Optimization of an Allysine-Targeted PET Probe for Quantifying Fibrogenesis in a Mouse Model of Pulmonary Fibrosis

Purpose Idiopathic pulmonary fibrosis (IPF) is a destructive lung disease with a poor prognosis, an unpredictable clinical course, and inadequate therapies. There are currently no measures of disease activity to guide clinicians making treatment decisions. The aim of this study was to develop a PET probe to identify lung fibrogenesis using a pre-clinical model of pulmonary fibrosis, with potential for translation into clinical use to predict disease progression and inform treatment decisions. Methods Eight novel allysine-targeting chelators, PIF-1, PIF-2, …, PIF-8, with different aldehyde-reactive moieties were designed, synthesized, and radiolabeled with gallium-68 or copper-64. PET probe performance was assessed in C57BL/6J male mice 2 weeks after intratracheal bleomycin challenge and in naïve mice by dynamic PET/MR imaging and with biodistribution at 90 min post injection. Lung hydroxyproline and allysine were quantified ex vivo and histological staining for fibrosis and aldehyde was performed. Results In vivo screening of probes identified 68 GaPIF-3 and 68 GaPIF-7 as probes with high uptake in injured lung, high uptake in injured lung versus normal lung, and high uptake in injured lung versus adjacent liver and heart tissue. A crossover, intra-animal PET/MR imaging study of 68 GaPIF-3 and 68 GaPIF-7 confirmed 68 GaPIF-7 as the superior probe. Specificity for fibrogenesis was confirmed in a crossover, intra-animal PET/MR imaging study with 68 GaPIF-7 and a non-binding control compound, 68 GaPIF-Ctrl. Substituting copper-64 for gallium-68 did not affect lung uptake or specificity indicating that either isotope could be used. Conclusion A series of allysine-reactive PET probes with variations in the aldehyde-reactive moiety were evaluated in a pre-clinical model of lung fibrosis. The hydrazine-bearing probe, 68 GaPIF-7, exhibited the highest uptake in fibrogenic lung, low uptake in surrounding liver or heart tissue, and low lung uptake in healthy mice and should be considered for further clinical translation.