Comparison of the hepatic metabolism of triazolam in wild-type and Cyp3a-knockout mice for understanding CYP3A-mediated metabolism in CYP3A-humanized mice in vivo.

1. To investigate cytochrome P450 3A (CYP3A)-mediated metabolism in vivo, plasma concentrations of triazolam (TRZ) are often monitored as a CYP3A marker in CYP3A-humanised mice. However, it has not been determined whether plasma concentrations of TRZ after intravenous administration can reflect hepatic CYP3A activity in CYP3A-humanised mice. 2. Firstly, we investigated the pharmacokinetics of TRZ in wild-type and Cyp3a-knockout (Cyp3a-KO) mice. Plasma concentration profiles of TRZ and alpha-hydroxy (OH) TRZ were very similar in wild-type and Cyp3a-KO mice. On the other hand, AUC of 4-OH TRZ in Cyp3a-KO mice was significantly lower than that in wild-type mice. Pregnenolone 16 alpha-carbonitrile (PCN) decreased the areas under the plasma concentration-time curves (AUCs) of TRZ and alpha-OH TRZ in both groups. There was no significant effect of PCN on AUC of 4-OH TRZ in Cyp3a-KO mice. 3. Next, we verified that AUC of 4-OH TRZ in CYP3A-humanised mice was higher than that in Cyp3a-KO mice, although the difference was not significant. 4. In conclusion, plasma concentrations of 4-OH TRZ, but not those of TRZ and alpha-OH TRZ, might reflect hepatic CYP3A activity in mice in vivo. These results provide important insights for in vivo studies using a CYP3A-humanised model.