Comparison of the CYP3A selective inhibitors CYP3cide, clobetasol, and azamulin for their potential to distinguish CYP3A7 activity in the presence of CYP3A4/5 .

The CYP3A7 enzyme accounts for ~50% of the total CYP content in fetal and neonatal livers and is the predominant CYP involved in neonatal xenobiotic metabolism. Additionally, it is a key player in healthy birth outcomes through the oxidation of dehydroepiandrosterone (DHEA) and DHEA-S (sulfate). The amount of the other hepatic CYP3A isoforms, CYP3A4 and CYP3A5, expressed in neonates is low, but highly variable, and therefore the activity of individual CYP3A isoforms is difficult to differentiate due to their functional similarities. Consequently, a better understanding of the contribution of CYP3A7 to drug metabolism is essential to identify the risk drugs may pose to neonates and developing infants. To distinguish CYP3A7 activity from CYP3A4/5, we sought to further characterize the selectivity of the specific CYP3A inhibitors CYP3cide, clobetasol, and azamulin. We utilized three substrate probes, dibenzylfluorescein, luciferin-PPXE, and midazolam, to determine the IC50 and metabolism-dependent inhibition (MDI) properties of the CYP3A inhibitors. Probe selection had a significant effect on the IC50 values and CYP inactivation across all inhibitory compounds and enzymes. CYP3cide and azamulin were both identified as MDIs and were most specific for CYP3A4. Contrary to previous reports, we found that CP was not an MDI of CYP3A5, but was more selective for CYP3A5 over CYP3A4/7. We further investigated CYP3cide and CP's ability to differentiate CYP3A7 activity in an equal mixture of recombinant CYP3A4, CYP3A5, and CYP3A7 and our results provide confidence of CYP3cide's and CP's ability to distinguish CYP3A7 activity in the presence of the other CYP3A isoforms. Significance Statement These findings provide valuable insight regarding in vitro testing conditions to investigate the metabolism of new drug candidates and help determine drug safety in neonates. The results presented here also clearly demonstrate the effect probe selection may have on CYP3A P450 inhibition studies.