A Novel Bifunctional Chelating Agent for Tyrosine-Specific Radiolabeling of Peptides and Proteins

Site-specific radiolabeling is utilized for the development of antibody- or peptide-based radiotheranostic agents. Although tyrosine can be exploited as one of the target residues for site-specific radiolabeling of peptides and proteins, a tyrosine-specific radiolabeling method has not been established. In this study, we newly designed and synthesized a novel bifunctional chelating agent, TBD-DO3A, consisting of a triazabutadiene (TBD) scaffold and metal chelator, 1,4,7,10-tetraazacyclododecane 1,4,7-triacetic acid (DO3A). Conjugation of TBD-DO3A with Ac-Tyr-NHMe followed by 111In-labeling afforded [111In]­In-Tyr-DO3A, which showed high-level stability in mouse plasma. Then, we selected the tyrosine-containing cyclic peptide c­(RGDyK) as a model ligand and synthesized [111In]­In-RYD. [111/natIn]­In-RYD showed in vitro binding properties for integrin αvβ3 equivalent to those of [111/natIn]­In-RKD, a lysine residue-labeled control compound. In in vivo biodistribution and SPECT/CT imaging studies using U87MG/PC-3 tumor-bearing mice, [111In]­In-RYD and [111In]­In-RKD were selectively accumulated and facilitated U87MG tumor visualization at 24 h postinjection. These results indicate that TBD-DO3A has fundamental properties as a bifunctional chelator for tyrosine-specific radiolabeling of peptides and proteins.