4-¹⁸F-Fluorobenzonitrile oxide ([¹⁸F]FBNO]) - A novel amine-reactive prosthetic group for radiolabeling

Objectives A direct addition of nucleophiles such as amines and thiols to 1,3-dipoles such as nitrile oxides has not yet been explored in radiochemistry. The aim of this work was to study the applicability of this reaction for radiofluorination. Consequently, ¹⁸F-labeled conjugates, including a novel PSMA-ligand, were prepared via addition of nitrogen or sulphur nucleophiles to [¹⁸F]FBNO. Additionally, first results of the in vitro evaluation of the PSMA-ligand are also presented.

Methods [¹⁸F]FBNO was generated in situ from the respective N-hydroxy-benzoimidoyl chloride ([¹⁸F]FBIC) and allowed to react with H-βAla-Phe-OMe, H-DUPA and Boc-Cys(H)-OMe in aq. EtOH at rt to give conjugates [¹⁸F]1, [¹⁸F]2 and [¹⁸F]3, respectively. Hydrolytic (pH = 2, 7, 9) and blood serum stability of [¹⁸F]2 and [¹⁸F]3 was determined at 37 ^○C. Cellular uptake of [¹⁸F]2 in PSMA-positive LNCaP C4-2 and PSMA-negative PC-3 prostate tumor cells was determined in the presence and absence of 2-PMPA (PSMA-inhibitor).

Results [¹⁸F]1-3 were prepared in 50-92% RCYs after 10 min incubation at pH = 8-9. Radiofluorinated amidoxime [¹⁸F]2 demonstrated excellent hydrolytic and blood serum stability (> 98% after 3 h incubation). In contrast, thiohydroxymate [¹⁸F]3 was unstable at pH 9 and only moderately stable in blood serum (35% after 3 h incubation). A high accumulation of [¹⁸F]2 in LNCaP C4-2 cells was blocked by 2-PMPA up to 95% (%ID/10⁵ cells: 2.31±0.25 vs. 0.15±0.05 and 5.5±0.3 vs. 0.2±0.05 after 1 and 4 h, respectively). On the contrary, the cellular uptake in PC-3 control cells was low.

Conclusions [¹⁸F]FBNO is a new easy accessible amine-reactive prosthetic group for rapid radiofluorination of peptides and small molecules under mild conditions. Consequently, it represents an interesting alternative to [¹⁸F]SFB and other ¹⁸F-labelled active esters. Furthermore, [¹⁸F]2 is a promising PET-tracer candidate for imaging of PSMA-positive tumors.

Research Support Supported by EFRE-ZIEL2 program (North Rhine-Westphalia, Germany).