Achiral-chiral LC/LC-MS/MS coupling for determination of chiral discrimination effects in phenprocoumon metabolism

Many physiological processes show a high degree of stereoselectivity, including the metabolism of xenobiotics as catalyzed by cytochrome P450 enzymes. An analysis of these chiral discrimination effects in drug metabolism is essential for an in-depth understanding of metabolic pathways that differ between enantiomers of a given chiral drug or metabolite thereof. Achiral chromatographic separation and structural identification followed by chiral analysis of metabolites from blood specimens usually requires a time-consuming multistage analytical technique. In an effort to optimize such a complicated analytical scheme, a novel two-dimensional online achiral-chiral liquid chromatography-tandem mass spectrometry (LC/LC-MS/MS) coupling method was developed by using a peak parking technique in combination with a makeup flow system. Metabolites were separated in the first dimension using a C18 reversed-phase system. A makeup eluent of water/methanol (95/5) was split into the flow before storing the metabolites separately on chiral cartridges. Subsequently, the metabolite enantiomers were eluted backward onto the analytical chiral column and separated, and the ratio of enantiomers was determined. The method was successfully validated with respect to limit of detection, linearity, intra- and interday accuracy, and precision. In the course of a human volunteer study investigating the influence of CYP (cytochrome) 2C9 genetic polymorphism on phenprocoumon (PPC) metabolism, we used this new two-dimensional online analytical technique for the analysis of PPC metabolites in plasma. The enantiomeric forms of 4'-, 6-, and 7-hydroxy-PPC metabolites as well as two novel metabolites were identified, and the ratio of the enantiomers was calculated. We found that the enantiomeric ratio for the different metabolites in the plasma sample of each measured individual differs markedly from a nearly 100% chiral discrimination for the two new putative metabolites. This new analytical coupling method possesses general utility in the analysis of chiral discrimination effects, particularly as it relates to pharmacokinetics and dynamics, a scientific field that is rapidly becoming an area of concern and interest.