Evaluation of 18F-FAPI-04 Imaging in Assessing the Therapeutic Response of Rheumatoid Arthritis

Purpose Fibroblast activating protein (FAP) is highly expressed in the synovial tissues of rheumatoid arthritis (RA) patients. The aim of this study was to determine the feasibility of PET imaging with an Al[ 18 F] F-NOTA-labeled FAP inhibitor 04( 18 F-FAPI-04) for the evaluation of arthritic progression and therapeutic response in experimental arthritis. Methods Fibroblast-like synoviocytes (FLSs) were obtained from patients with RA or osteoarthritis (OA), and the relationship between 18 F-FAPI-04 uptake and the inflammatory activity of RA FLSs was investigated. Collagen-induce arthritis (CIA) mice models were established and treated with methotrexate (MTX) or etanercept (ETC). Then, PET imaging was performed 24 h following 18 F-FAPI-04 injection. The imaging results were compared by assessing macroscopic arthritis scores and histological staining. Results 18 F-FAPI-04 uptake was obvious in RA FLSs that characterizing FAP activation. The higher the uptake of 18 F-FAPI-04, the more severity of the inflammatory phenotype in RA FLS. Furthermore, the uptake of 18 F-FAPI-04 in inflamed joints could be found even before the deformity of the parental joints could be observed by histological examination. Both MTX and ETC were effective in inhibiting the progression of arthritis in CIA mice was confirmed by macroscopic, histological, and radiographic pathology scores. Importantly, 18 F-FAPI-04 uptake declined accordingly in CIA models following MTX and ETC treatment. Conclusions These findings suggest that PET imaging of 18 F-FAPI-04 can be used to monitor treatment response in RA, and is more sensitive in disease speculation than macroscopic arthritis scoring.