Metabolism of clozapine by rat brain: the role of flavin-containing monooxygenase (FMO) and cytochrome P450 enzymes

Abstract

The atypical antipsychotic clozapine has been reported to be metabolised mainly to its N-oxide and N-demethylated products. Brain, the target organ of clozapine, is known to contain numerous drug-metabolising enzymes which could alter the local concentrations of the drug. The metabolism of clozapine was, therefore, studied in rat brain preparations. Clozapine N-oxide was the major metabolic pathway in rat brain. We characterised the N-oxygenation of clozapine by rat brain preparations. The Km and Vmax values were found to be 319.6 microM and 28.1 pmol/min/mg protein, respectively. The formation of clozapine N-oxide was shown to be inhibited by thiourea (a flavin-containing monooxygenase inhibitor) but not by ketoconazole, quinidine or furafylline. This finding suggests prominent involvement of FMO in the N-oxygenation of clozapine in rat brain. This conclusion was further confirmed by the observation that the formation of clozapine N-oxide is sensitive to heat treatment of the brain preparation and can be partially protected from thermal degeneration by the presence of an NADPH generating system. It was further observed that the rate of clozapine N-oxygenation was much higher at pH 8.5 than at pH 7.4. Taken together, the data suggest that N-oxygenation is the major metabolic pathway catalysed by rat brain and this reaction is catalysed mainly by FMO. As significant interindividual differences have been observed in brain FMO activities, these differences may contribute to the interindividual differences in patient response to clozapine.

Peony-Glycyrrhiza Decoction, an Herbal Preparation, Inhibits Clozapine Metabolism via Cytochrome P450s, but Not Flavin-Containing Monooxygenase in In Vitro Models

“…The final version may differ from this version. the oxygenation of CLZ (Fang et al, 1998;Fang, 2000;Tugnait et al, 1997;. In the present study, we revealed that, under physiological conditions, the production of CLZ metabolites, CLZ-N-oxide and norCLZ, was not different between partial and full activation of FMOs in the presence and absence of the coenzyme NADPH, respectively.…”

“…FMOs and CYPs share many similarities in tissue distribution, subcellular location and substrate specificity (Krueger et al, 2005). FMOs, in particular FMO3, may be also involved in the N-oxygenation of CLZ (Tugnait et al, 1997;Fang et al, 1998;Fang, 2000;Zhang et al, 2008).…”

Progress in Brain Penetration Evaluation in Drug Discovery and Development

“…Metabolizing enzymes, such as monoamine oxidase, flavin-containing monooxygenase, cytochrome P450, and glucuronosyltransferases have been identified in brain endothelial cells and brain tissue (el-Bacha and Minn, 1999;Fang, 2000;Gervasini et al, 2004;Strazielle et al, 2004). Hence, the stability of a compound in brain tissue needs to be examined in early drug discovery.…”

Genetic Polymorphisms of Human Flavin-Containing Monooxygenase 3: Implications for Drug Metabolism and Clinical Perspectives

“…Its structure and function and the implications of its polymorphisms have been widely studied [8,12,13]. This enzyme has a wide substrate specificity, including the dietary-derived tertiary amines trimethylamine, tyramine and nicotine; commonly used drugs including cimetidine, ranitidine, clozapine, methimazole, itopride, ketoconazole, tamoxifen and sulindac sulfide; and agrichemicals, such as organophosphates and carbamates [14][15][16][17][18][19][20][21][22].…”