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 (MRP) 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 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 using programmed gradient elution at 40 °C. 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 (r(2)≥0.9981) 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 5.5. A new extraction method was used in the study and an excellent recovery (average) values for MRP and NDM (94.4%, 106.6%, respectively) was obtained. The method was specific and sensitive as the limit of detection (LOD) were 0.17 for MRP and 0.15 ng/mL for NDM. This method was applied properly to plasma samples taken from patients receiving MRI (n = 62) treated with 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.