Comparison of EC50 values for concentration-dependent induction of CYP2B6 and CYP3A4 activities by prototypical inducers Rates of bupropion hydroxylation and testosterone 6β-hydroxylation were determined with microsomes isolated from human hepatocyte preparations treated daily for 3 days with 0.4 to 40 μM CLZ, 10 μM to 2 mM PB, 0.4 to 400 μM PHN, and 0.1 to 150 μM RIF. The relationship between enzyme activity and inducer concentration was fit with the Hill equation by nonlinear regression to recover estimates for the EC50 and sigmoidicity factor (γ). The EC50 values represent the mean ± S.D. from three individual livers (HL-N140, HL-N145, and HL-N147 for RIF and PB, and HL-N140, HL-N145, and HL-N148 for PHN and CLZ). Statistical significance of the differences in the EC50 values for CYP2B6 and CYP3A4 induction was determined using a two-tailed paired t test.


CYP2B6 EC50a

Clotrimazole 8.80 ± 5.89 5.38 ± 0.420
Phenobarbital 238 ± 52.6 142 ± 36.0*
Phenytoin 34.4 ± 11.9 19.2 ± 5.39
1.17 ± 0.511
0.370 ± 0.100
  • a EC50 values are reported in micromoles

  • * p < 0.05; p is the level of statistical significance of the differences between the EC50 values for CYP2B6 and CYP3A4 induction