Abstract 546: PARG and WEE1 inhibition in GI cancers: A novel synergistic, targeted therapeutic approach
Authors: Agostini Lebaron C. , Jain Aditi , Pishvaian Michael , Yeo Charles , Brody Jonathan 
Published: September 2020

Abstract
Colorectal carcinomas (CRC) and pancreatic ductal adenocarcinomas (PDAC) are the 2nd and 3rd leading causes of cancer-related deaths in the U.S. respectively. Recent work from our lab highlighted Poly ADP-Ribose Glycohydrolase (PARG) as a target in PDAC, and PARG inhibition (PARGi) sensitized PDAC cells to the DNA damaging agents, 5-FU and oxaliplatin. Other labs have shown that PARG inhibition causes replication stress in cancer cells. CRC and PDAC cells, while cellularly different, share a high frequency of KRAS and P53 inactivating mutations. High levels of replication stress (RS) has also been well documented in these tumor types. Targeting DNA Damage Response Kinases (DDRKs) in combination with chemotherapy (mostly DNA damage inducers like 5-FU, oxaliplatin, etc.) has gained recent interest in solid tumors but overlapping toxicities and tolerability have been issues clinically. Accordingly, we hypothesized that combined inhibition of PARG and DDRKs could yield synergistic effects in tumors with high replication stress while alleviating adverse events seen in other combination strategies. We determined in drug sensitivity assays that PARGi did not synergize with either the CHK1 inhibitor prexasertib or the ATR inhibitor VE-821 in our cell lines but did synergize with Wee1i (AZD1775) in both PDAC and CRC cell lines (Comb. Analysis, p&lt;.05). Additionally, we assessed long-term cell survival using colony formation assays, which yielded a dramatic reduction in cell survival for Panc1, MiaPaCa-2, CFPAC1 (PDAC cells), HCT116 (CRC cells) and SW480 (CRC cells) cell lines with the combination of PARGi and Wee1i as compared to single-agent controls (p&lt;.001). To validate the targeted synergy between AZD1775 and PARGi, we utilized genetic models of PARG and Wee1 silencing, including CRISPR PARG knockout cell lines, siRNA, and shRNA knockdown models. Genetic knockdown of Wee1 and PARG in PDAC and CRC cell lines yielded strikingly similar results in colony formation assays, with 2 and 12-fold changes respectively compared to Wee1i alone (P-value &lt; .0001). HCT116 CRISPR PARG knockouts yielded a 5-fold difference in Wee1i sensitivity in colony formation assays. Mechanistically, western blot analysis demonstrated PARGi and Wee1i led to &gt;2-fold increases in PARylation, yH2AX, and cleaved-caspase 3 as compared to single-agent controls. In conclusion, combined PARG - Wee1 inhibition is a novel targeting strategy in PDAC and CRC tumors that results in decreased cell viability. We are currently testing the efficacy of AZD1775 in PARG CRISPR-knockout tumors in in vivo xenograft models. Ongoing studies are focused on assessing the effect of Wee1i + PARGi on chromosomal aberrations, disruption in replication fork dynamics via DNA fiber labeling, and cell cycle dependencies.
Citation Format: Lebaron C. Agostini, Aditi Jain, Michael Pishvaian, Charles Yeo, Jonathan Brody. PARG and WEE1 inhibition in GI cancers: A novel synergistic, targeted therapeutic approach [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 546.