Fibroblast activation protein inhibitor positron emission tomography imaging in muscles of patients with idiopathic inflammatory myopathy.

OBJECTIVES

Mesenchymal stromal cells in muscles participate in regeneration following muscle injury. This study explored the potential of [18F]fibroblast activation protein inhibitor (FAPI)-42 PET targeting mesenchymal stromal cells to evaluate disease activity of idiopathic inflammatory myopathy (IIM).

METHODS

Patients with IIM (n = 26) were prospectively included and underwent [18F]FAPI-42 PET/CT and whole-body MRI between January 2023 and July 2023. Patients with malignancies were retrospectively included in the control group and only underwent [18F]FAPI-42 PET/CT (n = 28). [18F]FAPI-42 PET/CT images were evaluated using for avid-FAPI uptake and the target-to-background ratio (TBR). Whole-body MRI was evaluated for oedema, fatty infiltration and atrophy in 42 muscles in the IIM group. The global FAPI- and MRI-derived parameters were calculated for each patient. Clinical assessment of disease activity and muscle strength were collected.

RESULTS

Patients with IIM had significantly higher global FAPI-avid muscle ratios (0.68 [IQR: 0.45, 0.79] vs 0.06 [IQR: 0, 0.11], P < 0.001) and global muscle TBR (2.26 [IQR: 1.71, 2.75] vs 1.23 [IQR: 1.02, 1.52], P < 0.001) compared with controls. In the IIM group, the median TBR was higher in muscles with oedema than in those without (2.44 [IQR: 1.46, 3.27] vs 1.31 [IQR: 0.95, 1.99], P < 0.001). Global FAPI-avid muscle ratios significantly correlated with global oedema score (r = 0.833), muscle strength (r = -0.649), serum creatine kinase (r = 0.456) and disease activity index (r = 0.495-0.621).

CONCLUSION

Increased [18F]FAPI-42 uptake was associated with muscle oedema in IIM. FAPI-derived parameters correlated with IIM disease activity. [18F]FAPI-42 is a promising PET tracer for evaluating IIM disease activity.