Introduction Tumor treating fields (TTFields), a clinically applied anti-neoplastic treatment modality, are low intensity (eg, 1-3 V/cm) alternating electric fields within the intermediate frequency range (100-300 kHz). In this study, we evaluated whether TTFields can elicit an immune response against tumors and the potential of TTFields and anti-programmed cell death protein-1 (PD-1) combination therapy to serve as a viable treatment regimen.

Methods For evaluation of immunogenic cell death (ICD), cultured murine cells were treated with TTFields using the inovitroTM system. ICD was characterized by exposure of calreticulin (CRT) on the cell surface, secretion of ATP, and release of HMGB1. For detection of ER stress, phosphorylation of the translation initiation factor eIF2α was assessed. TTFields effect on autophagy was evaluated using electron microscopy and immunoblot and immunofluorescence evaluation of LC3. T-cells migration assays were performed using a modified Matrigel coated Boyden chamber. For in-vivo studies, mice were implanted with either orthotropic lung cancer or subcutaneous colon cancer and treated with TTFields, anti-PD-1, or a combination of the 2 modalities. Tumor volume was monitored and flow cytometry analysis was performed for phenotypic characterization of infiltrating T cells.

Results TTFields treatment promoted release of HMGB1 and ATP, and ER stress leading to CRT cell surface translocation. In T-cell migration assays, no significant difference was observed in the migration rates between untreated and TTFields-treated T cells. In both tumor In vivo models, TTFields plus anti-PD-1 combined treatment of tumor-bearing mice led to a significant decrease in tumor volume compared to anti-PD-1 alone or to the control group. In the lung cancer model, TTFields or anti-PD-1 alone had no effect on CD8+ and CD4+ cellular abundance, while TTFields plus anti-PD-1 combination showed a trend toward increased cell numbers. No significant changes in the levels of CD4+Foxp3+ regulatory T cells were found between the different treatment groups. The combined treatment of TTFields and anti-PD-1 led to a significant increase in IFN- γ production in cytotoxic CD8+ tumor infiltrating lymphocytes. Comparable results were obtained in the colon cancer model, where significant increases in CD8+ and CD4+ were observed following long duration treatment with TTFields plus anti-PD-1.

Conclusions Our results demonstrate the potential of TTFields therapy to induce immunogenic cell death resulting in improved efficacy of anti-PD-1 therapy in mouse cancer models. The combination of TTFields with immune checkpoint inhibitors is currently also being tested in a phase 3 clinical trial (LUNAR - NCT02973789).

Citation Format: Noa Kaynan, Tali Voloshin, Shiri Davidi, Yaara Porat, Anna Shteingauz, Mijal Munster, Rosa Schneiderman, Moshe Giladi, Uri Weinberg, Yoram Palti. Tumor treating fields (TTFields) elicit an anti-tumor immune response and in combination enhance anti-PD-1 treatment efficacy [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 956.