Abstract 1860: Tumor-treating fields (TTFields) effects on glioblastoma cells are augmented by mitotic checkpoint inhibition
Authors: Kessler Almuth F. , Frömbling Greta E. , Gross Franziska , Hahn Mirja , Dzokou Wilfrid , Ernestus Ralf-Ingo , Löhr Mario , Hagemann Carsten 
Published: August 2018

Abstract
Introduction. Tumor Treating Fields (TTFields) supplementing the standard therapy led to a significant increase in progression free and overall survival of glioblastoma (GBM) patients. These alternating electric fields with low intensity (1-3 V/cm) and intermediate frequency (100-300 kHz) disrupt cell division through inhibition of spindle-fiber formation. The spindle assembly checkpoint (SAC) diminishes therapeutic effects of spindle damaging agents by monitoring correct sister chromatid attachment to functional spindle microtubules and ensures their equal segregation. Inhibition of its key regulator, Monopolar spindle 1 (MPS1), combined with anti-mitotic drugs, led to a synergistic effect on GBM growth in mice. Therefore, we hypothesized that MPS1 inhibition may increase the efficacy of TTFields.
Methods. TTFields were applied using the inovitro system. U87 and GaMG human GBM cells (30,000) were treated with TTFields (200 kHz, 1.7 V/cm) and/or with 4 µM of the MPS1 inhibitor MPS1-IN-3 (IN-3). Cell numbers were evaluated after 24 h, 48 h and 72 h of treatment and also at 24 h and 72 h after end of treatment (eot). Alterations of the cell cycle were detected by FACS analysis, aberrant mitotic figures and malformed nuclei by immunofluorescence microscopy and apoptosis by Annexin V staining and TUNEL-assay after 72 h of treatment.
Results. In all experiments U87 and GaMG yielded similar results. The combination of TTFields and IN-3 caused a more pronounced effect on cell proliferation (78.6% decrease of U87 cell number vs. TTFields (P = 0.0337), 52.6% vs. IN-3 (P = 0.0205) after 72 h). The number of viable cells was reduced (62% less GaMG cells than seeded) and the ratio of dead/alive cells increased. Significantly less cells re-entered the G1 phase (P &lt; 0.0001), while the number of cells in G2/M- and sub-G1-phase was increased. Very distinct mitotic figures and aberrant nuclei were visible, leading to apoptosis in 44% of the U87 cells (TTFields 14%, P = 0.0002; IN-3 4%, P &lt; 0.0001). After discontinuation of TTFields treatment alone, it took 24 h for the cells to start recovery. In contrast, the number of cells treated with IN-3 and TTFields, further decreased by 92% at 72 h after eot (P = 0.0288).
Conclusions. TTFields are an approved new treatment modality for GBM. A combination of physically damaging the spindle apparatus by TTFields and chemical inhibition of the SAC led to earlier and prolonged effects, which significantly augment TTFields efficacy and even may bridge TTFields treatment interruption, promising a new targeted multimodal treatment Option.
Citation Format: Almuth F. Kessler, Greta E. Frömbling, Franziska Gross, Mirja Hahn, Wilfrid Dzokou, Ralf-Ingo Ernestus, Mario Löhr, Carsten Hagemann. Tumor-treating fields (TTFields) effects on glioblastoma cells are augmented by mitotic checkpoint inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1860.