Enantioselective analysis of citalopram and demethylcitalopram in human and rat plasma by chiral LC-MS/MS: application to pharmacokinetics.

Citalopram (CITA) is available as a racemic mixture and as a pure enantiomer. Its antidepressive action is related to the (+)-(S)-CITA and to the metabolite (+)-(S)-demethylcitalopram (DCITA). In the present investigation, a method for the analysis of CITA and DCITA enantiomers in human and rat plasma was developed and applied to the study of pharmacokinetics. Plasma samples (1 ml) were extracted at pH 9.0 with toluene:isoamyl alcohol (9:1, v/v). The CITA and DCITA enantiomers were analyzed by LC-MS/MS on a Chiralcel OD-R column. Recovery was higher than 70% for both enantiomers. The quantification limit was 0.1 ng/ml, and linearity was observed up to 500 ng/ml plasma for each CITA and DCITA enantiomer. The method was applied to the study of the kinetic disposition of CITA administered in a single oral dose of 20 mg to a healthy volunteer and in a single dose of 20 mg/kg (by gavage) to Wistar rats (n = 6 for each time). The results showed a higher proportion of the (-)-(R)-CITA in human and rat plasma, with S/R AUC ratios for CITA of 0.28 and 0.44, respectively. S/R AUC ratios of DCITA were 0.48 for rats and 1.04 for the healthy volunteer.