Table 2.

Effects of protein binding of warfarin enantiomers and metabolites on inhibition of VKORC1 activity

Inhibitor	fu plasma, %	fu media, %	[I]u/Kiu × 102 (relative %)
9S-warfarin	0.4	31	135 (100)
9R-warfarin	0.4	25	25 (18.5)
4′-OH warfarin	2.1	50	1.4 (1.0)
5-OH warfarin	0.54	18	0.005 (<0.01)
6-OH warfarin	3.4	47	2.8 (2.1)
7-OH warfarin	2.2	34	1.2 (0.9)
8-OH warfarin	2.3	25	0.004 (<0.01)
10-OH warfarin	0.69	47	6.4 (4.7)
Warfarin alcohols	0.97	37	19 (16.2)

Unbound fraction of VKORC1 inhibitors in human plasma (f_u^plasma) and HEK-C3 culture media (f_u^media) was measured. Unbound fractions were used to refine [I]/IC₅₀ calculations (Table 1) for in vivo and in vitro protein binding and calculate ([I]_u/K_iu) reflecting relative in vivo contributions of each drug to VKORC1 inhibition.