Selective radioprotection of normal tissues by bowman-birk proteinase inhibitor (BBI) in mice

Background and purpose: the efficacy of radiotherapy is limited by the response of normal tissues within the radiation field. The application of normal-tissue-specific radioprotectors may improve the therapeutic benefit of radiotherapy. The purpose of the present study was to explore the in vivo normal-tissue radioprotective potential of Bowman-Birk proteinase inhibitor (BBI), which acts as a normal-cell-specific radioprotector in vitro. Material and methods: the leg contracture assay in mice, a model system assessing radiation-induced fibrotic processes, was used. To determine whether BBI acts also as a radioprotector of tumors (i.e., FSA and FSAII), the tumor growth delay assay was used. Results: radiation induced leg contracture in mice with a maximum of about 8 mm at day 150 after irradiation. Treatment of mice with 100 mg/kg BBI before irradiation reduced leg contracture by > 4 mm, by about 50% (p < 0.05, t-test). Doses < 100 mg/kg were ineffective, and doses > 100 mg/kg did not further increase the degree of radioprotection. By contrast, BBI did not induce radioprotection of either TP53-mutated FSA or TP53-normal FSAII tumor xenografts in mice, which argues for normal-tissue-specific effect. Conclusion: BBI acts as a potent selective normal-tissue radioprotector in vitro and in vivo, apparently without protecting tumors, and thus has the potential to improve clinical radiotherapy. (orig.)