Development of a CE-LIF method for the determination of plasma levels of the antidepressant drug citalopram and its main metabolites

Citalopram ((1-[3-(dimethylamino)propyl]-1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-carbonitrile) is a recent antidepressant drug belonging to the SSRI (selective serotonin reuptake inhibitor) class. It is considered one of the most important and widely used drugs in this class, with high efficacy and good tolerability. Citalopram is mainly metabolised in the liver by the cytochrome P450 (CYP) system; the major metabolites are N-desmethylcitalopram and N,N-didesmethylcitalopram, both of which retain an important therapeutic activity. During chronic treatment, plasma levels of citalopram and its metabolites are usually in the same order of magnitude; for this reason, it is important to simultaneously determine the plasma concentrations of all three compounds, to obtain a more complete picture of the total efficacy and safety of the treatment, including the metabolic state of the patient. In order to carry out a reliable therapeutic drug monitoring of patients treated with citalopram, sensitive and selective methods are obviously needed. The proposed method is based on capillary electrophoresis coupled to laser induced fluorescence (CE-LIF). After a suitable sample pre-treatment of the biological samples by solid phase extraction (SPE), the analytes are derivatised with DTAF (dichlorotriazinyl-aminofluorescein), a fluorescein derivative that reacts with aliphatic amines emitting strongly when excited at 488 nm. After derivatisation, the analytes are separated in a fused silica capillary, using a background electrolyte composed of basic carbonate buffer and 0.1% PEG 4000. The preliminary results of the method are very promising in terms of extraction yields and selectivity. Assays are in progress to fully validate the method according to international guidelines, for subsequent application to the TDM of patients. Future developments will aim to make the method enantioselective, in order to enable the analysis of both enantiomers of the three analytes in biological fluids.