Fig. 5.

IC₅₀ shift plot for silybin A and silybin B. Human liver microsomes (0.1 mg/ml) were incubated first with silybin A (squares) or silybin B (triangles) (0.1–1000 μM) in the presence (closed symbols) or absence (open symbols) of NADPH (1 mM). The primary reaction mixture was diluted 10-fold to initiate the secondary reaction, which contained NADPH (1 mM) and (S)-warfarin (4 μM). (S)-Warfarin 7-hydroxylation activity in the presence of vehicle control (0.75% methanol, v/v) was 1.5 and 1.6 pmol/min/mg microsomal protein in the absence and presence, respectively, of NADPH. The inset depicts the effects of the known CYP2C9 mechanism-based inhibitor, tienilic acid (circles) (1–500 μM). Symbols denote means of duplicate incubations. Open symbols denote observed values when NADPH was absent from the primary reaction mixture; solid symbols denote observed values when NADPH was present in the primary reaction mixture. Curves denote nonlinear least-squares regression of observed values using WinNonlin (version 5.0.1).