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 <i>in vitro</i> binding properties for integrin α_vβ₃ equivalent to those of [^111/natIn]In-RKD, a lysine residue-labeled control compound. In <i>in vivo</i> 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.