The Influence of Specific Activity on the Biodistribution of ¹⁸F-rhPSMA-7.3: A Retrospective Analysis of Clinical PET 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 ¹⁸F-rhPSMA-7.3 in patients with prostate cancer. <b>Methods:</b> The biodistribution of ¹⁸F-rhPSMA-7.3 on PET/CT scans obtained with a high specific activity (median, 178.9 MBq/µg; <i>n</i> = 42) and a low specific activity (median, 19.3 MBq/µg; <i>n</i> = 42) was compared. <b>Results:</b> 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. <b>Conclusion:</b> A 10-fold decrease in specific activity has only minor effects on the biodistribution of ¹⁸F-rhPSMA-7.3. These findings suggest that ¹⁸F-labeled PSMA ligands can be centrally produced and shipped to PET clinics in a similar way to ¹⁸F-FDG.