Combining vascular and cellular targeting regimens enhances the efficacy of photodynamic therapy

Photodynamic therapy (PDT) can be designed to target either tumor vasculature or tumor cells by varying the drug-light interval. Photodynamic therapy treatments with different drug-light intervals can be combined to increase tumor response by targeting both tumor vasculature and tumor cells. The sequence of photosensitizer and light delivery can influence the effect of combined treatments.

The R3327-MatLyLu rat prostate tumor model was used in this study. Photosensitizer verteporfin distribution was quantified by fluorescence microscopy. Tumor blood flow changes were monitored by laser-Doppler system and tumor hypoxia was quantified by the immunohistochemical staining for the hypoxic marker EF5. The therapeutic effects of PDT treatments were evaluated by the histologic examination and tumor regrowth assay.

Fluorescence microscopic studies indicated that tumor localization of verteporfin changed from predominantly within the tumor vasculature at 15 min after injection, to being throughout the tumor parenchyma at 3 h after injection. Light treatment (50 J/cm(2)) at 15 min after verteporfin injection (0.25 mg/kg, i.v.) induced significant tumor vascular damage, as manifested by tumor blood flow reduction and increase in the tumor hypoxic fraction. In contrast, the vascular effect observed after the same light dose (50 J/cm(2)) delivered 3 h after administration of verteporfin (1 mg/kg, i.v.) was an initial acute decrease in blood flow, followed by recovery to the level of control. The EF5 staining revealed no significant increase in hypoxic fraction at 1 h after PDT using 3 h drug-light interval. The combination of 3-h interval PDT and 15-min interval PDT was more effective in inhibiting tumor growth than each individual PDT treatment. However, it was found that the combined treatment with the sequence of 3-h interval PDT before 15-min interval PDT led to a superior antitumor effect than the other combinative PDT treatments. Histologic studies confirmed that this combined treatment led to damage to both tumor vasculature and tumor cells. Importantly, the combined PDT treatment did not increase normal tissue damage and tissue recovered well at 60 days after treatment.

Our results suggest that targeting both tumor vascular and cellular compartments by combining a long-interval PDT with a short-interval PDT can be an effective and safe way to enhance PDT damage to tumor tissue.