Influence of CYP2D6 genotype on the disposition of the enantiomers of venlafaxine and its major metabolites in postmortem femoral blood.

Venlafaxine (VEN) is an antidepressant drug mainly metabolized by the cytochrome P450 (CYP) enzyme CYP2D6 to the active metabolite O-desmethylvenlafaxine (ODV). VEN is also metabolized to N-desmetylvenlafaxine (NDV) via CYP3A4. ODV and NDV are further metabolized to N,O-didesmethylvenlafaxine (DDV). VEN is a racemic mixture of the S- and R-enantiomers and these have in vitro displayed different degrees of serotonin and noradrenaline reuptake inhibition. The aim of the study was to investigate if an enantioselective analysis of VEN and its metabolites, in combination with genotyping for CYP2D6, could assist in the interpretation of forensic toxicological results in cases with different causes of deaths. Concentrations of the enantiomers of VEN and metabolites were determined in femoral blood obtained from 56 autopsy cases with different causes of death. The drug analysis was done by liquid chromatography tandem mass spectrometry (LC/MS/MS) and the CYP2D6 genotyping by PCR and pyrosequencing. The mean (median) enantiomeric S/R ratios of VEN, ODV, NDV and DDV were 0.99 (0.91), 2.17 (0.93), 0.92 (0.86) and 1.08 (1.03), respectively. 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 to 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.

Cytochrome P450-dependent disposition of the enantiomers of citalopram and its metabolites: in vivo studies in Sprague-Dawley and Dark Agouti rats.

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.

Stereoselective determination of venlafaxine and its three demethylated metabolites in human plasma and whole blood by liquid chromatography with electrospray tandem mass spectrometric detection and solid phase extraction.

A stereoselective method is described for simultaneous determination of the S- and R-enantiomers of venlafaxine and its three demethylated metabolites in human plasma and whole blood samples. This validated method involved LC/MS/MS with positive electrospray ionization and solid phase extraction. Chromatographic separation was performed on a 250 mm x 2.1mm Chirobiotic V column with a total run time of 35 min. In plasma, calibration curves were in the range of 1-1000 nM for the S- and R-enantiomers of venlafaxine and O-desmethylvenlafaxine, and 0.5-500 nM for N-desmethylvenlafaxine and N,O-didesmethylvenlafaxine. In whole blood the corresponding concentrations were 10-4000 and 5-2000 nM, respectively. The intra-day precision was <6.3% and the inter-day precision was <9.9% for plasma and <15% and <19% for whole blood. LLOQ ranged between 0.25 and 0.5 nM. No ion suppression/enhancement or other matrix effects were observed. The method was successfully applied for determination of venlafaxine and its metabolites in plasma from patients and whole blood samples from forensic autopsy cases.

Influence of blood loss on the pharmacokinetics of citalopram.

Extended blood loss results in several compensatory physiological mechanisms, including transfer of extravascular fluid into the blood circulation. If drugs are present in the body, this fluid exchange may imply that blood drug concentrations found in a trauma victim may differ from the concentrations present at the time of the trauma. To address this issue, an animal model was used to investigate the influence of blood loss on pre-existing levels of the antidepressant drug citalopram and its demethylated metabolites. Rats were administered citalopram either acutely (40 mg/kg, orally) or chronically (20 mg/kg daily, subcutaneously) for 6 days using osmotic pumps. In the experimental rats, blood loss was accomplished by withdrawing 0.8 mL blood at 10 min intervals during 70 min. In the control rats, blood was withdrawn at 0 and 70 min only. Blood, brain and lung drug concentrations were analyzed with an enantioselective HPLC method. In the chronically treated rats, the ratios between final and initial citalopram concentrations were 1.08 +/- 0.15 and 1.01 +/- 0.09 in the experimental rats and controls, respectively, indicating no major effect of blood loss. In contrast, acute oral administration resulted in increased ratios in the exsanguinated rats as compared to controls (1.84 +/- 0.50 versus 0.73 +/- 0.07; p = 0.0495). In conclusion, the observation of increased blood drug levels in the acute oral rats indicates that absorption of fluid from the gastrointestinal tract may be important in the intravascular refill. Further, in the interpretation of post-mortem blood levels of drugs, these physiological mechanisms should be taken into account.

Early-phase postmortem redistribution of the enantiomers of citalopram and its demethylated metabolites in rats.

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.