LAUSR

Ovarian cancer is the seventh-most common cancer with a high mortality rate among women. Radiation therapy (RT) is used in ovarian cancer for local relapse, but doses are limited due to concern for small bowel obstruction in patients who have had multiple surgeries and chemotherapeutic regimens. Therefore, it’s an ideal setting to improve the therapeutic window by combining RT and a radiosensitizer. Poly (ADP-ribose) polymerase-1 (PARP-1) is involved in the recognition of DNA damage and the facilitation of DNA repair by recognizing DNA single-strand breaks (SSB). Inhibition of PARP1, especially in tumors driven by BRCA1 or 2 mutations, has been shown to potentiate the DNA-damaging effects of radiation in vitro and in vivo and to increase tumor vasculature perfusion and oxygenation. Approximately half of ovarian cancers have germline or somatic BRCA gene mutation, epigenetic silencing or other mutations that affect homologous recombination (HR) competency resulting in DNA repair defects, making this tumor particularly susceptible to the radiation-sensitizing effects of PARP inhibitors. Here, we investigated the therapeutic effects of the PARP inhibitor olaparib in preclinical models of ovarian cancer. Basic BRCA1 and PARP1 protein and mRNA levels were characterized in a panel of ovarian cancer cell lines. The radiosensitizing effects of olaparib were tested on both HR-proficient and HR-deficient ovarian cancer cells by evaluating the colony formation, DNA damage, apoptosis, and PARP1 activity. Survival benefit of olaparib was also examined in xenograft models of ovarian cancer. Our results demonstrate that combination of olaparib with IR decreased colony formation, inhibited DNA damage (γH2AX) repair, and induced more apoptosis compared to olaparib or IR alone. In addition, HR-deficient cells were more sensitive to IR than HR-proficient cells in the presence of olaparib. Furthermore, olaparib inhibited PARP1 activity in vivo, significantly decreased tumor growth and increased overall survival when combined with IR in mice bearing subcutaneous xenografts of HR-deficient OVCAR8 cells while producing a relative modest effect on overall survival of mice bearing xenografts of HR-proficient SKOV3 cells. These results provide a preclinical rationale for improved treatment modalities using olaparib as an effective radiosensitizer in ovarian cancer, particularly in tumors with HR-deficiencies. Citation Format: Yue Bi, Ioannis Verginadis, Souvik Dey, linlang Guo, Lilie Lin, Yanfang Zheng, Constantinos Koumenis. Identifying the radiosensitizing effects of PARP inhibitor in ovarian cancer [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 5863. doi:10.1158/1538-7445.AM2017-5863