Targeting ATR and Chk1 kinases for cancer treatment: a new model for new (and old) drugs.

Trying to kill cancer cells by generating DNA damage is by no means a new idea. Radiotherapy and genotoxic drugs are routinely used in cancer therapy. More recent developments also explored the potential of targeting the DNA damage response (DDR) in order to increase the toxicity of radio- and chemo- therapy. Chk1 inhibitors have pioneered studies in this regard. Interestingly, early studies noted that Chk1 inhibitors were particularly toxic for p53-deficient cells. The model proposed for this observation was that this effect was due to the simultaneous abrogation of the G2 (Chk1) and G1 (p53) checkpoints. We here challenge this view, and propose a model where the toxicity of Chk1 inhibitors is rather due to the fact that these compounds generate high loads of replicative stress (RS) during S-phase, which are further boosted by the less restrictive S-phase entry found in p53-deficient cells. This new model implies that the particular toxicity of Chk1 inhibitors might not be restricted to p53-deficient cells, but could be extended to other mutations that promote a promiscuous S-phase entry. In addition, this rationale also implies that the same effect should also be observed for other molecules that target the RS-response (RSR), such as inhibitors of the Chk1-activating kinase ATR.