aromatic carbon atoms 2) Oxidation of olefins (C=C bonds) 3) Oxidation of Benzylic, Allylic carbon atoms & carbon atoms alpha to carbonyl & imines 4) Oxidation of aliphatic carbon atoms 5) Oxidation of alicyclic carbon atoms 03-12-2010 KLECOP, Nipani 17

6) Oxidation of carbon-heteroatom systems:

A. Carbon-Nitrogen system

 N- Dealkylation.

 Oxidative deamination  N-Oxide formation  N-Hydroxylation

B. Carbon-Sulfur system

 S- Dealkylation  Desulfuration  S-oxidation

C. Carbon-Oxygen systems(O- Dealkylation)

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7) Oxidation of Alcohol, Carbonyle and Acid functios.

8) Miscellaneous oxidative reactions .



Reductive reactions:

1) Reduction of Carbonyl functions.(aldehydes/ketones) 2) Reduction of alcohols and C=C bonds 3) Reduction of N-compounds (nitro,azo & N-oxide) 4) Miscellaneous Reductive reactions .

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

Hydrolytic reactions

1) Hydrolysis of Esters and Ethers 2) Hydrolysis of Amides.

3) Hydrolytic cleavage of non aromatic heterocycles 4) Hydrolytic Dehalogination 5) Miscellaneous hydrolytic reactions .

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

Oxidation of aromatic carbon atoms (aromatic hydroxylation):

R R Arene R OH Arenol (major) O H 2 O epoxide hydrase R Arene oxide (highly reactive electrophile) GSH 5-epoxide transferase R OH Dihyrdrodiol R OH OH OH SG Tissue toxicity in instances when glutathione is depleted.

 Glutathione conjugate (min.pro.)

E.g. Epoxides of Bromobenzene and Benzopyrene.

03-12-2010 KLECOP, Nipani OH Catechol(min. Pro.) 21



Oxidation of olefins (C=C bonds):

 Oxidation of non-aromatic C=C bonds is analogous to aromatic hydroxylation. i.e. it proceeds via formation of epoxides to yield 1,2 dihydrodiols.

N CONH 2 Carbamazepine 03-12-2010 O HO OH H 2 O N CONH 2 Carbamazepine-10,11 epoxide KLECOP, Nipani epoxide hydrase N CONH 2 Trans-10,11 dihydroxy



Oxidation of Benzylic Carbon Atoms:

 Carbon atoms attached directly to the aromatic ring are hydroxylated to corresponding Carbinols.

 If the product is a primary carbinol, it is further oxidized to aldehydes and then to carboxylic acids,  

E.g.Tolbutamide

A secondary Carbinol is converted to Ketone.

CHO COOH CH 3 CH 2 OH Alcohol dehydrogenase SO 2 NHCONHC 4 H 9 Tolbutamide 03-12-2010 SO 2 NHCONHC 4 H 9 Prmary carbinol Corresponding aldehyde KLECOP, Nipani Corresponding carboxylic acid.



Oxidation of Allylic carbon Atoms:

 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.

Allylic carbon atom OH 3' 2' O H 3 C O O H 3 C O HN HN N CH 3 N CH 3 O O 3'-Hydroxy Hexobarbital Hexobarbital 03-12-2010 KLECOP, Nipani 24



Oxidation of Carbon Atoms Alpha to Carbonyls and Imines:

 Several Benzodiazepines contain a carbon atom (C-3) alpha to both Carbonyl (C=0) and imino (C=N) function which readily undergoes Hydroxylation.



E.g. Diazepam

O N N 3 N N IC IC OH 03-12-2010 Diazepam KLECOP, Nipani 3-Hydroxy diazepam 25



Oxidation of Aliphatic Carbon Atoms (Aliphatic Hydroxylation):

 Terminal hydroxylation of methyl group yields primary alcohols which undergoes further oxidation to aldehydes and then to carboxylic acid.

