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

Abstract

10556 Background: 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 confirmed with the colony formation assay. Results: We found that SLFN11 mRNA and protein is expressed at high levels in EWS, and SLFN11 is also variably present in a subset of other pediatric solid tumor lines, including desmoplastic small round cell tumor, osteosarcoma, and rhabdomyosarcoma. In all tumor cells with detectable SLFN11 expression, viability was reduced by greater than 90% when exposed to 2Gy IR and 1-10nM TAL, whereas cells with undetectable levels of SLFN11 were 5-10 times less sensitive. Intriguingly, variation in the potentiation between specific PARPi and IR correlated with the ability to form drug-induced PARP-DNA adducts, with the strong PARP trapper TAL showing ~10-fold higher potency compared to the moderate trapper NIR, and ~300-fold more potency relative to the weak trapper VEL. Consistent with our PARPi findings, the toposiomerase 1 inhibitor irinotecan, which also forms DNA adducts, potentiated with IR similarly to TAL at concentrations < 10nM in tumor cells expressing detectable levels of SLFN11. Conclusions: SLFN11 is present in select pediatric solid tumors and may induce a DNA repair defect that is best exploited by combining low-doses of TAL and irinotecan with IR.