Abstract 3924: PARP inhibition as a radiosensitizing strategy to improve locoregional control in inflammatory breast cancer
Authors: Michmerhuizen Anna , Pesch Andrea , Moubadder Leah , Cameron Meleah , Zhang Amanda , Hirsh Nicole , Liu Meilan , Wilder-Romans Kari , Pierce Lori J. , Jagsi Reshma , Speers Corey 
Published: August 2019

Abstract
Purpose: Sustained locoregional control of disease is a significant issue in patients with inflammatory breast cancer (IBC) with local control rates &lt;70% at 5 years in these patients. Given the unsatisfactory outcomes, there is a clear need for intensification of local therapy, including radiation, for these patients. Inhibition of poly(adenosine diphosphate-ribose) polymerase 1 (PARP1), a DNA repair protein, represents a novel and promising strategy for radiosensitizing aggressive tumors. The purpose of this study was to investigate radiosensitization by PARP1 inhibition in IBC and to determine the mechanism of radiosensitization.
Methods: Pharmacologic PARP1 inhibitors, olaparib and veliparib, were used at various concentrations (10-5 to 10-12 M). Proliferation assays were used to determine the IC50s of these inhibitors in three independent IBC cell lines (SUM 149, SUM 190, MDA-IBC-3). Clonogenic survival assays were used to determine the radiosensitization in cell lines after pharmacologic PARP inhibition (PARPi). DNA damage was quantified using γH2AX staining in IBC cell lines. Transcriptomic, protein and signaling pathway changes were measured using RNA-Seq and RPPA, respectively. In vivo tumor growth was also measured in CB17-SCID mice with varying control and treatment groups (16-20 tumors/group) that included radiation (RT) alone, olaparib alone, and RT+olaparib.
Results: PARPi via olaparib and veliparib had limited single agent efficacy in all three independent IBC models with IC50 values &gt;10 µM. Despite limited single agent efficacy, sub micromolar concentrations of PARPi with RT led to significant radiosensitization with radiation enhancement ratios of 1.20-1.35 in the same three models (500 nM in SUM 190, 500 nM in MDA-IBC-3, 2 nM in SUM 149). This effect was partially dependent on BRCA 1/2 mutational status. Radiosensitization was due, at least in part, to delayed resolution of double strand DNA (dsDNA) breaks at 12, 16, and 24 hours in all models. Additionally, this effect is mediated, at least in part, by a non-homologous end joining (NHEJ) mechanism and was not dependent on homologous recombination (HR). RNA-Seq and RPPA protein pathway data identify key pathways that regulate the radiosensitization in these IBC models. Additionally, xenograft studies show significant differences in tumor doubling and tripling times between treated and untreated tumors.
Conclusion: Our study demonstrates that the PARP1 inhibition improves the therapeutic index of radiotherapy in IBC cell lines. Combined PARPi with RT leads to unresolved dsDNA breaks and decreased clonogen survival. These studies provide the rationale for a phase II randomized trial of RT +/- PARPi (olaparib) in women with IBC that has recently opened for accrual (SWOG 1706 - NCT03598257). This work was supported by a grant from Komen for the Cure and NIH T-32-GM007315.
Citation Format: Anna Michmerhuizen, Andrea Pesch, Leah Moubadder, Meleah Cameron, Amanda Zhang, Nicole Hirsh, Meilan Liu, Kari Wilder-Romans, Lori J. Pierce, Reshma Jagsi, Corey Speers. PARP inhibition as a radiosensitizing strategy to improve locoregional control in inflammatory breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3924.