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Therapeutic drug monitoring of mirtazapine in a routine outpatient setting in Asian psychiatric patients.

INTRODUCTION
Mirtazapine is an antidepressant that acts by enhancing serotonergic and noradrenergic neurotransmission. This study aimed to evaluate mirtazapine pharmacokinetic data from Korean psychiatric patients and to identify the potential factors affecting its steady-state concentration.


METHODS
A total of 337 samples of steady-state mirtazapine concentrations from 188 adult psychiatric outpatients were retrospectively evaluated. Serum mirtazapine concentrations were measured by high-performance liquid chromatography-tandem mass spectrometry.


RESULTS
Median mirtazapine concentration was 43.6 μg/L (164.37 nmol) at a daily dosage range of 7.5-60 mg. At the steady state, mirtazapine dose had a positive correlation with the drug concentration. Mean concentration-to-dose (C/D) ratio was 1.48 μg/L/mg/day (5.58 nmol/mg/day), which was higher than that in a previous study in Caucasian subjects. Age and paroxetine co-medication were positively associated with C/D ratio. Initial mirtazapine concentration and C/D ratio did not show an association with responsiveness in depressive patients.


DISCUSSION
This study presented the therapeutic drug monitoring data for mirtazapine and pharmacokinetic variations of mirtazapine in an Asian population. A further study could be helpful for clinical decision making based on the characteristics of patients.