Chemistry of drug metabolism


To depict and explicate the chemical science of drug metamorphosis a basic foundation of cognition is needed to understand the concepts.Metabolism is one of the methods for analyzing the consequence of drugs or xenobiotics on the organic structure. It is fundamentally a procedure of change overing lipotropic drugs into more hydrophilic drugs to diminish pharmacological consequence and increase subsequent hepatic or nephritic riddance. So it is basically a procedure of inactivation and detoxification of a drug and subsequent riddance of the metabolite formed.

The basic cognition involves the all clip classical reactions such as oxidization and decrease and those more advanced reactions including glucuronidation and sulfation. Despite energy being needed to drive such reactions to be in favor, metamorphosis can non happen without the complex nature of enzymes catalyzing the procedure. However, metamorphosis of drugs in homo is non entirely dependent on the enzymes entirely – it can be affected by natural micro vegetation in the little bowels. In an in vitro experiment conducted on Zantac, it was found that N-oxide was cleaved and is hence a beginning of drug metamorphosis. An change in the population of micro vegetations can impact the of drugs efficacy – this is a beginning of interaction between antibiotics and Microgynon® .

Furthermore, some drugs are bioactivated by metamorphosis to organize active metabolites with a desirable pharmacological map i.e. prodrugs. Unfortunately metamorphosis can transform an inactive drug or xenobiotic into a biologically active compound which can be carcinogenic to worlds. Phenol is a readily formed metabolite of benzene metamorphosis before catechol and hydroquinone 3,6 which poses a major wellness concern for worlds because it can do acute myelogenous leukemia 6

As the great establishing male parent of medicine Paracelsus one time said “all drugs are poison” . Therefore worlds and animate beings have adapted many mechanisms for detoxicating xenobiotics, and these procedures are divided into two stages – stage I and phase II. It is of import to bear in head that some stage II reactions can happen without stage I metabolism, but stage I and phase II reactions are complimentary and non reciprocally sole. This study describes the chemical reactions of drug metamorphosis and explains how they occur in vivo.

3.0 Phase 1

Phase 1 metamorphosis involves the direct enzyme activity on drugs – P450 isoform enzymes and esterases are responsible for decrease and hydrolysis of drugs severally. Each P450 isoenzyme ‘s familial look varies and can either be inhibited or induced. Knowledge of these drivers of metamorphosis is indispensable non merely to optimize the usage of drugs, cut down injury, maximise benefits in poly pharmaceutics but besides to function as a templet for fresh drug development10.

P450 and esterase enzymes are chiefly found in the liver. Phase I metabolism consists of 3 chief reactions: oxidization, decrease and hydrolysis.

3.1 Oxidation

3.1.1 P450 mono oxygenase system

3.1.2 Other oxidization reactions

3.2 Reduction reactions

Decrease reactions are chiefly interconversion reactions that occur in azo, nitro and epoxide groups and transition of carbonyl to its matching intoxicant. Decrease reactions are carried out in the organic structure by P450 isoenzymes, NADH/NADPH decrease systems, carbonyl reductase or aldo-ketone reductase.

Azo compounds are by and large used in pharmaceutical and decorative merchandises. Decrease of an azo group is a classical illustration of a decrease metabolic reaction. This reaction occurs in the presence of other enzymes and is inhibited in the presence of molecular O.

Mechanism of Azo decrease

Azo decrease can besides happen in the presence of NADH/NADPH system entirely within the pH scope 3.5-6.08. An azo group can either be reduced by 2 Hs to organize hydrozo compounds or 4 Hs to organize two aromatic aminoalkanes which normally consequences in a coloring material loss10

Mechanism of Nitro decrease

Nitro groups besides undergo decrease reactions and these are catalysed by the same NADP systems. 6 e- are donated to the NO2 to organize amine functional groups as in Chloromycetin. This so undergoes acetylation junction in stage II metamorphosis.

