Inhibition and stimulation of intestinal and hepatic CYP3A activity: studies in humanized CYP3A4 transgenic mice using triazolam.

CYP3A4 is an important determinant of drug-drug interactions. In this study, we evaluated whether cytochrome P450 3A knockout mice [Cyp3a(-/-)] and CYP3A4 transgenic (CYP3A4-Tg) mice can be used to study drug-drug interactions in the liver and intestine. Triazolam was used as a probe drug because it is a highly specific CYP3A substrate and not a P-glycoprotein substrate. Triazolam metabolism was profoundly reduced in Cyp3a(-/-) mice both in vitro and in vivo. In vitro studies revealed clear species differences in humans and mice, but triazolam metabolism in microsomes derived from CYP3A4-Tg "humanized" mice closely resembled that in human microsomes. It is interesting to note that studies with tissue-specific CYP3A4-Tg mice revealed that intestinal CYP3A4 has a major impact on oral triazolam exposure, whereas the effect of hepatic CYP3A4 was limited. To mimic a drug-drug interaction, we coadministered triazolam with the prototypical CYP3A inhibitor ketoconazole, which increased triazolam exposure in all the CYP3A-proficient mouse strains but not in Cyp3a(-/-) mice. We further found that the anticancer drug gefitinib is a potent stimulator of 1'-OH triazolam formation in vitro. It is noteworthy that we could also show in vivo stimulation of triazolam metabolism by gefitinib, resulting in a lower oral triazolam exposure. To our knowledge, this is the first in vivo example of direct stimulation of CYP3A4 activity after oral drug administration. Overall, this study illustrates how Cyp3a(-/-) and CYP3A4-Tg mice can be used to study drug-drug interactions. The data clarify that for drugs that are not P-glycoprotein substrates, intestinal metabolism also can be more important than hepatic metabolism after oral administration.