Synthesis and Preclinical Evaluation of Novel (68)Ga-Labeled (R)-Pyrrolidin-2-yl-boronic Acid-Based PET Tracers for Fibroblast Activation Protein-Targeted Cancer Imaging

Fibroblast activation protein (FAP) is a membrane-tethered serine protease overexpressed in the reactive stromal fibroblasts of >90% human carcinomas, which makes it a promising target for developing radiopharmaceuticals for the imaging and therapy of carcinomas. Here, we synthesized two novel (R)-pyrrolidin-2-yl-boronic acid-based FAP-targeted ligands: SB02055 (DOTA-conjugated (R)-(1-((6-(3-(piperazin-1-yl)propoxy)quinoline-4-carbonyl)glycyl)pyrrolidin-2-yl)boronic acid) and SB04028 (DOTA-conjugated ((R)-1-((6-(3-(piperazin-1-yl)propoxy)quinoline-4-carbonyl)-D-alanyl)pyrrolidin-2-yl)boronic acid). (nat)Ga- and (68)Ga-complexes of both ligands were evaluated in preclinical studies and compared to previously reported (nat)Ga/(68)Ga-complexed PNT6555. Enzymatic assays showed that FAP binding affinities (IC(50)) of (nat)Ga-SB02055, (nat)Ga-SB04028 and (nat)Ga-PNT6555 were 0.41 ± 0.06, 13.9 ± 1.29 and 78.1 ± 4.59 nM, respectively. PET imaging and biodistribution studies in HEK293T:hFAP tumor-bearing mice showed that while [(68)Ga]Ga-SB02055 presented with a nominal tumor uptake (1.08 ± 0.37 %ID/g), [(68)Ga]Ga-SB04028 demonstrated clear tumor visualization with ~1.5-fold higher tumor uptake (10.1 ± 0.42 %ID/g) compared to [(68)Ga]Ga-PNT6555 (6.38 ± 0.45 %ID/g). High accumulation in the bladder indicated renal excretion of all three tracers. [(68)Ga]Ga-SB04028 displayed a low background level uptake in most normal organs, and comparable to [(68)Ga]Ga-PNT6555. However, since its tumor uptake was considerably higher than [(68)Ga]Ga-PNT6555, the corresponding tumor-to-organ uptake ratios for [(68)Ga]Ga-SB04028 were also significantly greater than [(68)Ga]Ga-PNT6555. Our data demonstrate that (R)-(((quinoline-4-carbonyl)-d-alanyl)pyrrolidin-2-yl)boronic acid is a promising pharmacophore for the design of FAP-targeted radiopharmaceuticals for cancer imaging and radioligand therapy.