Conversion of carbonyl to corresponding intoxicants

Many different enzymes have been identified that catalyse carbonyl decrease of xenobiotics, but most of them catalyse other endogenous substances including sugars and prostaglandins7

Oracin, an antineoplastic drug with a pro-chiral C is metabolised by 11 ?-hydroxysteroid dehydrogenase type I in the microsomes. These metabolites are two-channel specific to organize DHO7 as shown below in figure 3.2.4. Much of what is known about Oracin metamorphosis is from stage II clinical tests as its non licensed for usage in chemotherapy yet.

Mechanism of epoxide decrease

This reaction is catalysed by microsomal epoxide hydrolase, a catalytic three that consists of His 431, Asp226 and Glu 404. Their activity is limited because of a narrow hydrophobic tunnel in the active site and H2O.

  • A H2O molecule ionises to organize a – Ohio and H+
  • OH attacks the oxirane ring and therefore opens it ensuing in formation of vicinal dihydrodiol.

This reaction is slow in vitro without acid but in this instance epoxide hydrolase catalyses the reaction. Vicinal glycols formed are more H2O soluble thereby ending genotoxic potency.


Most hydrolysis reactions occur at the ester and amide functional groups, with ester more prone to hydrolysis than amide. Amides are more stable than esters because N is similar to carbon in size, but less negatively charged than O so negatrons are pulled into the carbonyl ? negatron systems which stabilise its construction. The easiness of hydrolysis of esters is used in the development of prodrugs to avoid first base on balls metamorphosis, a major job in orally administered drugs.

In vivo hydrolytic metamorphosis of drugs occurs in the presence of enzymes present in assorted parts of the organic structure. Hydrolysis of drugs and xenobiotics is by and large carried out by esterases chiefly in the plasma and bowel and non by P450 systems. The blood, GI piece of land and liver have the highest hydrolysing capacity. The most important hydrolysing enzymes are carboxylesterases, cholinesterases, arylesterases and serine endopeptidases.

Carboxylesterase is one of the major esterases involved in drug metamorphosis and xenobiotic biotransformation of drugs with esters, amide and thioester functional groups. In figure 3.0 hydrolysis of ester bond consequences in benzoylecgonine, a carboxylic acid metabolite. But this is non the lone ester group nowadays in the construction. The group present following to the benzine can besides undergo metamorphosis to organize benzoic acid. Cocaine in the presence of heroine can bring forth the toxic metabolite cocaethylene in the presence of intoxicant, from attendant cocaine maltreatment.

Carboxylesterase exists in two different signifiers – hCE1 and hCE2. hCE1 is a more effectual metabolic enzyme which transports protein to the endoplasmic Reticulum and processes fatty acids and cholesterin in the liver alongside other cholesterin enzymes.

The general mechanism of drug hydrolysis in esters and amides is by nucleophilic acyl permutation reactions as shown in figure 3.2.6.

Minor structural differences exist between heroine and its metabolites, but their activity differs. Heroin ( diamorphine ) is converted by hydrolysis to 6-acetylmorphine and morphia. hCE1 chiefly cleaves the 3-acetyl linkage to organize 6-acetylmorphine. The 6-acetyl linkage is cleaved which subsequently forms morphia with a phenolic -OH and secondary allylic -OH.

Diloxanide furorate is a drug of pick and an antiparasitic agent for handling symptomless patients with E. histolytica cysts in the fecal matters and cryptosporidiosis, an acute enteric amoebiaosis in HIV patients. The drug is orally administered and extensively metabolised by gastro enteric esterase to organize diloxanide and furoic acid, thereby decreasing its effectivity. This job is modified by utilizing cyclodextrin that prevents inordinate hydrolysis of the drug.

Carboxylesterase ‘s ability to organize a stable complex enhances its presence in the blood and makes it ideal for handling cocaine overdose. It is besides considered that as an active site for drugs, this would do it ideal for drug find e.g. GB and VX gas.

