Photodynamic immune stimulation eradicates primary pigmented melanoma and induces long-lasting anti-tumor immunity

Abstract Background Melanoma is highly aggressive with limited options in advanced disease. Photodynamic therapy (PDT) has shown poor efficacy in pigmented melanoma due to restricted light penetration and intrinsic ROS resistance. A novel approach is investigated here. Methods Full-thickness (4 mm) S91 pigmented melanoma grown intradermally in syngeneic immunocompetent (DBA) mice was treated using dual photosensitizers targeting tumor cells and microvasculature, together with a topical optical clearing agent. Anti-tumor immune effects were investigated in multiple functional assays, and the mechanisms-of-action were explored. Results Primary tumor was eradicated in all mice, with 95% disease-free survival at 90 d, falling to 20% upon CD8 + T-cell depletion pre-treatment. The median survival in nude mice was 21 d. Treated DBA mice also rejected systemic tumor re-challenge at 30 d post treatment. There was delayed growth of untreated contralateral tumors and of tumors co-injected into naïve mice together with splenocytes from treated mice. Biomarker analysis showed rapid tumor infiltration by CD8+ and CD3+ T cells, reduced suppressive myeloid populations, and widespread immune activation. Gene expression profiling revealed enrichment of immune-related pathways and long-lasting memory. Conclusions Photodynamic immune stimulation treatment can destroy primary pigmented melanoma far beyond the depth of light penetration and has potent anti-tumor systemic efficacy.