Caninized PD-1 monoclonal antibody in oral malignant melanoma: efficacy and exploratory biomarker analysis.

BACKGROUND: Canine oral malignant melanoma (OMM) is a highly aggressive tumor, with several available treatment options, though few achieve durable response or complete remission. Because of its biological similarity to human mucosal melanoma, canine OMM represents a valuable spontaneous model for translational immunotherapy studies. Anti-programmed cell death protein 1 (PD-1) antibody therapy has shown promise in canine OMM; however, predictive biomarkers for treatment response and survival have not been identified. METHODS: We conducted a multicenter, prospective, investigator-initiated clinical trial to evaluate the safety and efficacy of a caninized anti-canine PD-1 monoclonal antibody (ca-4F12-E6) in 150 dogs with advanced OMM. The dogs were administered 3 mg/kg ca-4F12-E6 intravenously every 2 weeks. Treatment response was assessed using cRECIST V.1.0. Biomarker analyses included peripheral blood parameters, cytokine/chemokines, peripheral lymphocyte subsets, microsatellite instability (MSI), immunohistochemistry for immune cell and mismatch repair protein markers, and RNA sequencing of the tumor tissue. Associations with clinical outcomes were determined by logistic regression and Cox proportional hazards models. RESULTS: The overall response rate was 14.7%, with a best overall response rate of 16.7%. Treatment-related adverse events occurred in 40.0% of the dogs, which were primarily grade 1-3. Increased baseline white blood cell, neutrophil count, and C reactive protein levels were significantly associated with poor response, shorter progression-free survival, and reduced overall survival (OS). MSI-high tumors were associated with significantly prolonged OS compared with MSI-low/microsatellite stable tumors. Transcriptome analysis revealed differentially expressed genes and enriched immune-related pathways in responders, though limited by sample size. CONCLUSION: ca-4F12-E6 exhibited durable antitumor activity with manageable safety profile in dogs with OMM. Baseline systemic inflammatory markers and MSI status may serve as predictive biomarkers for clinical outcomes. The results support the use of canine OMM as a comparative model for human immuno-oncology and biomarker discovery.