4. Phase II Conjugation tract

The stage II junction tract is frequently a detoxification mechanism. It terminates drug pharmacological activity by altering or dissembling functional groups in the parent drug or stage I metabolite into a more ionic polar merchandise which aids elimination. The procedures that normally occur in stage II metamorphosis can be basically divided into 3 groups which are glucuronidation, sulfation and acetylation. The nature and functional group of a drug molecule will find which one of these procedures be in favour e.g. Datril undergoes both glucuronidation and sulfation, nevertheless at high doses glucuronidation predominates and at low doses sulfation predominate ( Airpine & A ; Choonara, 2009 ) .

4.1. Junction with sugars

Junction with assorted sugars is possible in nature, and fresh tracts for xenobiotic metamorphosis are discovered often ( Ikenakaa, Ishizakab, & A ; Miyabaraa, 2007 ) . However the most of import reaction in worlds is glucuronidation.

4.1.1 Glucuronidation

Glucuronidation is basically junction of a substrate with ?-D-glucuronic acid, shown in figure As the name suggests, glucuronic acid is a derivative of glucose with the 6th C being oxidised to a carboxylic acid group. This in combination with the many hydroxyl groups gives glucuronic acid a solubility of 1g/10mL in cold H2O, which the British Pharmacopeia would category as “freely soluble” ( British Pharmacopeia Commission, 2009 )

Glucuronic acid is present in vivo as the co-factor uridine 5′-diphosphate-glucuronic acid ( UDP-glucuronic acid ) . The reaction of UDP-glucuronic acid with a xenobiotic substrate is catalysed by the enzyme UDP-glucuronosyltransferase ( UGT ) ( Kaeferstein, 2009 ) , and an illustration of a glucuronidation reaction is shown in figure

Figure demonstrates how glucuronidation can happen with a xenobiotic incorporating an acceptor nucleophilic group ( for illustration COOH, SH or NH2, but in this instance OH ) ( Kaeferstein, 2009 ) ( Sakaguchi, Green, Stock, Reger, & A ; King, 2004 ) . The lone brace of negatrons on the hydroxyl group onslaughts at the 1st C of the pyranose ring, which is activated because of the next electron-withdrawing Os, in an SN2 nucleophilic permutation reaction. The UDP glycosidic bond is cleaved off owing to the good go forthing group belongingss of the phosphate group, and the xenobiotic has reacted with the glucuronic acid to organize a ?-D-glucopyranosiduronic acid conjugate. Note that the reaction is known to be SN2 because the formation of an intermediate leads to an inversion of stereochemistry at the anomeric C.

The ensuing glucuronide conjugate has improved solubility due to the hydroxyl and carboxylate groups, and is normally excreted in the piss, although there is grounds to propose that conjugates with a high molecular weight are eliminated in the gall. However the glucuronides undergo some of import reactions within the organic structure which affects their metamorphosis. A self-generated intramolecular reaction can take to esterification of the glucuronide, as shown in figure The freshly formed ester carbonyl is capable of responding with the N-terminal of a protein residue to organize a stable imine, i.e. this can take to irreversible protein binding. Alternatively, depending on which species the glucuronic acid is bound to, nucleophilic permutation can once more happen and the xenobiotic will respond with the N-terminal of the protein and renew free glucuronic acid ( Zamek-Gliszczynski, Hoffmaster, Nezasa, & A ; Brouwer, 2006 ) .

Pharmaceutical companies may hence seek to avoid planing drugs which are predicted to be metabolised by the glucuronidation tract, non merely to increase the half life of the drug by avoiding junction and elimination but besides to avoid the possible side-effects that can happen as a consequence of protein binding, such as cirrhosis of the liver.

Interestingly, glucuronidation can besides take non merely to metabolites that lose their curative usage and are toxic, but some glucuronides can go on to be pharmacologically active and may even be more powerful than their parent drug.

