PT - JOURNAL ARTICLE AU - Heather Eng AU - Elaine Tseng AU - Matthew A. Cerny AU - Theunis C. Goosen AU - R. Scott Obach TI - Cytochrome P450 3A Time-Dependent Inhibition Assays are Too Sensitive for Identification of Drugs Causing Clinically Significant Drug-Drug Interactions: A Comparison of Human Liver Microsomes and Hepatocytes and Definition of Boundaries for Inactivation Rate Constants AID - 10.1124/dmd.121.000356 DP - 2021 Jan 01 TA - Drug Metabolism and Disposition PG - DMD-AR-2021-000356 4099 - http://dmd.aspetjournals.org/content/early/2021/04/01/dmd.121.000356.short 4100 - http://dmd.aspetjournals.org/content/early/2021/04/01/dmd.121.000356.full AB - Time-dependent inhibition (TDI) of cytochrome P450 3A (CYP3A) is an important mechanism underlying numerous drug-drug interactions (DDI), and assays to measure this are done to support early drug research efforts. However, measuring TDI of CYP3A in human liver microsomes (HLM) frequently yields over-estimations of clinical DDI, and thus can lead to the erroneous elimination of many viable drug candidates from further development. In this investigation, 50 drugs were evaluated for TDI in HLM and suspended human hepatocytes (HHEP) in order to define appropriate boundary lines for the TDI parameter kobs at a concentration of 30 µM. In HLM, a kobs value of 0.002 min‑1 was statistically distinguishable from control; however, many drugs show kobs greater than this but do not cause DDI. A boundary line, defined by the drug with the lowest kobs that causes a DDI (diltiazem), was established at 0.01 min-1. Even with this boundary, of the 33 drugs above this value, only 61% cause a DDI (true positive rate). A corresponding analysis was done using HHEP; kobs of 0.0015 min-1 was statistically distinguishable from control, and the boundary was established at 0.006 min-1. Values of kobs in HHEP were almost always lower than in HLM. These findings offer a practical guide to the use of TDI data for CYP3A in early drug discovery research. Significance Statement Time-dependent inhibition of CYP3A is responsible for many drug interactions. In vitro assays are employed in early drug research to identify and remove CYP3A time-dependent inhibitors from further consideration. This analysis demonstrates suitable boundaries for inactivation rates to better delineate drug candidates for their potential to cause clinically significant drug interactions.