Steady-State Concentrations of Mirtazapine, N-Desmethylmirtazapine, 8-Hydroxymirtazapine and their Enantiomers in Relation to Cytochrome P450 2D6 Genotype, Age and Smoking Behaviour

Mirtazapine is a tetracyclic antidepressant drug available as a racemic mixture of S(+)- and R(−)-mirtazapine. These enantiomers have different pharmacological properties, and both contribute to the clinical and adverse effects of the drug. Cytochrome P450 (CYP) 2D6 has been implicated in the metabolism of S(+)-mirtazapine. However, the effect of CYP2D6 on serum concentrations of the enantiomers of mirtazapine and its metabolites has not been assessed in patients on long-term treatment. The main objective of the study was to evaluate the effect of the CYP2D6 genotype on enantiomeric steady-state trough serum concentrations of mirtazapine and its metabolites N-desmethylmirtazapine and 8-hydroxymirtazapine. The effects of sex, age and smoking behaviour were also assessed.

The study included 95 patients who had depression according to the Diagnostic and Statistical Manual of Mental Disorders — 4th Edition and were treated for 4 weeks with a daily dose of mirtazapine 30 mg. The serum concentrations of the enantiomers of mirtazapine and its metabolites were analysed by liquid chromatography-mass spectrometry, and the subjects were genotyped for CYP2D6 alleles *3, *4, *5 and *6 and gene duplication.

Three subjects (3%) were classified as ultrarapid metabolizers (UMs), 56 (59%) as homozygous extensive metabolizers (EMs), 30 (32%) as heterozygous EMs and 6 (6%) as poor metabolizers (PMs) of CYP2D6. The median trough serum concentrations of S(+)-mirtazapine were higher in PMs (59 nmol/L, p = 0.016) and in heterozygous EMs (39 nmol/L, p = 0.013) than in homozygous EMs (28 nmol/L). PMs and heterozygous EMs also had higher mirtazapine S(+)/ R(−) ratios (0.4) than homozygous EMs (0.3, p = 0.015 and 0.004, respectively). The S(+)- N-desmethylmirtazapine concentration was higher in PMs (16 nmol/L) than in homozygous EMs (7 nmol/L, p = 0.043). There was an association between the CYP2D6 genotype and the ratio between S(+)-8-hydroxymirtazapine and S(+)-mirtazapine, with a significantly higher ratio in homozygous EMs than in heterozygous EMs (0.11 vs 0.05, p = 0.007). The influence of the CYP2D6 genotype on S(+)-mirtazapine, the mirtazapine S(+)/ R(−) ratio and S(+)- N-desmethylmirtazapine remained significant after correction for the influence of sex, age and smoking. Smokers had significantly lower concentrations of S(+)-mirtazapine (23 vs 39 nmol/L, p = 0.026) and R(−)- N-desmethylmirtazapine (39 vs 51 nmol/L, p = 0.036) and a significantly lower mirtazapine S(+)/R(−) ratio (0.28 vs 0.37, p = 0.014) than nonsmokers, and the effect of smoking remained significant after multivariate analysis.