Background: To investigate the impact of genetic variability in CYP2D6, CYP3A5, and ABCB1 on steady-state serum concentrations of quetiapine and the active metabolite, N-desalkylquetiapine, in psychiatric patients.
Methods: Measured serum concentrations of quetiapine and N-desalkylquetiapine from patients with biobanked DNA samples were included retrospectively from a routine therapeutic drug monitoring database. The impact of CYP2D6, CYP3A5, and ABCB1 (345C>T) genotypes on dose-adjusted serum concentrations (C/D ratios) of quetiapine and N-desalkylquetiapine was investigated by multivariate mixed model analysis.
Results: In total, 289 patients with 633 serum measurements were included. In the multivariate analysis, mean C/D ratio of N-desalkylquetiapine was estimated to be 33% and 22% higher in inherent CYP2D6 poor metabolizers (P = 0.03) and heterozygous extensive metabolizers (P < 0.001), respectively, compared with inherent extensive metabolizers. The ABCB1 3435C>T polymorphism and CYP3A5 genotype had no significant influence on either of the substances in the present material.
Conclusions: Genetic variability in CYP2D6 contributes to the interindividual variability in steady-state serum concentrations of N-desalkylquetiapine. Although the metabolite exhibits relevant pharmacological activity, the quantitative effect of CYP2D6 genotype on serum concentration of N-desalkylquetiapine is probably of limited clinical relevance for quetiapine treatment.