Direct radiofluorination of a heat-sensitive antibody by Al-¹⁸F complexation

¹⁸F is the most widely used radionuclide for PET imaging. However, peptides and antibodies cannot be radiolabeled with fluorine-18 without the extra attachment of an auxiliary group, and these biomolecules are typically radiolabeled indirectly through two reaction steps. The Al-¹⁸F complex has recently been developed as a new radiolabeling technique; however, the harsh conditions required for Al-¹⁸F complexation have restricted its widespread application for heat-sensitive biomolecules. Here we report fully optimized Al-¹⁸F radiofluorination enabling successful radiolabeling under mild conditions. Six different macrocyclic chelators with different backbones (NOTA, DOTA, and NODA) and pendant arms were tested for efficient radiolabeling using the Al-¹⁸F complex. We also optimized the radiolabeling conditions to achieve radiofluorination at a lower temperature by changing the radioactive fluorine source, labeling buffer, and organic solvent. By employing a NODA backbone chelator bearing two acetate pendant arms and one benzyl-NCS moiety, >60% Al-¹⁸F complexation was achieved at 30 °C in ammonium acetate buffer (pH 5.5). An antibody conjugated with the NODA chelator was also successfully radiolabeled with ¹⁸F at 30 °C using Al-¹⁸F complexation without loss of immunoreactivity.