AN IMAGING STUDY ASSESSING FEASIBILITY AND RELIABILITY OF WHOLE-BODY 68Ga-FAPI PET-CT FOR STRUCTURAL OSTEOARTHRITS EVALUATION

INTRODUCTION The relevance of fibroblastic activation has been acknowledged for several rheumatologic diseases. In rheumatoid arthritis fibroblast-like synoviocyte cells show significant fibroblast activation protein inhibitor (FAPI) tracer uptake in a lesion-specific manner, which correlates with clinical disease activity. While osteoarthritis (OA) is considered a primary non-inflammatory disorder, phases of marked synovial activation are commonly seen during the disease course, which drives clinical and structural progression. OBJECTIVE To evaluate whether presence of OA as assessed by ordinal grading on whole body computed tomography (CT) is associated with 68 Ga fibroblast activation protein inhibitor (FAPI) positron emission tomography (PET) tracer uptake as a measure of fibroblastic activation. Secondary aim was to evaluate reliability. METHODS In a retrospective study design patients who had undergone 68 Ga-FAPI PET-CT for a spectrum of clinical reasons were included. Whole body CT was assessed for OA using the OsteoArthritis Computed Tomography‐Score (OACT) in multiple joints and the spine. Maximum standard uptake value (SUV max ) was determined correspondingly. Logistic regression and correlation analyses were used to describe associations between structural OA and 68 Ga FAPI PET activity. RESULTS 54 patients were included. Presence of OA in the acromio-clavicular joints (ACJ) was strongly associated with odds of SUV max being in the highest tertile. Increased odds were seen for one location of the cervical (OR 5.7, 95% confidence interval [1.6, 20.4]) and both locations of the lumbar spine (OR 8.3, [1.5, 46.6] and 5.0 [1.3, 18.7]). Moderate correlations were observed for OA severity and 68 Ga-FAPI PET uptake for most joints and spine. Reliability for OA scoring (mean w-kappa) was 0.62 (intra-reader) and 0.67 (inter-reader). Mean intraclass correlation coefficients for SUV max were 0.76 and 0.82, respectively (Table 1). CONCLUSION Presence of OA was associated with increased SUV max uptake for the ACJ and the cervical and lumbar spine. In this feasibility study CT-based OA scoring and 68 Ga-FAPI PET SUV max determinations could be achieved with good reliability. Future longitudinal work will have to elucidate whether FAPI uptake actually precedes morphologic joint pathology and whether FAPI-positive and FAPI-negative (or only mildly positive) joints exhibit different trajectories of disease progression, which may have implications for future fibroblast-targeting therapeutic approaches.