Validation of a total IC50 method which enables in vitro assessment of transporter inhibition under semi-physiological conditions.

1. Accurate predictions of clinical transporter-mediated drug-drug interactions (DDI) from in vitro data can be challenging when compounds have poor solubility and/or high nonspecific binding. Additionally, current DDI predictions for compounds with high plasma-protein binding assume that the unbound fraction in plasma is 0.01, if the experimental value is less than 0.01 or cannot be determined. This approach may result in an overestimation of DDI risk. To overcome these challenges, it may be beneficial to conduct inhibition studies under physiologically relevant conditions. 2. Here, IC50 values, determined in the presence of 4% bovine serum albumin approximating human plasma albumin concentrations, were successfully used to predict DDI for uptake transporters, OATP1B1/1B3, OCT1/2, OAT1/3 and MATE1/2K. 3. The IC50 values of reference inhibitors with 4% bovine serum albumin, considered total IC50, were comparable to the predicted values based on nominal IC50 values determined under protein-free conditions and unbound fraction in plasma. Calculation of R-total and Cmax/IC50,total values using total plasma exposure and total IC50 values explained the clinical DDI or absence of it for these inhibitors. 4. These results suggest that IC50 determinations in the presence of 4% albumin can be used, in the context of clinical total exposure, to predict DDI involving uptake transporters.