The Combination of ATM and Chk1 Inhibitors Induces Synthetic Lethality in Colorectal Cancer Cells

Simple Summary DNA damage response (DDR)-related proteins contribute to tumorigenesis, tumor progression, and drug resistance. Pharmacological inhibition of DDR-related proteins, such as checkpoint kinase 1 (Chk1), exerts antitumor effects but has serious side effects. Therefore, we attempted to specifically kill cancer cells with low concentrations of drugs by simultaneously inhibiting two DDR-related proteins. In this study, we demonstrated that the combined treatment with ataxia telangiectasia-mutated serine/threonine kinase (ATM) inhibitor (ATMi) and Chk1 inhibitor (Chk1i) exerts synergistic antitumor effects and induces cancer-specific synthetic lethality at low doses. The ATMi and Chk1i combination synergistically promotes CDK1 activation, resulting in the induction of apoptosis with enhanced cell cycle progression in cancer cells. Considering that the combined treatment exerts antitumor effects at one-tenth the concentrations described in previous reports, with high antitumor efficacy and few side effects it could be an effective treatment approach. Genetic abnormalities induce the DNA damage response (DDR), which enables DNA repair at cell cycle checkpoints. Although the DDR is thought to function in preventing the onset and progression of cancer, DDR-related proteins are also thought to contribute to tumorigenesis, tumor progression, and drug resistance by preventing irreparable genomic abnormalities from inducing cell death. In the present study, the combination of ataxia telangiectasia-mutated serine/threonine kinase (ATM) and checkpoint kinase 1 (Chk1) inhibition exhibited synergistic antitumor effects and induced synergistic lethality in colorectal cancer cells at a low dose. The ATM and Chk1 inhibitors synergistically promoted the activation of cyclin-dependent kinase 1 by decreasing the phosphorylation levels of T14 and Y15. Furthermore, the combined treatment increased the number of sub-G1-stage cells, phospho-histone H2A.X-positive cells, and TdT-mediated dUTP nick-end labeling-positive cells among colon cancer cells, suggesting that the therapy induces apoptosis. Finally, the combined treatment exhibited a robust antitumor activity in syngeneic tumor model mice. These findings should contribute to the development of new treatments for colorectal cancer that directly exploit the genomic instability of cancer cells.