Combination index of the concentration and in vivo antagonism activity of racemic warfarin and its metabolites to assess individual drug responses

The present study was undertaken to examine whether in vivo vitamin K epoxide reductase complex 1 (VKOR) “actual” antagonism activity, calculated by the concentrations and the reported anticoagulant activities of the R- and S-warfarin enantiomers and their metabolites, correlates with the weekly dose of warfarin. Five patients under palliative care were enrolled in our study and 20 serum samples were analyzed by an enantioselective high-performance liquid chromatography–ultraviolet detection method. In vivo VKOR inhibition activities of S-warfarin, R-warfarin, 7- and 10-hydroxywarfarin were calculated as the ratio of drug or metabolite concentration to the IC50. The mean drug concentrations (± SD) of S- and R-warfarin, 7-hydroxywarfarin and 10-hydroxywarfarin were 334 ± 154 ng/ml, 370 ± 115 ng/ml, 42 ± 15 ng/ml and 80 ± 44 ng/ml, respectively. Then, in vivo VKOR actual antagonism activities of S- and R-warfarin, 7-hydroxywarfarin and 10-hydroxywarfarin were calculated. Good correlation (R2 = 0.69–0.72) was obtained between the weekly warfarin dose and the ratios of INR/actual antagonism activity, while poor correlation was observed between the weekly warfarin dose and INR (R2 = 0.32) or the activities (R2 = 0.17–0.21). Actual antagonism activities along with the INR correlated well with the warfarin dose. This parameter may be useful for predicting or altering warfarin doses, although further verification in a larger study is required.