-The use of midazolam in early stages of pregnancy has resulted in a high incidence of birth defects; however, the underlying reason is unknown. We investigated expression changes of the CYP3A molecular species and focused on its midazolam metabolizing activity from the foetal period to adulthood. CYP3A16 was the only CYP3A species found to be expressed in the liver during the foetal period. However, CYP3A11 is upregulated in adult mice, but has been found to be downregulated during the foetal period and to gradually increase after birth. When CYP3A16 expression was induced in a microsomal fraction of cells used to study midazolam metabolism by CYP3A16, its activity was suppressed. These results showed that the capacity to metabolize midazolam in the liver during the foetal period is very low, which could hence result in a high incidence of birth defects associated with the use of midazolam during early stages of pregnancy.

Pregnane X receptor (PXR) is a nuclear receptor that exists in forms such as PXR1 and PXR2. Activated PXR1 heterodimerizes with retinoid X receptor to increase the transcription of cytochrome P450 3A (CYP3A), a drug-metabolizing enzyme, by binding to the PXR biding site in nuclei. PXR2 is a splice variant of PXR1, which is localized to the nuclei and down regulates the transcription of CYP3A by PXR1. The present study investigated the roles of PXR1 and PXR2 in the regulation of CYP3A11 expression in adult mouse organs and primary cultured hepatocytes. In the liver and small intestine, which show high expression of CYP3A11 mRNA, PXR1 mRNA was highly expressed while PXR2 mRNA expression was low. In the lung, kidneys, heart, and stomach, which show low expression of CYP3A11 mRNA, both PXR1 and PXR2 mRNA were expressed at high levels. In the muscle and spleen, which express little CYP3A11 mRNA, both PXR1 and PXR2 mRNA were expressed at low levels. In primary-cultured hepatocytes, PXR1 mRNA was expressed at high levels on day 0 of culture, and markedly decreased beginning on day 1. PXR2 mRNA was expressed at low levels on day 0, and gradually increased until day 5. CYP3A11 mRNA exhibited a different expression pattern from that of PXR1 or PXR2; expression peaked on day 0 and then gradually decreased. These results suggest that CYP3A11 expression is upregulated by PXR1 and downregulated by PXR2.