Administration of TGF-ß Inhibitor Mitigates Radiation-induced Fibrosis in a Mouse Model

BACKGROUND: Radiation-induced fibrosis is a long-term adverse effect of external beam radiation therapy for cancer treatment that can cause pain, loss of function, and decreased quality of life. Transforming growth factor beta (TGF-β) is believed to be critical to the development of radiation-induced fibrosis, and TGF-β inhibition decreases the development of fibrosis. However, no treatment exists to prevent radiation-induced fibrosis. Therefore, we aimed to mitigate the development of radiation-induced fibrosis in a mouse model by inhibiting TGF-β. QUESTION/PURPOSES: Does TGF-β inhibition decrease the development of muscle fibrosis induced by external beam radiation in a mouse model? METHODS: Twenty-eight 12-week-old male C57BL/6 mice were assigned randomly to three groups: irradiated mice treated with TGF-βi, irradiated mice treated with placebo, and control mice that received neither irradiation nor treatment. The irradiated mice received one 50-Gy fraction of radiation to the right hindlimb before treatment initiation. Mice treated with TGF-c (n = 10) received daily intraperitoneal injections of a small-molecule inhibitor of TGF-β (1 mg/kg) in a dimethyl sulfoxide vehicle for 8 weeks (seven survived to histologic analysis). Mice treated with placebo (n = 10) received daily intraperitoneal injections of only a dimethyl sulfoxide vehicle for 8 weeks (10 survived to histologic analysis). Control mice (n = 8) received neither radiation nor TGF-β treatment. Control mice were euthanized at 3 months because they were not expected to exhibit any changes related to treatment. Mice in the two treatment groups were euthanized 9 months after radiation, and the quadriceps of each thigh was sampled. Masson's trichome stain was used to assess muscle fibrosis. Slides were viewed at 10 × magnification using bright-field microscopy, and in a blinded fashion, five representative images per mouse were used to quantify fibrosis. The mean ± SD fibrosis pixel densities in the TGF-βi and radiation-only groups were compared using Mann-Whitney U tests. The ratio of fibrosis to muscle was calculated using the mean fibrosis per slide in the TGF-βi group to standardize measurements. Alpha was set at 0.05. RESULTS: The mean (± SD) percentage of fibrosis per slide was greater in the radiation-only group (1.2% ± 0.42%) than in the TGF-βi group (0.14% ± 0.09%) (odds ratio 0.12 [95% CI 0.07 to 0.20]; p < 0.001). Among control mice, mean fibrosis was 0.05% ± 0.02% per slide. Mice in the radiation-only group had 9.1 times the density of fibrosis as did mice in the TGF-βi group. CONCLUSION: Our study provides preliminary evidence that the fibrosis associated with radiation therapy to a quadriceps muscle can be reduced by treatment with a TGF-β inhibitor in a mouse model. CLINICAL RELEVANCE: If these observations are substantiated by further investigation into the role of TGF-β inhibition on the development of radiation-induced fibrosis in larger animal models and humans, our results may aid in the development of novel therapies to mitigate this complication of radiation treatment.