In vivo imaging of activated macrophages by 18F-FEDAC, a TSPO targeting PET ligand, in the use of biologic disease-modifying anti-rheumatic drugs (bDMARDs).

Rheumatoid arthritis (RA) is a chronic disease with systemic inflammation resulting in destruction of multiple articular cartilages and bones. Activated macrophage plays a pivotal role during the disease course and has been one of main targets to inhibit inflammatory reaction of RA by using biological disease-modifying anti-rheumatic drugs (bDMARDs). 18F-FEDAC is one of PET imaging agents targeting TSPO, which is overexpressed in activated macrophages. The aim of this study was to evaluate the roles of 18F-FEDAC PET as an in vivo imaging of activated macrophages on etanercept (ETN), a TNF-antagonist as one of bDMARDs in collagen induced arthritis mice. In RAW 264.7 cells, the expressions of TSPO as well as iNOS and infiltrated nucleus of NF-κB were induced by activation with lipopolysaccharide and interferon-gamma. TSPO expression was slightly attenuated by ETN treatment, not by methotrexate (MTX) as a cytotoxic agent. However, cell uptake of 18F-FEDAC did not show significant changes according to both of the treatments. Similarly in CIA mice, 18F-FEDAC uptake in inflamed paws on PET imaging did not show significant changes during both of the treatments, contrary to the uptake decrease of 18F-FDG, a glucose analog to reflect metabolic or active inflammatory activity. Interestingly, when we divided joints according to the degree of 18F-FEDAC uptake before ETN treatment, the joints of high 18F-FEDAC uptake showed better response to ETN than the joints with low 18F-FEDAC uptakes. In case of 18F-FDG, there was no such kinds of patterns. We can speculate that 18F-FEDAC PET imaging may identify activated macrophage-induced arthritis because that 18F-FEDAC can reflect activated macrophages, which is the therapeutic target of ETN by TNF antagonistic effect. Thus, in vivo imaging using 18F-FEDAC may be used as a predictor of therapeutic effects among those kinds of bDMARDs having anti-inflammatory actions to inhibit activated macrophage.