Fluorescent substrates of sister-P-glycoprotein (BSEP) evaluated as markers of active transport and inhibition: evidence for contingent unequal binding sites.
OBJECTIVE
Although sister-P-glycoprotein (SPGP, BSEP) is closely related to P-glycoprotein, it is much more selective in distribution and substrate recognition. Moreover, because inhibition or lack of BSEP function has severe consequences including cholestasis, hepatotoxicity, exposure to toxic xenobiotics, and drug interactions, in vitro methods are necessary for quantifying and characterizing specific inhibition of BSEP. Therefore, the objective is to discern a method and quantitatively characterize several example BSEP inhibitors.


METHODS
With fluorescent markers having been used successfully to evaluate and quantify inhibition of P-gp-mediated transport, this study evaluates several compounds for specific cell retention caused by BSEP inhibitors. In addition to the several compounds asserted to be BSEP inhibitors, the compounds suggested to be BSEP substrates might also inhibit BSEP competitively. Retained fluorescence of possible BSEP substrates was measured by a flow cell cytometer using transfected cells presenting the BSEP transporter specifically and abundantly.


RESULTS
Several compounds were shown to inhibit BSEP active transport of the fluorescent substrates dihydrofluorescein and bodipy. The inhibition potency was quantified (i.e., cyclosporin A IC50 approximately 7 microM), revealing incongruent relative sensitivities among the substrate markers, with H2FDA generally the most sensitive of the series of substrate markers evaluated.


CONCLUSIONS
The inconsistent sensitivities of the transport markers (H2FDA and bodipy) were reminiscent of the apparent multiple binding site behaviors observed for P-gp and could indicate opposing and unequal yet interacting binding sites akin to those of P-gp. Nonetheless, notable differences between P-gp and BSEP in marker substrate recognition/transport were apparent despite the observed overlap in xenobiotic recognition and transport. Thus far the most potent inhibitors seem to be cyclosporin, tamoxifen, and valinomycin. There are likely to be much more potent inhibitors, and other substrates also may be more sensitive to inhibition of transport.
Drug Metabolism and Pharmacokinetics, Schering-Plough Research Institute, Lafayette, New Jersey 07848, USA.
Er-jia
EJ
Wang
Christopher N
CN
Casciano
Robert P
RP
Clement
William W
WW
Johnson
D000074020:ATP Binding Cassette Transporter, Subfamily B, Member 11; D018528:ATP-Binding Cassette Transporters / Q000037:antagonists & inhibitors* / Q000378:metabolism* / Q000502:physiology; D001665:Binding Sites / Q000187:drug effects / Q000502:physiology; D001693:Biological Transport, Active / Q000187:drug effects / Q000502:physiology*; D015415:Biomarkers; D002478:Cells, Cultured; D004347:Drug Interactions / Q000502:physiology; D005456:Fluorescent Dyes; D006801:Humans
C093491:ABCB11 protein, human; D000074020:ATP Binding Cassette Transporter, Subfamily B, Member 11; D018528:ATP-Binding Cassette Transporters; D015415:Biomarkers; D005456:Fluorescent Dyes