The Influence of Specific Activity on the Biodistribution of ¹⁸F-rhPSMA-7.3: A Retrospective Analysis of Clinical Positron Emission Tomography Data

Abstract

We investigated whether the time between synthesis and injection and the resulting decrease in specific activity affects the normal organ and tumor uptake of the PSMA ligand, 18 F-rhPSMA-7.3, in patients with prostate cancer. Methods: The biodistribution of 18 F-rhPSMA-7.3 on PET/CT scans performed with a high specific activity (median=178.9MBq/µg, n=42) and a low specific activity (median=19.3MBq/µg, n=42) were compared. Results: Tracer uptake by the parotid gland, submandibular gland and spleen was moderately, but significantly lower in the "low specific activity" group than in the "high specific activity" group (median SUV mean 16.7 vs. 19.2; 18.1 vs. 22.3, and 7.8 vs. 9.6, respectively). No other statistically significant differences were found for normal organs or tumor lesions. Conclusion: A 10-fold decrease in specific activity has only minor effects on the biodistribution of 18 F-rhPSMA-7.3. These findings suggest that 18 F-labeled PSMA ligands can be centrally produced and shipped to PET clinics in a similar way to 18 Ffluorodeoxyglucose.

Impact of the molar activity and PSMA expression level on [18F]AlF-PSMA-11 uptake in prostate cancer

“…However, as the molar activity of a PSMA PET tracer decreases over time, it would be beneficial to evaluate the effect on both tumor uptake and non-target uptake in humans. In a retrospective analysis of patients who underwent [ 18 F]rhPSMA-7.3 PET/CT, only minor effects on biodistribution were found for a tenfold decrease of MA app , except for the salivary glands and spleen, which showed significantly lower uptake for the lower molar activity 35 . Overall, these results encourage a prospective clinical trial using [ 18 F]AlF-PSMA-11 comparing a high to medium MA app .…”

Radiolabeled PSMA Inhibitors

“…This was confirmed in healthy volunteers determining biodistribution and radiation dosimetry data (NCT03995888) and in a larger cohort of patients [ 84 , 195 , 196 , 197 ]. The molar activity of [ 18 F]Ga-rh-PSMA-7.3 had a minor influence on the biodistribution [ 198 , 199 ]. Three clinical trials using [ 18 F]Ga-rh-PSMA-7.3 are either ongoing or will commence in the near future (NCT04978675/NCT04186819/NCT04186845).…”

PSMA PET/CT: joint EANM procedure guideline/SNMMI procedure standard for prostate cancer imaging 2.0

“…Liver uptake is also higher with [ 18 F]F-rhPSMA-7.3 than with 68 Ga-PSMA-11 and excretion is mainly via the urinary tract. However, retention in the urinary system is usually low at the time of imaging and can be further lowered by application of furosemide [21,109].…”