A potent and selective Polo-like kinase 1 (Plk1) Inhibitor (GSK461364) induces cell cycle arrest and growth inhibition of cancer cell

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Polo-like kinase 1 (Plk1) is a conserved ser/thr kinase that plays a critical role during multiple stages of cell cycle progression. In humans, Plk1 is expressed primarily during late G2 and M phases, where it appears to regulate many processes involved in mitotic entry and progression. Plk1 overexpression is strongly associated with cancer and has been correlated with poor prognosis in a broad range of human tumor types. Several studies have demonstrated the essential role of Plk1 in mitotic events, and many protein substrates have been identified. For example, overexpression of Plk1 by transfection results in multinucleation and can also override the G2 arrest checkpoint induced by DNA damage. It has also been shown to have specific roles in centrosome maturation, microtubule dynamics, chromatid separation and cytokinesis. In addition to its mitotic regulation, Plk1 has been reported to phosphorylate the p53 tumor-suppressor causing inhibition of pro-apoptotic p53 function. Thus, Plk1 is a prime target for drug discovery in proliferative diseases such as cancer. We identified a potent and selective thiophene amide Plk1 inhibitor, GSK461364. This compound was tested for anti-proliferative activity against >120 cancer cell lines and potently inhibited the proliferation of greater than 83% of them with IC50s lower than 50nM and 91% with IC50 lower than 100nM. GSK461364 had no measurable effect on human non proliferating normal cells. Detailed studies of the effects of this compound on mitosis and the cell cycle demonstrated that the cell cycle arrest in mitosis is the mechanism of action. Interestingly, the precise nature of the mitotic arrest was concentration-dependent. Concentrations of >300 nM caused a G2 delay followed by entry into mitosis and a prometaphase arrest. Concentrations between 300 and 10 nM caused mitotic arrest with effects ranging from severely perturbed mitotic spindles (250nM) to relatively normal spindles with misaligned chromosomes (10nM). At concentrations less than 10 nM, the effects on mitosis were modest but there was evidence that cytokinesis was sometimes inhibited. Mitotic arrest due to prolonged Plk1 inhibition leads to cell death by mitotic catastrophe, or aberrant mitotic exit frequently characterized by severe micronucleation. Thus the mechanism of action of GSK461364 is in agreement with the reported roles of Plk1 in late G2 and M phases of the cell cycle and suggests that different thresholds of kinase activity may be required for distinct mitotic processes. The favorable properties of this compound make it an ideal candidate for further development for the treatment of cancer.