Morphine-6-glucuronide ( M6G ) is one such illustration. M6G and morphia are both powerful anodynes – M6G, despite holding been conjugated with a big polar molecule, still binds strongly to ? opioid receptors to supply hurting alleviation to the same extent as morphia. Morphine-3-glucuronide, another metabolite, binds preferentially to NMDA receptors alternatively, and causes allodynia, myoclonus and ictuss ( the side-effects associated with opiate use ) . Morphine and codeine are so far the lone known illustrations of glucuronides with high activity ( Kaeferstein, 2009 ) .

4.2. Glutathione junction

Glutathione serves as a substrate for electrophilic drugs because of the nucleophilic thiol mediety on the cysteine residue ( therefore glutathione can be referred to in reaction tracts as merely GSH ) . GSH junction hence involves a nucleophilic onslaught of the sulfur atom onto drugs with electrophilic C atoms, i.e. those bound to good go forthing groups such as halogens, sulfate and nitro, every bit good as activated C atoms in pealing labored systems such as epoxides and ?-lactones ( Zamek-Gliszczynski, Hoffmaster, Nezasa, & A ; Brouwer, 2006 ) .

Conjugation leads to a thioether bond being formed between GSH and the drug molecule. Following this reaction, conjugates are typically metabolised farther to give more polar molecules which are better excreted in the piss and gall ( Zamek-Gliszczynski, Hoffmaster, Nezasa, & A ; Brouwer, 2006 ) .

Figure 4.2.4 shows the possible biotransformation reactions of a glutathione conjugate. Transpeptidase and protease convert glutamate to NH2 and take glycine, severally. NH2 is so a mark for N-acetylation ( mentioned in subdivision 4.4 ) .

Alternatively, two molecules of glutathione can respond together to organize a disulfide span, in the procedure donating H atoms to cut down another molecule. This is normally utilised in vivo when glutathione acts as an antioxidant ( Forman, Zhang, & A ; Rinna, 2009 ) , but besides plays a portion in drug metamorphosis as seen in the denitrification of the antianginal drug, glyceryl trinitrate ( GTN ) in figure 4.2.5 ( Ji, Anderson, & A ; Bennett, 2009 ) .

To repeat, GSH reacts with extremely electrophilic species in the organic structure. This prevents drugs with electrophilic groups from assailing of import nucleophilic Centres in biological molecules, such as Deoxyribonucleic acid and proteins, which could take to toxicity. This is explored farther in subdivision 5 where the effects of deficient glutathione junction of paracetamol metabolites are looked at.

4.3. Sulfation

Sulfation is one of the classical procedures of stage II metamorphosis. It allows the biotransformation of legion xenobiotics and metabolites from stage 1 ( shown in figure 4.3.1 ) to be sulphate conjugates.

This gives protection against toxicity or the possible toxic effects from the legion xenobiotics and metabolites non being conjugated. It besides produces more polar, more H2O soluble metabolites, which means they are more easy and readily excreted in piss or gall. The sulphate conjugate possesses such advantageous belongingss by holding a low pKa, leting an increased aqueous solubility and elimination. It is an of import reaction for drugs and endocrines that contain the phenolic functional group to be metabolised by junction to a sulfate group – illustrations include steroid endocrines, catecholamines, neurotransmitters, tetraiodothyronine, bile acids and phenolic drugs.

Examples of drugs and xenobiotics with a phenolic group attached:

The chemical science behind the sulfation junction reaction emphasizes the of import key characteristics of the system. This includes the two enzymes sulfatase and sulfotransferase, alongside the co factor 3?-phosphoadenosine 5?-phosphosulfate ( 3?-phosphoadenylylsulfate, PAPS ) which plays an of import function in sulfation junction. The handiness of PAPS and its precursor inorganic sulfate determines the reaction rate as the entire sum of sulfate is limited and can be readily used up. PAPS is formed enzymatically by ATP and inorganic sulfate. The enzyme sulfotransferase transfers the active sulfate from PAPS to the xenobiotic or a stage 1 metabolite organizing the sulfate conjugate ( VL & A ; Verdugo D, 2004 ) . Sulphate junction is a reaction chiefly of phenols and to a lesser extent intoxicants to organize extremely ionic polar sulfates. Sulphate junction is besides of import for steroids because steroid sulfates are non capable of adhering to their receptor and so this reduces its biological activity. Sulfation of intoxicant generates a good departure group and can be an activation procedure for intoxicants to bring forth a reactive electrophilic species.

