Synthesis and Preclinical Evaluation of Novel F-labeled Glu-urea-Glu-based PSMA Inhibitors for Prostate Cancer Imaging: a Comparison with F-DCFPyl and F-PSMA-1007

Background: Due to its high and consistent expression in prostate cancer (PCa), the prostate-specific membrane antigen (PSMA) represents an ideal target for molecular imaging and targeted therapy using highly specific radiolabeled PSMA ligands. To address the continuously growing clinical demand for F-labeled PSMA-probes, we developed two novel Glu-urea-Glu-(EuE)-based inhibitors, EuE-k-F-FBOA (1) and EuE-k-β-a-F-FPyl (2), both with optimized linker structure and different F-labeled aromatic moieties. The inhibitors were evaluated in a comparative preclinical study with F-DCFPyl and F-PSMA-1007.

Results: Radiolabeling procedures allowed preparation of (1) and (2) with high radiochemical yields (67 ± 7 and 53 ± 7%, d.c.) and purity (> 98%). When compared with F-DCFPyl (IC = 12.3 ± 1.2 nM) and F-PSMA-1007 (IC = 4.2 ± 0.5 nM), both metabolically stable EuE-based ligands showed commensurable or higher PSMA affinity (IC = 4.2 ± 0.4 nM (1), IC = 1.1 ± 0.2 nM (2)). Moreover, 1.4- and 2.7-fold higher internalization rates were observed for (1) and (2), respectively, resulting in markedly enhanced tumor accumulation in LNCaP-tumor-bearing mice ((1) 12.7 ± 2.0% IA/g, (2) 13.0° ± 1.0% IA/g vs. 7.3 ± 1.0% IA/g (F-DCFPyl), 7.1 ± 1.5% IA/g (F-PSMA-1007), 1 h p.i.). In contrast to (1), (2) showed higher kidney accumulation and delayed clearance kinetics. Due to the high hydrophilicity of both compounds, almost no unspecific uptake in non-target tissue was observed. In contrast, due to the less hydrophilic character (logP = - 1.6) and high plasma protein binding (98%), F-PSMA-1007 showed uptake in non-target tissue and predominantly hepatobiliary excretion, whereas, F-DCFPyl exhibited pharmacokinetics quite similar to those obtained with (1) and (2).

Conclusion: Both F-labeled EuE-based PSMA ligands showed excellent in vitro and in vivo PSMA-targeting characteristics. The substantially higher tumor accumulation in mice compared to recently introduced F-PSMA-1007 and F-DCFPyl suggests their high value for preclinical studies investigating the effects on PSMA-expression. In contrast to (2), (1) seems to be more promising for further investigation, due to the more reliable F-labeling procedure, the faster clearance kinetics with comparable high tumor uptake, resulting therefore in better high-contrast microPET imaging as early as 1 h p.i.