Abstract LB-A13: Targeting poly-ADP ribose glycohydrolase: pursuing a synthetic lethal therapeutic strategy to treat all pancreatic cancers (2018)

Abstract This study lays the groundwork to target poly-ADP ribose glycohydrolase (PARG), a DNA repair enzyme that counteracts PARP-1 activity and is upregulated in response to PARP inhibitor (PARPi) treatment. The goal of this study is to optimize PARPi-based therapies for all pancreatic cancer patients, irrespective of their DNA repair status (i.e., homologous repair (HR) deficiency). Metastatic pancreatic ductal adenocarcinoma (PDA) is a lethal disease with an average survival of less than one year, which highlights the need for novel treatment regimens. Though PARPi therapy offers a personalized strategy in a subset of tumors with HR defects, initially responsive patients typically become resistant to therapy. Therefore, identifying and combating PARPi resistance mechanisms is a rising concern. We have previously shown that PARG is a critical player in mediating resistance; therefore we evaluated PARG as a candidate target in PDA. Importantly, we demonstrated that the pro-oncogenic protein, HuR (ELAVL1), upregulates PARG specifically in tumor microenvironment niches (e.g., drug exposure), thus providing a unique, cancer-specific therapeutic window for targeting PARG. We evaluated a series of potent first-in-class, cell-active small molecule PARG inhibitors; PDDX-01, PDDX02 and PDDX-04 in pre-clinical models of PDA. These PARGi effectively inhibited PARG activity, caused persistence of PAR polymers, increased accumulation of γH2AX foci and inhibited short and long-term growth of PDA cell lines. Similar to PARPi agents, PARG inhibition was also shown to be synthetic lethal; DNA damage repair (DDR)- deficient cell lines were more sensitive than the DDR- proficient PDA cell lines. This was further validated in isogenic colorectal cell lines with varying DNA repair functionality; DLD1 lines with BRCA2 (+/+, +/-, -/-) and RKO lines with FANCC (+/+, +/-, -/-) modulation. Using chromatin tethering assays, these PARGis effectively trapped PARP1 on DNA, alone as well as combination with the PARPi olaparib, and a DNA damaging agent oxaliplatin. We also developed and characterized a PARG inhibition model in vivo in a DNA- repair proficient PDA line, MIA PaCa2. The doxycycline-inducible shPARG knockdown was validated for PARG inhibition, PAR accumulation and increased PARPi sensitivity in vitro. Complementing xenograft experiments were performed wherein 3 million cells were injected per flank in nude female athymic mice. At an average tumor volume of 50mm3, respective groups were fed DOX- chow to induce PARG knockdown and treated with olaparib intraperitoneally at 50mg/kg five times a week. PARG inhibition by doxycycline induction significantly decreased tumor volumes (50% decrease, p-value 0.0165), which was further enhanced with olaparib treatment (70% decrease, p- value 0.0004), when compared with control arms. Taken together, these studies help define and break a potent PARPi resistance mechanism and also validate pursuing PARG as a valuable “druggable” target in both HR-proficient and deficient PDA cells. Citation Format: Saswati N. Chand, AnnJosette Ramirez, Aditi Jain, Avinoam Nevlar, Cinthya Yabar-Lowder, Joseph A. Cozzitorto, Dominic I. James, Ian D. Waddell, Charles J. Yeo, Michael J. Pishvaian, Jordan M. Winter, Jonathan R. Brody. Targeting poly-ADP ribose glycohydrolase: pursuing a synthetic lethal therapeutic strategy to treat all pancreatic cancers [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr LB-A13.