P18.20.B INHIBITION OF BCL-XL EMPOWERS THE ANTINEOPLASTIC ACTIVITY OF TTFIELDS IN MALIGNANT GLIOMA IN VITRO

Abstract BACKGROUND The anti-apoptotic Bcl-2 family proteins Bcl-2, Bcl-xL and Mcl-1 are important negative regulators of apoptosis and are frequently dysregulated in malignant glioma. In this study, we examined whether Bcl-xL inhibition enhances the biological activity of Tumor Treating Fields (TTFields) in different glioma models in vitro. MATERIAL AND METHODS We performed MTT-assays to examine effects of the concomitant treatment on the viability of different malignant glioma cell models. Tumor spheroids were used as a model to examine effects of the concomitant treatment in a 3-dimensional setting. Staining with annexin V/propidium iodide followed by flow cytometry was done to assess pro-apoptotic effects. Specific protein expression of caspases, and protein members of the Bcl-2 family were determined by Western blot analyses. RESULTS Bcl-xL inhibition had synergistic inhibitory effects on the cell viability of PC40 and PC128 malignant glioma primary cultures when concomitant with TTFields in 2- and 3-dimensional settings. In addition, flow cytometric analyses revealed that a simultaneous treatment with the Bcl-xL inhibitor A-1331852 and TTFields significantly increased the fraction of annexin V-positive (apoptotic) glioma cells. On the molecular level, these observations were associated with enhanced cleavage of effector caspase 3 and reduced expression of the anti-apoptotic Bcl-2 family proteins, Bcl-2 and Mcl-1. CONCLUSION These data suggest that Bcl-xL inhibition enhances the susceptibility of malignant glioma cells towards TTFields. This effect seems to be, at least in part, due to a caspase-dependent cell death mechanism. Further studies are warranted.