68Ga-NODAGA-indole, an allysine-reactive PET probe for molecular imaging of actively progressive pulmonary fibrogenesis (2018)

260 Objectives: Tissue fibrosis is the accumulation of extracellular matrix molecules that make up scar tissue as a consequence of chronic tissue injury and accounts for nearly half of the deaths in the industrialized world.1 There is an unmet need to develop effective methods for noninvasive detection of actively progressive fibrogenesis. Noninvasive methods that can distinguish active fibrogenesis from stable scar tissue provide indispensable insights for monitoring disease activity or therapeutic responses.Allysine-rich collagen, which promotes its crosslinking is a marker of active fibrogenesis.2,3 Here, we report a novel 68Ga-NODAGA based PET probe that targets allysine residues in active pulmonary fibrogenesis. 
Methods: The allysine-reactive PET probe 68Ga-NODAGA-indole was synthesized by conjugation of a macrocyclic chelator (NODAGA) to a functionalized indole that rapidly forms hydrolytically stable condensation products with aldehydes at physiological pH, followed by labeling with 68Ga. The non-reactive analogue 68Ga-NODAGA-carboline was also synthesized, incapable of undergoing condensation reactions with aldehydes. Both probes were evaluated in the bleomycin mouse (BM) model of pulmonary fibrosis and in healthy animals. In vivo studies include biodistribution and small animal PET-CT imaging after intravenous injection of the active probe 14 days after bleomycin whereas the inactive probe was injected 13 days after bleomycin. Additionally, characterization of collagen/ allysine content and histology was performed. Pharmacokinetics of the probes were evaluated in healthy mice. 
Results: The active probe, 68Ga-NODAGA-indole, and the control probe, 68Ga-NODAGA-carboline, showed similar low, nonspecific uptake in the lungs of control mice. In the well-established bleomycin mouse model of pulmonary fibrosis, active probe 68Ga-NODAGA-indole showed 3-fold increased uptake in fibrotic lungs relative to the control probe, 68Ga-NODAGA-carboline, applied in the same disease model. The control probe, 68Ga-NODAGA-carboline, showed similar uptake in the lungs of both fibrotic and control mice.

Conclusion: We demonstrated that a small molecule PET probe can be applied for the molecular imaging of active fibrogenesis.

Research Support: This project is supported by the National Institutes of Health (EB009062, HL131907, RR029495). JW is supported by the German Research Foundation (DFG).

References:
1. Friedman SL, Sheppard D, Duffield JS, Violette S. Therapy for fibrotic diseases: nearing the starting line Sci Transl Med.2013, 5:167sr161.
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3. Waghorn PA, Jones CM, Rotile NJ, Koerner SK, Ferreira DS, Chen HH, Probst CK, Tager AM, Caravan P. Molecular Magnetic Resonance Imaging of Lung Fibrogenesis with an Oxyamine-Based Probe Angew Chem Int Ed Engl.2017, 56:9825-9828.