Individual differences in in vitro and in vivo metabolic clearances of the antipsychotic drug olanzapine from non-smoking and smoking Japanese subjects genotyped for cytochrome P4502D6 and flavincontaining monooxygenase 3

Objective The antipsychotic olanzapine is reportedly metabolized by inducible human cytochrome P450 (CYP) 1A2 and variable copy‐number CYP2D6 and polymorphic flavin‐containing monooxygenase 3 (FMO3) in different pathways. We investigated individual differences in the metabolite formation and clearance of olanzapine in vitro and in vivo. Methods Human liver microsomal olanzapine oxidation activities were evaluated, and plasma concentrations of olanzapine were determined in 21 Japanese patients (mean age: 50 years, range: 32–69 years, 14 male and 7 female, including 6 smokers) genotyped for CYP2D6 (*1, *5, and *10) and FMO3 (E158K, C197fsX, R205C, V257M, E308G, and R500X). Results Furafylline (a CYP1A2 inhibitor), quinidine (a CYP2D6 inhibitor), and heat treatment (inactivates FMO3) suppressed liver microsomal metabolic clearance of olanzapine by approximately 30%. Olanzapine N‐demethylation and N‐oxygenation were found to be catalyzed by CYP1A2 and CYP2D6 and by CYP2D6 and FMO3, respectively, in experiments using liver microsomes and recombinant enzymes. Plasma concentrations and clearance of olanzapine were not affected by CYP2D6 or FMO3 genotypes or smoking behavior. Conclusions Olanzapine clearance was not affected by CYP2D6 or FMO3 genotypes or smoking behavior as a single factor under the present conditions because olanzapine clearance is mediated by multiple enzymes involved in two major and one minor pathways. Copyright © 2016 John Wiley & Sons, Ltd.