PD-1 Blockade Combined with Heated Intraperitoneal Chemotherapy Improves Outcome in Experimental Peritoneal Metastases from Colonic Origin in a Murine Model

Background: Heated intraperitoneal chemotherapy (HIPEC) was shown to induce immunogenicity of peritoneal metastases from colorectal cancer (PM-CRC) by induction of immunogenic cell death. We aimed to explore whether the addition of a checkpoint inhibitor would augment the effect of HIPEC in an experimental murine model of PM-CRC. Methods: PM-CRC was established in C57BL mice by intraperitoneal inoculation of MC38 colon cancer cells. HIPEC was administered using the closed technique with mitomycin C (MMC). Clinical and immunological parameters were compared between animals treated with HIPEC alone and those treated with HIPEC + anti-programmed death receptor-1 (aPD-1). Results: MMC-based HIPEC increased the overall survival of animals compared with sham-treated animals (22.8; 95% confidence interval [CI] 21.14–24.53 vs. 18.9 days; 95% CI 17.6–20.3, p < 0.001). The extent of peritoneal disease as measured by the modified peritoneal carcinomatosis index was also reduced by HIPEC. This clinical benefit was accompanied by increased infiltration of CD8⁺, CD68⁺, and CD20⁺ cells into tumor metastases in HIPEC-treated animals compared with sham-treated animals. We identified heat shock protein (HSP) 90 as a potential immunogenic cell death protein whose expression is increased under HIPEC conditions (fold change: 2.37 ± 1.5 vs. 1 without HIPEC, p < 0.05). Combined HIPEC + PD-1 treatment ameliorated survival compared with HIPEC alone and sham treatment (24.66; 95% CI 20.13–29.2 vs. 19; 95% CI 15.85–22.14 and 14.33 days; 95% CI 9.6–19.04, respectively; p = 0.008). This clinical effect was accompanied by increased CD8⁺ tumor infiltration. Conclusions: HIPEC induced the expression of immunogenic cell death signals that can support an anti-tumor immune response. This response can be further exploited by a checkpoint inhibitor.