H 3 C H C CH 3 C H 2 Ibuprofen 03-12-2010 CH 3 C H COOH KLECOP, Nipani H 3 C OH C CH 3 C H 2 CH 3 C H COOH Tertiary alcohol metabolite 26



Oxidation of Alicyclic Carbon Atoms (Alicyclic Hydroxylation):

O  Cyclohexane (alicyclic) and piperidine (non-aromatic heterocyclic) rings are commonly found in a number of molecules.

 

E.g. Acetohexamide and minoxidil respectively.

Such rings are generally hydroxylated at C-3 or C-4 positions.

H 2 N H 2 N N N N N 4' O N N OH H 2 N Minoxidil 03-12-2010 KLECOP, Nipani H 2 N 4'-Hydroxy Minoxidil 27



Oxidation of Carbon-Heteroatom Systems:

 Biotransformation of C-N, C-0 and C-S system proceed in one of the two way: 1. Hydroxylation of carbon atom attached to the heteroatom and subsequent cleavage at carbon heteroatom bond.



E.g. N-, O- and S- dealkylation, oxidative deamination and desulfuration.

2. Oxidation of the heteroatom itself.



E.g. N- and S- oxidation.

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N 

Oxidation of Carbon-Nitrogen System:



N-Dealkylation:

 Mechanism of N-dealkylation involve oxidation of α carbon to generate an intermediate carbinolamine which rearranges by cleavage of C-N bond to yield the N dealkylated product and the corresponding carbonyl of the alkyl group.

OH H O C NH C N C + H H Carbonyl N-Dealkylated metabolite Carbinolamine 03-12-2010 29

 A tertiary nitrogen attached to different alkyl groups undergoes dealkylation by removal of smaller alkyl group first.



Example:

 Secondary aliphatic amine

E.g. Methamphetamine.

 Tertiary aliphatic amine  Tertiary alicyclic amine  Amides 03-12-2010

E.g. imipramine E.g. hexobarbital E.g. Diazepam

KLECOP, Nipani 30



N-Hydroxylation:-

 Converse to basic compounds that form N-oxide, N hydroxy formation is usually displayed by non-basic nitrogen atoms such as amide Nitrogen.



E.g. Lidocaine

CH 3 N H O C C 2 H 5 C H 2 N C 2 H 5 CH 3 Lidocaine 03-12-2010 KLECOP, Nipani CH 3 N O C C 2 H 5 C H 2 N C 2 H 5 OH CH 3 N- Hydroxy Lidocaine 31



Oxidation of Carbon-Sulfur Systems:



S-Dealkylation:

 The mechanism of S-Dealkylation of thioethers is analogous to N-dealkylation .IT

 proceed via α-carbon hydroxylation.

The C-S bond cleavage results in formation of a thiol and a carbonyl product.



E.g

.

6-Methyl mercaptopurine.

SCH 2 OH SCH 3 SH N N N N H 6-Methyl Mercaptopurine 03-12-2010 N N N N H Hydroxylated intermediate KLECOP, Nipani N N N N H 6-Mercaptopuri9ne + HCHO 32



Desulfuration

:  This reaction also involves cleavage of carbon-sulfur bond (C=S).

 The product is the one with C=0 bond. Such a desulfuration reaction is commonly observed in thioamides such as thiopental.

Thiopental KLECOP, Nipani Pentobarbital 33



S-Oxidation:

 Apart from S-dealkylation, thioethers can also undergo S oxidation reaction to yield sulfoxides which may be further oxidized to sulfones several phenothiazines.



E.g

. Chlorpromazine undergo S-oxidation

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

Oxidation of Carbon-Oxygen Systems:



O-Dealkylation:

 This reaction is also similar to N-Dealkylation and proceeds by α-carbon hydroxylation to form an unstable hemiacetal or hemiketal intermediate.

 Which spontaneously undergoes C-0 bond cleavage to form alcohol and a carbonyl moiety.

