Determination of the antidepressant paroxetine and its three main metabolites in human plasma by liquid chromatography with fluorescence detection.

A high-performance liquid chromatographic method has been developed for the determination in human plasma of the specific serotonin reuptake inhibitor (SSRI) antidepressant paroxetine and its three main metabolites (M1, M2, M3). Fluorescence detection was used, exciting at lambda = 294 nm and monitoring emission at lambda = 330 nm for paroxetine (lambda exc = 280 nm, lambda em = 330 nm for M1 and M2; lambda exc = 268 nm, lambda em = 290 nm for M3). Separation was obtained on a reversed-phase C18 column using a mobile phase composed of 67% aqueous phosphate at pH 2.5 and 33% acetonitrile. Imipramine (lambda exc = 252 nm, lambda em = 390 nm) was used as the internal standard. A careful pre-treatment of plasma samples was developed, using solid-phase extraction with C8 cartridges (50 mg, 1 mL). The calibration curves were linear over a working range of 2.5-100 ng/mL for paroxetine and of 5-100 ng/mL for for all metabolites. The limit of detection (LOD) was 1.2 ng/mL for PRX and 2.0 ng/mL for the metabolites. The method was applied with success to plasma samples from depressed patients undergoing treatment with paroxetine. Hence the method seems to be suitable for the therapeutic drug monitoring of paroxetine and its main metabolites in depressed patients' plasma.