Determination of unbound warfarin enantiomers in human plasma and 7-hydroxywarfarin in human urine by chiral stationary-phase liquid chromatography with ultraviolet or fluorescence and on-line circular dichroism detection

Enantiomers of warfarin and 7-hydroxywarfarin in human plasma and urine, respectively, were determined by high-performance liquid chromatography using a cellulose-derivative column with UV or fluorescent detection, and their absolute configuration was determined simultaneously by a circular dichroism spectropolarimeter connected in series. Enantiomers of warfarin and its major metabolites [i.e., ( R )-6-hydroxywarfarin, ( S )-7-hydroxywarfarin and ( RS )-warfarin alcohol] were well resolved. The method was precise and sensitive: within- and between-day coefficients of variation were <9.6% for warfarin enantiomers in plasma and <7.1% for 7-hydroxywarfarin enantiomers in urine, respectively, and the lower detection limits were 20 ng/ml for ( R )-warfarin, 40 ng/ml for ( S )-warfarin, 2.5 ng/ml ( R )-7-hydroxywarfin and 4.5 ng/ml for( S )-7-hydroxywarfarin in 0.5 ml of both plasma and urine. The ultrafiltration technique was used for determining unbound concentrations of warfarin enantiomers in plasma using [ 14 C]warfarin enantiomers resolved by the present HPLC system. Clinical applicability of the method was evaluated by determining unbound concentrations of warfarin enantiomers in five consecutive plasma samples obtained from a patient exhibiting an unstable anticoagulant response to warfarin (4 mg/day, p.o.). Results indicated that the present method would be useful in clarifying factors responsible for a large intra- and inter-patient variability in warfarin effects with regard to unbound plasmaenantiomer pharmacokinetics.