Development and full validation of six inhibition assays for five major cytochrome P450 enzymes in human liver microsomes using an automated 96-well microplate incubation format and LC–MS/MS analysis

Substrate inhibition assays for five of the major CYP enzymes (phenacetin for CYP1A2, diclofenac for CYP2C9, (S)-mephenytoin for CYP2C19, dextromethorphan for CYP2D6 and midazolam and testosterone for CYP3A4) in human liver microsomes were developed. Fully automated incubations were conducted in a 96-well format under optimized enzyme kinetic conditions. Metabolites of probe substrates were analyzed with rapid LC–MS/MS methods. The assays were fully validated following the procedure for validating bioanalytical methods recommended by regulatory agencies. Quality control samples and a positive control CYP inhibitor were included in each assay. The IC50 values determined for typical CYP inhibitors were reproducible and consistent with those reported in the literature. The high quality and throughput of these assays make them ideally suited for providing information for decision making in late drug discovery and early development and for providing labeling input for new drug registrations.