Fluorine-18 labeled PSMA-targeted phosphoramidate inhibitors as potential PET imaging agents for prostate cancer

Methods: Succinimidyl [ 18 F]fluorobenzoate ([ 18 F]SFB) prosthetic group was prepared in the Neptis radiosynthesis unit. SFB conjugation to the three analogues was carried out at pH 9.5, 40 °C for 15 min, purified by reversed phase HPLC and concentrated for in vitro and in vivo studies. In vitro cell uptake in CWR22Rv1 (PSMA+) and PC3 (PSMA-) and internalization in CWR22Rv1 (PSMA+) cells were conducted at 1 and 2 h post incubation at 37 °C and 5% CO2. MicroPET/CT imaging at 2 h and biodistribution studies at 1 and 2 post injection were performed using nude mice with CWR22Rv1 (PSMA+) and PC3 (PSMA-) xenograft. Diffraction quality crystals of hGCPII (extracellular domain of human GCPII; amino acids 44-750) / AH-TG97 and hGCPII/ AH2-TG97 were prepared. X-ray structures of the two complexes were determined. Results: SFB conjugation to the PSMA inhibitor analogues was accomplished in 50-60% radiochemical yields. Significant cell uptake (2-28.2 fold) was seen in the CWR22Rv1 cells versus the PC3 cells at 1h and 2h incubation times. Internalization of the cell associated activity ranged from 80.6-84.9% and 84.294.2% at 1 and 2h, respectively. In vivo tumor uptake ranged between 1.6-3.2 %ID/g with retention longer than 2h. Minimal uptake in the non-target organs and rapid clearance from blood was noted. High uptake was observed in the mice kidneys as expected due to high PSMA expression (human kidneys do not express PSMA). X-ray crystallography data corroborate the interaction of the phosphoramidate analog with PSMA.