Effect of BEL on RIF-induced and basal CYP3A4 activity. (A) BEL inhibits RIF-induced CYP3A4 activity in the human primary hepatocytes. CYP3A4 activity was analyzed by the luminescent cytochrome P450-Glo CYP3A4 assays in the human primary hepatocytes after treatment with vehicle, DMSO, RIF, BEL ± RIF, or KET ± RIF as indicated for 24 hours. Results are presented as the fold change over DMSO treatment. Data represent the mean ± S.D. from four independent experiments performed on single-donor hepatocytes. P < 0.05; compared with DMSO alone (#) or RIF alone (*) by ANOVA with Dunnett’s multiple-comparisons test. (B) BEL does not inhibit the basal CYP3A4 activity in the human primary hepatocytes. DMSO, BEL, or KET was added to the hepatocytes only during the assay period to determine their direct effects on CYP3A4 activity. Results are presented as the fold change over DMSO treatment. Data represent the mean ± S.D. values from four independent experiments performed on single-donor hepatocytes. #P < 0.05; compared with DMSO alone by ANOVA with Dunnett’s multiple-comparisons test.