Vascular-targeted
photodynamic therapy (VTP) takes advantage of intravascular excitation of a
photosensitizer (PS) to produce cytotoxic
reactive oxygen species (ROS). These ROS are potent mediators of vascular damage inducing rapid local
thrombus formation, vascular occlusion, and tissue
hypoxia. This light-controlled process is used for the eradication of solid
tumors with Pd-
bacteriochlorophyll derivatives (Bchl) as PS. Unlike classical
photodynamic therapy (
PDT),
cancer cells are not the primary target for VTP but instead are destroyed by treatment-induced
oxygen deprivation. VTP initiates acute local
inflammation inside the illuminated area accompanied by massive
tumor tissue death. Consequently, in the present study, we addressed the possibility of immune response induction by the treatment that may be considered as an integral part of the mechanism of VTP-mediated
tumor eradication. The effect of VTP on the host immune system was investigated using
WST11, which is now in phase II clinical trials for
age-related macular degeneration and intended to be evaluated for
cancer therapy. We found that a functional immune system is essential for successful VTP. Long-lasting systemic antitumor immunity was induced by VTP involving both cellular and humoral components. The antitumor effect was cross-protective against mismatched
tumors, suggesting VTP-mediated production of overlapping
tumor antigens, possibly from endothelial origin. Based on our findings we suggest that local VTP might be utilized in combination with other anticancer
therapies (e.g.,
immunotherapy) for the enhancement of host antitumor immunity in the treatment of both local and disseminated disease.