An exploratory pre-clinical study of the combination of hyperthermia and checkpoint inhibitors

BACKGROUND AND PURPOSE: This was a pre-clinical study designed to investigate the potential of combining hyperthermia with checkpoint inhibitors. MATERIAL AND METHODS: . Hyperthermia (39.5-42.5 °C for 30-120 min) was applied following immersion of the tumor bearing leg into a circulating water bath. The checkpoint inhibitors were anti-CTLA-4/PD-1/PD-L1 antibodies injected intraperitoneally (4 x 10 mg/kg). Tumor growth was monitored daily and the treatment endpoint was TGT5 (time for tumors to regrow to 5 times treatment volume). RESULTS: None of the antibodies alone affected TGT5 when compared to controls. However, while neither the anti-PD-1 nor anti-PD-L1 antibodies influenced heat response, an enhanced effect was found with the anti-CTLA-4 antibody. This was most apparent when tumors were heated once at 42.5 °C for 1-hour on day 0 and the antibody administered on days 1, 4, 8 and 11 or 1, 2, 3 and 4 after heating. However, the responses were heterogenous with some animals treated with heat and anti-CTLA-4 showing an increase in TGT5 compared to heat alone, while others showed no change. Mechanistic studies of tumor blood perfusion and infiltrating lymphocytes in tissue sections showed a substantial decrease reaching a nadir 1-3 days after heating, with recovery by 7-10 days. CONCLUSIONS: Under specific conditions, there appeared to be an enhanced anti-tumor effect with the hyperthermia-checkpoint inhibitor combination, but these effects were often sporadic. Mechanisms for the interaction between heat and the anti-CTLA-4 antibody remain unclear.