Stereoselective HPLC-assay for citalopram and its metabolites

The racemic selective serotonin reuptake inhibitor citalopram (CIT) is incr easingly used for depressive disorders. As both enantiomers of CIT differ i n pharmacologic activity and demethylated metabolites might contribute to t he antidepressent action of CIT, a stereoselective assay for all active com pounds is needed, but will require that pharmacokinetic/pharmacodynamic rel ationships be investigated. A stereoselective high-performance liquid chrom atography (HPLC)-assay (Chirobiotic V column) with UV-detection (lambda = 2 40 nm) for R- and S-CIT as well as both enantiomers of desmethyl(DM)-CIT an d 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 c oefficients 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 stea dy 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 d etectable as K-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 b oth enantiomers of CIT and DM-CIT as well as between parent drug and primar y metabolite for the R-enantiomers. Apparently R/S- and metabolic ratios in creased 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 (act ive) metabolites which can be applied for assessing pharmacokinetic paramet ers and evaluating putative relationships to clinical (side) effects.