A small inhibitor of the interaction between Bax and Bcl-XL can synergize with methylprednisolone to induce apoptosis in Bcl-XL-overexpressing breast-cancer cells

Purpose. To identify inhibitors of the interaction between Bax and Bcl-X-L. Methods. Using an assay based on biosensor technology, we screened a chemical library of 10,000 compounds for inhibitors of the interaction between Bax and Bcl-X-L. Using cell-culture systems we tested active compounds for their ability to induce apoptosis in Bcl-X-L-overexpressing MCF7 cells and increase the sensitivities of the cells to apoptosis-inducing drugs [vincristine sulphate, dexamethasone, cycloheximide and 6alpha-methylprednisolone (MP)]. Results. A single compound, 2',4',5',7'-tetrabromofluorescein (A5), from the library was found to inhibit this interaction efficiently. Several structural analogues of A5 were tested and two of these [4',5'-dibromofluorescein (A9) and 3,4,5,6-tetrabromofluorescein (A11)] were found to be active, and their activities were confirmed by an independent in vitro pull-down assay. These active compounds were observed to induce apoptosis in Bcl-X-L-overexpressing MCF7 cells. Moreover, two of the compounds (A5 and A11) appeared to increase the sensitivities of the cells to MP. A more rigorous test using the isobologram technique showed that there is a synergistic cytotoxic effect between A11 and MP. Conclusions. We have identified a small inhibitor of the interaction between Bax and Bcl-X-L that can synergize with methylprednisolone to induce apoptosis in Bcl-X-L-overexpressing breast-cancer cells.