Objectives: (S)-4-(3-[18F] Fluoropropyl)-L-glutamic acid (18F-FSPG) is a novel radiopharmaceutical for PET imaging. It is a glutamate analogue with a good dosimetric and safety profile. We present data on 5 healthy volunteers, 5 subjects with brain cancer, and 5 subjects with head and neck cancer.

Methods: For the healthy volunteers, 8 (± 10%) mCi of 18F-FSPG was given IV and 7 PET and 3 CT scans per subject were obtained 0-2 hours post-injection (p.i.). From the 60 minute p.i. scan, we measured mean standardized uptake values (SUV) for 16 normal organs/regions within the body. A subset of these regions was analyzed over all 7 scans to evaluate the kinetics of this radiotracer as well. For the cancer subjects, the same imaging protocol was followed. Seven of the 10 cancer subjects also had a clinical 18F-FDG PET/CT scan within 1 month of the FSPG scan. SUV measurements and visual comparison of the 18F-FSPG and 18F-FDG scans were done to assess relative efficacy and differences between these tracers. Significant SUV differences between the tracers were evaluated with an unpaired T-test.

Results: There was similar distribution of the radiotracer in all 15 subjects, with primary clearance through the kidneys. On the 60 minute p.i. scan, there was very low-grade background activity in the majority of normal regions analyzed. Significant activity was visualized throughout the pancreas. Quantitatively, all malignant lesions were positive on 18F-FSPG but with a range of uptake. Compared to the 18F-FDG scan, the 18F-FSPG scan showed lower absolute SUV values in the lesions, but higher tumor-to-background levels such that, visually, the FSPG scans were easier to evaluate than FDG.

Conclusions: Understanding the physiological uptake of this promising new tracer throughout the body is critical for improved interpretative accuracy and understanding potential applications. This data suggests that 18F-FSPG may be suitable for a wide variety of malignancies except for pancreas, kidneys and bladder. Markedly lower background uptake than 18F-FDG, especially in the brain, appears to be beneficial. These results are promising and warrant further analysis in a larger cohort of subjects.

Instituição: Stanford University; Piramal Imaging Gmbh; Bayer Healthcare; Universität München.