The role of radical derivatives of high reactivity in the radiosensitizing action of Photofrin II

The in vitro and in vivo efficacy of Photofrin II as a selective radiosensitizing agent has recently been demonstrated, the mechanism of action still being unknown. We aimed to investigate the mechanism of action of Photofrin II and the role played by radiolytically generated cytotoxic species. RT4 human bladder carcinoma cell lines were seeded and incubated with Photofrin II and mannitol, with Photofrin II alone, and with no added agent. The cells were treated with ionizing radiation (6 Gy). The numbers of colonies in the respective groups were compared. In addition, the influence of mannitol and histidine on cell survival after Photofrin-incubation and irradiation (0-1-2-4-6 Gy) was evaluated by a tetrazolium-based colorimetric assay in an ELISA reader. RT4 cells, which were exposed to 6 Gy radiation after incubation with Photofrin II, showed a significantly lower survival rate compared to cells exposed to the same radiation dose in the absence of Photofrin II (p < 0.05 in a Student t-test). The addition of mannitol as a scavenger for OH radicals exerted a protective effect especially against the radiosensitizing action of Photofrin. Increased cell survival was observed when mannitol and histidine were present in addition to Photofrin II. The mechanism of radio sensitization by Photofrin II appears to involve the formation of radical derivatives of high reactivity, such as OH radicals.