Increased vascular permeability is an important hallmark of many diseases, including cancer, cerebral ischemia, and severe inflammatory disorders. In this regard, the noninvasive assessment of pathologically increased vascular permeability in vivo is of great interest. In this study, the potential of albumin- and transthyretin-binding radioligands was evaluated for imaging of vascular hyperpermeability. For this purpose, the bleomycin-induced lung injury model was used as a model of inflammation-associated vascular leakage. The plasma protein-binding ligands, which bind to albumin (DOTA-PPB-01) and transthyretin (DOTA-PPB-03), were radiolabeled and used for nuclear imaging and biodistribution studies. In this regard, 177Lu was employed as a surrogate nuclide for detailed preclinical investigations, including single-photon emission computed tomography (SPECT) studies, whereas 44Sc was proposed as a radionuclide for positron emission tomography (PET), which may be relevant for future clinical translation. Mice were administered with these radioligands 6-9 days after intratracheal instillation of bleomycin or saline. Bleomycin-treated mice developed pronounced lung inflammation with enhanced vascular permeability that was reflected in significantly increased lung size and weight due to edema and infiltration with inflammatory cells. Biodistribution studies revealed significantly higher accumulation of 177Lu-DOTA-PPB-01 in injured lungs as compared to lungs of control animals at all investigated time points (4-48 h p.i.). The best contrast was achieved at late time points (16.1 ± 2.91% IA/g vs 2.03 ± 1.22% IA/g, 48 h p.i.) when the blood activity levels were ∼7.5% IA/g. Injection of 177Lu-DOTA-PPB-03 also resulted in increased lung accumulation in bleomycin-treated mice at all investigated time points (2-8 h p.i.). The pharmacokinetics was significantly faster, however, resulting in good contrast already at 8 h p.i. (4.32 ± 0.85% IA/g vs 1.06 ± 0.10% IA/g) when blood activity levels were ∼2% IA/g. The absolute lung accumulation of 177Lu-DOTA-PPB-03 was significantly lower than that of 177Lu-DOTA-PPB-01. PET/CT scans performed with 44Sc-DOTA-PPB-01 distinguished injured from healthy lungs only at late time points (20 h p.i.), whereas 44Sc-DOTA-PPB-03 already allowed the differentiation at 4 h p.i. due to its faster clearance. The investigated radioligands, 44Sc/177Lu-DOTA-PPB-01 and 44Sc/177Lu-DOTA-PPB-03, hold promise for the visualization of vascular leakage in a variety of pathological conditions. 44Sc would be the radionuclide of choice for clinical application as it can be stably coordinated with a DOTA chelator and enables PET imaging over extended periods.
Keywords: 177Lu; 44Sc; albumin; bleomycin; in vivo labeling; lung injury; transthyretin; vascular leakage.