Warfarin metabolites: Stereochemical aspects of protein binding and displacement by phenylbutazone

The in vitro human serum albumin binding characteristics of the enantiomers of the major metabolites of warfarin [6‐hydroxywarfarin (6‐HW), 7‐hydroxywarfarin (7‐HW), (S)‐warfarin alcohols [(S,S)‐ and (S,R)‐WA], and (R,S)‐warfarin alcohol [(R,S)‐WA]] have been studied, using a stereospecific HPLC assay. Warfarin metabolites are less bound both within plasma and a 40 g/liter solution of human serum albumin than the enantiomers of warfarin. The reduced warfarin metabolites have a lower fraction unbound [1.33% for (S,R)‐WA, 2.09% for (S,S)‐WA, and 1.04% for (R,S)‐WA] than hydroxylated metabolites [3.24% for (R)‐6‐HW, 4.26% (S)‐6‐HW, 4.49% for (R)‐7‐HW and 4.27% for (S)‐7‐HW] to HSA. Phenylbutazone produced a concentration‐dependent increase in the unbound fraction of all metabolites. It was possible to predict the unbound fraction of warfarin metabolites based on the unbound fraction of warfarin enantiomers. © 1993 Wiley‐Liss, Inc.