Targeting the IGF1R/PI3K/AKT Pathway Sensitizes Ewing Sarcoma to BET Bromodomain Inhibitors

Inhibitors of the bromodomain and extra-terminal domain (BET) family proteins modulate EWS-FLI1 activities in Ewing sarcoma. However, the efficacy of BET inhibitors as a monotherapy was moderate and transient in preclinical models. The objective of this study was to identify the mechanisms mediating intrinsic resistance to BET inhibitors and develop more effective combination treatments for Ewing sarcoma. Using a panel of Ewing sarcoma cell lines and patient-derived xenograft lines (PDX), we demonstrated that IGF1R inhibitors synergistically increased sensitivities to BET inhibitors and induced potent apoptosis when combined with BET inhibitors. Constitutively activated AKT significantly protected Ewing sarcoma cells against BET inhibitors, suggesting that IGF1R regulates responsiveness to BET inhibitors mainly through the PI3K/AKT pathway. Although two Ewing sarcoma cell lines were resistant to IGF1R inhibitors, they retained synergistic response to a combination of BET inhibitors and mTOR inhibitors, suggesting that BET proteins, when IGF1R is not functional, cross-talk with its downstream molecules. Furthermore, the combination of a BET inhibitor and an IGF1R inhibitor induced potent and durable response in xenograft tumors, whereas either agent alone was less effective. Taken together, our results suggest that IGF1R and the downstream PI3K/AKT/mTOR kinase cascade mediate intrinsic resistance to BET inhibitors in Ewing sarcoma. These results provide the proof-of-concept for combining BET inhibitors with agents targeting the IGF1R pathway for treating advanced Ewing sarcoma.