Genetic influence on enantiomeric drug disposition : Focus on venlafaxine and citalopram

Sammanfattning: A molecule that is not identical to its mirror image is said to be chiral. A racemic mixture, or a racemate, is one that has equal amounts of S- and R-enantiomers of a chiral molecule. Two examples of frequently prescribed racemic drugs are the antidepressants venlafaxine (VEN) and citalopram (CIT). The R-enantiomer of VEN is a potent inhibitor of serotonin and noradrenaline reuptake, while the S-enantiomer is more selective in inhibiting serotonin reuptake. CIT is a selective serotonin reuptake inhibitor and the S-enantiomer is responsible for this effect. The R-enantiomer of CIT is therapeutically inactive, but displays other effects or side-effects. Due to the potential of different pharmacological and toxicological activities of the VEN and CIT enantiomers, it is of great interest to investigate the individual enantiomers of these drugs, concerning both pharmacokinetics and pharmacodynamics. For this purpose, it is necessary to develop stereoselective bioanalytical methods. A major clinical problem in the use of many drugs is the inter-individual variability in drug metabolism and response. Genetic variations contribute to this variability, including e.g. polymorphisms in the cytochrome P450 (CYP) enzymes. Approximately 7% of all Caucasians lack the polymorphic isoenzyme CYP2D6 and these individuals are classified as poor metabolisers. Both VEN and CIT are partly metabolised by CYP2D6. However, it is not completely known how CYP2D6 deficiency may influence the in vivo pharmacokinetics of these drugs, especially regarding the enantiomeric disposition. The overall aim of this thesis was to study the relationship between pharmacokinetics and pharmacogenetics for VEN and CIT, with emphasis on enantiomeric drug disposition in different biomatrices. In Paper I, a validated liquid chromatography-tandem mass spectrometry (LC/MS/MS) method for enantioselective determination of VEN and its three major metabolites was developed and applied in plasma from patients and whole blood samples from forensic autopsy cases. In Papers II and III, the genetic influence on enantiomeric drug disposition in serum and brain following administration of racemic CIT and VEN to Sprague-Dawley and Dark Agouti rats was studied. The female Sprague-Dawley and Dark Agouti rats are considered the animal counterparts of the human extensive and poor metaboliser CYP2D6 phenotypes, respectively. Significant quantitative strain-related differences in the pharmacokinetics of CIT and VEN, and their metabolites, were observed. The results indicate that the CYP2D enzymes display a significant impact on the stereoselective metabolism of these drugs. The findings also highlight the importance of comparing different rat strains when conducting experimental pharmacokinetic studies. In Paper IV, the relation between CYP2D6 genotype and the disposition of the enantiomers of VEN and its metabolites in femoral blood from forensic autopsy cases was studied. A substantial variation in the relationship between the S- and R-enantiomers of VEN, and metabolites, was found. In individuals lacking two functional CYP2D6 alleles, a low enantiomeric S/R VEN ratio was strongly related to a high S/R ratio for the main metabolite O-desmethylvenlafaxine. Hence, by using enantioselective analysis of VEN and O-desmethylvenlafaxine, it is possible to predict if a person is a poor metaboliser genotype/phenotype for CYP2D6.