Mechanism of sulfation junction – an electrophilic permutation reaction:

  • The O of the OH has a negative inductive consequence on the benzine pealing so it withdraws negatrons towards it doing it a more reactive nucleophile
  • It attacks the electrophilic sulfur of the sulfate group of PAPS
  • The H of the OH bond foliages in exchange for the sulfate group and UDP acts as a good departure group
  • This forms the sulfate conjugate which is soluble and readily excreted via the kidneys

4.4. Acetylation Junction

Acetylation is besides an of import reaction in stage II metamorphosis as the bulk of drugs contain a primary amine functional group. It is a major path for the biotransformation of hydrazine and aromatic aminoalkanes. This means that acetylation of the arylamine or stage 1 metabolites can happen more easy to cut down their biological activity ( Garcia-Galan & A ; Diaz-Cruz, 2008 ) . The restriction of acetylation is that it produces conjugates that are less H2O soluble ( Zamek-Gliszczynski, Hoffmaster, Nezasa, & A ; Brouwer, 2006 ) every bit good as it does non work for drugs incorporating secondary aminoalkane groups. The purpose of acetylation is to change over the primary aminoalkane mediety into an amide because amides are more stable as peptide bonds are more immune to hydrolysis. Like glucuronidation and sulfation this reaction is extremely specific because of the nature of the enzyme involved. The chief participants of acetylation junction are N-acetyltransferase and the carbon monoxide factor ethanoyl group Coenzyme which is a thioester. The reaction undergoes electrophilic permutation similar to Friedal-Craft acylation. The NH2 attached to the aromatic ring makes it much more reactive and electron donating. NAT helps to reassign the ethanoyl group group ( CH3CO ) obtained from Co enzyme A ( CH3COSCoA ) to conjugate with the drug at the aminoalkane site organizing the amide bond. H-SCo-enzyme Acts of the Apostless as a good departure group.

Mechanism of acetylation junction:

  • The lone brace of the N of the primary aminoalkane of sulphonamide onslaught the carbonyl C of the acetyl group of the ethanoyl group coenzyme A. In this reaction N acts as a nucleophile, donating the brace of negatrons to the electrophilic carbonyl C. The carbonyl C ( ?+ ) is activated by the negatron retreating O ( ?- ) doing it more susceptible to nucleophilic onslaught.
  • This forms a impermanent tetrahedral intermediate, which falls back to organize an amide bond and SH-CoA Acts of the Apostless as a go forthing group.
  • As a consequence the acetyl junction of sulphonamide is formed, and this is readily excreted via the kidneys.

4.5 Stereo selectivity

Stereo selectivity is classed as a cardinal facet of drug metamorphosis of all time since the tragic instance of the drug thalidomide. This has provided a broader cognition on the apprehension of drugs and xenobiotics and besides the importance of their stereochemistry belongingss.

As mentioned in subdivision 4.1.1 ( glucuronidation ) , drug metamorphosis may take to stereochemistry inversion of substrates during the assorted reactions that occur. An illustration of how the apprehension of stereochemistry in xenobiotic metamorphosis has practical applications can be seen with the non-steroidal anti-inflammatory drug isobutylphenyl propionic acid.

It has been found that in vitro, merely the S-isomer is pharmacologically active in suppressing Cox enzymes. However in vivo the metamorphosis of isobutylphenyl propionic acid is complex, affecting glucuronidation at the acyl group and hydroxylation at the 2 and 3 places, but most significantly the metamorphosis of the 2 enantiomorphs differs because there is a unidirectional enzymatic transition of the R-isomer to the active S-isomer. ( Chang, et al. , 2008 ) . The metamorphosis of isobutylphenyl propionic acid is summarised in figure 4.5.2.

