Stereoselective HPLC-assay for citalopram and its metabolites

The racemic selective serotonin reuptake inhibitor citalopram (CIT) is increasingly used for depressive disorders. As both enantiomers of CIT differ in pharmacologic activity and demethylated metabolites might contribute to the antidepressent action of CIT, a stereoselective assay for all active compounds is needed, but will require that pharmacokinetic/pharmacodynamic relationships be investigated. A stereoselective high-performance liquid chromatography (HPLC)-assay (Chirobiotic V column) with UV-detection (lambda = 240 nm) for R- and S-CIT as well as both enantiomers of desmethyl(DM)-CIT and didesmethyl(DDM)-CIT was developed. The calibration range was linear from 5 to 200 ng/mL and the lower limit of detection averaged 2 ng/mL. Based on three quality controls (10, 75, and 150 ng/mL) the intraday and interday coefficients of variation ranged from 1.3% to 9.0% for CIT and from 2.2% to 10.3% for DM-CIT and DDM-CIT. The assay was used to analyze the trough steady state plasma levels of all 6 agents in 16 elderly patients treated daily with 20 to 40 mg CIT. For the dose of 20 mg (n = 14) mean values +/- SD of R- (and S-CIT) averaged 36.2+/-15.0 (27.4+/-13.1) ng/mL, respectively (mean R/S-ratio: 1.4+/-0.4), for R- (and S-DM-CIT) 7.2+/-3.1 (7.7+/-3.8) ng/mL, respectively (R/S-ratio: 1.0+/-0.3) whereas DDM-CIT was only detectable as R-enantiomer in 8 cases (13.2+/-12.1 ng/mL; range: 2.2-36.2 ng/mL). Significant (p < 0.01) linear correlations could be found between both enantiomers of CIT and DM-CIT as well as between parent drug and primary metabolite for the R-enantiomers. Apparently R/S- and metabolic ratios increased with dose; this might indicate that stereoselective disposition of CIT and DM-CIT is concentration-dependent. The present assay allows a rapid, sensitive, and reliable stereoselective determination of CIT and its (active) metabolites which can be applied for assessing pharmacokinetic parameters and evaluating putative relationships to clinical (side) effects.