An indolocarbazole inhibitor of human checkpoint kinase (Chk1) abrogates cell cycle arrest caused by DNA damage

Many cancer therapies cause DNA damage to effectively kill proliferating tu mor cells; however, a major limitation of current therapies is the emergenc e of resistant tumors following initial treatment. Cell cycle checkpoints a re involved in the response to DNA damage and specifically prevent cell cyc le progression to allow DNA repair. Tumor cells can take advantage of the G (2) checkpoint to arrest following DNA damage and avoid immediate cell deat h, This can contribute to acquisition of drug resistance. By abrogating the G(2) checkpoint arrest, it may be possible to synergistically augment tumo r cell death induced by DNA damage and circumvent resistance. This requires an understanding of the molecules involved in regulating the checkpoints. Human Chk1 is a recently identified homologue of the Schizosaccharomyces po mbe checkpoint kinase gene, which is required for G(2) arrest in response t o DNA damage, Chk1 phosphorylates the dual specificity phosphatase cdc25C o n Ser-216, and this may be involved in preventing cdc25 from activating cdc 2/cyclinB and initiating mitosis. To further study the role of Chk1 in G(2) checkpoint control, we identified a potent and selective indolocarbazole i nhibitor (SB-218078) of Chk1 kinase activity and used this compound to asse ss cell cycle checkpoint responses. Limited DNA damage induced by gamma-irr adiation or the topoisomerase I inhibitor topotecan was used to induce G(2) arrest in HeLa cells. In the presence of the Chk1 inhibitor, the cells did not arrest following gamma-irradiation or treatment with topotecan, but co ntinued into mitosis, Abrogation of the damage-arrest checkpoint also enhan ced the cytotoxicity of topoisomerase I inhibitors. These studies suggest t hat Chk1 activity is required for G(2) arrest following DNA damage.