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

This two-part preclinical study aims to evaluate prostate specific membrane antigen (PSMA) as a valuable target for expression-based imaging applications and to determine changes in target binding in function of varying apparent molar activities (MA) of [F]AlF-PSMA-11. For the evaluation of PSMA expression levels, male NOD/SCID mice bearing prostate cancer (PCa) xenografts of C4-2 (PSMA+++), 22Rv1 (PSMA+) and PC-3 (PSMA-) were administered [F]AlF-PSMA-11 with a medium MA (20.24 ± 3.22 MBq/nmol). SUV and SUV values were respectively 3.22 and 3.17 times higher for the high versus low PSMA expressing tumors (p < 0.0001). To evaluate the effect of varying MA, C4-2 and 22Rv1 xenograft bearing mice underwent additional [F]AlF-PSMA-11 imaging with a high (211.2 ± 38.9 MBq/nmol) and/or low MA (1.92 ± 0.27 MBq/nmol). SUV values showed a significantly increasing trend with higher MA. Significant changes were found for SUV and SUV between the high versus low MA and medium versus low MA (both p < 0.05), but not between the high versus medium MA (p = 0.055 and 0.25, respectively). The effect of varying MA was more pronounced in low expressing tumors and PSMA expressing tissues (e.g. salivary glands and kidneys). Overall, administration of a high MA increases the detection of low expression tumors while also increasing uptake in PSMA expressing tissues, possibly leading to false positive findings. In radioligand therapy, a medium MA could reduce radiation exposure to dose-limiting organs with only limited effect on radionuclide accumulation in the tumor.