Improved In Vitro-In Vivo Correlation by Using the Unbound-Fraction-Adjusted IC50 for Breast Cancer Resistance Protein Inhibition

One function of the blood-brain barrier (BBB) is the efflux of xenobiotics by breast cancer resistance protein (BCRP), and inhibition of BCRP can cause unexpected central nervous system toxicity. Despite the importance of BCRP inhibition and the associated risk of BBB penetration in vivo, there has been little investigation of it to date. In this study, inhibition of BCRP-mediated transport was assessed by in vitro assay in the presence of bovine serum albumin (BSA) to change the unbound inhibitor concentrations, and the in vitro-in vivo correlation (IVIVC) at the BBB was evaluated. The IC50 values of BCRP inhibitors were determined in vitro with and without BSA and the inhibitors were categorized into two groups. One group of compounds had little risk of inhibiting BCRP because of their low unbound concentrations. In contrast, the other group has the potential to facilitate BBB penetration by inhibiting BCRP. In the IVIVC approach, brain concentrations and the brain-to-plasma ratio were better correlated with the ratio of the unbound plasma concentration at steady-state to the unbound-fraction-adjusted IC50. We have found a way to obtain a better in vitro-in vivo correlation for BCRP-mediated transport.