Usefulness of 18F-FPP-RGD2 PET in pathophysiological evaluation of lung fibrosis using a bleomycin-induced rat model.

PURPOSE: Integrins αv are key molecules in the pathogenesis of fibrosis in multiple organs. To assess the potential utility of integrin αvß3 imaging for idiopathic pulmonary fibrosis (IPF), we evaluated an 18F-FPP-RGD2 PET probe in a rat model of bleomycin-induced lung fibrosis. METHODS: Pulmonary fibrosis was induced by single intratracheal instillation of bleomycin (3 mg/rat). Positron emission tomography (PET)/computerized tomography scans were performed 4 weeks after bleomycin administration using 18F-FPP-RGD2. Total distribution volume (VT) was estimated using one-tissue/two-compartment, two-tissue/three-compartment models, and Logan graphical analysis (Logan plot; t* = 30 min). Plasma-free fractions were estimated from images of the left ventricle. Correlation between Logan VT and lung pathology was assessed by Spearman's rank correlation. RESULTS: Histopathological evaluation demonstrated the development of fibrosis in IPF-model group. Integrin αv protein expression and lung radioactivity were higher in IPF-model group compared with control group. The lung radioactivity of 18F-FPP-RGD2 rapidly reached the peak after administration and then gradually decreased, whereas left ventricular radioactivity rapidly disappeared. Logan graphical analysis was found to be suitable for 18F-FPP-RGD2 kinetic analysis in the IPF-model lung. Logan VT values for 18F-FPP-RGD2 were significantly higher in IPF rats compared with control rats and strongly correlated with lung fibrosis, pathology, integrin αv protein expression, and oxygen partial pressure. CONCLUSION: Our findings demonstrate that the integrin αvß3 PET probe 18F-FPP-RGD2 can detect pathophysiological changes in lungs, including fibrosis accompanying upregulated integrin αv of IPF-model rats. These findings support the utility of 18F-FPP-RGD2 PET imaging for the pathophysiological evaluation of pulmonary fibrosis.