The Combination of <scp>PARP</scp> and Topoisomerase 1 Inhibitors Improves Radiation Therapy for Ewing Sarcoma

Although primary tumor control rates after surgery and/or radiation therapy (RT) are generally high in patients with Ewing sarcoma (EWS), those with unresectable tumors have failure rates approaching 30% and experience poorer outcomes. Additionally, although metastatic site irradiation is associated with improved survival, dose, and volume effects influence the long-term toxicity risk. Consequently, it is important to identify novel systemic agents to enhance the therapeutic ratio of RT. Given the reported DNA damage response deficits in EWS, we hypothesized that PARP inhibitors (PARPis) would preferentially potentiate radiation relative to standard-of-care (SOC) chemotherapeutics. We investigated primary and recurrent SOC drugs and PARPis with varied trapping potential in combination with radiation in EWS cell lines. At physiologically relevant concentrations, the strong PARP trapper talazoparib (TAL) potentiated radiation to a greater extent than did SOC or other PARPis, although the magnitude of the effect was modest. The radiosensitizing effect of TAL was mediated through the induction of DNA double-strand breaks, rather than through the catalytic inhibition of PARP1. Drug + RT combinations were further tested in vivo by using orthotopic xenograft models of EWS treated with image-guided fractionated radiation. The addition of RT to the combination of TAL plus irinotecan (IRN), a recently evaluated clinical regimen for relapsed pediatric solid tumors, significantly prolonged survival and reduced tumor burden in all EWS-treated mice. This triplet therapy (TAL + IRN + RT) was feasible and yielded responses in several patients with EWS and may represent a useful salvage strategy in recurrent or progressive disease.