Intraoperative Photodynamic Therapy Combined with Hyperthermic Intraperitoneal Chemotherapy: Synergistic Efficacy in a Novel C57BL/6 Mouse Model of Colorectal Cancer-Derived Peritoneal Metastases

Peritoneal metastases from colorectal cancer (CRC) origin present a significant therapeutic challenge due to their aggressive progression associated with extremely poor prognosis and limited treatment options. Cytoreductive surgery (CRS), photodynamic therapy (PDT) and especially the aggressive hyperthermic intraperitoneal chemotherapy (HIPEC) are emerging locoregional therapies with potential synergistic effects with limited efficacy in eliminating microscopic residual disease. However, the efficacy of combining these therapeutic modalities remains underexplored, and robust preclinical models are still lacking. This study investigates the synergistic efficacy of intraoperative PDT combined with HIPEC in a novel C57BL/6 murine model of CRC-derived peritoneal metastases. A syngeneic CRC peritoneal metastasis murine model was established in C57BL/6 mice using MC38 + luciferase cells (cell line). Mice were injected intraperitoneally and were randomized into five groups: (1) control (no treatment), (2) control two (hematoporphyrin photo synthesizer- only, no treatment efficacy), (3) PDT-only (intraperitoneal hematoporphyrin photo synthesizer followed by 630 nm laser irradiation, 5 mins), (4) HIPEC-only (intraoperative mitomycin C, 41°C, 12 minutes), and (5) HIPEC+PDT (dual combination therapy). Tumor burden was assessed via bioluminescence imaging and histopathology. Survival analysis was performed using Kaplan-Meier curves. Molecular mechanisms were evaluated through, HE staining, immunohistochemistry (IHC) for apoptosis (TUNEL assay), proliferation (Ki-67), and cytokine profiling. The HIPEC+PDT group demonstrated a 70% reduction in tumor burden compared to controls (p < 0.01), significantly outperforming monotherapy groups (HIPEC-only: 40% reduction, PDT-only: 35% reduction; p < 0.05). Median survival in the HIPEC+PDT group was 60 days, compared to 30 days in controls (p < 0.001). IHC analysis revealed increased apoptosis and decreased proliferation in the HIPEC+PDT group, alongside upregulation of pro-inflammatory cytokines (e.g., IL-6, TNF-α) in the tumor microenvironment. No significant toxicity was observed in the combined treatment group. Additionally, PDT suppressed peritoneal fibrosis and adhesions, potentially mitigating long-term treatment complications. This study provides compelling preclinical evidence that intraoperative PDT combined with HIPEC exhibits potent synergistic efficacy in colorectal peritoneal metastasis (CRPM). The dual-modality approach enhances tumor cell eradication, prolongs survival, and mitigates adverse peritoneal remodeling. These findings warrant further translational research and clinical investigation into HIPEC-PDT as a next-generation therapeutic strategy for peritoneal metastases in colorectal cancer.