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

Radiohybrid PSMA (rhPSMA) ligands, a new class of theranostic prostate-specific membrane antigen (PSMA)-targeting agents, feature fast F synthesis and utility for labeling with radiometals. Here, we assessed the biodistribution and image quality of F-rhPSMA-7 to determine the best imaging time point for patients with prostate cancer. In total, 202 prostate cancer patients who underwent a clinically indicated F-rhPSMA-7 PET/CT were retrospectively analyzed, and 12 groups based on the administered activity and uptake time of PET scanning were created: 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 extra long, ≥111 min). For quantitative analyses, SUV and organ- or tumor-to-background ratio were determined for background, healthy organs, and 3 representative tumor lesions. Qualitative analyses assessed overall image quality, nonspecific blood-pool activity, and background uptake in bone or marrow using 3- or 4-point scales. In quantitative analyses, SUV showed a significant decrease in the blood pool and lungs and an increase in the kidneys, bladder, and bones as the uptake time increased. SUV showed a trend to increase 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 or uptake time. In qualitative analyses, the overall image quality was stable along with the uptake time, but the proportion rated to have good image quality decreased as the administered activity increased. All other qualitative image parameters showed no significant differences for the administered activities, but they showed significant trends with increasing uptake time: less nonspecific blood activity, more frequent background uptake in the bone marrow, and increased negative impact on clinical decision making. The biodistribution of F-rhPSMA-7 was similar to that of established PSMA ligands, and tumor uptake of F-rhPSMA-7 was stable across the administered activities and uptake times. An early imaging time point (50-70 min) is recommended for F-rhPSMA-7 PET/CT to achieve the highest overall image quality.