Radiation-induced lung injury (RILI) is a significant dose limiting complication of thoracic radiation for lung, breast, and esophageal cancer. Strategies for increasing the therapeutic index of radiation involve the use of radiosensitizing agents. We investigated the potential of M867 to sensitize non-small cell lung cancer (NSCLC) to radiation in vivo, while assessing its protective effects in normal lung parenchyma. H460-Luc2 cells were implanted into the mediastinum of athymic nude mice, which were separated into four treatment groups: control, M867, radiation therapy (RT) or combination. H460-Luc2 cell cultures were treated in parallel. Tumor growth was followed using bioluminescence imaging. Immunohistochemistry staining was used to detect phospho-Smad2/3 and cleaved caspase-3 expression. Western blot was done for the detection of cleaved caspase-3 and phospho-Smad2/3. TUNEL assays were used to measure apoptosis. M867+RT group had significantly increased tumor growth inhibition relative to either treatment alone (p=0.02). M867+RT was associated with increased levels of apoptosis (p<0.01), but combination treatment was associated with a decrease in caspase-dependent apoptosis relative to RT alone (p<0.01). We found that this increase in apoptosis in the M867+RT group was due to caspase-independent cell death. Based on early biomarker analyses of phospho-Smad 2/3 and cleaved caspase-3, M867+RT had a radio-protective effect on normal lung parenchyma. M867 may increase the therapeutic ratio of RT by enhancing the radiosensitivity of NSCLC while mitigating RILI. Further research is warranted to examine the late effects of lung injury and to study differences in the mechanism of action of M867 on lung cancer and normal tissue.
Keywords: H460-Luc2; M867; NSCLC; caspase; orthotopic mouse model; radiation.