Disposition of warfarin enantiomers and metabolites in patients during multiple dosing with rac-warfarin

1. The disposition of warfarin enantiomers and metabolites has been studied in 36 patients receiving chronic rac-warfarin therapy, titrated to approximately the same anticoagulant response. 2. A stereoselective h.p.l.c. assay was employed to determine the concentrations of (R)- and (S)-warfarin, (R,S)-warfarin alcohol and (S)-7-hydroxywarfarin in plasma and 24 h urine samples. The concentrations of (R)-7-hydroxywarfarin, (S,S)-warfarin alcohol and (R)-6- and (S)-6-hydroxywarfarin were also determined in urine samples. The fractions unbound of warfarin enantiomers were determined using equilibrium dialysis. 3. Wide variability was observed in daily dose requirements (mean 6.1 mg; range: 2.5-12 mg), in plasma concentrations of (S)-warfarin (0.48 mg l(-1); 0.11-1.02 mg l(-1)), (R)-warfarin (0.87 mg l(-1); 0.29-1.82 mg l(-1)), (R,S)-warfarin alcohol (0.31 mg l(-1); 0.02-0.72 mg l(-1)) and (S)-7-hydroxywarfarin (0.25 mg l(-1); 0.07-0.37 mg l(-1)) and the percentage unbound of (S)-warfarin (0.53%; 0.29%-0.82%) and (R)-warfarin (0.54%; 0.26%-0.96%). 4. The mean plasma clearances of warfarin enantiomers were 7.5 1 day-1 per 70 kg (2.5-22.1) for (S)-warfarin and 3.6 1 day-1 per 70 kg (1.6-8.8) for (R)-warfarin. There was a significant correlation between the estimated formation clearance of (S)-7-hydroxywarfarin and the clearance of (S)-warfarin, which accounted for much of the variability in the latter.(ABSTRACT TRUNCATED AT 250 WORDS)