Review. Antioxidant gene therapeutic approaches to normal tissue radioprotection and tumor radiosensitization.

Administration of manganese superoxide dismutase-plasmid liposomes (MnSOD-PL) has been demonstrated to provide local radiation protection to the lung, esophagus, oral cavity, urinary bladder and intestine. Radiation protection has been shown to be mediated in part by MnSOD stabilization of the antioxidant pool including glutathione and total thiols within cells and in normal tissues. In experiments to determine whether organ-specific radioprotection would also protect orthotopic tumors, mice with Lewis lung carcinoma orthotopically placed at the carina or in other experiments with mice with cheek pouch placed SCCVII orthotopic squamous cell tumors demonstrated paradoxical and beneficial tumor radiosensitization following intratracheal or intraoral MnSOD-PL, respectively. The mechanism of MnSOD-PL tumor radiosensitization may involve a difference in redox balance between tumors and normal tissues. Differences in handling radiation-induced oxidative stress between tumors and normal tissues can provide a fundamental basis to design new cancer therapeutic agents which can exploit differences between normal tissue and tumor mechanisms of handling the oxidative stress of ionizing irradiation damage.