Cryopreserved human hepatocytes suspended in human plasma (HHSHP) represent an integrated metabolic environment for predicting drug-drug interactions (DDIs). In this study, 13 CYP3A reversible and/or time-dependent inhibitors (TDIs) were incubated with HHSHP for 20 min over a range of concentrations after which midazolam 1′-hydroxylation was used to measure CYP3A activity. This single incubation time method yielded IC₅₀ values for the 13 inhibitors. For each CYP3A inhibitor-victim drug pair, the IC₅₀ value was combined with total average plasma concentration of the inhibitor in humans, fraction of the victim drug cleared by CYP3A, and intestinal availability of the victim drug to predict the ratio of plasma area under the curve of the victim drug in the presence and absence of inhibitor. Of 52 clinical DDI studies using these 13 inhibitors identified in the literature, 85% were predicted by this method within 2-fold of the observed change, and all were predicted within 3-fold. Subsequent studies to determine mechanism (reversible and time-dependent inhibitors) were performed by using a range of incubation periods and inhibitor concentrations. This system differentiated among reversible inhibitors, TDIs, and the combination of both. When the reversible and inactivation parameters were incorporated into predictive models, 65% of 52 clinical DDIs were predicted within 2-fold of the observed changes and 88% were within 3-fold. Thus, HHSHP produced accurate DDI predictions with a simple IC₅₀ determined at a single incubation time regardless of the inhibition mechanism; further if needed, the mechanism(s) of inhibition can be identified.

• Received September 20, 2010.
• Accepted January 6, 2011.

• Copyright © 2011 by The American Society for Pharmacology and Experimental Therapeutics