Coordination-Mediated Synthesis of Purification-Free Bivalent Tc-Labeled Probes for in Vivo Imaging of Saturable System.

In the synthesis of technetium-99m (Tc) labeled target-specific ligands, the presence of a large excess of unlabeled ligands over Tc in the injectate hinders target accumulation of Tc-labeled ligands by competing for target molecules. To circumvent the problem, we recently developed a concept of the metal coordination-mediated multivalency, and proved the concept with a Tc-labeled trivalent compound [Tc(CO)(CN-RGD)]. In this study, D-penicillamine (Pen) was selected as a chelating molecule and a cyclic RGDfK peptide was conjugated to Pen via a hexanoic linkage (Pen-Ahx-c(RGDfK)). Tc complexation reaction, and the stability, integrin αβ binding affinity, and biodistribution of the Tc-labeled probe were investigated to evaluate the applicability of the concept to bivalent probes. Tc-[Pen-Ahx-c(RGDfK)] was obtained over 95% radiochemical yields under low Pen-Ahx-c(RGDfK) concentration (50 μM). Tc-[Pen-Ahx-c(RGDfK)] showed approximately 10-times higher integrin αβ binding affinity than the monovalent compounds, Pen-Ahx-c(RGDfK) and c(RGDyV). In biodistribution studies, the tumor accumulation of Tc-[Pen-Ahx-c(RGDfK)] was decreased to 77% and 43% of HPLC-purified (Pen-Ahx-c(RGDfK)-free) Tc-[Pen-Ahx-c(RGDfK)] by the presence of 5 nmol of unlabeled Pen-Ahx-c(RGDfK) and Re-[Pen-Ahx-c(RGDfK)], respectively. Tc-[Pen-Ahx-c(RGDfK)] provided tumor image without removing unlabeled ligand, while a Tc-labeled monovalent probe prepared from a monovalent ligand could not. These findings indicate the availability of the design concept to prepare Tc-labeled bivalent probes with a variety of Tc core and other metallic radionuclides of clinical relevance.