Synthesis and pre-clinical evaluation of a new class of high-affinity 18F-labeled PSMA ligands for detection of prostate cancer by PET imaging

Current clinical imaging of PSMA-positive prostate cancer by positron emission tomography (PET) mainly features Ga-68-labeled tracers, notably [Ga-68]Ga-PSMA-HBED-CC. The longer half-life of fluorine-18 offers significant advantages over Ga-68, clinically and logistically. We aimed to develop high-affinity PSMA inhibitors labeled with fluorine-18 as alternative tracers for prostate cancer. Six triazolylphenyl ureas and their alkyne precursors were synthesized from the Glu-urea-Lys PSMA binding moiety. PSMA affinity was determined in a competitive binding assay using LNCaP cells. The [F-18]triazoles were isolated following a Cu(I)-catalyzed click reaction between the alkynes and [F-18]fluoroethylazide. The F-18-labeled compounds were evaluated in nude mice bearing LNCaP tumors and compared to [Ga-68]Ga-PSMA-HBED-CC and [F-18]DCFPyL. Biodistribution studies of the two tracers with the highest imaged-derived tumor uptake and highest PSMA affinity were undertaken at 1 h, 2 h and 4 h post-injection (p.i.), and co-administration of PMPA was used to determine whether uptake was PSMA-specific. F-18-labeled triazolylphenyl ureas were prepared with a decay-corrected RCY of 20-40 %, > 98 % radiochemical and chemical purity, and specific activity of up to 391 GBq/mu mol. PSMA binding (IC50) ranged from 3-36 nM. The position of the triazole influenced tumor uptake (3 > 4 > 2), and direct conjugation of the triazole with the phenylurea moiety was preferred to insertion of a spacer group. Image-derived tumor uptake ranged from 6-14 %ID/g at 2 h p.i., the time of maximum tumor uptake; uptake of [Ga-68]Ga-PSMA-HBED-CC and [F-18]DCFPyL was 5-6 %ID/g at 1-3 h p.i., the time of maximum tumor uptake. Biodistribution studies of the two most promising compounds gave maximum tumor uptakes of 10.9 +/- 1.0 % and 14.3 +/- 2.5 %ID/g, respectively, as compared to 6.27 +/- 1.44 %ID/g for [Ga-68]Ga-PSMA-HBED-CC. Six [F-18]triazolylphenyl ureas were prepared in good radiochemical yield. Compounds showed PSMA-specific uptake in LNCaP tumors as high as 14 % ID/g, more than a 2-fold increase over [Ga-68]Ga-PSMA-HBED-CC. The facile and high-yielding radiosynthesis of these F-18-labeled triazoles as well as their promising in vitro and in vivo characteristics make them worthy of clinical development for PET imaging of prostate cancer.