Objectives: Vascular adhesion protein-1 (VAP-1) plays a key role in recruiting leukocytes into sites of inflammation. VAP-1 is stored in intracellular granules of endothelial cells, but upon inflammation it is rapidly translocated to the endothelial cell surface. Using a phage display approach, we have recently discovered that sialic acid-binding Ig-like lectin 9 (Siglec-9) is a leukocyte ligand for VAP-1 and a 5-deoxy-5-[¹⁸F]fluororibose-labeled Siglec-9 motif containing peptide ([¹⁸F]FDR-Siglec-9) specifically detects VAP-1 in vasculature at sites of inflammation. This study investigated the uptake of a novel [¹⁸F]FDR-Siglec-9 tracer in inflamed atherosclerotic plaques in mice.

Methods:

Atherosclerotic LDLR^-/-ApoB^100/100(n=12) and C57BL/6N control mice (n=9) were intravenously injected with 20 MBq of [¹⁸F]FDR-Siglec-9. The aorta and other tissues were excised 25 min later and measured with gamma counter to clarify biodistribution of radioactivity. In addition, the aorta was studied in more detailed by autoradiography, histologicy and immunohistochemistry. The specificity of tracer binding was studied with in vitro competitive assay with human atherosclerotic lesions.

Results: The aorta uptake of [¹⁸F]FDR-Siglec-9 was significantly higher in the LDLR^-/-ApoB^100/100 mice (0.80±0.36 %IA/g) compared to control mice (0.44±0.15 %IA/g, P=0.01). The autoradiography demonstrated significantly higher radioactivity concentration in atherosclerotic plaques compared to healthy vessel wall or adjacent adventitia. Plaque-to-wall ratio was 1.9±0.3 (p=0.003) and plaque-to-adventitia ratio was 2.4±0.6 (p=0.004).  Competition with excess of unlabeled peptide decreased tracer binding by 8-folds in human atherosclerotic lesions. The expression of VAP-1 in human and mouse plaques was confirmed by positive anti-VAP-1 staining.

Conclusion: VAP-1 targeting [¹⁸F]FDG-Siglec-9 peptide binds to the mice and human atherosclerotic plaques  and it might be promising and novel PET tracers for detection of plaque inflammation.