Photodynamic therapy for melanoma: efficacy and immunologic effects

Malignant melanoma is one of the fastest growing cancers and if it cannot be completely surgically removed the prognosis is bleak. Melanomas are known to be particularly resistant to both chemotherapy and radiotherapy. Various types of immunotherapy have however been investigated with mixed reports of success. Photodynamic therapy (PDT) has also been tested against melanoma, again with mixed effects as the melanin pigment is thought to act as both an optical shield and as an antioxidant. We have been investigating PDT against malignant melanoma in mouse models. We have compared B16F10 melanoma syngenic to C57BL/6 mice and S91 Cloudman melanoma syngenic to DBA2 mice. We have tested the hypothesis that S91 will respond better than B16 because of higher expression of immunocritical molecules such as MHC-1, tyrosinase, tyrosinase related protein-2 gp100, and intercellular adhesion molecule-1. Some of these molecules can act as tumor rejection antigens that can be recognized by antigen-specific cytotoxic CD8 T cells that have been stimulated by PDT. Moreover it is possible that DBA2 mice are intrinsically better able to mount an anti-tumor immune response than C57BL/6 mice. We are also studying intratumoral injection of photosensitzers such as benzoporphyrin monoacid ring A and comparing this route with the more usual route of intravenous administration.