Visualization of activated macrophages using a translocator protein ligand, 18F-FEDAC, in a rheumatoid arthritis model

Objectives Early diagnosis is a major challenges for rheumatology, and activated macrophages play an important roles in an early rheumatoid arthritis (RA) pathogenesis. Translocator protein (TSPO) is abundant in activated macrophages, thus can be used for the target of biomarker of inflammation. 18F-FEDAC (N-Benzyl-N-methyl-2-[7,8-dihydro-7-(2-[18F]fluoroethyl)-8-oxo-2-phenyl-9H-purin-9-yl] acetamide) is one of the radiolabeled ligand that recognizes TSPO. In this study, we evaluated the efficacy of 18F-FEDAC as a potential PET tracer for TSPO in a mouse RA model.

Methods RAW 264.7 mouse macrophages were activated with lipopolysaccharide (LPS). Expression level of TSPO mRNA and protein measured by quantitative PCR and western blotting. 18F-FEDAC uptake was measured using gamma counter, and a traditional TSPO ligand PK11195 was used for competition assay. We used collagen induced arthritis (CIA) mouse model as a rheumatoid arthritis animal model for in vivo study. The arthritis incidence was evaluated using an appearance of mouse paws and 99mTc-MDP SPECT/CT. Small animal PET/CT images were acquired using 500 μCi 18F-FEDAC, and PK11195 was 10 minutes pretreated for blocking. We performed H&E staining and TSPO targeted immune fluorescence in normal and arthritic mouse paw tissues.

Results mRNA and protein levels of TSPO expression were 3.9 fold and 2.3 fold higher in activated RAW 264.7 than inactivated RAW 264.7. Uptake of 18F-FEDAC in activated RAW 264.7 cells was 1.5 fold higher than that in non-activated cells. In addition, uptake of 18F-FEDAC in activated RAW 264.7 was successfully blocked by a PK11195. In CIA model, swelling was started 28 days after modeling and fully swelled around at 40 days after modeling. In 99mTc-MDP SPECT images also showed high signals in inflammatory paws. At 1 h after injection of 18F-FEDAC, radiotracer uptake in arthritic paws were significantly increased at 2 to 3 folds more than that in normal paws. PK11195 also successfully blocked in in vivo images. In arthritic mouse paw tissue showed cartilage damages and cells infiltration in bone tissues. The TSPO expression was higher in arthritic paw tissue than normal one.

Conclusions We demonstrated a specific binding of 18F-FEDAC to activated macrophages which have abundant TSPO. 18F-FEDAC PET/CT showed strong uptake in arthritis areas. Our results indicated that 18F-FEDAC-PET images have a potential to visualize RA activity by targeting TSPO.