Abstract 5197: Targeting PARP-1 to deliver alpha-particles to cancer chromatin
Authors: Puentes Laura , Xu Kuiying , Hou Catherine , Mach Robert H. , Maris John M. , Pryma Daniel A. , Makvandi Mehran 
Published: July 2017

Abstract
Introduction: Neuroblastoma (NB) is a radiosensitive pediatric cancer that develops in the sympathetic nervous system and typically affects children under the age of 10. High-risk NB is associated with a 40% 5-year survival rate. Nuclear enzyme poly (ADP-ribose) polymerase 1 (PARP-1) is overexpressed in high-risk NB, making it an attractive target for alpha-particle therapy. Alpha-particles have a short path length and high linear energy transfer, causing dense ionizations across DNA, inducing double stranded breaks that result in cell death. The purpose of this study was to explore a newly developed radiotherapeutic ([211At]MM4) that combines the targeting potential of a small molecule PARP inhibitor (PARPi) with the cytotoxic effects of 211At in high-risk NB.
Methods: In vitro cytotoxicity was performed in a panel of high-risk NB cell lines to evaluate the relative potency of [211At]MM4. Next, DNA damage was assessed by measuring gH2AX foci formation at 1, 4, and 24 hrs after [211At]MM4 treatment. In parallel, PARP-1 expression was measured in response to therapy and cleaved PARP-1 was quantified to assess apoptosis. Cell cycle analysis was performed after treatment to identify therapy related effects. The in-vivo biodistribution of [211At]MM4 was performed alongside ex-vivo autoradiography. Tumor cytology for PARP-1, gH2AX, and Ki-67 was performed in response to therapy. Finally, in-vivo therapy experiments were performed.
Results: Cytotoxicity data indicated a significant reduction in cell viability following treatment for all six NB cell lines, in-vitro co-incubation studies confirmed the specificity of [211At]-MM4 to its drug target. Immunofluorescence analysis showed a dose-dependent increase in both gH2AX and PARP-1 expression. The in-vivo biodistribution of [211At]MM4 revealed rapid tumor targeting at 1 hr and clearance from all tissues at 4 hrs. Ex-vivo autoradiography showed a tumor-muscle ratio greater than 6. Tumor cytology revealed DNA damage measured by gH2AX and PARP-1 expression increased following treatment. Small colonies of proliferating tumor cells were detected after treatment using Ki-67 staining. In-vivo therapy efficacy studies revealed low fractionated doses were tolerable and resulted in significant delay in tumor regrowth.
Conclusion: [211At]MM4 is a novel alpha-emitting radiotherapeutic that specifically targets nuclear PARP-1 overexpression in neuroblastoma and incites double-stranded breaks in cancer DNA. The cytotoxic effects of [211At]MM4 have been experimentally validated both in-vitro and in-vivo; the results of these experiments confirm the therapeutic potential of [211At]MM4 as a viable treatment option for high-risk neuroblastoma.
Citation Format: Laura Puentes, Kuiying Xu, Catherine Hou, Robert H. Mach, John M. Maris, Daniel A. Pryma, Mehran Makvandi. Targeting PARP-1 to deliver alpha-particles to cancer chromatin [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5197. doi:10.1158/1538-7445.AM2017-5197