Characterization of Human Cytochrome P450s Involved in the Bioactivation of Clozapine

Abstract

Clozapine is known to cause hepatotoxicity in a small percentage of patients. Oxidative bioactivation to reactive intermediates by hepatic cytochrome P450s (P450s) has be proposed as a possible mechanism. However, in contrast to their role in formation of N-desmethylclozapine and clozapine N-oxide, the involvement of individual P450s in the bioactivation to reactive intermediates is much less well characterized. The results of the present study show that 7 of 14 recombinant human P450s were able to bioactivate clozapine to a glutathione-reactive nitrenium ion. CYP3A4 and CYP2D6 showed the highest specific activity. Enzyme kinetical characterization of these P450s showed comparable intrinsic clearance of bioactivation, implicating that CYP3A4 would be more important because of its higher hepatic expression, compared with CYP2D6. Inhibition experiments using pooled human liver microsomes confirmed the major role of CYP3A4 in hepatic bioactivation of clozapine. By studying bioactivation of clozapine in human liver microsomes from 100 different individuals, an 8-fold variability in bioactivation activity was observed. In two individuals bioactivation activity exceeded N-demethylation and Noxidation activity. Quinidine did not show significant inhibition of bioactivation in any of these liver fractions, suggesting that CYP2D6 polymorphism is not an important factor in determining susceptibility to hepatotoxicity of clozapine. Therefore, interindividual differences and drug-drug interactions at the level of CYP3A4 might be factors determining exposure of hepatic tissue to reactive clozapine metabolites.

Metabolomics reveals the formation of aldehydes and iminium in gefitinib metabolism

“…Extensive studies have shown that iminium ion intermediate from nicotine covalently binds to cellular macromolecules [34]. In addition, iminium has been trapped in the metabolism of nefazodone [35] and clozapine [36]. Liver toxicities caused by both drugs have been reported in clinical practice and iminium intermediates participated in these adverse effects [37].…”

The Aggravation of Clozapine-Induced Hepatotoxicity by Glycyrrhetinic Acid in Rats

“…CLZinduced hepatotoxicity was considered to be related to the chemically reactive metabolites during CLZ metabolism (3,4,30,31). Our results demonstrated that GA enhanced CLZinduced hepatotoxicity, accompanied by inhibition of CLZ metabolism indexed as formation of CLZ's two metabolites norCLZ and CLZ Noxide.…”

“…induced hepatotoxicity by GA may be correlated to alteration in activities of CYP450s. Although CYP3A4 in human liver was reported to be involved in the demethylation and Noxidation of CLZ (32), CLZ showed a very lower affinity to CYP3A4 with K m values over 100 mM (5,30). Compared to CYP3A4, CLZ showed a much higher affinity to CYP2C9 (K m = 15 mM), CYP1A2 (K m = 25 mM), CYP2C19 (K m = 13 mM), and CYP2D6 (K m = 25 mM) (5).…”

“…The metabolism mediated by CYP1A2 was often accompanied by production of the chemically reactive metabolites (37). CLZ was reported to be metabolized to generate electrophilic reactive metabolites such as iminium ions (38) and nitrenium ions (4) via P450mediated metabolic bioactivation including CYP1A1, CYP1A2, CYP2D6, CYP2J2, and CYP3A4 (30). CLZinduced liver damage was reported to associate with its bioactivation in liver (3).…”

Atypical Neuroleptic Malignant Syndrome in Patients Treated with Aripiprazole and Clozapine: A Case-Series Study and Short Review

“…Clozapine and aripiprazole are both metabolized by the cytochrome P 2D6 (CYP2D6) enzyme in the liver, which means there is a possible competitive effect on the CYP2D6 binding site (see Table 1). Clozapine might delay the metabolism of aripiprazole and increase aripiprazole plasma levels [29][30][31]. Also, the combination of aripiprazole and clozapine might result in competition between aripiprazole and the active metabolite of clozapine, N-desmethylclozapine, which acts as a partial agonist of D2 and D3 receptors; the affinity of aripirazole is superior to N-desmethylclozapine on dopamine receptors [32].…”