Quantitative and Qualitative Analyses of Biodistribution and PET Image Quality of Novel Radiohybrid PSMA, 18F- rhPSMA-7, in Patients with Prostate Cancer

Objectives: Radiohybrid PSMA (rhPSMA) ligands, a new class of theranostic PSMA-targeting agents, feature fast (18)F synthesis and utility for labelling with radiometals. Here, we assessed the biodistribution and image quality of (18)F-rhPSMA-7 to determine the best imaging time point for patients with prostate cancer (PCa). Methods: A total of 202 PCa patients who underwent a clinically indicated (18)F-rhPSMA-7 PET/CT were retrospectively analyzed, and 12 groups were created based on the administered activity and uptake time of PET scanning; 3 administered activities (low: 222-296 MBq, moderate: 297-370 MBq, and high: 371-444 MBq) and 4 uptake time points (short: 50-70 min, intermediate: 71-90 min, long: 91-110 min, and longest: >/= 111 min). For quantitative analyses, mean standardized uptake value (SUVmean) and organ/tumor-to-background ratio(ratio-SUVmean) were determined for background, healthy organs, and for three representative tumor lesions. Qualitative analyses assessed overall image quality, non-specific blood pool activity, and background uptake in bone/marrow using 3 or 4-point scales. Results: In quantitative analyses, the mean SUV values showed a significant decrease in the blood pool and lungs, and an increase in the kidneys, bladder, and bones, as the uptake time increased. The mean SUV values demonstrated a trend increasing in the blood pool and bones, as the administered activity increased. However, no significant differences were found in 377 tumor lesions with respect to the administered activity and uptake time. In qualitative analyses, the overall image quality was stable along with the uptake times, but the proportion rated to have good image quality showed a decrease as the administered activity increased. All other qualitative image parameters showed no significant differences for the administered activities, but they showed significant trends along with increasing uptake times; less non-specific blood activity, more frequent background uptake in the bone marrow, and increased negative impact on the clinical decision making. Conclusion: The biodistribution of (18)F-rhPSMA-7 was similar to that of established PSMA-ligands, and tumor uptake of (18)F-rhPSMA-7 was stable across the administered activity and uptake time. An early imaging time point (50-70 min) is recommended for (18)F-rhPSMA-7 PET/CT to achieve the highest overall image quality.