Sequential combinations of chemotherapeutic agents with BH3 mimetics to treat rhabdomyosarcoma and avoid resistance

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in childhood and adolescence. Refractory/relapsed RMS patients present a bad prognosis that combined with the lack of specific biomarkers difficult the development of new therapies. We here utilize dynamic BH3 Profiling (DBP), a functional predictive biomarker that measures net changes in mitochondrial apoptotic signaling, to identify anti-apoptotic adaptations upon treatment. We use this information to guide the use of BH3 mimetics to specifically inhibit BCL-2 pro-survival proteins, defeat resistance and avoid relapse to therapy. Indeed, we found that BH3 mimetics that selectively target BCL-xL and MCL-1 synergistically enhance the effect of the clinically used chemotherapeutic agents vincristine and doxorubicin in RMS cells. We validated this strategy in vivo using a RMS patient-derived xenograft (PDX) model and observed a reduction on tumor growth with a tendency to its stabilization with the sequential combination of vincristine and the MCL-1 inhibitor S63845. Finally, we identified the molecular mechanism by which RMS cells acquire resistance to vincristine: through the anti-apoptotic protein MCL-1 for which we observed an enhanced binding between MCL-1 and BID after drug exposure, which is suppressed by the sequential addition of S63845. In conclusion, our findings validate the use of DBP as a functional assay to predict treatment effectiveness in RMS and provide a rationale for BH3 mimetic combination with chemotherapeutic agents to avoid tumor resistance, improve treatment efficiency and decrease undesired secondary effects.