7-[18F]Fluoro-8-azaisatoic Anhydrides: Versatile Prosthetic Groups for the Preparation of PET Tracers

18F-Fluorination of sensitive molecules is often challenging, but can be accomplished under suitably mild conditions using radiofluorinated prosthetic groups (PGs). Herein, 1-alkylamino-7-[18F]fluoro-8-azaisatoic anhydrides ([18F]AFAs) are introduced as versatile 18F-labeled building blocks that can be used as amine-reactive or "click chemistry" PGs. [18F]AFAs were efficiently prepared within 15 min by "on cartridge" radiolabeling of readily accessible trimethylammonium precursors. Conjugation with a range of amines afforded the corresponding 2-alkylamino-6-[18F]fluoronicotinamides in radiochemical conversions (RCCs) of 15-98%. In addition, radiolabeling of alkyne- or azide-functionalized precursors with azidopropyl- or propargyl-substituted [18F]AFAs using Cu-catalyzed click cycloaddition afforded the corresponding conjugates in RCCs of 44-88%. The practical utility of the PGs was confirmed by the preparation of three 18F-labeled PSMA ligands in radiochemical yields of 28-42%. Biological evaluation in rats demonstrated excellent in vivo stability of all three conjugates. In addition, one conjugate ([18F]JK-PSMA-15) showed favorable imaging properties for high-contrast visualization of small PSMA-positive lesions.

Next Generation Copper Mediators for the Efficient Production of 18 F-Labeled Aromatics

Cu-mediated radiofluorination is a versatile tool for the preparation of ¹⁸ F-labeled (hetero)aromatics. In this work, we systematically evaluated a series of complexes and identified several generally applicable mediators for highly efficient radiofluorination of aryl boronic and stannyl substrates. Utilization of these mediators in nBuOH/DMI or DMI significantly improved ¹⁸ F-labeling yields despite use of lower precursor amounts. Impressively, application of 2.5 μmol aryl boronic acids was sufficient to achieve ¹⁸ F-labeling yields of up to 75 %. The practicality of the novel mediators was demonstrated by efficient production of five PET-tracers and transfer of the method to an automated radiosynthesis module. In addition, (S)-3-[¹⁸ F]FPhe and 6-[¹⁸ F]FDOPA were prepared in activity yields of 23±1 % and 30±3 % using only 2.5 μmol of the corresponding boronic acid or trimethylstannyl precursor.

Evaluation of 3-l- and 3-d-[18F]Fluorophenylalanines as PET Tracers for Tumor Imaging

Simple Summary

The early detection and treatment of malignant brain tumors can significantly improve the survival time and life quality of affected patients. Whereas positron emission tomography (PET) with O-(2-[¹⁸F]fluoroethyl)tyrosine ([¹⁸F]FET) offers improved diagnostic accuracy compared to other imaging methods, there is still a need for PET tracers with better tumor-specificity. A higher protein incorporation rate, as well as a higher affinity for the amino acid transporter LAT1, could provide probes with superior image quality compared to [¹⁸F]FET. The aim of the present study was a preclinical evaluation of the two enantiomeric phenylalanine (Phe) analogues, 3-l- and 3-d-[¹⁸F]fluorophenylalanine ([¹⁸F]FPhes), as possible alternatives to [¹⁸F]FET. Based on promising in vitro evaluation results, the radiolabeled amino acids were studied in vivo in two subcutaneous and one orthotopic rodent tumor xenograft models using µPET. The results show that 3-l- and 3-d-[¹⁸F]FPhe enable high-quality visualization of tumors with certain advantages over [¹⁸F]FET, making them promising candidates for further preclinical and clinical evaluations.

Abstract

Purpose: The preclinical evaluation of 3-l- and 3-d-[¹⁸F]FPhe in comparison to [¹⁸F]FET, an established tracer for tumor imaging. Methods: In vitro studies were conducted with MCF-7, PC-3, and U87 MG human tumor cell lines. In vivo µPET studies were conducted in healthy rats with/without the inhibition of peripheral aromatic l-amino acid decarboxylase by benserazide pretreatment (n = 3 each), in mice bearing subcutaneous MCF-7 or PC-3 tumor xenografts (n = 10), and in rats bearing orthotopic U87 MG tumor xenografts (n = 14). Tracer accumulation was quantified by SUV_max, SUV_mean and tumor-to-brain ratios (TBrR). Results: The uptake of 3-l-[¹⁸F]FPhe in MCF-7 and PC-3 cells was significantly higher relative to [¹⁸F]FET. The uptake of all three tracers was significantly reduced by the suppression of amino acid transport systems L or ASC. 3-l-[¹⁸F]FPhe but not 3-d-[¹⁸F]FPhe exhibited protein incorporation. In benserazide-treated healthy rats, brain uptake after 42–120 min was significantly higher for 3-d-[¹⁸F]FPhe vs. 3-l-[¹⁸F]FPhe. [¹⁸F]FET showed significantly higher uptake into subcutaneous MCF-7 tumors (52–60 min p.i.), while early uptake into orthotopic U87 MG tumors was significantly higher for 3-l-[¹⁸F]FPhe (SUV_max: 3-l-[¹⁸F]FPhe, 107.6 ± 11.3; 3-d-[¹⁸F]FPhe, 86.0 ± 4.3; [¹⁸F]FET, 90.2 ± 7.7). Increased tumoral expression of LAT1 and ASCT2 was confirmed immunohistologically. Conclusion: Both novel tracers enable accurate tumor delineation with an imaging quality comparable to [¹⁸F]FET.