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

Objectives Prostate-specific membrane antigen (PSMA), a transmembrane protein commonly found on the tumor cell surface of late-stage, androgen-independent and metastatic prostate cancer, is an ideal biomarker for staging and targeted therapy. Previously reported FB-LW-54 (IC50 = 14 nM) is an 18F labelled irreversible phosphoramidate PSMA inhibitor, which demonstrated promising in vivo results in LNCaP tumor bearing mice [1]. Three more stable phosphoramidate-based PSMA inhibitor analogues with similar binding affinity as FB-LW-54 (TG97, AH-TG97 and AH2-TG97) were synthesized for radiolabeling with 18F-flourobenzamide (18FB) for PET imaging applications.

Methods Succinimidyl [18F]fluorobenzoate (SFB) prosthetic group was prepared successfully either manually in lab or the Neptis box. SFB conjugation to the three analogues was carried out at pH 9.5, 37 °C for 10 min, purified by reversed phase HPLC and concentrated for in vivo studies. PET imaging (1 and 2 h) and biodistribution studies (1, 2 h and blocked with 250 ug LW-54) were performed using nude mice with 22RV1 (PSMA+) and PC3 (PSMA-) xenograft.

Results SFB conjugation to the PSMA inhibitor analogues was accomplished in 27-45% radiochemical yields. Tumor uptake ranged between 1.7-2.4 %ID/g and can retain up to 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 its normal expression. But PSMA expression in human kidney has not been identified. PSMA specificity was demonstrated in the tumor and kidneys by LW-54 blocking (56-93% uptake decrease).

Conclusions We have successfully radiolabeled three PSMA-targeted analogues for mice xenografts ([18F]FB-TG97, [18F]FB-AH-TG97 and [18F]FB-AH2-TG97) and demonstrated their specificity. [18F]FB-AH-TG97 exhibited the highest tumor uptake with exceptional clearance in 22RV1 tumor bearing mice.

Research Support Supported by DOD W81XWH-11-1-0464 (SFD) and W81XWH-11-1-0191 (LW) NIH 5R01CA140617 (CB).