Fibroblast activation imaging in rheumatoid arthritis: evaluating disease activity and treatment response using [18F]FAPI PET/CT.

Objectives: To evaluate the utility of fibroblast activation protein (FAP) imaging using [¹⁸F]FAPI PET/CT for assessing disease activity and monitoring treatment response in rheumatoid arthritis (RA).

Materials and methods: A prospective, single-center study was conducted with 42 RA patients and 8 healthy controls. Participants underwent clinical assessment and [¹⁸F]FAPI PET/CT imaging. Semiquantitative parameters, including standardized uptake value maximum (SUV_max), lesion-to-muscle ratio (LMR), [¹⁸F]FAPI-avid lesion volume (FLV), and total lesion FAP expression (TLF), were analyzed. Disease activity was stratified by the disease activity score-28 using C-reactive protein (DAS28-CRP) into three groups (low, moderate, and high). Correlations between PET/CT parameters and disease acitivity were examined, and follow-up imaging was performed in ten patients to assess treatment response.

Results: [¹⁸F]FAPI PET/CT identified 201 RA-affected joints with significantly higher SUV_max in positive joints compared to controls (median 3.6 vs 2.4; p < 0.01). High disease activity was associated with increased SUV_max (median 5.6 vs 3.3 vs 2.8; p < 0.05), TBR (median 8.4 vs 3.0 vs 3.6; p < 0.05), FLV (162.2 vs 11.5 vs 9.8; p < 0.01), and TLF (569.1 vs 27.3 vs 22.7; p < 0.01). Positive correlations were observed between DAS28-CRP and both FLV (r = 0.50, p < 0.01) and TLF (r = 0.51, p < 0.01). Follow-up imaging revealed significant associations between changes in disease activity and FAP burden.

Conclusion: [¹⁸F]FAPI PET/CT is a promising tool for non-invasive assessment of RA disease activity. It provides quantitative insights into fibroblast activation, offering potential to enhance disease management and therapeutic planning.

Key points: Question What is the potential of fibroblast activation imaging using [¹⁸F]FAPI PET/CT for assessing RA disease activity and monitoring treatment response? Findings ¹⁸F]FAPI PET/CT effectively identifies joint involvement and quantifies fibroblast activation, correlating closely with clinical disease activity in RA. Clinical relevance [¹⁸F]FAPI PET/CT provides a non-invasive, comprehensive assessment of RA activity, and possible cutoff values for identifying high disease activity.