Abstract 1973: Targeting PARG in pancreatic cancer: Implications for synthetic lethal therapeutic strategies
Authors: Chand Saswati N. , Ramirez AnnJosette , Jain Aditi , Nevler Avinoam , Yabar-Lowder Cinthya , Cozzitorto Joseph A. , James Dominic I. , Jordan Allan , Smith Kate M. , Waddell Ian , Yeo Charles J. , Winter Jordan M. , Brody Jonathan R. 
Published: August 2018

Abstract
Metastatic pancreatic ductal adenocarcinoma (PDA) has an average survival of less than one year. There is a pressing need to identify patient subgroups for treatment with novel targeted agents and the necessity to combat increasing incidence of therapeutic resistance. In a previous study, we identified that poly(ADP) ribose glycohydrolase (PARG) is a critical player in mediating resistance to PARP inhibitor (PARPi); therefore targeting PARG is a strategy to enhance PARPi therapy in PDA and can be optimized to benefit patients with or without homologous repair (HR) deficiencies. We developed and characterized multiple PARG inhibition models in both DNA- repair proficient (MIA PaCa-2) and deficient (Hs766t) PDA lines; doxycycline-inducible shPARG knockdown, CRISPR- mediated PARG knockout and small molecule inhibition via a series of potent first-in-class, cell-active PARG inhibitors (PARGi). Our data show that PARG inhibition is synthetic lethal with DNA damage repair deficiency in PDA cells. This was further validated in isogenic colorectal cell lines with varying DNA repair functionality: DLD1 lines with BRCA2 (+/+, +/-, -/-) and RKO lines with FANCC (+/+, +/-, -/-). We have also shown that PARG inhibition enhances PARPi sensitivity through increased accumulation of DNA damage, apoptosis and persistence of detrimental PARylation. Moreover, PARP1 was trapped on the chromatin in response to both PARPi treatment as well as DNA damaging agents such as oxaliplatin. Complementary xenograft experiments were performed wherein MIA.shPARG cells were injected 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 100mg/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. Similar results were obtained when DOX-fed mice with MIA.shPARG cells were treated with olaparib at 50mg/kg. Furthermore, in an attempt to mimic and break long-term in vivo PARPi resistance, doxycycline-mediated PARG inhibition was induced in the olaparib treatment arm on day 56 (with established tumors, and exposed to olaparib for 3weeks i.e. 15 injections). This resulted in a significant decrease in tumor volume when compared to control untreated arm (46% decrease, p-value 0.0025) and the olaparib only treatment arm (25% decrease, p-value 0.0124). We are currently validating these results in a DDR-deficient HST.shPARG cell line, as well as with CRISPR knockouts of PARG. Together these studies validate PARG as a therapeutically relevant and “druggable” target in both HR-proficient and deficient PDA cells, and lays the groundwork to optimize PARPi-based as well as other DNA targeted therapies in the treatment of PDA.
Citation Format: Saswati N. Chand, AnnJosette Ramirez, Aditi Jain, Avinoam Nevler, Cinthya Yabar-Lowder, Joseph A. Cozzitorto, Dominic I. James, Allan Jordan, Kate M. Smith, Ian Waddell, Charles J. Yeo, Jordan M. Winter, Jonathan R. Brody. Targeting PARG in pancreatic cancer: Implications for synthetic lethal therapeutic strategies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1973.