Targeting the multidrug resistance-conferring ABC transporters overexpressed in cancer cells: in vitro optimization and in vivo validation

Attilio Di Pietro
First-Class CNRS Research Director Institute of Protein Biology and Chemistry 2009 Certification of the group by the National League against Cancer Lyon, France

Abstract:

In addition to P-glycoprotein/ABCB1, for which some third-generation inhibitors are under advanced clinical development, two more-recently discovered ATP-binding cassette (ABC) transporters are strongly involved in tumor resistance to anticancer drugs by pumping out the chemotherapeutics: breast cancer resistance protein/ABCG2 and multidrug resistance (associated) protein/ABCC1. Screening of various series of flavonoids and derivatives as inhibitors of mitoxantrone efflux from ABCG2-transfected HEK293 human cells by flow cytometry, and as chemosensitizers of cell proliferation, allowed us to establish 3D-Quantitative Structure-Activity Relationships (3D-QSARs). The pharmacophoric molecular model built for such ABCG2-specific inhibitors, which appear to be non-competitive, was quite different from those previously reported for ABCB1, whereas both transporters display strongly-overlapping spectra for transported drug substrates. The most potent ABCG2 inhibitors, which were active in vitro at submicromolar concentrations, were indeed efficient in vivo on SCID mice, xenografted with the same human ABCG2-transfected cells, in chemosensitizing tumor growth to irinotecan, an ABCG2 substrate. These flavonoidic inhibitors therefore constitute good drug candidates, with low intrinsic toxicity, to be used as adjuvants to conventional chemotherapeutics. Taking advantage that ABCC1 has the peculiar ability to efflux either glutathione conjugates or free glutathione together with hydrophobic drugs such as vincristine, we identified verapamil derivatives and other types of flavonoids for their ability to induce a fast and massive efflux of intracellular glutathione from ABCC1-overexpressing cells, as either transfected or drug-selected lines, leading to selective cell death through apoptosis. These modulators retained in vivo activity, on SCID mice bearing human xenografts. Such apoptogenic compounds might constitute a new, promising, type of anticancer drugs operating through an original strategy aimed at selectively eliminating multidrug-resistant tumors expressing the ABCC1 transporter.