Subclinical synovitis detected by macrophage PET, but not MRI, is related to short-term flare of clinical disease activity in early RA patients: an exploratory study

INTRODUCTION: Residual subclinical synovitis can still be present in joints of rheumatoid arthritis (RA) patients despite clinical remission and has been linked to ongoing radiological damage. The aim of the present study was to assess subclinical synovitis by positron emission tomography (PET; macrophage tracer (11)C-(R)-PK11195) in early RA patients with minimal disease activity without clinically apparent synovitis (MDA); and its relationship with clinical outcome and magnetic resonance imaging (MRI), respectively. METHODS: Baseline PET and MRI of hands/wrists were performed in 25 early MDA RA patients (DAS 44 < 1.6; no tender/swollen joints) on combined DMARD therapy. PET tracer uptake (semi-quantitative score: 0–3) and MRI synovitis and bone marrow edema (OMERACT RAMRIS) were assessed in MCP, PIP and wrist joints (22 joints/patient; cumulative score). RESULTS: Eleven of 25 patients (44 %) showed enhanced tracer uptake in ≥ 1 joint. Fourteen of these 25 (56 %) patients developed a flare within 1 year: 8/11 (73 %) with a positive, and 6/14 (43 %) with a negative PET. In the latter, in 5/6 patients flare was located outside the scan region. Median cumulative PET scores of patients with a subsequent flare in the hands or wrists were significantly higher than those of patients without a flare (1.5 [IQR 0.8–5.3] vs 0.0 [IQR 0.0–1.0], p = 0.04); significance was lost when all flares were considered (1.0 [IQR 0.0–4.0] vs 0.0 [IQR 0.0–1.0], p = 0.10). No difference in cumulative MRI scores was observed between both groups. CONCLUSIONS: Positive PET scans were found in almost half of early RA patients with MDA. Patients with a subsequent flare in hand or wrist had higher cumulative PET scores but not MRI scores, suggesting that subclinical arthritis on PET may predict clinical flare in follow-up.