The synthesis of a Tc-labeled tetravalent targeting probe upon isonitrile coordination to Tc for enhanced target uptake in saturable systems.

The presence of excess unlabeled ligands in the injectate hinders the target uptake of Tc-labeled targeting vectors. To address the issue, we previously developed a chemical design which provides a Tc-labeled trivalent RGD probe upon CN-βAla-Gly-Gly-c(RGDfK) (L) coordination to [Tc][Tc(CO)] core at pH 6.0. In this study, we extended our coordination mediated synthesis of the trivalent RGD probe to that of a tetravalent one. Our initial attempts reacting L with [Tc][Tc(CO)] core at pH 8.0 failed to provide [Tc][Tc(CO)(L)] due to the formation of multiple side products. A γ-aminobutylic acid (GABA) based isonitrile ligand CN-GABA-Gly-Gly-c(RGDfK) (L), on the other hand, avoided the side reaction and selectively provided [Tc][Tc(CO)(L)] (Tc-[L]) at pH 8.0. Tc-[L] exhibited higher binding affinity to integrin αβ than its unlabeled ligand, and visualized U87MG tumor without tedious post-labeling purification. These results indicate that the metal coordination-mediated syntheses of Tc-labeled multivalent probes have been successfully applied to a tetravalent one, which would allow a wider range of choices for designing novel Tc-labeled multivalent probes of high target uptake.