Impact of the molar activity and PSMA expression level on [(18)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(app)) of [(18)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 [(18)F]AlF-PSMA-11 with a medium MA(app) (20.24 ± 3.22 MBq/nmol). SUV(mean) and SUV(max) 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(app), C4-2 and 22Rv1 xenograft bearing mice underwent additional [(18)F]AlF-PSMA-11 imaging with a high (211.2 ± 38.9 MBq/nmol) and/or low MA(app) (1.92 ± 0.27 MBq/nmol). SUV values showed a significantly increasing trend with higher MA(app). Significant changes were found for SUV(mean) and SUV(max) between the high versus low MA(app) and medium versus low MA(app) (both p < 0.05), but not between the high versus medium MA(app) (p = 0.055 and 0.25, respectively). The effect of varying MA(app) was more pronounced in low expressing tumors and PSMA expressing tissues (e.g. salivary glands and kidneys). Overall, administration of a high MA(app) 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(app) could reduce radiation exposure to dose-limiting organs with only limited effect on radionuclide accumulation in the tumor.