Beneficial Effects of Combining an Immune Checkpoint Inhibitor With Proton Radiation, OXi4503, or Hyperthermia in a Murine Solid Tumor Model

<h3>Background/Aim:</h3> This pre-clinical study investigated the combination of various treatments with a checkpoint blocker in an immunologically unresponsive tumor. <h3>Materials and Methods:</h3> Experiments were performed using C3H mammary carcinoma mice, where tumors of various sizes (44-467 mm³) were treated with either proton radiation (up to 20 Gy), OXi4503 (4×50 mg/kg), or hyperthermia (42.5°C for 60 min), with or without anti-CTLA-4 (4×10 mg/kg). The endpoint was the time to reach 1,000 mm³. Mechanistic studies focused on CD4/CD8 expression, assessed in histological sections up to 10 days after irradiation. <h3>Results:</h3> The median time for untreated tumors to reach 1,000 mm³ was unaffected by anti-CTLA-4 alone. Proton radiation, OXi4503, or hyperthermia alone increased the tumor growth time at all tumor sizes when compared to untreated tumors. Combining proton radiation or OXi4503 with anti-CTLA-4 resulted in additional tumor inhibition, which was more pronounced in smaller tumors (44-131 mm³). Anti-CTLA-4 had no additional effect on tumor response to hyperthermia regardless of tumor size at treatment. Expression of CD4/CD8 levels decreased 1 day after irradiation before recovering to pre-treatment levels on day 5. <h3>Conclusion:</h3> Overall, combining anti-CTLA-4 with either proton radiation or OXi4503, but not with hyperthermia, enhanced growth inhibition of C3H mammary carcinoma compared to that seen with each treatment alone, suggesting potential clinical relevance.