Overcoming cell size escape from tumor treating fields using a varying frequency treatment paradigm in vitro.

Abstract

e22134 Background: TTFields Therapy at 200 kHz received FDA approval for the treatment of patients with recurrent glioblastoma (GBM) based on the results of a phase III clinical trial. Radiological responses were observed in 14% of the treated patients. The lack of radiological response in the remaining patients suggests that GBM cells may escape the effect of TTFields over time. The goals of the present study were to identify possible routes by which cancer cells can escape the antimitotic effect of TTFields and to explore ways to overcome such escape mechanisms. Methods: Measurements of cell size before and after 72 hours TTFields application revealed a significant volume increase in 10 cancer cell lines (17% - 101%). A2780 ovarian cancer cultures (cell volume 2.0+0.2 pL) exhibit maximal inhibition by TTFields at 200 kHz. Twenty four hour application of 200 kHz TTFields led to a 70+29% increase in average cell volume (p<0.05). FACS analysis showed that TTFields application to A2780 cells led to a 44% increase in the G2 population due to the antimitotic effect of TTFields. However, the increase in the G2 proportion is not sufficient to explain the increase in cell volume seen after TTFields application. Furthermore, after an additional 24 hours of treatment, the fraction of cells in the G2 population returned to its original percentage (30%), while the increase in cell volume remained high (67+26%). Results: Previous publications demonstrated an inverse relationship exists between cell size and optimal TTFields frequency. Thus an increase in cell size may allow cells to avoid the maximal TTFields effect. To attempt to overcome such cell size escape we decreased the frequency of the TTFields from 200 to 150 kHz after 24 hours treatment to coincide with the increase in cell volume. We found that varying the frequency from 200 to 150 kHz increased the inhibitory effect of TTFields compared to continuous treatment at 200Khz (37+10% and 57+16% decrease in cell count, respectively; p=0.043). Conclusions: This is the first evidence of an escape mechanism from the antimitotic effect of TTFields. The results presented provide a simple way to restore treatment efficacy by varying TTFields frequency in parallel to the change in cell volume.