A simple and reliable method for quantifying plasma concentrations of tetracyclic antidepressants using monolithic silica solid-phase extraction tips

Three tetracyclic antidepressants, mianserin, mirtazapine, and setiptiline, were extracted from human plasma samples using a new micropipette solid-phase extraction tip containing a monolithic silica gel packing material, the MonoTip C18. The plasma samples (0.1 ml), which contained mianserin, mirtazapine, and setiptiline, were mixed with 330 μl of distilled water and 50 μl of 0.5 M glycine–sodium hydroxide buffer solution (pH 8). After centrifugation, the supernatant was extracted onto the C18 phase of the tip (pipette tip volume, 200 μl) by 25 sequential aspirating/dispensing cycles. The three analytes retained in the C18 phase were eluted with methanol by 5 aspirating/dispensing cycles. The eluate was subjected to measurements by gas chromatography (GC) with nitrogen–phosphorus detection (NPD). The recoveries of the three antidepressants were 84.6–99.6% and the limits of detection for each drug were between 2.4 and 15 ng/ml of plasma. The maximum intraday and interday coefficients of variation for the drugs was below 10.6%. Regression equations for the three drugs showed excellent linearity in the range of 50–5000 ng/ml for mianserin and mirtazapine and 50–10 000 ng/ml for setiptiline. The antidepressants were stable for at least 12 h at 4°C, 4 weeks at −80°C, and through three freeze–thaw cycles in plasma. The validated method was successfully used to quantify the plasma concentrations of mirtazapine in a human subject after oral administration of the drug. Although we achieved analyte detection with GC-NPD, the present monolithic micropipette extraction method for tetracyclic antidepressants seems to be very useful for combining it with GC–mass spectrometry (MS) and/or liquid chromatography–tandem MS to secure high simplicity, purification, reproducibility, and sensitivity.