OH H R-O-CH 2 R' Ether 03-12-2010 R-O-CH-R' Hemiacetal KLECOP, Nipani R-OH Alcohol + O=C-R' Aldehyde/ketone 35



Oxidation of Alcohol, Carbonyl and Carboxylic Acid

 In case of ethanol, Oxidation to acetaldehyde is reversible and further oxidation of the latter to acetic acid is very rapid since Acetaldehyde is highly toxic and should not accumulate in body.

CH 3 CH 2 OH Ethanol 03-12-2010 alcohol dehydrogenase Aldehyde CH 3 CHO Acetaldehyde KLECOP, Nipani dehydrogenase CH 3 COOH Aceticacid 36



Miscellaneous oxidative reactions:



Oxidative aromatization /dehydrogenation:

 E.g. Metabolic aromatization of drugs is nifedipine.

NO 2 COOCH 3 CH 3 NO 2 COOH CH 2 OH NH N CH 3 COOCH 3 Nitedipine 03-12-2010 KLECOP, Nipani CH 3 COOCH 3 Pyridine metabolite 37



Oxidative Dehalogenation:

 This reaction is common with halogen containing drugs such as chloroform.

 Dehalogenation of this drug yields phosgene which may results in electrophiles capable of covalent binding to tissue.

Cl Cl Cl H Chloroform 03-12-2010 oxi.

-HCL Cl O Cl Phosgene KLECOP, Nipani Covalent binding to tissues.



Reductive reaction:

 Bioreductions are also capable of generating polar functional group such as hydroxy and amino which can undergo further biotransformation or conjugation.

 

Reduction of carbonyls:

Aliphatic aldehydes : 

E.g. Chloral hydrate

Cl 3 C-CHOH 2 O Cl 3 C-CH 2 OH Chloral hydrate Trichloroethanol 03-12-2010 KLECOP, Nipani 39



Aliphatic ketones: E.g. Methadone.

H 2 C H C H 3 C O C 2 H 5 N CH 3 CH 3 H 2 C H 3 H C C OH C 2 H 5 CH 3 N CH 3 Methadone Methadol 

Aromatic Ketone: E.g. Acetophenone.

O CH 3 OH C H CH 3 Acetophenone 03-12-2010 KLECOP, Nipani Methyl phenyl carbinol 40



Reduction of alcohols and C=C:

 These two reductions are considered together because the groups are interconvertible by simple addition or loss of a water molecule. Before an alcohol is reduced it is dehydrated to C=C bond.



Example – bencyclane .

(antispasmodic) O (CH 2 ) 3 N(CH 3 ) 2 Bencyclane 03-12-2010 OH Bencyclanol KLECOP, Nipani Benzylidine cycloheptane 41



Reduction of N-compounds:

 Reduction of nitro groups proceeds via formation of nitro so and hydroxyl amine intermediates to yield amines.

RNO 2 R-N=O R-NHOH RNH 2 Nitro Nitroso Hydroxylamine Amine  For

E.g. Reduction of Nitrazepam.

H N O H N O N O 2 N N H 2 N Nitrazepam 7-Amino metabolite.

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

R-N=N-R' R-NH-NH-R' RNH 2 + NH 2 R' Azo Hydrazo Amines 

E.g. Prontosil .

NH 2 NH 2 H 2 N N N SO 2 NH 2 H 2 N NH 2 + H 2 N SO 2 NH 2 Prontosil 1,2,4-Triamino benzene sulfanilamide  It is reduced to active Sulfanilamide.

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

Miscellaneous reductive reactions:



Reductive Dehalogenation:

 This reaction involves replacement of halogen attached to the carbon with the H-atom.

E.g. Halothane.

