Abstract B11: YM155 synergizes with Plk1 inhibitor volasertib in killing of non-small cell lung cancer
Authors: Hong Mei , Ren Mingqiang , Cowell John , Hao Zhonglin 
Published: August 2014

Abstract
Non-small cell lung cancer (NSCLC) remains the leading cause of cancer death in the US. There is a pressing need for more regimens that are both effective and well tolerated by the typical metastatic lung cancer population. The synthetic lethal approach is emerging as an attractive strategy in mining such regimens. Volasertib is a highly selective and potent small molecule inhibitor of polo-like kinase 1(Plk1) that is essential for cell cycle progression. Its toxicity is mainly hematological and reversible. Single agent performance is less than optimal. In a search for novel agents that boost the anti-neoplastic effects of Plk1 inhibitor volasertib, we found that a YM155 and volasertib combination at the nano mole concentration potently inhibited growth of various NSCLC cell lines than either drug alone could achieve in vitro. Flow cytometry analysis showed that the combination regimen is much more potent in inducing apoptosis than either alone. Volasertib normally arrests cells at G2/M. Cells treated with both drugs initially arrest in G2 and then distribute equally in both G1/S and G2/M suggesting that fewer cells escaped to G1 from the polo arrest compared to volasertib treatment alone in which the majority of them were able to transit to G1 from G2 phase. Western blot analysis demonstrated that addition of YM155 to volasertib prevented accumulation of survivin and cyclin B, which is consistent with a cell cycle arrest before the end of mitosis and cytokinesis when compared to volasertib alone. All these data suggest that YM155 induces cell cycle arrest. However, YM155, originally developed as a survivin inhibitor, only caused modest suppression of survivin expression and could not explain the degree of cell growth inhibition it caused. Further studies demonstrated DNA damage of cancer cells when they are treated with YM155, which is evidenced by induction of phospho-γH2AX, a marker for DNA double strand break (DSB). In line with induction of a DNA damage response in cancer cells, known DNA damage response transducers ATM, ATR, CHK1, CHK2, p53 and BRCA1 are phosphorylated following drug treatment. Meanwhile, expression of CIP1, BAX and KARP1 are induced at the mRNA level as determined by quantitative real time PCR. Therefore, YM155 suppresses cancer cell growth by causing DNA damage and illicits a DNA damage response, cell cycle arrest and programmed cell death. Furthermore, cells with wild type p53 (H661) seem to be more sensitive to YM155 induced growth arrest than cells with p53 deletion (H1299, H520), which is consistent with a role of p53 in mediating programmed cell death after DNA damage. Polo-like kinase is known to play an essential role in recovery from checkpoint-triggered cell cycle arrest. We speculate that volasertib synergizes with YM155 by preventing recovery from DNA damage checkpoint triggered cell cycle arrest. On the other hand, since YM155 causes DNA damage, fewer cells arrested in G2 by volasertib may be able to escape from the polo arrest when their DNAs are severely damaged. Currently, we are testing the two hypotheses. Combination of the two inhibitors deserves close scrutiny in the clinical arena if future studies in an animal model support this prediction.
Citation Format: Mei Hong, Mingqiang Ren, John Cowell, Zhonglin Hao. YM155 synergizes with Plk1 inhibitor volasertib in killing of non-small cell lung cancer. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Synthetic Lethal Approaches to Cancer Vulnerabilities; May 17-20, 2013; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(5 Suppl):Abstract nr B11.