A biomarker-guided approach to combining PARP inhibitors with radiotherapy in pediatric solid tumors.

10556Background: Ewing sarcoma (EWS) expresses high levels of Schlafen-11 (SLFN11). SLFN11 disrupts checkpoint maintenance and may serve as a biomarker to assess sensitivity to Poly (ADP-ribose) polymerase 1 and 2 inhibitors (PARPi). The goal of this study is to evaluate SLFN11 protein expression in a panel of pediatric solid tumors and correlate levels of protein with sensitivity to PARP inhibition combined with ionizing radiation (IR), a component of therapy for many pediatric solid tumors and a potent inducer of DNA damage. Methods: SLFN11 mRNA and protein levels were assessed by quantitative RT-PCR, and immunohistochemistry, Western blot, and immunofluorescence microscopy, respectively. PARPi included: talazoparib (TAL), niraparib (NIR), veliparib (VEL), and olaparib (OLA). Approximately 30 minutes after addition of systemic therapy, graded doses of radiation were delivered and viability across a panel of pediatric solid tumor cell lines was measured using the ATP-based Cell TiterGlo assay and confirm...