Abstract 2986: Partial inhibition of Plk1 is cytotoxic despite normal spindle structure

Abstract Polo-like kinase 1 (Plk1) is a multifunctional mitotic kinase that has emerged as an attractive target for cancer chemotherapy. Frequently over-expressed in tumors, Plk1 is absent in the surrounding non-proliferating tissue. Furthermore, Plk1 depletion in tumor cell lines produces monopolar spindles and mitotic arrest that ultimately leads to cell death. However, it is unclear whether monopolar spindles mediate cytotoxicity at physiologic drug concentrations. We used a chemical genetic system to test the effect of partial inhibition of Plk1 in human cells. We discovered that partial inhibition of Plk1 produces a cytotoxic response in the absence of a prolonged mitotic delay. We hypothesized that partial inhibition disrupts mitosis via alternate mechanisms from the archetypal monopolar spindle-induced prometaphase arrest. We performed time-lapse microscopy experiments after exposure to low (0-1 μM) and high (10 μM) concentrations of the ATP analog, 3-MBPP1. As expected, 90% (53/60) of untreated cells successfully completed mitosis, whereas all cells (28/28) treated with 10 μM exhibited a monopolar-spindle induced prometaphase arrest. Surprisingly, 29-40% of cells treated with 3-MBPP1 exhibited a transient prometaphase delay, followed by impaired anaphase segregation and frequent cytokinesis failure. Indirect immunofluorescence demonstrates aberrations in chromosome and microtubule dynamics, which we are further characterizing. Partial inhibition of Plk1 leads to cytotoxicity by a spindle-independent mechanism. These results are important for developing Plk1-targeted therapies. First, they demonstrate that continuous dosing of an oral Plk1 inhibitor is likely to act by a different mechanism from intermittent high doses of an intravenous drug. Second, partial inhibition may provide a more physiologically relevant background to study function of kinase inhibitor activity in the treatment of cancer. Finally, identification of the responsible substrate may reveal the most sensitive biomarker for kinase inhibition in patient samples. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2986. doi:10.1158/1538-7445.AM2011-2986