Abstract 1056: Volasertib in combination with radiotherapy: The perfect match in non-small cell lung cancer
Authors: Bossche Jolien Van den , Pauw Ines De , Lambrechts Hilde , Merlin Céline , Deben Christophe , Deschoolmeester Vanessa , Specenier Pol , Pauwels Patrick , Peeters Marc , Lardon Filip , Wouters An 
Published: July 2017

Abstract
Background: Polo-like kinase 1 (Plk1), a master regulator of mitosis and the DNA damage response, is overexpressed in several human malignancies, making it a promising therapeutic target. Monotherapy of Plk1 inhibition has shown only a moderate effect in clinical trials, indicating the need to combine with other therapies. Remarkably, Plk1 inhibition arrests cancer cells in mitosis, which is the most radiosensitive cell cycle phase. Hence, we are the first to investigate the effect of volasertib, the lead agent in category of Plk1 inhibitors at the moment, on the radiosensitivity of a panel of non-small cell lung cancer (NSCLC) cell lines with a different p53 background under both normal and reduced oxygen conditions.
Material and methods:
Three isogenic NSCLC cell lines (i.e. A549 (p53 wt); A549-NTC (non template control, p53 wt); and A549-920 (p53 deficient)) and one TP53 mutant cell line (NCI-H1975, R273H mutation) were included. Cell survival after volasertib monotherapy (0-85 nM, 24h) was assessed using the sulforhodamine B assay. The effect of volasertib (0-20 nM, 24h) on cell cycle distribution was determined flow cytometrically using the Vindelov method. The clonogenic assay (24h 0-10 nM volasertib followed by 0-8 Gy irradiation) was performed to evaluate the radiosensitizing effect of volasertib. Cells were incubated under normoxia or hypoxia (1% O2) during the treatment period. Data analysis was done using WinNonlin and FlowJo software.
Results: Plk1 inhibition by volasertib established a dose-dependent growth inhibition in all cell lines under both normoxia and hypoxia. Cell survival was significantly influenced by the p53 status, with a reduced sensitivity to volasertib in p53 deficient/mutant cells compared to p53 wild type cells (p&lt;0.001). Except for the TP53 mutant NCI-1975 cell line, IC50-values were significantly higher in hypoxic cells compared to their normoxic counterparts (p&lt;0.001). Treatment with increasing concentrations of volasertib induced a strong G2/M phase block (p&lt;0.001), which was most pronounced in p53 deficient A549-920 cells, accompanied by a significant decrease in the number of G1 and S phase cells (p&lt;0.001). Under hypoxia, a mitotic arrest was only detected when high volasertib concentrations were used. Intriguingly, the radiosensitizing effect of the Plk1 inhibitor was more pronounced in p53 wild type cells than in p53 deficient cells. For example, the dose enhance factor (DEF) for volasertib treatment ranged from 1.60 to 2.13 in A549-NTC cells and from 1.18 and 1.22 in A549-920 cells, respectively. Importantly, radiosensitivity was retained when cells were treated and irradiated under hypoxia.
Conclusion: Our in vitro data confirm the therapeutic potential of volasertib in NSCLC cells. Pretreatment with the Plk1 inhibitor enhanced radiosensitivity, especially in cells with functional p53, suggesting a potential role for p53 in radiosensitization by volasertib.
Citation Format: Jolien Van den Bossche, Ines De Pauw, Hilde Lambrechts, Céline Merlin, Christophe Deben, Vanessa Deschoolmeester, Pol Specenier, Patrick Pauwels, Marc Peeters, Filip Lardon, An Wouters. Volasertib in combination with radiotherapy: The perfect match in non-small cell lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1056. doi:10.1158/1538-7445.AM2017-1056