Influence of FMO1 and 3 polymorphisms on serum olanzapine and its N-oxide metabolite in psychiatric patients

The widely used antipsychotic drug, olanzapine (OLA) shows large interindividual variability in metabolic clearance. Although the role of the enzymes CYP1A2, CYP2D6 and UGT1A4 has been extensively explored, little is known about the in vivo role of flavin-containing monooxygenases (FMOs) catalyzing the N-oxidation of OLA in vitro. We investigated the influence of FMO1 and 3 polymorphisms on the steady state serum concentrations of OLA and its N-oxide metabolite in 379 patients. The upstream FMO1*6 was associated with increased dose-adjusted serum OLA concentrations (C/Ds; P=0.008), an effect further enhanced by FMO1rs7877C&gt;T in smokers. The influence of FMO3 polymorphisms was limited to variability in OLA N-oxide. Homozygous carriers of FMO3rs2266780A&gt;G (p.E308G) displayed 50% lower C/D of OLA N-oxide compared with subjects homo- or heterozygous for the A-variant (P&lt;0.003). Our data support the role of FMO3 in the N-oxidation of OLA and implicate for the first time the contribution of FMO1 and its functional *6 variant in OLA disposition.</p

UGT1A4*3 Encodes Significantly Increased Glucuronidation of Olanzapine in Patients on Maintenance Treatment and in Recombinant Systems

Olanzapine, a world leader in antipsychotic drugs, is used in the treatment of schizophrenia and bipolar disorder. There is considerable interpatient variability in its hepatic clearance. Polymorphic glucuronidation of olanzapine by uridine diphosphate glucuronosyltransferase 1A4 (UGT1A4) was investigated retrospectively in patient samples taken for routine therapeutic drug monitoring (TDM) and in recombinant metabolic systems in vitro. Multivariate analyses revealed that patients who were heterozygous as well as those who were homozygous for the UGT1A4*3 allelic variant had significantly higher concentrations of the major metabolite olanzapine 10-N-glucuronide in serum (+38% (P = 0.011) and +246% (P &lt; 0.001), respectively). This finding was in line with the significant increases in glucuronidation activity of olanzapine observed with recombinant UGT1A4.3 (Val-48) as compared with UGT1A4.1 (Leu-48) (1.3-fold difference, P &lt; 0.001). By contrast, serum concentrations of the parent drug were not significantly influenced by UGT1A4 genotype. Our findings therefore indicate that UGT1A4-mediated metabolism is not a major contributor to interpatient variability in olanzapine levels. However, with respect to other drugs for which UGT1A4 has a dominant role in clearance, increased glucuronidation encoded by UGT1A4*3 might impact the risk for subtherapeutic drug exposure.</p