Background Electrochemotherapy (ECT) exhibits high therapeutic effectiveness in the clinic, achieving up to 80% local tumor control but without a systemic (abscopal) effect. It was proposed that ECT elicits in situ vaccination; therefore, we investigated its immunological effects. Moreover, we designed a combination therapy consisting of ECT via intratumoral application of bleomycin, oxaliplatin or cisplatin with peritumoral gene electrotransfer of a plasmid encoding interleukin-12 (p. t. IL-12 GET). Our hypothesis was that p. t. IL-12 GET potentiates the effect of ECT on local and systemic levels and the potentiation varies depending on tumor immune status.

Materials and Methods The combination therapy was tested in three immunologically different tumor models: B16F10 malignant melanoma, 4T1 mammary carcinoma and CT26 colon carcinoma (U34401-1/2015/7, U34401-3/2022/11). Growth of primary treated tumors and of distant untreated tumors, mimicking a systemic disease, was followed. After the therapy, cytological and histological analyses were performed to detect the types of cell death and immunologically important biomarkers as tumor immune infiltrate, the expression of MHC-1, PD-L1 and danger signals.

Results ECT induced immunogenic cell death, changes in the expression of cell markers such as MHC-1 and PD-L1 and other immunologically important danger signals. Moreover, it attracted effector immune cells intratumorally.In poorly immunogenic B16F10 melanoma, IL-12 potentiated the antitumor effect of ECT with biologically equivalent low doses of cisplatin, oxaliplatin or bleomycin. The most pronounced potentiation was observed after ECT using cisplatin, resulting in a complete response rate of 38% and an abscopal effect. Compared to B16F10 melanoma, better responsiveness to ECT was observed in more immunogenic 4T1 mammary carcinoma and CT26 colorectal carcinoma. In both models, p. t. IL-12 GET did not significantly improve the therapeutic outcome of ECT using any of the chemotherapeutic drugs.

Conclusions Electrochemotherapy induces immunologically important changes intratumorally and the effectiveness of the combination therapy depends on tumor immune status. ECT was more effective in more immunogenic tumors, but GET exhibited a greater contribution in less immunogenic tumors. Thus, the selection of the therapy, namely, either ECT alone or combination therapy with p. t. IL-12 GET, should be predominantly based on tumor immune status.

Disclosure Information K. Ursic Valentinuzzi: None. U. Kamensek: None. S. Kos: None. S. Kranjc Brezar: None. M. Cemazar: None. G. Sersa: None.