Factors Affecting Steady-state Plasma Concentrations of Enantiomeric Mirtazapine and its Desmethylated Metabolites in Japanese Psychiatric Patients

Abstract

This study evaluated the effects of the CYP2D6*10 genotype on steady-state plasma concentrations of enantiomeric mirtazapine (MIR) and N-desmethylmirtazapine (DMIR) in Japanese patients. Subjects were 77 Japanese patients treated with racemic MIR. Steady-state plasma concentrations of MIR and DMIR enantiomers were measured using stereoselective liquid chromatography. Polymerase chain reaction was used to determine the CYP2D6 genotypes. After correcting for dose and body weight, smokers (n=15) had significantly lower S-(+)-MIR than nonsmokers (n=55) (15.1±17.8 vs. 23.9±17.8 ng/mL/mg/kg, Kruskal-Wallis test, p=0.034). One-way analysis of variance revealed that CYP2D6*10 homozygotes had significantly higher corrected plasma concentrations of S-(+)-MIR than the no-variant allele group (p=0.034). Multiple regression analysis revealed a significant positive correlation between the number of CYP2D6*10 alleles and corrected plasma concentrations of S-(+)-MIR. These results yielded the following final model: corrected plasma concentration of S-(+)-MIR=15.9+7.30×(number of CYP2D6*10 alleles) (R=0.279, p=0.023, coefficient of determination (R(2))=0.078). Homozygous CYP2D6*10 alleles and smoking have a significant impact on the metabolism of S-(+)-MIR in Japanese patients.