Effects of selected flavonolignans and 7-O-methylated analogues on CYP2C9-mediated (S)-warfarin 7-hydroxylation in human liver microsomes (HLM). Incubation mixtures consisted of HLM (0.1 mg/mL), (S)-warfarin (4 μM), flavonolignan or 7-O-methylated analogue (10 or 100 μM; hatched and solid bars, respectively), and potassium phosphate buffer (0.1 M, pH 7.4). Reactions were initiated by the addition of NADPH (1 mM) and were terminated after 30 min with ice-cold MeOH containing 4-chlorowarfarin as internal standard. Activity in the presence of vehicle control (0.75% methanol, v/v) was 2.5 ± 0.1 pmol/min/mg microsomal protein. Bars and error bars denote means and SDs, respectively, of triplicate incubations. *p < 0.05, 10 versus 100 μM; #p < 0.05, flavonolignan versus 7-O-methylated analogue at 100 μM (two-way ANOVA with Bonferroni adjustment).