Combined Inhibition of the Ataxia‐telangiectasia mutated and Rad3‐related Pathway and the G9a Methyltransferase Synergize to Reduce Pancreatic Cancer Cell Growth

Abstract

Pancreatic ductal adenocarcinoma (PDAC) is the third leading cause of cancer‐related deaths in the United States with a devastating overall five‐year survival rate of only 10%. Consequently, there is an urgent need to identify innovative therapeutic strategies. PDAC exhibits chronic replication stress (RS), becoming reliant on a proficient Ataxia‐telangiectasia mutated and Rad3‐related (ATR) pathway to adapt to high RS levels in cells. This also presents an exploitable therapeutic vulnerability in tumors via strategies that enhance the endogenous RS to trigger replication catastrophe and cell death. Our laboratory is focused on utilizing epigenetic inhibitors in this context as novel therapeutic approaches. Recently, we have shown that combined inhibition of the major ATR effector, CHK1, and the G9a methyltransferase leads to replication catastrophe, providing an effective therapeutic approach for PDAC by priming replication failure through the epigenome rather than genomic damage. In the current study, we sought to investigate additional targets in the RS response pathway through direct ATR inhibition combined with G9a inhibition and perform mechanistic studies on DNA damage response checkpoints and cell cycle. We treated multiple PDAC cell lines with a combination of the ATR inhibitor, AZD6738, and either BRD4770 or UNC0642 to inhibit G9a at various concentrations to determine the effect on cell viability and cytotoxicity by live‐cell analysis. We find that the AZD6738‐BRD4770 and AZD6738‐UNC0642 combinations display synergistic effects to disrupt the RS response and inhibit PDAC growth. Overall, our work provides quantitative insights into a novel therapeutic opportunity for PDAC and expands the universality of a synergistic interaction resulting from simultaneous targeting of the ATR‐CHK1 RS pathway and the epigenomic regulator G9a.