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 regulates a key cell regulatory node for maintaining genomic integrity by preventing replication fork collapse. ATR inhibition has been shown to increase replication stress resulting in DNA double strand breaks (DSBs) and cancer cell death, and several 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 synergistically 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. Further, 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 to single-agent M4344, suggesting a novel and potentially broad combination approach to cancer therapy.