Effects of combination metformin and nintedanib therapy in a mouse model of irradiation-induced pulmonary fibrosis

Radiation-induced lung injury, a well-recognized complication of thoracic radiotherapy, can progress to pulmonary fibrosis, sharing pathological features with idiopathic pulmonary fibrosis. Although nintedanib is widely used as an antifibrotic agent, the therapeutic benefit of combination strategies remains partially understood. Therefore, in this study, we aimed to evaluate the effects of combined metformin and nintedanib treatment in a mouse model of irradiation-induced lung fibrosis. Female C57BL/6 mice were subjected to localized irradiation of 90 Gy in the left lung and treated daily with metformin, nintedanib or both drugs for 6 weeks. Lung fibrosis was assessed using gross examination, hydroxyproline quantification, histopathological scoring and quantitative image analysis. Changes in fibrosis- and injury-related proteins were examined by western blotting, including transforming growth factor-β (TGF-β), a profibrotic cytokine; SMAD family member 7 (SMAD7), an inhibitory regulator of TGF-β signaling; and surfactant protein D (SP-D), a lung epithelial injury-associated protein. Nintedanib-containing treatments improved histological lung injury in irradiated mice, including reduced collagen deposition, low pathological scores and preserved air space structure. Hydroxyproline content was also lower in the combination treatment group than in the other groups. The nintedanib-containing treatments suppressed irradiation-induced upregulation of transforming growth factor-β expression, and SMAD7 expression was selectively upregulated following combined treatment. In addition, elevated SP-D level following irradiation was partially reduced following nintedanib administration. These findings suggest that combined metformin and nintedanib treatment may provide a broader protective profile against radiation-induced pulmonary fibrosis, particularly in association with SMAD7 upregulation, although further studies are required to clarify whether the combination provides significant advantages over nintedanib monotherapy.