SUVs Versus Dynamic Pharmacokinetic [18F]Fluoro-Polyethylene Glycol-Folate Uptake Parameters in Joints of Rheumatoid Arthritis Patients at Baseline and at 4 Weeks of Antitumor Necrosis Factor Therapy

Quantitative assessment of rheumatoid arthritis (RA) activity using [18F]fluoro-polyethylene glycol (PEG)-folate PET/CT scans may prove a useful noninvasive therapeutic response assessment tool to evaluate antitumor necrosis factor therapy in RA patients. This study aims to assess [18F]fluoro-PEG-folate kinetics through a metabolite-corrected plasma input model and to investigate comparisons with simplified quantitative PET outcome measures. Methods: Dynamic [18F]fluoro-PEG-folate PET/CT scans were obtained for 6 patients for a total of 11 scans, 6 before and 5 after treatment. These scans were analyzed using conventional pharmacokinetic models. In addition, SUVs were calculated at intervals of 10-40, 20-50, 30-60, and 40-60 min after injection for comparison and imaging window optimization. Results: [18F]fluoro-PEG-folate kinetics in joints of RA patients were best described using the reversible pharmacokinetic 2-tissue compartment model with a volume of distribution (VT ) mean of 1.0 (±0.5). VT values correlated between arterial and venous samples at both baseline (P < 0.001, r2 = 0.96) and 4 wk after antitumor necrosis factor treatment (P < 0.001, r2 = 0.75), both at intervals of 30-60 and 40-60 min. Changes in VT behavior during treatment could not be accurately assessed because of limited available data, but observed changes in the linear association slope may indicate changed kinetic behavior. Conclusion: The most optimal kinetic model for [18F]fluoro-PEG-folate uptake in joints of RA patients was the reversible 2-tissue compartment model. The associations between VT and a simplified SUV interval of 30-60 min allow us to quantify tracer uptake without the need for a full cross-sectional pharmacokinetic evaluation at the time of imaging. Further research will be required to accurately assess the change in tracer behavior between time points and the use of simplified assessment of changes of tracer uptake in joints over time.