Experimental Investigation of Lung Toxicity after Radiation Therapy Combined with Immune Checkpoint Inhibitors

Abstract

The combination of radiation therapy (RT) with immune checkpoint inhibitors (ICIs) has resulted in prolonged survival in patients with locally advanced lung cancer (NSCLC). However, RT and ICIs carry the risk of inflammatory and irreversible lung damage and might have synergistic effects on these adverse events. Importantly, only little is known about the risk of enhanced side effects of the lung and which factors modulate such toxicity. In addition, there is a need for sensitive diagnostics for the early detection of lung injury after combined radioimmunotherapy. Recently, experimental x-ray dark-field radiography was able to detect radiation-induced lung injury earlier than conventional radiography in mice. The right thorax of mice was irradiated with unfractionated RT (1x15 Gy or 1x30 Gy) or fractionated RT (5x9 Gy). In addition, indicated experimental groups received ICIs (a-PD1, a-CTLA4 or combination) for one month. After four months, the mice were analyzed using different methods to quantify lung damage: lung coefficient as surrogate marker for lung fibrosis, histopathological staining of fibrosis, conventional x-ray, and dark-field radiography. We found enhanced signs of lung fibrosis (lung coefficient and histopathological score) after unfractionated RT in combination with ICIs. Surprisingly, combination of ICIs with RT resulted in opposite effects. Specifically, concomitant combination of ICIs with fractionated RT resulted in reduced lung coefficients and lower histopathological signs of lung fibrosis in mice treated in this way. Importantly, in vivo x-ray dark-field radiographs showed the same trend as the ex vivo assessment of the lungs. To the best of our knowledge, this is the first experimental study to find that the combination of RT with ICIs has an impact on the risk of pulmonary fibrosis. Strikingly, ICIs (a-PD1 + a-CTLA4) combined with unfractionated RT resulted in increased lung damage, while concomitant use of ICIs (a-PD1 + a-CTLA4) with fractionated RT resulted in reduced lung toxicity. In the field of radioimmunotherapy, such studies are of great value for the interpretation of clinical studies and of importance for the design of further clinical trials.