Purpose: The aim of this study was to synthesize and preclinically evaluate an 18F-PSMA positron emission tomography (PET) tracer. Prostate-specific membrane antigen (PSMA) specificity, biodistribution, and dosimetry in healthy and tumor-bearing mice were determined.
Methods: Several conditions for the labeling of 18F-PSMA-11 via 18F-AlF-complexation were screened to study the influence of reaction temperature, peptide amount, ethanol volume, and reaction time. After synthesis optimization, biodistribution and dosimetry studies were performed in C57BL6 mice. For proof of PSMA-specificity, mice were implanted with PSMA-negative (PC3) and PSMA-positive (LNCaP) tumors in contralateral flanks. Static and dynamic microPET/computed tomography (CT) imaging was performed.
Results: Quantitative labeling yields could be achieved with >97 % radiochemical purity. The 18F-PSMA-11 uptake was more than 24-fold higher in PSMA-high LNCaP than in PSMA-low PC3 tumors (18.4 ± 3.3 %ID/g and 0.795 ± 0.260 %ID/g, respectively; p < 4.2e-5). Results were confirmed by ex vivo gamma counter analysis of tissues after the last imaging time point. The highest absorbed dose was reported for the kidneys. The maximum effective dose for an administered activity of 200 MBq was 1.72 mSv.
Conclusion: 18F-PSMA-11 using direct labeling of chelate-attached peptide with aluminum-fluoride detected PSMA-expressing tumors with high tumor-to-liver ratios. The kidneys were the dose-limiting organs. Even by applying the most stringent dosimetric calculations, injected activities of up to 0.56 GBq are feasible.
Keywords: 18F; Dosimetry; PET; PSMA; Preclinical; Prostate cancer.