By :A. Ashwan Kumar Pharmaceutical Chemistry 09171s0201
Definitiony Biotransformation of drugs is defined as the
conversion of the drug molecule from one chemical form to another.
Biotransformation may result in: a. Pharmacological inactivation of drugs, i.e. it results in formation of metabolites with liitle or no pharmacologic activity; e.g. conversion of Phenytoin to p- hydroxy phenytoin. b. Yield metabolites with equal activity; e.g. conversion of phenylbutazone to oxyphenbutazone. c. Rarely lead to toxicologic activation of drugs, i.e. it results in formation of metabolites with high tissue reactivity; e.g. conversion of paracetamol to reactive metabolites that cause hepatic necrosis. d. Pharmacological activation; e.g. conversion of enalapril to enalaprilat e. Change in Pharmacologic activity; e.g.conversion of diazepam(tranquilizer) to oxazepam(anticonvulsant).3
Metabolic transformation of drugs occur between
absorption of drugs and its renal elimination. The reactions fall into 2 categories:y y
Phase I reactions Phase II reactions
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 or asynthetic reactions.5
Oxidative Reactions Oxidative reactions are the most important and most common metabolic
reactions. Almost all the drugs that undergo phase I biotransformations undergo oxidation 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:
Since only one oxygen atom from the molecular oxygen is incorporated in the
product formed, the mixed function oxidases are also called as monooxygenases. The multienzyme mixed function oxidase system located in the endoplasmic reticulum of hepatic cells, is composed of an electron transfer chain consisting of 3 Components. A heme protein known as cytochrome P-450, which is actually a family of enzymes. It
is 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 flavoprotein known as cytochrome P-450 reductase which is NADPH 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 to facilitate electron transfer from NADPH to cytochrome
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 cases catechols and glutathione conjugates.
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 1,2dihydrodiols. E.g. Conversion of carbamazepine to carbamazepine-10,11-epoxide; the latter is converted to corresponding trans-10,11-dihydrodiol.
Monosubstituted benzene derivatives can be hyroxylated at ortho-, meta- or parapostions but para-hydroxylated product is most common e.g. Conversion of acetanilide to paracetamol and phenylbutazone to oxyphenbutazone
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.
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
CH3 O HN N
CH3 O HN N
CH3 O Hexobarbital
CH3 O 3'-Hydroxy barbital12
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 carbon atom(called as -1 oxidation).H5C2 HOCH2 H5C2 CH2 CH COOH H5C2 H5C2 CH2 CH OH 4-hydroxy Valproic acid (major product) CH2 CH2 CH COOH 5-hydroxy Valproic acid
H5C2 CH2 Valproic acid
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 Nsubstituted amides and hydrazines. Mechanism involves oxidation of -carbon to generate an intermediate carbinolamine which rearranges by cleavage of C-N bond to yield the Ndealkylated product and the corresponding carbonyl of the alkyl group.
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 links amino group to the larger portionof the drug molecule.
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 decarboxylation also undergo deamination.CH3 CH2 Amphetamine CH NHCH2 CH2 Phenyl Acetone CH3 CH O + NH3
c).N-Oxide Formationy 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 arey Aliphatic amines-e.g. imipramine y Alicyclic amines-e.g. nicotine y Nitrogen atoms of aromatic heterocycles-e.g. trimethoprim y 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 atoms such as amide nitrogen, e.g. Lidocaine.
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 covalent tissue binding, thus occurs resulting in hepatotoxacity.
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
c). Desulfuration: The reaction also involves cleavage of carbon-sulfur bond. The product is the one with c=o bond. E.g. Thioamides- thiopental
8.Oxidation of C-O Systems:a).O-Dealkylation: This reaction is similar to N-dealkylation and proceeds by -carbon hydroxylation to from an unstable hemiacetal or hemiketal intermediate which spontaneously undergoes C-O bond cleavage to form alcohol and a carbonyl moiety.
E.g. Phenacetin to Paracetamol
9.Oxidation of Alcohol, Carbonyl and Carboxylic acid: 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.
Reductive reactions They are capable of generating polar functional groups like hydroxy and amino
which further undergo biotransformation reactions or conjugation. A number of reductive reaction are exact opposite of oxidation. for example:
Such reactions may be catayzed by the same enzyme (true reversible reaction) or by
different enzymes (apparent reversible reaction). Since reversible reactions usually lead to conversion of inactive metabolites into active drug, they may result in delay of drug removal from the body and hence prolongation of action
1.Reduction of Carbonyls (Aldehydes and ketones): Depending on their reactibity towards reduction, carbonyls can be
divided 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 inundergoing
reduction is 1>2>3 i.e. aliphatic aldehydes and ketones undergo extensive reduction where as ester,acids and amides are least reactive.
