Determination of mirtazapine and desmethyl mirtazapine in human plasma by a new validated HPLC ultraviolet method with a simple and reliable extraction method: Application to therapeutic drug monitoring study by 62 real patient plasma

Determination of mirtazapine during psychopharmacotherapy in biological fluids is essential to achieve successful therapy, to avoid toxicity related to drug interactions, genetic variability and poor compliance. A new, rapid and sensitive high-performance liquid chromatography method has been developed in human plasma for the determination of mirtazapine (MRP) and N-desmethylmirtazapine (NDM) that is an active metabolite. The separation was achieved on a reverse phase C18 250 x 4.6 mm i.d., ODS-3 column at 40°C, using a programmed gradient elution. 20 mM potassium phosphate buffer (pH 3.9), acetonitrile and triethylamine (75.0:24.9:0.1, v/v/v) were used as mobile phase A. Mobile phase B consisted of absolute acetonitrile. Clozapine was used as an internal standard. The method showed linearity with good determination coefficients (r2>0.998) for each analyte. Intra-day and interday assay precisions (RSD%) were found less than 3.4 and 2.9 for MRP and NDM respectively. The intra-day and interday accuracy (RE%) of the method were calculated between (-2.8) and 3.5. A new extraction method was used in the study and an excellent efficiency values for MRP and NDM (94.4%, 106.6%; respectively) was obtained. The method was specific and sensitive as the limits of detection were 0.17 for MRP and 0.15 ng/mL for NDM. This method was applied to patients who received MRZ (n=62) at the 15-30 mg/day. The obtained and statistically evaluated plasma MRP and NDM levels which were 28.6±13.8 and 12.3±6.5 (mean±SD). The described procedure is relatively simple, precise, and applicable for routine therapeutic drug monitoring, especially in psychiatry clinics and toxicology reference laboratories.