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However, a substantial variation in the relationship between the S-and R-enantiomers of VEN and metabolites was evident (S/R ratios ranging from 0.23-17.6). In six cases, a low S/R VEN ratio (mean 0.5) was associated with a high S/R ODV ratio (mean 11.9). Genotyping showed that these individuals carried two inactive CYP2D6 genes indicating a poor metabolizer phenotype. From these data we conclude that enantioselective analysis of VEN and ODV can predict if a person is a poor metabolizer genotype/phenotype for CYP2D6. Knowledge of the relationship between the S-and R-enantiomers of this antidepressant drug and its active metabolite is also important since the enantiomers display different pharmacodynamic profiles.

The aim of this study was to investigate the early-phase postmortem redistribution of the enantiomers of citalopram (CIT) and its metabolites demethylcitalopram (DCIT) and didemethylcitalopram (DDCIT) in a rat model. Furthermore, we wanted to examine the role of the lungs as a reservoir of postmortem drug release and to investigate the influence of storage temperature (21 degrees C vs. 4 degrees C) on postmortem changes. Rats were administered a single CIT dose of 100 mg/kg (s.c.), and heart blood and lung samples were collected antemortem and 15 min postmortem for enantioselective high-performance liquid chromatographic analysis. About three times higher blood drug and metabolite levels were observed in the postmortem rats than in the antemortem rats (p < 0.0001). Refrigeration at 4 degrees C did not prevent, but significantly reduced, the postmortem increase in heart blood CIT levels as compared to the concentrations in the rats stored at 21 degrees C (p < 0.05). The lung drug concentrations were lower postmortem than antemortem (p < 0.05). The enantiomeric (S/R) concentration ratios of CIT and metabolites in blood and lungs were of similar magnitude before and after death. The parent-drug-to-metabolite ratios for CIT/DCIT were unchanged after death. In conclusion, this study shows that heart blood CIT and metabolite levels increase rapidly after death. Further, a fall in postmortem CIT concentrations in the lungs was observed, indicating that the lungs seemed to represent one major source of drug release during early-phase postmortem redistribution.

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The female Sprague-Dawley (SD) and Dark Agouti (DA) rats are considered the animal counterparts of the human extensive and poor metabolizer cytochrome P450 (CYP) 2D6 phenotypes, respectively. The aim of this work was to study possible rat strain differences in the steady-state pharmacokinetics of the (+)-(S)- and (-)-(R)-enantiomers of citalopram and its demethylated metabolites. A chronic drug treatment regimen (15 mg/kg daily) was implemented for 13 days in separate groups of SD (n = 9) and DA (n = 9) rats by using osmotic pumps. The concentrations of citalopram and two major metabolites in serum and two brain regions were analyzed by an enantioselective high-performance liquid chromatography assay. Higher serum and brain levels of citalopram and demethylcitalopram, but lower levels of didemethylcitalopram, were observed in DA rats when compared with SD rats. The enantiomeric (S/R) concentrations ratios of citalopram were lower in the DA rats when compared with the SD rats (0.53 ± 0.05 vs. 0.80 ± 0.03, P < 0.001), indicating a possibly decreased capacity in the metabolism of the (-)-(R)-enantiomer in the DA rats. This study shows that CYP2D deficiency results in steady-state pharmacokinetic differences of the enantiomers of citalopram and its metabolites.