68Ga-FAPI and 18F-NaF PET/CT in psoriatic arthritis: a comparative study.

Objectives: As fibroblast-like synoviocyte activation and bone formation are associated with psoriatic arthritis (PsA), positron emission tomography (PET) using the tracers of 68Ga-fibroblast activation protein inhibitor (FAPI) and 18F-sodium fluoride (NaF) may sensitively detect the disease. In this prospective study, we aimed to evaluate the performance of 68Ga-FAPI PET/CT in PsA and to compare it with 18F-NaF PET/CT.

Methods: Sixteen participants (female 7/16, age 42.31 ± 10.66 years) with PsA were prospectively enrolled and underwent dual-tracer PET/CT, clinical assessment and ultrasonography. PET/CT images were scored for PET-positive lesions at the peripheral joints, entheses, and axial joints.

Results: The positivity rate of 68Ga-FAPI in peripheral joints is higher than that in entheses and axial joints (21.84% vs 12.15% vs 0%), whereas high positivity rates of 18F-NaF in peripheral joints, entheses, and axial joints were observed (85.23%, 78.13%, and 75%, respectively). The disease activity score 28 was higher in the PET-positive than in the PET-negative group with 68Ga-FAPI (5.25 ± 1.84 vs 2.55 ± 0.94, p= 0.037), but not with 18F-NaF. Besides, the PET joint count at 68Ga-FAPI PET/CT was positively correlated with the TJC (r = 0.604, p= 0.017), SJC (r = 0.773, p= 0.001), DAS28-CRP (r = 0.556, p= 0.032), PASDAS (r = 0.540, p= 0.038) and PsASon13 (r = 0.701, p= 0.005), while no correlation was observed in 18F-NaF PET/CT.

Conclusion: The positivity rates of 68Ga-FAPI- and 18F-NaF PET/CT were different in patients with PsA in peripheral joints, entheses, and axial joints. The extent of joint involvement at 68Ga-FAPI PET/CT correlated with clinical and ultrasound variables as well as disease activity.