Warfarin is an oral anticoagulant that requires frequent therapeutic drug monitoring due to a narrow therapeutic window, considerable interindividual variability in drug response, and susceptibility to drug-drug and drug-diet interactions. Enantiomeric separation and quantification of warfarin enantiomers and clinically important major hydroxylation metabolites are essential for drug interaction studies and phenotypic characterization of CYP2C9 and CYP3A4, the major cytochrome P450 (CYP) enzymes involved in warfarin metabolism. Here, we describe the development and validation of a chiral high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS)-based quantification of R-warfarin, S-warfarin, S-7-hydroxywarfarin (the major CYP2C9 metabolite) and (9R;10S)-10-hydroxywarfarin (the CYP3A4 metabolite) in human plasma. Simple protein precipitation-based extraction showed good recovery of analytes (82.9 - 96.9%). The developed method exhibited satisfactory intra-day and inter-day accuracy and precision. The lower limits of detection were 0.25 nM (or ~0.08 ng/mL) for the warfarin enantiomers and 0.1 nM (or ~0.04 ng/mL) for S-7-hydroxywarfarin and (9R;10S)-10-hydroxywarfarin using only 50 µL plasma during extraction. The validated method was successfully applied to analyze plasma samples obtained from a healthy human subject who enrolled in a clinical drug interaction study involving warfarin.
Keywords: Chiral Separation; HPLC-MS/MS; Hydroxywarfarin; Protein precipitation extraction; Warfarin.