Targeting Chk1 protein kinase is a valid anti cancer strategy to enhance the cytotoxic effects of radio and chemotherapy. Recent evidence disclosed a role of Chk1 in unperturbed cell proliferation and survival, implying that Chk1 inhibitors could also be effective as single agents in tumors with a specific genetic background. The effect of Chk1 inhibition on a wide panel of cancer cell lines has been herein conducted by using a specific and selective Chk1 inhibitor PF-00477736. The results showed that 2 cancer cell lines out of 16 analyzed (the ovarian cancer cell line OVCA-432 and the breast cancer cell line SK-BR3) were very sensitive to PF-00477736 treatment. To identify genes in synthetic lethality with Chk1, which may justify the sensitivity to Chk1 inhibitor of some cancer cell lines, a high-throughput screening using a siRNA library directed against 719 human protein kinases, in combination with either scramble or Chk1 siRNA, was undertaken. The ovarian cancer cell line OVCAR-5, resistant to Chk1 inhibition, was used to perform the screening. After 2 independent screenings and the validation procedure using specific esiRNAs, and the three siRNAs of each target (used in the screening as a pool), the Wee1 tyrosine kinase was identified as the most significant gene in synthetic lethality with Chk1. Treatment with both PF-00477736 and a Wee1 inhibitor (MK-1775) confirmed the remarkable synergistic effect in various human cancer cell lines (breast, ovarian, colon, prostate). Same results have been obtained with AZD-7762 Chk1 inhibitor. Isogenic cellular systems differing only for the status of p53 (the colon carcinoma cancer cell lines HCT-116 p53 +/+ and p53 -/- and the ovarian cancer cell lines A2780 and A2780/E6) showed a comparable and strong synergistic effect of the combined treatment. Interestingly, in the normal fetal human lung cell line MRC-5 the effect of the combined treatment was additive. Time lapse experiments and an evaluation of the ability of cells to form colonies, suggested that cells treated with the two inhibitors almost completely lost the ability to proliferate and failed to perform a proper cell division. A detailed molecular analysis showed that the combined treatment causes cells to undergo premature mitosis before the end of DNA replication and with damaged DNA, leading to a massive cell death by apoptosis. Taken together these preclinical data address a new promising combined targeted anticancer strategy that deserves clinical investigation.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4699. doi:1538-7445.AM2012-4699