Inflammation is related to many diseases, such as atherosclerosis, rheumatoid arthritis and metabolic diseases. Vascular adhesion protein-1 (VAP-1) is an endothelial adhesion molecule involved in leukocyte trafficking cascades from blood circulation to the sites of inflammation. In normal condition, VAP-1 is stored in intracellular granules. During inflammation it is rapidly translocated from the intracellular storage granules to the endothelial cell surface. Siglec-9 is a leukocyte ligand of VAP-1 and Siglec-9 motif containing peptide can be used as a positron emission tomography (PET) tracer for in vivo imaging of inflammation-related diseases. 

For this study, radiolabeled Siglec-9 was evaluated for feasibility as a tracer in imaging orthopaedic implant infection in rats, synovitis in rabbits, turpentine oil-induced inflammation in rats and atherosclerosis in mice. Dynamic PET imaging was performed using [68Ga]DOTA-Siglec-9 or [18F]FDR-Siglec-9. After PET imaging various tissues were collected for ex vivo measurements with a gamma counter. Inflamed tissues were further studied with the digital autoradiography and histological staining. The expression of luminal VAP-1 in inflamed tissues was also studied by means of immunohistochemical stainings. 

Inflammation in different experimental settings were clearly visualized with [68Ga]DOTA-Siglec-9 PET. The [18F]FDR-Siglec-9 uptake in atherosclerotic mouse model and an acute sterile inflammation in a turpentine oil-induced rat model were comparable with [68Ga]DOTA-Siglec-9. Anti-inflammatory therapy with VAP-1 inhibitor (LJP1586) reduced macrophages in atherosclerotic plaques in mice. The expression of luminal VAP-1 in inflamed tissue was verified with fluorescence-based immunohistochemistry. 

In conclusion, [68Ga]DOTA-Siglec-9 and [18F]FDR-Siglec-9 are promising imaging agents for in vivo imaging of inflammation. [68Ga]DOTA-Siglec-9 was able to detect inflammation already in its early stage. VAP-1 is a promising target for both anti-inflammatory therapy and molecular imaging of inflammation.