A proposed simple screening method to determine relative contributions of CYP3A4 and CYP3A5 to drug metabolism in vitro

The cytochrome P450 (CYP) 3A family of enzymes metabolize the majority of clinically used drugs. CYP3A4 and CYP3A5 are the two major CYP3A isoforms, but exhinbit different substrate specificity. The aim of this study was to establish a simple screening method to determine the relative contributions of CYP3A4 and CYP3A5 to drug metabolism in vitro. A screening method was developed based on competitive inhibition using luciferin-PPXE (L-PPXE), a luminogenic CYP3A substrate. CYP3cide, tacrolimus, and midazolam were selected as standard compounds metabolized by CYP3A4 or CYP3A5. Nine clinically-used drugs were evaluated for their abilities to inhibit luminescence resulting from L-PPXE metabolism. Appropriate reaction conditions for the screening method were determined using recombinant CYP3A4 and CYP3A5. A significant decrease in luminescence resulting from L-PPXE metabolism by CYP3A4 and CYP3A5 was observed only for drugs reported to be metabolized by CYP3As. The substrate specificities of CYP3A4 or CYP3A5 for the proposed CYP3A substrates using our screening method were consistent with those of previous reports or available drug information from pharmaceutical companies. The reaction volume for this method was 50 μL, and the time required for the entire procedure was 70 min. Furthermore, this screening can be performed using a single tube with minimal training. Through the establishment of our screening method in the present study, we are sure it is useful to determine the relative contributions of CYP3A4 and CYP3A5 to drug metabolism in vitro.