A representative example of compounds undergoing redutive reactions is given
2.Reduction of Alcohols and carbon carbon double bonds: These two reactions 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, e.g. bencyclane.
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.
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
HydrolysisThe reaction does not involve change in the oxidation
state of the substrate. The reaction results in the large chemical change in the substrate due to loss of large fragments in the substrate. The hydrolytic enzymes that metabolize xenobiotics are the ones that also act on endogenous substances.
1.Hydrolysis of Esters : Esters on hydrolysis yield alcohol and carboxylic acid.the reaction is catalyzed by esterases. E.g.
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.
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 metabolites of 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 possess 3 characteristics: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.1.
Reactions involved1. 2. 3. 4. 5.
Glycoside conjugation - glucuronidation Sulfate - sulfation Glutathione (G-SH) Methylation AcylationAcetylation ii. Amino acid conjugation iii. Deacetylationi.
6. Phosphate conjugation36
Glucuronidation:TYPES OF GLUCURONIDES FORMED y o-glucuronides:- xenobiotics with hydroxyl and /or carboxyl functions form o-glucuronides (ether glucuronides , ester glucuronides) y N-glucuronides :- xenobiotics with amide , amine and sulfonamide form N-glucuronides( y S-glucuronides :-xenobiotics with thiols (SH) form S-glucuronides (thioether glucuronides) y C-glucuronides :- xenobiotics with nucleophillic carbon atoms form C-glucuronides37
Formation of glucuronide:a-D-glucose-1-phosphate + UTPPyro phosphorylase
UDPG + 2NAD* + H2O
UDPGA + 2NADH + 2H*
RX-Glucuronic acid + UDP
X HO OH OH O OH OH C O 2H O OH OH O OH P H
R O N O P O N C O 2H X O OH OH OH R
G lu c o s e
U D P -G lu c u r o n a te HO OH
Glucuronidation of phenol
SULFATIONy It occurs in 2 stepsATP + SO42ATP-sulfurylase/Mg+2 APS + PPi APS-Phosphokinase/Mg+2 APS + ATP PAPS + RXH sulphotransferase RX-SO3 + PAP
PAPS + ADP
y X = O, NH; PAPS = 3 -phospho adenosine- 5 phosphosulfate y APS = adenosine- 5 - phosho sulfate y E.g. Phenols(Paracetamol), alochols, arylamines.40
Sulfation of phenol and toluene
Conjugation with Alpha Amino Acidsy It is a limited extent reaction because of limited availability of amino acids.RCOOH + ATP RCOAMP + CoASH RCOSCoA + H2N-R'-COOH RCOAMP + H2O RCOSCoA + AMP RCONH-R'COOH + CoASH
y R = CH2 (if glycine) GLUTAMINE)
or >CH-CH2-CH2-CONH2 (IF
Examples of drugs forming glycine or glutamine conjugates y Aliphatic acids iso propoxyacetic acid y Alicyclic acids cholic acid y Aryl acids salicylic acid y Heterocyclic aryl acids nicotinic acid
Conjugation with Glutathione and Mercapturic Acid formationStrong nucleophile
SHR -X + HS HN
O N H O
C O 2H C O 2H NH2
O R S HN N H
C O 2H R C O 2H NH2 S HN
O G luta thio ne con ju ga te
O M e rca p tu ric a cid d er.
Acetylation:y Acetate derived from acetyl coenzyme A conjugates with several drugs including isoniazid, hydralazine and procainamide. Acetylating activity resides in the cytosol and is widely distributed, occurring in leukocytes and gastrointestinal cells as well as in liver, in which it is present in reticuloendothelial rather than parenchymal cells. y Eg: histamine, procainamide, PABA, sulphanilamide, isoniazidOH HOOC NH2 + CH3COSCoA N-acetyl transferase HOOC OH NHCOCH3
Methylation:. Methylation of substrate proceeds in two steps 1) synthesis of activated coenzyme s-adenosyl methionine(SAM), the donor of methyl group,from L-methionine and ATP 2) transfer of methyl group from SAM to the substrates containing a free amino , hydroxyl or thiol groups . Eg;phenols priamry aliphatic amines thiols morphine norephidrine 6-mercaptopurine
MiscellaneousOther conjugation reactions include y Conjugation with cyanide y Conjugation with ribose y Conjugation with taurine
References:y Richard B. Silverman, The Organic Chemistry of drug
design and drug action, Pg.no: 405-479. y William & lemke ,Foye s Principles of Medicinal Chemistry 6th edition ,Pg.no:65-126. y S.N.Pandeya Textbook of Medicianal Chemistry Vol-1, Pg.no:31-62. y Wilson and Gisvold s Textbook of Organic Medicinal pharmaceutical Chemistry.