Dual Checkpoint Abrogation Is Synergistic in KRAS-Mutant Cancer

In response to genotoxic stress, the DNA damage response, which is governed primarily by the ATM–CHK2, ATR–CHK1, and p38–MK2 (also known as MAPKAPK2) checkpoint effector pathways, becomes activated in order to slow cell-cycle progression and allow time for DNA repair. The CHK1 and MK2 pathways converge on inhibition of cell division cycle 25 (CDC25)–mediated activation of cyclin-dependent kinases, prompting Dietlein and colleagues to hypothesize that simultaneous small-molecule inhibition of CHK1 and MK2 may synergistically silence the DNA damage checkpoint. To systematically characterize combinatorial drug–inhibitor relationships, 96 cancer cell lines were screened with various concentrations of the CHK1 inhibitor PF477736 and the MK2 inhibitor PF3644022, and PreCISE (predictor of chemical inhibitor synergistic effects) software was used to calculate synergism scores based on GI₅₀ drug curves. Synergistic effects between PF477736 and PF3644022 were observed in 33 of 96 cell lines and were correlated with mutations in the KRAS or BRAF oncogenes or deletion of the tumor suppressor gene CDKN2A. Combined CHK1/MK2 inhibition prevented CDC25B phosphorylation, promoted apoptosis, and reduced long-term survival specifically in KRAS-, BRAF- or CDKN2A-altered cell lines, but had little effect in nonsynergistic lines or as single agents. Mechanistically, KRAS-driven cells were characterized by tonic activation of CHK1 and MK2, indicative of oncogene-induced genotoxic stress, and cotreatment with PF477736 and PF3644022 enhanced DNA damage and triggered mitotic catastrophe in synergistic cell lines. Combined CHK1/MK2 inhibition suppressed KRAS-mutant tumor growth in vivo and extended overall survival in mice. Furthermore, human KRAS-mutant tumors exhibited chronic activation of CHK1 and MK2, and patient-derived KRAS- or BRAF-mutant cancer cells displayed exquisite sensitivity to combined CHK1/MK2 inhibition ex vivo. Together, these results suggest that KRAS- and BRAF-mediated oncogenic stress confers dependency on the CHK1/MK2 checkpoint and provide a rationale for combinatorial inhibition of CHK1 and MK2 as a therapeutic strategy in KRAS- or BRAF-driven tumors.

Dietlein F, Kalb B, Jokic M, Noll EM, Strong A, Tharun L, et al. A synergistic interaction between Chk1 and MK2 inhibitors in KRAS-mutant cancer. Cell 2015;162:146–59.

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