The synthesis of a 99m Tc-labeled tetravalent targeting probe upon isonitrile coordination to 99m Tc I for enhanced target uptake in saturable systems.

The presence of excess unlabeled ligands in the injectate hinders the target uptake of 99m Tc-labeled targeting vectors. To address the issue, we previously developed a chemical design which provides a 99m Tc-labeled trivalent RGD probe upon CN-βAla-Gly-Gly-c(RGDfK) (L β ) coordination to [ 99m Tc][Tc(CO) 3 ] + 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 β with [ 99m Tc][Tc(CO) 3 ] + core at pH 8.0 failed to provide [ 99m Tc][Tc(CO) 2 (L β ) 4 ] + due to the formation of multiple side products. A γ-aminobutylic acid (GABA) based isonitrile ligand CN-GABA-Gly-Gly-c(RGDfK) (L G ), on the other hand, avoided the side reaction and selectively provided [ 99m Tc][Tc(CO) 2 (L G ) 4 ] + ( 99m Tc-[L G ] 4 ) at pH 8.0. 99m Tc-[L G ] 4 exhibited higher binding affinity to integrin α v β 3 than its unlabeled ligand, and visualized U87MG tumor without tedious post-labeling purification. These results indicate that the metal coordination-mediated syntheses of 99m Tc-labeled multivalent probes have been successfully applied to a tetravalent one, which would allow a wider range of choices for designing novel 99m Tc-labeled multivalent probes of high in vivo target uptake.