A). After treated with rifampin (10 μM), phenobarbital (400 μM), hyperforin (0.5 μM), carbamazepine (50 μM), or vehicles (0.1% v/v) for 48 hr, human hepatocytes (from three different liver donors) were incubated with either midazolam (2 μM) for 30 min or 25OHD3 (2 μM) for 4 hr. The rate of 1′-hydroxylation of midazolam was correlated with the concentration of 4β,25(OH)2D3 using linear regression. B). Correlation between the ratio of plasma 4β,25(OH)2D3/25OHD3 and oral midazolam clearance for pre-RIF and post-RIF treatment periods. Open circle: constitutive condition; closed circle: rifampin-induced condition. Oral midazolam clearance was estimated using the Pharsight WinNonlin. Correlation coefficients and corresponding p-values were calculated according to the null hypothesis.