To cite this article: Rulcova A, Prokopova I, Krausova L, Bitman M, Vrzal R, Dvorak Z, Blahos J, Pavek P. Stereoselective interactions of warfarin enantiomers with pregnane X nuclear receptor in gene regulation of major drug-metabolizing cytochrome P450 enzymes. J Thromb Haemost 2010; 8: 2708-17.See also Miller GP. Warfarin therapy: how the less interesting half just got interesting. This issue, pp 2705-7.Summary. Background: Warfarin, an antagonist of vitamin K, is an oral coumarin anticoagulant widely used to control and prevent thromboembolic disorders. Warfarin is clinically available as a racemic mixture of R-and S-warfarin. The Senantiomer has three to five times greater anticoagulation potency than its optical congener. Recently, vitamin K 2 function has been proposed via the pregnane X receptor (PXR) in osteocytes. PXR acts as a xenobiotic sensor that controls expression of many genes involved in drug/xenobiotic metabolic clearance. Objective: The aim was to examine whether enantiomers of warfarin stereoselectively interact with PXR to upregulate main drug/xenobiotic-metabolizing enzymes of the cytochrome P450 superfamily. Methods: Interactions of warfarin enantiomers with PXR were tested by gene reporter assays and time-resolved fluorescence resonance energy transfer technology (TR-FRET) ligand binding assay. Up-regulation of PXR-target gene mRNAs by warfarin enantiomers was studied using semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) in primary human hepatocytes. Results: We found that R-warfarin interacts with the PXR nuclear receptor. Consistently, R-warfarin significantly induced CYP3A4 and CYP2C9 mRNAs in cultures of primary human hepatocytes or in LS174T intestinal cells. On the other hand, S-warfarin is a less potent inducer of PXR-target genes in human hepatocytes and activates PXR only at supraphysiological concentrations. In addition, we showed that racemic 10-and 4¢-hydroxywarfarins are also highly potent PXR ligands and inducers of CYP3A4 and CYP2C9 mRNA in human hepatocytes. Conclusion: We showed that R-warfarin can significantly up-regulate major drug-metabolizing enzymes CYP3A4 and CYP2C9 in the liver and thus may cause drug-drug interactions (DDI) with coadministered drugs. The results warrant reconsideration of racemic warfarin usage in clinics.