For this ground drug makers typically produce a racemic mixture of isobutylphenyl propionic acid for disposal to patients, since the R-isomer will be converted within the organic structure, and bring forthing an enantiomerically pure sample would be needlessly expensive.

5. Micellaneous

Amino acerb junction is of import for metabolising, solubilising and extinguishing carboxylic acids through the piss because it produces really soluble conjugates.

Amino acerb junction mechanism e.g. benzoic acid ( Xu, et al. , 2007 ) :

  • The carboxylic group of the benzoic acid is foremost activated by ATP to the AMP ester
  • This is so converted to the corresponding coenzyme A thioester with CoASH.
  • These first two stairss are catalysed by acyl Coenzyme A synthase enzyme
  • The appropriate amino acid N-acyltransferase so catalyses the condensation of amino acid and Coenzyme A thioester to organize the amino acid conjugate.

Methylation junction: Even though it is non a common reaction for most drugs and xenobiotics, it is deserving adverting methylation because it is the most common biochemical reaction for endogenous compounds such as catecholamines ( Strous, et al. , 2009 ) . Methylation plays a cardinal function in the inactivation of aminoalkanes such as noradrenaline, 5-hydroxytryptamine, Dopastat and histamine, and is besides involved in the biogenesis of adrenaline and melatonin. A beginning of methyl comes from the high energy nucleotide S- adenosylmethionine ( SAM ) which is transported by cathecol-O- methyltransferase. However, it has been reported that methylated conjugates do non hold improved H2O solubility ( a similar disadvantage to acetylation ) .

Methylation mechanism – the nucleophilic permutation of noradrenaline:

  • The lone brace on the negatively charged O of noradrenaline ( R-OH ) attacks the CH3 of SAM
  • The bond between the sulfur and C interruptions ( S-C )

Drug Toxicity

The toxicity associated with acute paracetamol overdose is due to its metamorphosis processes. In the human organic structure, paracetamol is largely metabolised – 30 % by the sulfation tract, 60 % via glucuronidation and the staying 10 % being either excreted unchanged in the piss or undergoing CYP450-dependent oxidization as shown in figure 5.3 to organize N-acetyl-p-benzoquinoneimine ( NAPQI ) ( Airpine & A ; Choonara, 2009 ) .

NAPQI contains an electronically activated ring system, capable of assailing nucleophilic molecules such as N atoms in cellular supermolecules and doing cell harm. However NAPQI will sooner assail the more nucleophilic sulfur atom of glutathione and therefore will besides undergo stage II metamorphosis to organize inactive conjugates – a conventional sum-up of the metamorphosis of paracetamo

In overdose state of affairss, the glutathione supply is used up as it is conjugated with the inordinate NAPQI in the system. This leaves the remainder of the NAPQI free to adhere irreversibly to proteins in hepatic liver cells ( since P450 metamorphosis occurs preponderantly in the liver ) and this cause liver mortification. Without the detoxification capacity of the liver, the human organic structure will typically decease within 2 hebdomads ( Airpine & A ; Choonara, 2009 ) .

With the chemical science of paracetamol metamorphosis in head, it is easier to understand why some patients are classed as “high-risk” and therefore more susceptible to paracetamol overdose:

  • Recent intoxicant ( ethyl alcohol ) ingestion causes initiation of the P450 enzyme involved in the formation of the NAPQI molecule ; this leads to an increased measure of NAPQI being produced and hence the organic structure ‘s supply of glutathione for junction is more quickly used up taking to toxicity. Other drugs which induce the same P450 enzymes will hold the same consequence.
  • Eating upsets such as anorexia nervosa lead to a hapless diet and hence reduced synthesis of glutathione in vivo, so NAPQI detoxification junction can be overwhelmed at lower concentrations of paracetamol ingestion.

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