Dmd052290 1859..1866

Organic anion transporting polypeptide (OATP) 1B1 plays an important role in the hepatic uptake of many drugs, and the evaluation of OATP1B1-mediated drug-drug interactions (DDIs) is emphasized in the latest DDI (draft) guidance documents from U.S. and E.U. regulatory agencies. It has been suggested that some OATP1B1 inhibitors show a discrepancy in their inhibitory potential, depending on the substrates used in the cell-based assay. In this study, inhibitory effects of 14 compounds on the OATP1B1-mediated uptake of the prototypical substrates [H]estradiol-17b-glucuronide (E2G), [H]estrone-3-sulfate (E1S), and [ H]sulfobromophthalein (BSP) were studied in OATP1B1-transfected cells. Inhibitory potencies of tested compounds varied depending on the substrates. Ritonavir, gemfibrozil, and erythromycin caused remarkable substrate-dependent inhibition with up to 117-, 14-, and 13-fold difference in their IC50 values, respectively. Also, the clinically relevant OATP inhibitors rifampin and cyclosporin A exhibited up to 12and 6-fold variation in their IC50 values, respectively. Regardless of the inhibitors tested, the most potent OATP1B1 inhibition was observed when [H]E2G was used as a substrate. Mutual inhibition studies of OATP1B1 indicated that E2G and E1S competitively inhibited each other, whereas BSP noncompetitively inhibited E2G uptake. In addition, BSP inhibited E1S in a competitive manner, but E1S caused an atypical kinetics on BSP uptake. This study showed substratedependent inhibition of OATP1B1 and demonstrated that E2G was the most sensitive in vitro OATP1B1 probe substrate among three substrates tested. This will give us an insight into the assessment of clinically relevant OATP1B1-mediated DDI in vitro with minimum potential of false-negative prediction.