Kit-like 18F-labeling using triazole-linked conjugates for [18F]aluminum monofluoride complexation

Objectives: For about 10 years, numerous ligands have been developed and tested for their suitability for 18F-labeling in the form of [18F]aluminum monofluoride. Initially, the well-known macrocyclic ligands NOTA and NODA [1,2] with high temperature reactions were used. Under current approaches, open chain ligands, derived from ethylene diamine, provide a much faster complex formation at lower temperatures (<40°C) [3]. The use of a terminal alkyne linker, as one of these new approaches, also enables the conjugation to target molecules via copper catalyzed azide-alkyne cycloaddition (CuAAC) as the final step in ligand synthesis. Furthermore, these triazole-containing ligands offer interesting additional applications, besides kit-like 18F-labeling, they also can be used for the stable fixation of other radiometal ions preferring a six-fold coordination. Radionuclides like 68Ga, 111In or 64Cu show promising results with these ligands and are under current investigation.
Methods: Ligand L1-L3 synthesis was carried out via CuAAC with subsequent product characterization. Radiolabeling reactions with the ligands L1-L3 were carried out under comparable conditions for different radionuclides: 0.1 M sodium acetate buffer pH 4-5, 10 µg ligand, 20 min mixing at room temperature with [68Ga]GaCl3, [111In]InCl3, [64Cu]CuCl2 or freshly prepared [18F]AlFCl2-solution. Examples for the ligand synthesis, radiolabeling experiments with different radionuclides, stability studies and initial in vitro and in vivo studies will be presented in this work.
Results: The [68Ga]Ga-L1, [64Cu]Cu-L1, and [18F]AlF-L1 complexes show comparable in vivo behavior 5 min after single intravenous injection in NMRI nu/nu mice. The gallium and fluorine labeled species show faster renal elimination than the copper compound. The [68Ga]Ga-L1 complex also shows higher stability in serum than the corresponding copper complex [64Cu]Cu-L1. The 18F-labeled species show very low defluorination in vivo and therefore high stability.
Conclusion: The incorporation of the ligand via CuAAC opens up many new possibilities for the radiolabeling of peptides and small antibody-derived proteins by 18F as coordinated [18F]aluminum monofluoride. Additional options are given due to the flexibility of the triazole-containing conjugates for further radionuclides like 68Ga, 64Cu and 111In.