Introduction Tumor Treating Fields (TTFields) are a clinically applied anti-neoplastic treatment modality delivered via noninvasive application of low-intensity, intermediate-frequency (100-500 kHz), alternating electric fields. TTFields had been shown to inhibit the growth of different cancerous cells, both in vitro and in vivo, by interrupting cancer cell mitosis. TTFields were recently reported to alter cellular membrane structure of glioblastoma cells, thus rendering them more permeable. In the current study, we characterized TTFields-induced cellular permeability in several cancerous cell lines including breast adenocarcinoma, uterine sarcoma and glioma.

Methods Flow cytometry analysis of intracellular 7-aminoactinomycin D (7-AAD; fluorescent DNA marker of cell viability) accumulation was used to quantify the percentages of cells showing increased cellular permeability following TTFields application. The optimal TTFields frequency for membrane permeability was evaluated using a frequency scan of TTFields in the range of 100-500 kHz. Kinetics were determined using different 7-AAD exposure times. Changes in intracellular accumulation of anthracycline chemotherapeutics (doxorubicin and mitoxantron) were evaluated in chemotherapy-sensitive and chemotherapy-resistant cells.

Results TTFields induced cellular permeability to 7-AAD, which is typically excluded by intact cells and was detected in all cancer cell lines tested with the highest permeability frequency of up to 80% of treated cells. The optimal frequency for TTFields induced cellular permeability was found to be different from the optimal cytotoxic frequency. Moreover, measurements of kinetics revealed that TTFields-induced permeability is transient and is effective only during application of TTFields. In combination experiments, TTFields improved intracellular accumulation of chemotherapeutic agents, which may explain the enhanced efficacy of combination treatment of TTFields with different chemotherapies. Furthermore, the combined treatment also facilitated the accumulation of chemotherapeutics in chemotherapy-resistant cells, and restored it to the level observed in the chemotherapy-sensitive cancer cells.

Conclusions This study demonstrates that TTFields can transiently increase cancer cell permeability in vitro with an optimal frequency that is variable from the frequency that is used to induce cancer cell cytotoxicity. Moreover, this effect is reversible and cellular permeability is restored to normal state function upon TTFields treatment cessation. Future studies will include in vivo experiments to validate TTFields-induced permeability and to evaluate the efficacy of TTFields and chemotherapy combinations in tumor models that are both sensitive and resistant to chemotherapy.

Citation Format: Tali Voloshin, Yaara Porat, Noa Kaynan, Anat Klein-Goldberg, Rom Paz, Alexandra Volodin, Moshe Giladi, Uri Weinberg, Yoram Palti. Tumor treating fields (TTFields) induce cancer cell permeability in vitro [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5848.