The checkpoint kinase 1 (Chk1) is a key component of the DNA damage response, a molecular network deputed to maintain genome integrity. Nevertheless, cancer cells aberrantly exploit these circuits to overcome chemotherapy-induced cytotoxicity. Chk1 inhibitors have been developed as a chemopotentiating strategy and different molecular mechanisms underlying the synergism with chemotherapeutics have been uncovered. The monotherapy with Chk1 inhibitors seems to be endowed with antitumor activity against cancer cells characterized by specific defects in the DNA damage machinery or characterized by elevated levels of oncogene-induced replication stress. In this biological framework Chk1 neutralization represents a synthetic lethality-based therapeutic approach. Moreover, a dual targeting of the DNA damage machinery has been proposed envisioning the association of Chk1 abrogation with poly-ADP ribose polymerase inhibitors. The spectrum of antitumor properties of Chk1 antagonists is completed by the activity against cancer stem cells, the prominent tumorigenic population that is equipped to survive stressful conditions through multiple and interconnected mechanisms. Although the clinical development of the first generation of Chk1 antagonists was hindered by off-target effects and an unfavorable pharmacokinetic profile, a new wave of early clinical trials with more selective compounds are currently being carried out. To this end, the identification of predictive biomarkers and an in-depth characterization of molecular circuits governed by Chk1 are issues that need to be addressed for sharpening the therapeutic potential of Chk1 inhibitors.
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