Determination of the antidepressant mirtazapine and its two main metabolites in human plasma by liquid chromatography with fluorescence detection

A high-performance liquid chromatographic method for the determination in human plasma of the recent noradrenergic and specific serotonergic
antidepressant (NaSSA) mirtazapine and its two main metabolites, N-desmethylmirtazapine and 8-hydroxymirtazapine, has been developed.
Fluorescence detection was used, exciting at ? = 290 nm and monitoring emission at ? = 370 nm. Separation was obtained by using a reversedphase
column (C8, 250mm×4.6mm I.D., 5?m) and a mobile phase composed of 75% aqueous phosphate buffer containing triethylamine at pH
3.0 and 25% acetonitrile. Melatonin was used as the internal standard. A careful pre-treatment of plasma samples was developed, using solid-phase
extraction with phenyl cartridges (100 mg, 1 mL). The calibration curves were linear over a working range of 5-150 ngmL-1 for mirtazapine and
of 2.5-75.0 ngmL-1 for N-desmethylmirtazapine and 8-hydroxymirtazapine. The limit of quantitation (LOQ) was 2.5 ngmL-1 and the limit of
detection (LOD) was 1.25 ngmL-1 for all analytes. The method was applied with success to plasma samples from depressed patients undergoing
treatment with mirtazapine. Precision data, as well as accuracy results, were satisfactory and no interference from other drugs was found. Hence
the method is suitable for therapeutic drug monitoring of mirtazapine and its metabolites in depressed patients' plasma.