Interindividual Variation in Relative CYP1A2/3A4 Phenotype Influences Susceptibility of Clozapine Oxidation to Cytochrome P450-Specific Inhibition in Human Hepatic Microsomes

Abstract

ABSTRACT:The atypical antipsychotic drug clozapine (CLZ) is effective in a substantial number of patients who exhibit treatment-resistance to conventional agents. CYP1A2 is generally considered to be the major enzyme involved in the biotransformation of CLZ to its N-demethylated (norCLZ) and N-oxygenated (CLZ N-oxide) metabolites in liver, but several studies have also implicated CYP3A4. The present study assessed the interplay between these cytochrome P450s (P450s) in CLZ biotransformation in a panel of hepatic microsomal fractions from 14 individuals. The relative activity of P450s 1A2 and 3A4 in microsomes was found to be a major determinant of the relative susceptibility of norCLZ formation to inhibition by the P450-selective inhibitors fluvoxamine and ketoconazole. In contrast, the activity of CYP3A4 alone was correlated with the susceptibility of CLZ N-oxide formation to inhibition by these agents. These findings suggest that both P450s may be dominant CLZ oxidases in patients and that the relative activities of these enzymes may determine clearance pathways. In vivo assessment of CYP1A2 and CYP3A4 activities, perhaps by phenotyping approaches, could assist the optimization of CLZ dosage and minimize pharmacokinetic interactions with coadministered drugs.The atypical antipsychotic agent clozapine (CLZ) is effective in many patients who are resistant to treatment with conventional antipsychotic drugs, such as the phenothiazines and butyrophenones. However, its wider use is limited by interindividual variation in efficacy and toxicity. In vitro evidence suggests that the biotransformation of CLZ to its major metabolites N-desmethyl-CLZ (norCLZ) and CLZ N-oxide is catalyzed by hepatic cytochromes P450 (P450s) and flavin-containing monooxygenases (FMOs) (Pirmohamed et al., 1995). NorCLZ formation has been attributed to CYP1A2 and CYP3A4 (Pirmohamed et al., 1995;Linnet and Olesen, 1997) and CYP1A2 and CYP3A4, as well as the FMOs, catalyze CLZ N-oxygenation in vitro (Pirmohamed et al., 1995;Tugnait et al., 1997).There is wide interindividual variation in serum concentrations of CLZ and its active metabolite norCLZ, and the potential for pharmacokinetic drug-drug interactions in psychotic patients is high because of the likelihood of concurrent drug treatment. There have been numerous clinical reports that coadministration of alternate substrates and inhibitors of CYP1A2, such as the selective serotonin reuptake inhibitor fluvoxamine, fluoroquinolone antibacterials, and caffeine, inhibit CLZ clearance and mediate toxic interactions (Hiemke et al., 1994;Jerling et al., 1994). However, it has also been reported that drugs such as fluoxetine, paroxetine, and erythromycin, which inhibit CYP3A4 rather than CYP1A2, may also precipitate toxicity (Centorrino et al., 1996;Wetzel et al., 1998). The factors that determine individual susceptibility to CYP1A2 and CYP3A4 inhibition that leads to clinical toxicity are presently unclear.In a proportion of patients who receive CLZ therapeutic failure may occur because of rapid el...

Characterization of Human Cytochrome P450s Involved in the Bioactivation of Clozapine

“…The K M and V max values for DMCLZ and CLZ-NO are somewhat higher than previously determined in the literature (Eiermann et al, 1997;Tugnait et al, 1999;Zhang et al, 2008), although higher values (.300 mM) have also been reported for N-oxide formation (Eiermann et al, 1997;Tugnait et al, 1999). In addition, our results are in agreement with Zhang et al (Zhang et al, 2008), who showed that K M values for DMCLZ and CLZ-NO are similar. When comparing CLZ, clozapine; CLZ-NO, clozapine N-oxide; DMCLZ, N-desmethylclozapine; GSH, glutathione; n.d., not detectable; P450, cytochrome P450.…”

“…In vivo studies indicate a major role of CYP1A2 in the pharmacokinetics of CLZ . Furthermore, the involvement of polymorphic CYP2D6 and CYP2C19 in formation of DMCLZ and CLZ-NO has been reported (Fischer et al, 1992;Zhang et al, 2008). However, no association has been found between CLZ pharmacokinetics and debrisoquine (CYP2D6) or (S)-mephenytoin (CYP2C19) metabolizer status Arranz et al, 1995).…”

“…Incubations with recombinant human P450 were performed at CLZ concentrations of 10 and 100 mM, as was done previously Eiermann et al, 1997;Linnet and Olesen, 1997;Fang et al, 1998;Tugnait et al, 1999;Olesen and Linnet, 2001;Zhang et al, 2008;Dragovic et al, 2010). Duplicate incubations were performed in 100 mM potassium phosphate buffer (pH 7.4) at a final volume of 200 ml.…”

“…CYP1A2) and N-oxidation (CYP3A4 . CYP1A2, CYP2D6) of CLZ, whereas the other P450s showed only low or no activity Eiermann et al, 1997;Linnet and Olesen, 1997;Fang et al, 1998;Tugnait et al, 1999;Olesen and Linnet, 2001;Zhang et al, 2008). Figure 4 shows the relative activities of individual recombinant human P450s in bioactivation of CLZ using GSH and potassium cyanide as trapping agents.…”

“…Several studies have been performed to identify the role of individual P450s in the oxidative metabolism of CLZ to its major metabolites, N-desmethylclozapine (DMCLZ; C-2) and clozapine N-oxide (CLZ-NO; C-1) ( Fig. 1) Eiermann et al, 1997;Linnet and Olesen, 1997;Fang et al, 1998;Tugnait et al, 1999;Olesen and Linnet, 2001;Zhang et al, 2008). The combined results of these in vitro studies show that both CYP1A2 and CYP3A4 are playing major roles in the biotransformation of CLZ to these two metabolites.…”