Selective Inhibition of ATM-dependent Double-strand Break Repair and Checkpoint Control Synergistically Enhances the Efficacy of ATR Inhibitors

Ataxia telangiectasia and Rad3-related protein (ATR) kinase regulate a key cell regulatory node for maintaining genomic has been shown to increase replication stress resulting in DNA inhibitors are under clinical investigation for cancer therapy. However, activation of cell-cycle checkpoints controlled by ataxia telangiectasia-mutated (ATM) kinase could minimize the lethal consequences of ATR inhibition and protect cancer cells. Here, we investigate ATR-ATM functional relationship and potential therapeutic implications. In cancer cells with functional ATM and p53 signaling, selective suppression of ATR catalytic activity by M6620 induced G1-phase arrest to prevent S-phase entry with unrepaired DSBs. The selective ATM inhibitors, M3541 and M4076, suppressed both ATM-dependent cell-cycle checkpoints, and DSB repair lowered the p53 protective barrier and extended the life of ATR inhibitor-induced DSBs. Combination treatment amplified the fraction of cells with structural chromosomal defects and enhanced cancer cell death. ATM inhibitor syner-gistically potentiated the ATR inhibitor efficacy in cancer cells in vitro and increased ATR inhibitor efficacy in vivo at doses that did not show overt toxicities. Furthermore, a combination study in 26 patient-derived xenograft models of triple-negative breast cancer with the newer generation ATR inhibitor M4344 and ATM inhibitor M4076 demonstrated substantial improvement in efficacy and survival compared with single-agent M4344, sug-gesting a novel and potentially broad combination approach to cancer therapy.