A framework for optimized reaction phenotyping of the seven major hepatic P450 isoforms in human liver microsomes using six mechanism-based inactivators and one reversible inhibitor.

Cytochrome P450 (CYP) enzymes play a central role in drug metabolism. Understanding the contribution of individual CYP isoforms, known as CYP reaction phenotyping, is essential for elucidating metabolic pathways and predicting drug-drug interactions. Reversible inhibitors are commonly used in human liver microsome (HLM)-based CYP reaction phenotyping. However, selecting appropriate inhibitor concentrations is challenging as experimental results will vary depending on the HLM concentration, substrate concentration, and incubation duration selected. These factors collectively influence the observed inhibitory potency, often complicating data interpretation. The use of mechanism-based inactivators (MBIs) of P450 enzyme can potentially overcome these issues. However, the lack of standardized rigorously evaluated protocols and concerns about off-target inhibition have limited the broader application of MBIs for reaction phenotyping. This study systematically evaluated assay conditions for the use of P450 isoform-selective MBIs across the 7 major hepatic P450 isoforms (CYP1A2, 2B6, 2C8, 2C9, 2C19, 2D6, and 3A4) to establish an HLM concentration-independent, universal protocol for CYP reaction phenotyping. Inactivation kinetics were determined, and potential off-target P450 inhibition of each MBI across all 7 P450 isoforms was assessed. Optimized assay conditions were established for each P450 isoform-MBI pair, including HLM concentration, MBI concentration, and incubation time. Under these optimized assay conditions, all selected MBI demonstrated potent and selective inhibition of their target P450 isoforms, demonstrating ≥90% activity loss and minimal off-target inhibition for most MBIs (none >16%). Overall, the established protocol provides a robust and reproducible framework for HLM-based P450 reaction phenotyping using MBIs. This approach enhances the specificity of P450 isoform contribution assessments and supports improved drug-drug interaction prediction in early drug development. SIGNIFICANT STATEMENT: Mechanism-based inactivators offer a powerful approach for defining individual P450 isoform contributions. This study systematically optimized mechanism-based inactivator assays across all major hepatic P450 isoforms in human liver microsomes (HLMs), establishing a unified protocol that balances inactivator concentration, HLM level, and incubation duration to achieve ≥90% target inhibition with minimal off-target effects. The resulting framework improves assay specificity, enhances reliability of P450 reaction phenotyping in HLM, and supports more accurate prediction of metabolic pathways and drug-drug interaction risk.