Manipulating Pharmacokinetics of Purification-Free ^99mTc-Labeled Bivalent Probes for In Vivo Imaging of Saturable Targets

Abstract

The accumulation of 99m 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-coordinationmediated synthesis of a bivalent 99m 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 [ 99m Tc]Tc-[(Pen-Hx-c(RGDfK)) 2 that possessed much higher integrin α v β 3 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 99m Tc-Pen 2 -based bivalent probe. The Hx linkage in Pen-Hx-c(RGDfK) was replaced with acetyl-D-serine-D-serineglycine (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 99m Tc-labeled compound was generated at low ligand concentration. However, Pen-Hx-sss-c(RGDfK) provided the objective bivalent 99m Tc-labeled probe in high radiochemical yields at a concentration similar to that of Pen-Hx-c(RGDfK). [ 99m Tc]Tc-[Pen-Hx-sss-c(RGDfK)] 2 also possessed stability and integrin α v β 3 binding affinity similar to those of [ 99m Tc]Tc-[Pen-Hx-c(RGDfK)] 2 . As a result, [ 99m Tc]Tc-[Pen-Hx-sssc(RGDfK)] 2 exhibited tumor and abdominal radioactivity levels similar to and significantly lower than those of [ 99m Tc]Tc-[Pen-Hxc(RGDfK)] 2 . 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 99m 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 99m Tc-Pen 2 -based bivalent probes for imaging saturable targets of the body.

Development of a hydroxamamide-based bifunctional chelating agent to prepare technetium-99m-labeled bivalent ligand probes

“…To date, several chelating groups, such as D-penicillamine 23 , 24 and isocyanide 25 , 26 , have been reported to form multivalent complexes with 99m Tc. These chelates reacted with 99m Tc effectively at a low concentration of the precursors, but their reaction temperature was high (100–120 °C).…”

Development of a Hydroxamamide-Based Bifunctional Chelating Agent to Prepare Technetium-99m-Labeled Bivalent Ligand Probes

“…To date, several chelating groups, such as D-penicillamine 22,23 and isocyanide 24,25 , have been reported…”

Beyond Small Molecules: Antibodies and Peptides for Fibroblast Activation Protein Targeting Radiopharmaceuticals

“…In contrast, small peptides typically exhibit favorable PK profiles characterized by rapid clearance from tissues and efficient tumor penetration [ 105 , 187 , 188 , 189 , 190 , 191 , 192 ]. As such, peptide-based radiopharmaceuticals have emerged as the most promising candidates for molecular imaging in the clinical setting [ 193 , 194 , 195 , 196 , 197 , 198 , 199 , 200 , 201 , 202 , 203 , 204 , 205 , 206 ]. Cyclic peptides, in particular, offer enhanced structural stability and resistance to enzymatic degradation compared with linear peptides, greatly enhancing their efficacy in radiological diagnosis and therapy [ 207 , 208 , 209 , 210 , 211 , 212 , 213 , 214 , 215 ].…”