Br CF 3 H CF 3 -CH 3 CF 3 -COOH Cl Halothane 1,1,1- Trifluroethane Trifluroacetic acid 

Reduction of sulfur containing functional groups:

C 2 H 5 N C 2 H 5 S S S S N C 2 H 5 C 2 H 5 C 2 H 5 N C 2 H 5 S SH

E.g. Disulfuram

Dsulfiram Diethyldithiocarbamic acid 03-12-2010 KLECOP, Nipani 44



Hydrolytic reactions:

1. The reaction does not involve change in the state of oxidation of substrate.

2. The reaction results in a large chemical chain in the substrate brought about by loss of relatively large fragments of the molecule.



Hydrolysis of esters and ethers:

 Esters on hydrolyisis yield alcohol and carboxylic acid.

The reaction is catalyzed by esterases .

O R-C-OR' 03-12-2010 O R-C-OH KLECOP, Nipani + R'-OH 45



Organic acid esters:

 Esters with a large acidic group :

E.g. Clofibrate

Cl COOC 2 H 5 O CH 3 CH 3 Cl COOH O CH 3 CH 3 + C 2 H 5 OH Clofibrate Free acid metan  Esters with a large acidic and alcoholic group: H 2 C C H 2 COOCH 2 CH 2 -N(CH 3 ) 2 COOCH 2 CH 2 -N(CH 3 ) 2 Pseudo Choline sterase CH 2 COOH CH 2 COOH

E.g. Succinylcholine

+ 2 HOCH 2 CH 2 N(CH 3 ) 2 Suceinylcholine succinic acid choline 03-12-2010 KLECOP, Nipani 46



Inorganic acid esters:



Phosphates: E.g. Stilbestrol diphosphate

OH O P OH O C 2 H 5 C 2 H 5 OH O P OH O H O C 2 H 5 Stilbestrol diphosphate stilbestrol C 2 H 5 OH + 2 H 3 PO 4 

Sulfates: E.g. Isopropyl methanesulfonate

H 3 C CH 3 O O S CH 3 H 3 C CH 3 OH + H O O S CH 3 H O H O Isopropyl methnesulfonate isopropanol methanesulfonic acid 03-12-2010 KLECOP, Nipani 47



Hydrolysis of amides:

 The reactions catalyzed by amides, involves C-N cleavage to yield carboxylic acid and amine .

O O R-C-NHR' R-C-OH + R'NH 2 

primary amide with aliphatic substituent on N-atom:

H 2 N

E.g. Procainamide

O N H C 2 H 5 C H 2 C H 2 N C 2 H 5 H 2 N O + OH H 2 N C 2 H 5 C H 2 C H 2 N C 2 H 5 Procanamide PABA 03-12-2010 KLECOP, Nipani 48



Secondary amide with aromatic Substituent on N-atom: E.g. Lidocaine

CH 3 O C 2 H 5 N H CH 3 C H 2 N C 2 H 5 CH 3 NH 2 CH 3 + HOOC C 2 H 5 C H 2 N C 2 H 5 Lidocaine 2,6 Xylidine N, N- Diethylglycine 

Tertiary amide: E.g. Carbamazepine

N CONH 2 N H Carbamazepine Iminostilbene 03-12-2010 KLECOP, Nipani 49

 

Hydrolytic cleavage of non aromatic heterocyclics:

Four – membered lactams: E.g. Penicillins

C H 2 O H N O N S CH 3 CH 3 COOH C H 2 O H N H O O N S CH 3 CH 3 COOH Penicillin G Penicinoic acid metabolite 

Five – member lactams: E.g. Succinimides

O N CH 3 O COOH O NH 2 Phensuximide Phenyl succinamic acid 03-12-2010 KLECOP, Nipani 50



Hydrolytic dehalogenation:

 Chlorine atoms dehalogenated attached to aliphatic carbons are easily.



E.g. Dichloro diphenyl trichloro ethane

H -HCL H Cl Cl Cl Cl CCl 3 CCl 2 DDT DDE 

Miscellaneous hydrolytic reactions:

 Include hydration of epoxides and arene oxides, hydrolysis of Sulfonylureas, Carbamates, Hydroxamates and alpha Glucuronide and sulfate conjugates 03-12-2010 KLECOP, Nipani 51

Phase 2 Reactions

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Synthetic Conjugation

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Phase II

• Phase II - combines functional group of compound with endogenous substance

E.g.

