Abstract 2814: Potent and selective small molecule inhibitors of polo-like kinase 1: Biological characterization

Polo-like kinase 1 (Plk1) is one of the emerging new generation of anti-mitotic targets. Plks are a family of serine-threonine kinases involved in the regulation of mitosis and maintenance of DNA integrity. Plk1 is required for almost every step of mitosis: promotes mitotic entry, contributes to centrosome maturation, regulates kinetochore assembly, contributes to spindle assemble checkpoint and initiates mitotic exit. Besides promoting proliferation, Plk1 overexpression contributes to oncogenesis by promoting chromosome instability and aneuploidy. Overexpression of Plk1 is associated with tumor development and many cancers have elevated Plk1 levels compared to surrounding normal tissues. Numerous studies have shown that Plk1 expression levels correlate with disease progression, invasiveness and poor patient prognosis. Furthermore, pre-clinical studies have demonstrated that cancer cell proliferation is blocked in vitro and in vivo via inhibition of Plk1 using siRNA or small molecule inhibitors. Collectively, these observations support the selection of Plk1 as an attractive target for cell cycle-directed cancer therapy and several small molecule inhibitors are currently being investigated in clinical trials. Using a de novo ligand design approach we have identified pyrimidodiazepinone small molecules, which are ATP-competitive, highly selective and potent Plk1 inhibitors. These molecules have good drug-like properties, oral bioavailability and efficacy in mouse xenograft models. To characterize the cell sensitivity toward the Plk1 inhibitors we have used washout and outgrowth assays in a panel of oesophageal, lung and colon cancer cell lines. The anti-proliferative activity of Plk1 inhibitors was correlated with the Plk1 expression level, p53 and Ras mutational status. Generally, we found that cell lines harbouring p53 mutations are more sensitive towards these inhibitors. Time-lapse microscopy and flow cytometry studies of RKO cell line (WT p53) treated with Plk1 inhibitors demonstrated that a proportion of cells arrested just before the G2-/-M transition and were able to resume normal proliferation upon compound removal. These results suggest that Plk1 inhibitors should have better therapeutic window when used for treatment of tumors with mutant p53, p53 mutational status could be used as a patient stratification marker, and tumor types with high frequency of p53 mutations, such as oesophageal cancer, could be suitable therapeutic indications for Plk1 inhibitors. 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 2814. doi:1538-7445.AM2012-2814