Determination of mirtazapine and its main active metabolite in human plasma by HPLC with fluorescence detection

Mirtazapine (1,2,3,4,10,14b-hexahydro-2-methyl-pyrazino[2,1-a]-pyrido[2,3-c][2-benzazepine], MRT), is a novel 'NaSSA' antidepressant which selectively interacts with noradrenergic and serotonergic receptors. The usual dose range is 30-60 mg/day, administered as Remeron® tablets. MRT undergoes hepatic metabolism by means of CYP450 enzymes, which produces a major active metabolite, N-demethyl-MRT. Mirtazapine is quite effective against depression, and due to the induced sedation usually improves depressed patients’ insomnia. Side effects, however, such as weight gain, sedation, headache, dizziness and jaundice have been reported, therefore the determination of MRT and N-demethyl-MRT levels in human plasma is useful for therapeutic drug monitoring purposes. Since the analytes are fluorescent, an HPLC method with fluorescence detection has been developed for the analysis of these compounds in human plasma. Baseline separation of the analytes is achieved using a reversed phase column (C8, 250x3.0 mm I.D.; 5 µm) and a mobile phase composed of a pH 2.5 phosphate buffer and acetonitrile, with detection wavelengths of 290 nm (excitation) and 370 nm (emission). Under these conditions, the analytes and the internal standard (melatonin) are detected within 9 minutes. Plasma sample pre-treatment is carried out by means of a solid-phase extraction procedure, using phenyl cartridges (50 mg, 1 mL). The method has been applied to the analysis of some plasma samples from depressed patients, and the plasma levels of MRT and N-demethyl-MRT thus obtained are in good agreement with those obtained by another method based on capillary electrophoresis.