[Determination of doxepin and nordoxepin in plasma by solid-phase extraction coupled with liquid chromatography-mass spectrometry (PRiME MCX-UPLC-HRMS)].

Objective: To establish a rapid method for the determination of doxepin and its metabolite nordoxepin residues in plasma samples by PRiME mixed cation-exchange solid-phase extraction coupled with ultra-performance liquid chromatography-high resolution mass spectrometry (PRiME MCX-UPLC-HRMS) . Methods: Plasma samples were extracted with 2% (V/V) aqueous phosphoric acid and purified by a PRiME MCX SPE cartridge. The target compounds were separated on an aurorasil MMP-G column (100 mm×2.1 mm, 3 μm) and gradient elution was performed with the mobile phases 5.0 mmol/L ammonium formate -0.1% (V/V) aqueous formic acid and methanol, and were determined in electrospray positive ionization mode (ESI(+)) and full mass scan-data dependent ms(2) scan (Full mass-ddMS(2)) mode, and the internal standard method was used for quantification. The limits of detection (LODs) and limits of quantification (LOQs) were calculated at signal-to-noise ratios of 3 and 10, respectively. Results: The linear ranges of doxepin and its metabolite nordoxepin were 1.0-100.0 ng/ml with correlation coefficients of 0.9988-0.9999. The LODs and LOQs for doxepin and its metabolite nordoxepin were determined to be 0.18-0.22 ng/ml and 0.54-0.66 ng/ml, respectively. The spiked recoveries of doxepin and its metabolite nordoxepin in plasma at the four concentration levels of 1.0 ng/ml, 10.0 ng/ml, 50.0 ng/ml and 80 ng/ml ranged from 90.8% to 105%, with relative standard deviations (RSDs) of 1.31%-4.59%. Conclusion: The method was rapid, sensitive and accurate and was suitable for the monitoring and analysis of doxepin and its metabolite nordoxepin in plasma samples from clinical patients.