title: 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 N-oxidation 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.
authors.affiliation: Division of Molecular Toxicology, Leiden/Amsterdam Center for Drug Research, Faculty of Sciences, Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands.
authors.forename: Sanja
authors.initials: S
authors.lastname: Dragovic
authors.forename: Patrina
authors.initials: P
authors.lastname: Gunness
authors.forename: Magnus
authors.initials: M
authors.lastname: Ingelman-Sundberg
authors.forename: Nico P E
authors.initials: NP
authors.lastname: Vermeulen
authors.forename: Jan N M
authors.initials: JN
authors.lastname: Commandeur
mesh_terms: D014150:Antipsychotic Agents / Q000378:metabolism* / Q000633:toxicity; D001711:Biotransformation; D003024:Clozapine / Q000378:metabolism* / Q000633:toxicity; D019389:Cytochrome P-450 CYP2D6 / Q000378:metabolism; D051544:Cytochrome P-450 CYP3A / Q000378:metabolism; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D003640:Dealkylation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005978:Glutathione / Q000378:metabolism; D006801:Humans; D007527:Isoenzymes; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000201:enzymology*; D008862:Microsomes, Liver / Q000201:enzymology; D015394:Molecular Structure; D010084:Oxidation-Reduction; D011994:Recombinant Proteins / Q000378:metabolism
chemical_list: D014150:Antipsychotic Agents; D065607:Cytochrome P-450 Enzyme Inhibitors; D004791:Enzyme Inhibitors; D007527:Isoenzymes; D011994:Recombinant Proteins; D003577:Cytochrome P-450 Enzyme System; D019389:Cytochrome P-450 CYP2D6; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human; D005978:Glutathione; D003024:Clozapine