Glucuronicacid, Sulfuric acid, Amino Acid, Acetyl.

 Products usually very hydrophilic  The final compounds have a larger molecular weight.

53 03-12-2010 KLECOP, Nipani

Enzymes

• Glucuronosyl Transferases • Sulfotransferases (ST) • Acetyltransferase • Methylases

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How We Get To Phase 2

• • Most of the drugs do not become polar upon phase 1 reactions.

• The Body is left with a plan to further metabolize the Drugs

Goal of Phase 2 :

Make substances more soluble that couldn’t be done in the Phase 1 reactions.

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Synthetic Reactions / Phase II

• 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,-NH 2 ,- 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 total loss of pharmacologic activity so called as a true detoxification reaction 03-12-2010 KLECOP, Nipani 56

Phase II

• Glucuronide Conjugation • Methylation • Acetylation • Sulfate Conjugation • Conjugation With Alpha Amino Acids • Glutathione Conjugation • Glycine Conjugation • Cyanide Conjugation 03-12-2010 KLECOP, Nipani 57

• Very important Synthetic reactions carried out by Uredine Di Phosphate Glucuronosyl Transferase.

• Hydroxyl and Carboxylic acid groups are easily combined with Glucuronic acid.

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

Glucuronide formation occurs in 2 steps:-

Synthesis of an activated coenzyme uridine-5’- diphospho -alpha D- Glucuronic acid (UDPGA) from UDP- glucose (UDPG).

Pyrophosphorylase



-D-Glucose-1-phosphate + UTP UDPG + Ppi UDPG +2NAD + H 2 O

UDPG - Dehydrogenase

UDPGA +2NADH + 2H +

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Transfer of the glucuronyl moiety from UDPGA to the substrate RXH in presence of enzyme UDP-

UDP-Glucuronyl transferase

glucuronyl transferase to form the conjugate.

UDPGA + RXH Where,

X = O, COO, NH or S

RX Glucuronic Acid +UDP

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COOH COOH COOH Benzoic acid OH OH

UDPGA

OH UDP

Benzoic Acid

OH OH OH

Glucuronide Benzoic acid

Ex.

Chloramphenicol, Morphine, Salicylic Acid, Paracetamol 03-12-2010 KLECOP, Nipani 61

• Common, minor pathway.

• Methyltransferases – -CH 3 transfer to

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1. Synthesis of an activated coenzyme S adenosyl methionine(SAM), the donor of methyl group, from methionine and ATP.

L-

Methionine Adenosyl Transferase

L – Methionine + ATP SAM + PPi + Pi

2. Transfer of the methyl group from SAM to the substrate in presence of nonmicrosomal enzyme methyl transferase.

RXH + SAM

Methyl Transferase

RX-CH3 + S – Adenosyl Homocysteine

Ex.

Morphine, Nicotine, Histamine 03-12-2010 KLECOP, Nipani 63

 Major route of biotransformation for aromatic amines, hydrazine.

 Generally decreases water solubility  Enzyme: - N- Acetyltransferase (NAT) R – NH

R – NH – COCH

03-12-2010 KLECOP, Nipani 64

COOH OH CH 3 COS COA NH 2 Paraaminosalicyclic Acid Acetyl Co enzyme

Ex.

Histamine, PAS, PABA 03-12-2010 KLECOP, Nipani COOH OH NHCOCH3 NH 2 N- Acetylated PAS 65

• Sulfotransferases are widely-distributed enzymes • Cofactor is 3’-phosphoadenosine-5’-phosphosulfate (PAPS) • Produce highly water-soluble eliminated in urine, bile sulfate esters, • R – OH R – O – SO

03-12-2010 KLECOP, Nipani 66

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 + SO 4 2-

ATP Sulfurylase/Mg ++

APS + Ppi APS + ATP

03-12-2010

APS Phosphokinase/Mg ++

PAPS + ADP

KLECOP, Nipani 67

2. Transfer of sulfate group from PAPS to the substrate RXH in presence of enzyme Sulfotransferase and subsequent liberation of 3’- phosphoadenosine-5’-phosphate(PAP).

