A novel solid phase extraction sample preparation method for sensitively determining doxepin and N-nordoxepin in human plasma and its application in a bioequivalence study in healthy Chinese volunteers

Abstract

Doxepin, a tricyclic antidepressant (TCA), is widely used in the treatment of depressive disorder and anxiety. There are some liquid chromatography-tandem mass spectrometry (LC-MS/MS) methods that have been reported for detecting doxepin, but inadequacies in recovery and cumbersome sample preparation obstruct the pharmacokinetics study. Therefore, we aimed to develop and validate a rapid sample preparation method based on solid-phase extraction (SPE) for the precise quantification of doxepin and its metabolites. Chromatography separation was performed on a Waters ACQUITY UPLC BEH C18 column (2.1 × 100 mm, 1.7 μm) and a mobile phase consisting of 70% of mobile phase A (0.1% formic acid and 10 mM ammonium formate) and 30% mobile phase B (0.1% formic acid in acetonitrile) at a flow rate of 0.4 mL min⁻¹ in the step gradient elution conditions. The lower limits of quantification for doxepin and N-nordoxepin were 4 pg mL⁻¹ and 2 pg mL⁻¹, respectively. This method was validated with satisfactory results including good precision and accuracy. A rapid, sensitive, and specific LC-MS/MS method was developed and validated for the determination of doxepin in human plasma. This method could be applied for determining doxepin and N-nordoxepin concentrations in plasma that could be useful for bioequivalence study of 3 mg doxepin.