Fibroblast activation imaging in rheumatoid arthritis: evaluating disease activity and treatment response using [F]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), 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 in positive joints compared to controls (median 3.6 vs 2.4; p < 0.01). High disease activity was associated with increased SUV (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 [F]FAPI PET/CT for assessing RA disease activity and monitoring treatment response? Findings F]FAPI PET/CT effectively identifies joint involvement and quantifies fibroblast activation, correlating closely with clinical disease activity in RA. Clinical relevance [F]FAPI PET/CT provides a non-invasive, comprehensive assessment of RA activity, and possible cutoff values for identifying high disease activity.