Quantitative prediction of hepatic CYP3A activity using endogenous markers in healthy subjects after administration of CYP3A inhibitors or inducers.

Accurate prediction of cytochrome P450 (CYP) 3A activity in the early stage of drug development and in clinical practice is important. This study aimed to evaluate the previously constructed CYP3A activity prediction model after administration of CYP3A inhibitors and inducers and to modify the model for better prediction of CYP3A activity. Healthy male subjects received the following study drugs during three study periods: midazolam alone (control phase); midazolam with 200 mg of itraconazole (CYP3A inhibition phase); and midazolam with 150 mg of rifampicin (CYP3A induction phase). We quantified the concentrations of several endogenous CYP3A markers in both urine and plasma using gas chromatography-mass spectrometry. The urinary markers, including 6ß-hydroxy (OH)-cortisol/cortisol, 6ß-OH-cortisone/cortisone, 16α-OH-dehydroepiandrosterone (DHEA)/DHEA, 16α-OH-androstenedione (A-dione)/A-dione and 7ß-OH-DHEA/DHEA, were significantly correlated with midazolam clearance in both the CYP3A inhibition and induction phases. We constructed a statistical prediction model after integrating data from a previous study to predict midazolam clearance as follows: Ln(midazolam clearance) = 2.5545 + 0.3988 × ln(7ß-OH-DHEA/DHEA) + 0.1984 × ln(16α-OH-DHEA/DHEA) + 0.5031 × ln(6ß-OH-cortisol/cortisol) - 0.1261 [ln(7ß-OH-DHEA/DHEA) × ln(6ß-OH-cortisol/cortisol)] (r2 = 0.75). We suggest that quantitating endogenous markers in vivo coupled with the statistical prediction model may be useful for predicting CYP3A parameters.