Application of the semi-simultaneous bioavailability approach to hepatic and intestinal CYP3A phenotyping with midazolam

Background. Determination of hepatic and intestinal CYP3A activity is important because CYP3A substrates generally show substantial variability in plasma concentrations after oral administration due to the variation of hepatic and intestinal metabolism. The goals of this research were: (1) to determine whether indices of hepatic and intestinal extraction of the CYP3A probe drug midazolam are not different when determined using the semi-simultaneous bioavailability approach as compared with the traditional method, and (2) to evaluate the hepatic and intestinal metabolism of midazolam as determined by the semi-simultaneous approach in the presence and absence of ketoconazole. Methods. The pharmacokinetics of midazolam were assessed in twelve healthy volunteers after administration of midazolam 5 mg orally followed at 6 hours by 2 mg intravenously. Concentration-time data were fitted to a combined oral-intravenous infusion model by nonlinear regression (semi-simultaneous approach). Data from the semi-simultaneous approach were compared with that obtained after the same midazolam doses given individually, one week apart (traditional approach). The effect of ketoconazole on the pharmacokinetics of midazolam was also determined in four healthy volunteers by the semi-simultaneous approach. Results. There were no significant differences in bioavailability (0.340 +/- 0.100 versus 0.343 +/- 0.094, p = 0.73), hepatic extraction ratio (0.269 +/- 0.064 versus 0.267 +/- 0.077, p = 0.88), and intestinal extraction ratio (0.534 +/- 0.135 versus 0.531 +/- 0.124, p = 0.87) between the semi-simultaneous and traditional approaches. The area under the concentration-time curve (AUC) of midazolam was markedly increased with ketoconazole inhibition. The AUC increase was more pronounced with oral dosing (8.1-fold) than with intravenous administration (3.7-fold). Ketoconazole markedly increased the average bioavailability of midazolam by 2.4-fold from 0.343 to 0.838. This increase in bioavailability was consistent with decreases in average hepatic and intestinal extraction ratios to 0.082 (3.7-fold) and 0.086 (5.7-fold), respectively. Conclusion. The pharmacokinetic parameter estimates for midazolam determined using the semi-simultaneous bioavailability approach were not different from those determined using the traditional method. The semi-simultaneous approach also yielded expected, marked changes in the parameter estimates due to ketoconazole inhibition. Therefore, the semi-simultaneous bioavailability approach using midazolam appears to be a suitable method to measure hepatic and intestinal CYP3A activity, and to detect changes in CYP3A activity due to enzyme inhibition.