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

This was a pre-clinical study designed to investigate the potential of combining hyperthermia with checkpoint inhibitors.

C3H mammary carcinomas, grown in the right rear foot of CDF1 mice, were treated when tumors reached 200 mm³. 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).

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.

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.