| Summary: | Abstract Background The identification of biomarkers predicting the treatment response of rheumatoid arthritis (RA) is important. [68 Ga]Ga-FAPI-04 showed markedly increased uptake in the joints of patients with RA. The purpose of this study is to investigate whether [68 Ga]Ga-FAPI-04 PET/CT can be a predictor of treatment response in RA. Results Nineteen patients diagnosed with RA in the prospective cohort study were finally enrolled. Both total synovitis uptake (TSU) and metabolic synovitis volume (MSV) in [68 Ga]Ga-FAPI-04 and [18F]FDG PET/CT of the responders were significantly higher than those in non-responders according to Clinical Disease Activity Index (CDAI) and Simplified Disease Activity Index (SDAI) response criteria at 3-months’ follow-up (P < 0.05). The PET joint count (PJC) detected in [68 Ga]Ga-FAPI-04 and [18F]FDG PET/CT were also significantly higher in CDAI responders than non-responders (P = 0.016 and 0.045, respectively). The clinical characteristics of disease activity at baseline did not show significant difference between the responders and non-responders, except CRP (P = 0.035 and 0.033 in CDAI and SDAI response criteria, respectively). The baseline PJCFAPI, TSUFAPI and MSVFAPI > cutoff values in [68 Ga]Ga-FAPI-04 PET/CT successfully discriminated CDAI and SDAI responders and non-responders at 3-months’ follow-up. Conclusion [68 Ga]Ga-FAPI-04 uptake at baseline were significantly higher in early responders than those in non-responders. Trial registration ClinicalTrials. NCT04514614. Registered 13 August 2020, https://register.clinicaltrials.gov/prs/app/action/SelectProtocol?sid=S000A4PN&selectaction=Edit&uid=U0001JRW&ts=2&cx=-x9t7cp
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