Late treatment with imatinib mesylate ameliorates radiation-induced lung fibrosis in a mouse model

Abstract

Background: We have previously shown that small molecule PDGF receptor tyrosine kinase inhibitors (RTKI) can drastically attenuate radiation-induced pulmonary fibrosis if the drug administration starts at the time of radiation during acute inflammation with present but limited effects against acute inflammation. To rule out interactions of the drug with acute inflammation, we investigated here in an interventive trial if a later drug administration start at a time when the acute inflammation has subsided--has also beneficial antifibrotic effects.

Methods: Whole thoraces of C57BL/6 mice were irradiated with 20 Gy and treated with the RTKI imatinib starting either 3 days after radiation (during acute inflammation) or two weeks after radiation (after the acute inflammation has subsided as demonstrated by leucocyte count). Lungs were monitored and analyzed by clinical, histological and in vivo non-invasive computed tomography as a quantitative measure for lung density and lung fibrosis.

Results: Irradiation induced severe lung fibrosis resulting in markedly reduced mouse survival vs. non-irradiated controls. Both early start of imatinib treatment during inflammation and late imatinib start markedly attenuated the development of pulmonary fibrosis as demonstrated by clinical, histological and qualitative and quantitative computed tomography results such as reduced lung density. Both administration schedules resulted in prolonged lifespans. The earlier drug treatment start resulted in slightly stronger beneficial antifibrotic effects along all measured endpoints than the later start.

Conclusions: Our findings show that imatinib, even when administered after the acute inflammation has subsided, attenuates radiation-induced lung fibrosis in mice. Our data also indicate that the fibrotic fate is not only determined by the early inflammatory events but rather a complex process in which secondary events at later time points are important. Because of the clinical availability of imatinib or similar compounds, a meaningful attenuation of radiation-induced lung fibrosis in patients seems possible.

Figure 1

(a) Kaplan-Meier analysis of mouse survival following thoracic irradiation and imatinib treatment. Death was considered complete (cause-specific due to radiation) in all cases except those of planned euthanasia for histological assessment, which were considered as censored. Radiation (20 Gy) reduced survival (P < 0.001 vs. the control as reported in (24)). Imatinib treatment increased mouse survival if administration started as late as 2 weeks after radiation (P < 0.02 vs. radiation) and if started early within 3 days after radiation (P < 0.01 as reported in (24)). The earlier start of drug treatment tended to be more effective in prolonging survival than later start of drug treatment, but this difference was not significant (P > 0.1). (b) Bodyweight follow-up after thoracic irradiation and imatinib treatment. Five mice were randomly selected in each group and weighed every two weeks. Mean ± SE was presented. * P < 0.01 vs. the RT only group; # P < 0.01 vs. the control group.

Figure 2

(a) High resolution computed tomography (CT) as a non-invasive tool for qualitative and quantitative longitudinal monitoring of pulmonary fibrosis progression in mice. Representative CT scans showing progression of pulmonary fibrosis in mice after 20 Gy whole thorax irradiation (RT), and treatment with imatinib treatment starting 3 days and 2 weeks after radiation (RT). Fibrosis is characterized by diffuse bilateral areas of "ground-glass" attenuation and intralobular reticular opacities. (b) Quantitative lung density values derived from CT scans. The same 5 randomly chosen mice in each treatment group were examined in a longitudinal study by CT every 2 weeks. 8 regions of interest (ROI) were randomly selected in the lungs and the lung density (in Hounsfield Units (HU)) was determined for each ROI. Mean ± SE are presented. * P < 0.01 vs. the RT only group; # P < 0.01 vs. the control group.

Figure 3

(a) Photomicrographs of H&E stained mouse lung tissue sections from a) control mice, b) irradiated mice (20 Gy, RT) and mice treated with imatinib starting 3 days (c) or 2 weeks (d) after thoracic irradiation. Leukocytes infiltration was marked with asterisk. (b) Photomicrographs of H&E stained mouse lung tissue sections at 16 weeks from a) control mice, b) irradiated mice (20 Gy, RT) and mice treated with imatinib starting 3 days (c) or 2 weeks (d) after thoracic irradiation. Fibroblasts were marked with arrow. (c) Photomicrographs of H&E stained lung tissue sections at 20 weeks from a) control mice, b) irradiated mice (20 Gy, RT) and mice treated with imatinib starting 3 days (c) or 2 weeks (d) after thoracic irradiation. Collagen depositions were marked with #.

Figure 4

(a) Quantitative analysis of leukocyte numbers as inflammation parameter. Bars are mean ± SE. * p < 0.05 vs. controls. §p < 0.05 vs. radiation only for both early and late imatinib schedules. (b) Quantitative analysis of septal thickness as fibrotic parameter presenting deposition of extracellular collagen. Bars are mean ± SE. * p < 0.05 vs. controls. §p < 0.05 vs. radiation only, for both early and late imatinib schedules.