The protein kinase C inhibitor Gö6976 is a potent inhibitor of DNA damage-induced S and G2 cell cycle checkpoints.

In response to DNA damage, cells arrest progression through the cell cycle at either G(1), S, or G(2). We have reported that UCN-01 (7-hydroxystaurosporine) abrogates DNA damage-induced S and G(2) arrest and enhances cytotoxicity selectively in p53 mutant cells, thus providing a potential, tumor-targeted therapy. Unfortunately, UCN-01 binds avidly to human plasma proteins, limiting bioavailability. Because UCN-01 also inhibits protein kinase C (PKC), we screened other PKC inhibitors, expecting them to be unable to abrogate arrest. However, Gö6976 potently abrogated S and G(2) arrest and enhanced the cytotoxicity of the topoisomerase I inhibitor SN38 only in p53-defective cells. Importantly, Gö6976 was nearly as potent at abrogating S and G(2) arrest in human serum, a property not possessed by UCN-01. Cell viability studies demonstrated that Gö6976 was impressively nontoxic as a single agent. Analysis of proteins that regulate cell cycle arrest suggested that both drugs inhibit the checkpoint kinases Chk1 and/or Chk2. Additionally, Gö6976 abrogated S and G(2) arrest at a concentration substantially lower than that required to inhibit PKC; UCN-01 did not demonstrate this selectivity for checkpoint inhibition. These properties make Gö6976 a promising candidate for preclinical and clinical studies.