• clozapine;
  • human liver microsomes;
  • CYP1A2;
  • CYP3A4

Aims Clozapine (CLZ), an atypical neuroleptic with a high risk of causing agranulocytosis, is metabolized in the liver to desmethylclozapine (DCLZ) and clozapine N-oxide (CLZ-NO). This study investigated the involvement of different CYP isoforms in the formation of these two metabolites.

Methods Human liver microsomal incubations, chemical inhibitors, specific antibodies, and different cytochrome P450 expression systems were used.

Results Km and Vmax values determined in human liver microsomes were lower for the demethylation (61±21 &mgr;m, 159±42 pmol min−1 mg protein−1 mean±s.d.; n=4), than for the N-oxidation of CLZ (308±1.5 &mgr;m, 456±167pmol min−1 mg protein−1; n=3). Formation of DCLZ was inhibited by fluvoxamine (53±28% at 10 &mgr;m ), triacetyloleandomycin (33±15% at 10 &mgr;m ), and ketoconazole (51±28% at 2 &mgr;m ) and by antibodies against CYP1A2 and CYP3A4. CLZ-NO formation was inhibited by triacetyloleandomycin (34±16% at 10 &mgr;m ) and ketoconazole (51±13% at 2 &mgr;m ), and by antibodies against CYP3A4. There was a significant correlation between CYP3A content and DCLZ formation in microsomes from 15 human livers (r=0.67; P=0.04). A high but not significant correlation coefficient was found for CYP3A content and CLZ-NO formation (r=0.59; P=0.09). Using expression systems it was shown that CYP1A2 and CYP3A4 formed DCLZ and CLZ-NO. Km and Vmax values were lower in the CYP1A2 expression system compared to CYP3A4 for both metabolic reactions.

Conclusions It is concluded that CYP1A2 and CYP3A4 are involved in the demethylation of CLZ and CYP3A4 in the N-oxidation of CLZ. Close monitoring of CLZ plasma levels is recommended in patients treated at the same time with other drugs affecting these two enzymes.