PAPS + RxH X= O,NH

Sulfotransferase

Rx-SO 3 + PAP Examples of compounds undergoing sulfation are:  Phenol Paracetamol , Salbutamol  Alcohols Aliphatics C-1 to C-5  Arylamines Aniline 03-12-2010 KLECOP, Nipani 68

• Alternative to glucuronidation • Two principle pathways – -COOH group of substrate conjugated with -NH 2 of Glycine, Serine, Glutamine, requiring CoA activation •

E.g.

conjugation of benzoic acid with Glycine to form Hippuric acid – Aromatic -NH 2 or NHOH conjugated with -COOH of Serine, Proline, requiring ATP activation 03-12-2010 KLECOP, Nipani 69

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 donor coenzyme is activated and not the substrate.

RCOOH + ATP

Acetyl Synthetase

RCOAMP + H 2 O + Ppi

Acyl CoA Transferase

RCOAMP + CoA-SH RCSCoA + AMP

2. Acylation of the alpha- amino acid by the acyl CoA in presence of enzyme N-acyl transferase.

RCSCoA + NH 2 -R’-COOH

03-12-2010

N-Acetyl transferase

KLECOP, Nipani

RCONH-R’COOH + CoA- SH

• Glutathione-

-transferase catalyzes conjugation with glutathione • Glutathione is tripeptide of Glycine, Cysteine, Glutamic acid 03-12-2010 KLECOP, Nipani 71

Glutathione-S-transferase

03-12-2010

γ Glutamyl transferase

KLECOP, Nipani 72

Glycine Cysteinyl glycinase

N- acetylase

03-12-2010

Mercapturic Acid Derivative

KLECOP, Nipani 73

Glycine Conjugation

 Salicylates and other drugs having carboxylic acid group are conjugated with Glycine.

 Not a major pathway of metabolism

Cyanide Conjugation

 Conjugation of cyanide ion involves transfer of sulfur atom from thiosulfate to the cyanide ion in presence of enzyme rhodanese to form inactive thiocyanate.

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+ CN

rhodanese

KLECOP, Nipani

SCN + SO

3 2-

Biotransformation-Conclusion

• Change the Xenobiotics to a form that can be eliminated from the body • Change the Xenobiotics to a less biologically active form • Bioactivation to more toxic forms can also occur • Synthetic Phase II reactions are carried out by other enzymes.

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

Factor affection of Biotransformation of drug:

 The Therapeutic efficacy, Toxicity and Biological half life of drug depends on metabolic rate and the factor that influence metabolic rate are:

1) Physicochemical property of drug.

) chemical factors:

   a. Induction of drug metabolizing enzyme.

b. Inhibition of drug metabolizing enzyme c. Environmental chemicals.

Biological factors.

   A. Species differences.

B. Strain differences.

C. Sex differences.

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 D.

Age.

 E.

Diet.

 F.

Altered pharmacologic factors:

   i Pregnancy.

ii Hormonal imbalance.

iii Disease state.

 G.

Temporal factors:

 I Circadian rhythm .

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References

• Biopharmaceutics & Pharmacokinetics by D.M.

Brahmankar, S. B. Jaswal, Vallabh Prakashan, Pg 111-158.

• Biopharmaceutics & Pharmacokinetics by Milo Gibaldi, 4 th edition, Pg.no. 203.

• Text book of Biopharmaceutics & pharmacokinetics, Dr.Sobha Rani R. Hiremath, Prism Books Pvt Ltd, Bangalore, 2000 Pg.no. 157-166.

• • www.google.com

www.pharmacology.com

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03-12-2010

Cell No: 0091 9742431000 E-mail: [email protected]

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