AbstractNon-invasive imaging of arthritis activity in rheumatoid arthritis (RA) patients using macrophage PET holds promise for early diagnosis and therapeutic response monitoring. Previously obtained results with macrophage tracer (R)-[¹¹C]PK11195 were encouraging, but the imaging signal could be further improved by reduction of background uptake. Recently, the novel macrophage tracer [¹⁸F]fluoro-PEG-folate was developed. This tracer showed excellent targeting of the folate receptor β on activated macrophages in synovial tissue in a preclinical arthritic rat model. We performed three substudies to investigate the biodistribution, potential for imaging arthritis and kinetic properties of [18F]fluoro-PEG-folate in RA patients. Firstly, biodistribution demonstrated fast clearance of [¹⁸F]fluoro-PEG-folate from heart and blood vessels and no dose limiting uptake in organs. Secondly, [¹⁸F]fluoro-PEG-folate showed uptake in arthritic joints with significantly lower background and hence significantly higher target-to-background ratios as compared to reference macrophage tracer (R)-[¹¹C]PK11195. Lastly, dynamic scanning demonstrated fast tracer uptake in affected joints, reaching a plateau after 1 minute, co-existing with a rapid blood clearance. In conclusion, this first in man study demonstrates the potential of [¹⁸F]fluoro-PEG-folate to image arthritis activity in RA with favourable imaging characteristics of rapid clearance and low background uptake, that allow for detection of inflammatory activity in the whole body.