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 [ values, respectively. Also, the clinically relevant OATP inhibitors rifampin and cyclosporin A exhibited up to 12-and 6-fold variation in their IC 50 values, respectively. Regardless of the inhibitors tested, the most potent OATP1B1 inhibition was observed when [ 3 H]E 2 G was used as a substrate. Mutual inhibition studies of OATP1B1 indicated that E 2 G and E 1 S competitively inhibited each other, whereas BSP noncompetitively inhibited E 2 G uptake. In addition, BSP inhibited E 1 S in a competitive manner, but E 1 S caused an atypical kinetics on BSP uptake. This study showed substratedependent inhibition of OATP1B1 and demonstrated that E 2 G 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.

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The human CYP1A1 and CYP1A2 genes on chromosome 15 are orientated head-to-head and are separated by a 23-kilobase (kb) intergenic spacer region. Thus, the possibility exists for sharing common regulatory elements contained in the spacer region responsible for transcriptional activation and regulation of the CYP1A1 and CYP1A2 genes. In the present study, a reporter gene construct containing Ϫ22.4 kb of the 5Ј-flanking region of the CYP1A2 gene was found to support ␤-naphthoflavone (BNF) and 3-methylchoranthrene (3-MC)-mediated transcriptional activation. The responsive region was also functional in directing activation of the CYP1A1 promoter, indicating that the region works bidirectionally to govern transcriptional activation of both CYP1A1 and CYP1A2. To simultaneously evaluate transcriptional activation of both genes, a dual reporter vector was developed in which the spacer region was inserted between two different reporter genes, firefly luciferase and secreted alkaline phosphatase. Transient transfection of the dual reporter vector in HepG2 cells revealed increases in both reporter activities after exposure of the cells to BNF and 3-MC. Deletion studies of the spacer region indicated that a region from Ϫ464 to Ϫ1829 of the CYP1A1 gene works bidirectionally to enhance the transcriptional activation of not only CYP1A1 but also CYP1A2. In addition, a negative bidirectional regulatory region was found to exist from Ϫ18,989 to Ϫ21,992 of the CYP1A1 gene. These data established that induction of human CYP1A1 and CYP1A2 is simultaneously controlled through bidirectional and common regulatory elements.