Manipulating Pharmacokinetics of Purification-Free Tc-Labeled Bivalent Probes for In Vivo Imaging of Saturable Targets.

The accumulation of Tc-labeled probes targeting saturable systems of the body is hindered by the presence of a large excess of unlabeled ligands needed to ensure high radiochemical yields in a short reaction time. To address the issue, we recently reported a novel concept of a metal-coordination-mediated synthesis of a bivalent Tc-labeled probe from a monovalent ligand using d-penicillamine (Pen) as a chelating molecule and c(RGDfK) as a model targeting device. The Pen-conjugated c(RGDfK) via a hexanoate linkage (Pen-Hx-c(RGDfK)) provided a bivalent [Tc]Tc-[(Pen-Hx-c(RGDfK)) that possessed much higher integrin αβ binding affinity than Pen-Hx-c(RGDfK) and visualized a murine tumor without purification. However, high radioactivity levels were observed in the abdominal regions, which necessitated improved pharmacokinetics of the probes for practical applications. In this study, a pharmacokinetic (PK) modifier was introduced to manipulate the pharmacokinetics of the Tc-Pen-based bivalent probe. The Hx linkage in Pen-Hx-c(RGDfK) was replaced with acetyl-d-serine-d-serine-glycine (Ac-ssG) or hexanoyl-d-serine-d-serine-d-serine (Hx-sss) to prepare Pen-Ac-ssG-c(RGDfK) or Pen-Hx-sss-c(RGDfK). Pen-Ac-ssG-c(RGDfK) impaired the complexation ability of Pen-Hx-c(RGDfK), and a monovalent Tc-labeled compound was generated at low ligand concentration. However, Pen-Hx-sss-c(RGDfK) provided the objective bivalent Tc-labeled probe in high radiochemical yields at a concentration similar to that of Pen-Hx-c(RGDfK). [Tc]Tc-[Pen-Hx-sss-c(RGDfK)] also possessed stability and integrin αβ binding affinity similar to those of [Tc]Tc-[Pen-Hx-c(RGDfK)]. As a result, [Tc]Tc-[Pen-Hx-sss-c(RGDfK)] exhibited tumor and abdominal radioactivity levels similar to and significantly lower than those of [Tc]Tc-[Pen-Hx-c(RGDfK)]. These findings indicate the incorporation of a tripeptide PK modifier to Pen-Hx-c(RGDfK) preserved the complexation ability and improved the pharmacokinetics of the resulting Tc-labeled bivalent probe without impairing the targeting ability. Thus, the [Pen-Hx-(PK modifier)-(targeting device)] would constitute a basic formulation for preparing the Tc-Pen-based bivalent probes for imaging saturable targets of the body.