A validated liquid chromatographic tandem mass spectrometric method for the determination of mirtazapine and demethylmirtazapine in human plasma: application to a pharmacokinetic study

A new liquid chromatographic tandem mass spectrometric method for the determination of mirtazapine and demethylmirtazapine in human plasma has been developed and fully validated. The article describes in detail the bioanalytical procedure and summarizes the validation results obtained. The samples were extracted using liquid-liquid extraction with a mixture of 1-chlorobutane/isopropanol/ethyl acetate (88:2:10, (v/v/v)). The chromatographic separation was performed on a reversed-phase XTerrra MS C₈ column ( i.d.; 3.5 μm particle size) using a mobile phase consisting of 0.010 M ammonium formate (pH 7.8) and acetonitrile (35:65, (v/v)), pumped at a flow rate of 0.80 ml min⁻¹. The analytes were detected using a Finnigan LCQ advantage ion-trap mass spectrometer with positive electrospray ionization in selected reaction monitoring (SRM) mode. Tandem mass spectrometric detection enabled the quantitation of both compounds down to 0.10 ng ml⁻¹. Calibration graphs were linear (r better than 0.990, n=11), in concentration ranges 0.10 to 200 ng ml⁻¹ for mirtazapine demethylmirtazapine. The intra- and inter-day R.S.D. values were less than 14.8 and 16.6% for mirtazapine and demethylmirtazapine, respectively. The method was successfully applied to a kinetic study in order to assess the main pharmacokinetic parameters of mirtazapine and demethylmirtazapine.