Characterization of physiologic ¹⁸F FSPG uptake in healthy volunteers

Purpose: To evaluate the normal biodistribution and kinetics of (S)-4- (3-[18F]fluoropropyl)-L-glutamic acid (¹⁸F FSPG) in healthy volunteers and to compare 18F FSPG mean and maximum standardized uptake values (SUV_mean and SUV_max, respectively) with those of 18F fluorodeoxyglucose (FDG) across a variety of organs. Materials and Methods: This protocol was reviewed and approved by all appropriate regulatory authorities. An 8-mCi (610%) dose of ¹⁸F FSPG was given to five subjects (three women, two men), and seven whole-body positron emission tomography (PET) scans were performed 5, 10, 20, 30, 45, 150, and 240 minutes after injection. Regions of interest were analyzed on the resultant ¹⁸F FSPG images to evaluate the kinetics of this radiotracer. The images obtained 45 minutes after injection were used to measure SUV_mean and SUV_max in additional regions of the body. These values were compared with similar values obtained with ¹⁸F FDG PET published previously. Descriptive statistics, including average and standard deviation across the five subjects, were used. ¹⁸F FSPG SUV_mean and SUV_max were compared. Results: On the ¹⁸F FSPG images obtained 45 minutes after injection, there was only low-grade background activity in the majority of analyzed regions. Prominent activity was seen throughout the pancreas. Clearance of the radiotracer through the kidneys and collection in the bladder also were seen. SUV quantification shows notable differences between ¹⁸F FSPG and ¹⁸F FDG in the pancreas (¹⁸F FSPG SUV_mean, 8.2; ¹⁸F FDG SUV_mean, 1.3), stomach (¹⁸F FSPG SUV_max, 3.6; ¹⁸F FDG SUV_max, 1.6), and brain (¹⁸F FSPG SUV_mean, 0.08; ¹⁸F FDG SUV_mean, 7.8). The kinetic data showed rapid clearance of the radiotracer from the blood pool and most organs, except the pancreas. Conclusion: ¹⁸F FSPG is a PET radiopharmaceutical characterized by rapid clearance from most healthy tissues, except the pancreas and kidneys. A consistent biodistribution pattern was observed with low background uptake. The physiologic uptake of this new radiotracer throughout the body is described in more detail, which is important for improved interpretative accuracy and understanding potential clinical applications.