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BACKGROUND: Poly(ADP-ribose) glycohydrolase (PARG) is responsible poly(ADP-ribose) (PAR) catabolism, which is synthesized by poly(ADP-ribose) polymerases (PARPs) at the site of DNA single-strand breaks (SSB). Faulty PAR formation or disintegration inhibits SSB repair. PARP inhibitors (PARPi) exploit the synthetic lethality between SSB repair and homologous recombination (HR) double strand break (DSB) repair. The most effective PARPi traps PARP at the SSB site, causing a double strand break during DNA synthesis that the cell cannot repair. The trapping of PARP can also be achieved through PARG inhibition. Due to the high rate of HR defects in epithelial ovarian cancer, the aim of this study is to evaluate the effect of a PARG inhibitor (PARGi) on epithelial ovarian cancer cell lines, alone and in combination with PARPi or cisplatin. METHODS: PARG protein levels were assessed in 74, unselected, snap-frozen, tumors kept in our bio-bank by western blotting and immuno-histochemistry (IHC), as well as in the TCGA database. PARG was knocked down in two cell lines resistant to PARP inhibition, OVCAR3 and SKOV3. The knockdowns were subjected to cell migration and cell proliferation assays, as well as single cell analysis. BRCA1 mutated and BRCA1 wild-type cell lines were exposed to clinically relevant doses of PARGi, PARPi, or cisplatin, separately or in various combination. Therapeutic efficacy was assessed using colony formation assay. Western Blotting was used to detect the levels of PARG and PARP proteins in the cell lines. RESULTS: PARG mRNA was expressed in 30% of tumors in TCGA. PARG protein was also detected in 30% of the biobank tumors analyzed. PARG knockdowns exhibited reduced rates of cellular proliferation and significant G2/M cell cycle arrest. In PARG over expressed ovarian cancer cell lines, PARG inhibitor similarly reduced cell migration (OVCAR3 and SKOV3: 75% reduction, SNU-251: 48% reduction). CONCLUSION: PARG inhibition appears to be a viable, complementary strategy to PARP inhibition in HR-deficient cancers. The effect of BRCA1 on the efficacy of PARG inhibition, as well as various treatment combinations, are currently under investigation. Citation Format: Emad Matanes, Tahira Baloch , David Octeau, Roy Kessous, Liron Kogan, Ido Laskov, Walter Gotlieb, Amber Yasmeen. INHIBITION OF PARG, SENSITIZES OVARIAN CANCER CELLS TO PARP INHIBITORS AND DNA DAMAGING AGENTS [abstract]. In: Proceedings of the 12th Biennial Ovarian Cancer Research Symposium; Sep 13-15, 2018; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2019;25(22 Suppl):Abstract nr NT-117.

BACKGROUND: Ovarian cancer is the most lethal gynecologic cancer. High grade serous ovarian cancer (HGSC) is the most common and deadly histological subtype. The current standard treatment protocol involves primary debulking surgery followed by platinum-based combination chemotherapy. PARP inhibitors(PARPi) are the first approved personalized treatments used in BRCA1-mutated recurrent ovarian cancer patients and have shown promising clinical results. Previously published data in collaboration with Dr. Witcher's lab, we described a significant reduction in PARP1 protein levels in patients after giving standard carboplatinum-paclitaxel chemotherapy that is effecting the clinical efficacy of PARP inhibitors in clinical trials. PARP inhibitors are currently administered after standard chemotherapy, when PARP levels are the lowest which was clearly shown in the previous published paper. Applying novel strategy and following the sequence of administration, giving PARP inhibitors first followed by standard chemotherapy might improve response rates. This study aims to evaluate this strategy (in vitro) in the pre-clinical models. METHODS: BRCA1 mutated (UWB1.287, SNU-251), epigenetically silenced (OVCAR8), and wild-type BRCA1 (OVCAR3, SKOV3, A2780P & A2780R) cell lines were exposed to clinically relevant doses of PARPi, either followed by standard chemotherapy, or the inverse sequence. Therapeutic efficacy was assessed using colony formation assay. Apoptotic index was evaluated by cell cycle analysis and apoptotic assays using flow cytometry. Western Blotting was used to detect the levels of relevant apoptotic and cell cycle proteins. RESULTS: Exposure to PARPi prior to standard chemotherapy sensitized BRCA1 mutated or epigenetically silenced BRCA1 cell lines to lower doses of Cisplatin (CP) or Paclitaxel (PT). Similarly, pre-treatment with PARPi prior to chemotherapy induced apoptosis more effectively in the same cell lines. Similar results were observed in BRCA1 wild-type cell lines and cell lines in which BRCA1 functionality was restored. CONCLUSION: Pre-treatment of cell lines with PARPi followed by standard chemotherapy is more efficient (in vitro) in inhibiting growth and inducing apoptosis than the present sequence of chemotherapy followed by PARPi. Citation Format: Tahira Baloch, Roy Kessous, David Octeau , Liron Kogan, Ido Laskov, Michael Witcher, Walter H. Gotlieb and Amber Yasmeen. SEQUENTIAL THERAPEUTIC TARGETING OF OVARIAN CANCER HARBORING DYSFUNCTIONAL BRCA1 [abstract]. In: Proceedings of the 12th Biennial Ovarian Cancer Research Symposium; Sep 13-15, 2018; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2019;25(22 Suppl):Abstract nr NT-118.