Background Cancer maintenance, metastatic dissemination and drug-resistance are sustained by cancer stem cells (CSCs). Triple negative breast cancer (TNBC) is the breast cancer subtype with the highest numbers of CSCs and poorest prognosis. Here, we aimed to identify potential drugs targeting CSCs to be further employed in combination with standard chemotherapy in TNBC treatment.
Methods The anti-CSC efficacy of up to 17 small-drugs was tested in TNBC cell lines using cell viability assays on differentiated cancer cells and CSCs. Then, the effect of 2 selected drugs (8-quinolinol -8Q- and niclosamide -NCS-) in the cancer stemness hallmarks were evaluated using mammosphere growth, cell invasion, migration and anchorage-independent growth assays. Changes in the expression of stemness genes upon 8Q or NCS treatment were also evaluated. Moreover, the potential synergism of 8Q and NCS with PTX on the CSC proliferation and on stemness-related signaling pathways was evaluated using TNBC cell lines, CSC-reporter sublines, and CSCenriched mammospheres. Finally, the efficacy of the NCS in combination with PTX was analyzed in vivo using an orthotopic mice model of MDA-MB-231 cells.
Results Among all tested drug candidates, 8Q and NCS showed remarkable specific anti-CSC activity in terms of CSC viability, migration, invasion and anchorage independent growth reduction in vitro. Moreover, specific 8Q/PTX and NCS/PTX ratios at which both drugs displayed a synergistic effect in different TNBC cell lines were identified. The solely use of PTX increased the relative presence of CSCs in TNBC cells, whereas the combination with 8Q and NCS counteracted this pro-CSC activity of PTX whilst significantly reducing cell viability. In vivo, the combination of NCS with PTX reduced tumor growth, and limited the dissemination of the disease by reducing the circulating tumor cells and the incidence of lung metastasis.
Conclusions The combination of 8Q and NCS with PTX at established ratios inhibits both, the proliferation of differentiated cancer cells and the viability of CSCs, opening a way to more efficacious TNBC treatments.