SPECT imaging of pulmonary vascular disease in bleomycin-induced lung fibrosis using a vascular endothelium tracer.

Background: Pulmonary hypertension (PH) complicating idiopathic pulmonary fibrosis (IPF) is associated to worse outcome. There is a great need for a non-invasive diagnostic modality to detect and evaluate the severity of pulmonary vascular disease (PVD). Tc-PulmoBind is a novel imaging agent that binds to the adrenomedullin (AM) receptor on the pulmonary microvascular endothelium. SPECT imaging employing the endothelial cell tracer Tc-PulmoBind was used to assess PVD associated with lung fibrosis.

Methods: Rats with selective right lung bleomycin-induced fibrosis were compared to control rats. SPECT imaging was performed after three weeks with Tc-PulmoBind and Tc-macroaggregates of albumin (MAA). PH and right ventricular (RV) function were assessed by echocardiography. Lung perfusion was evaluated by fluorescent microangiography. Lung AM receptor expression was measured by qPCR and by immunohistology. Relevance to human IPF was explored by measuring AM receptor expression in lung biopsies from IPF patients and healthy controls.

Results: The bleomycin group developed preferential right lung fibrosis with remodeling and reduced perfusion as assessed with fluorescent microangiography. These rats developed PH with RV hypertrophy and dysfunction. Tc-PulmoBind uptake was selectively reduced by 50% in the right lung and associated with reduced AM receptor expression, PH and RV hypertrophy. AM receptor was co-expressed with the endothelial cell protein CD31 in alveolar capillaries, and markedly reduced after bleomycin. Quantitative dynamic analysis of Tc-PulmoBind uptake in comparison to Tc-MAA revealed that the latter distributed only according to flow, with about 60% increased left lung uptake while left lung uptake of Tc-PulmoBind was not affected. Lung from human IPF patients showed important reduction in AM receptor expression closely associated with CD31.

Conclusions: SPECT imaging with Tc-PulmoBind detects PVD and its severity in bleomycin-induced lung fibrosis. Reduced AM receptor expression in human IPF supports further clinical development of this imaging approach.