Coordination-Mediated Synthesis of 67Ga-Labeled Purification-Free Trivalent Probes for in Vivo Imaging of Saturable Systems

The Chemical Scaffold of Theranostic Radiopharmaceuticals: Radionuclide, Bifunctional Chelator, and Pharmacokinetics Modifying Linker

Therapeutic radiopharmaceuticals have been researched extensively in the last decade as a result of the growing research interest in personalized medicine to improve diagnostic accuracy and intensify intensive therapy while limiting side effects. Radiometal-based drugs are of substantial interest because of their greater versatility for clinical translation compared to non-metal radionuclides. This paper comprehensively discusses various components commonly used as chemical scaffolds to build radiopharmaceutical agents, i.e., radionuclides, pharmacokinetic-modifying linkers, and chelators, whose characteristics are explained and can be used as a guide for the researcher.

Aryl isocyanide derivative for one-pot synthesis of purification-free 99mTc-labeled hexavalent targeting probe

INTRODUCTION

^99mTc-labeled hexavalent probes can be readily synthesized by the coordination of six equivalent isocyanide ligands towards Tc^I, and alkyl isocyanide ligands have been extensively used for preparing such probes. However, high ligand concentration (>1 mM) is generally required due to their insufficient coordination ability to Tc^I.

METHODS AND RESULTS

In this study, we revealed that aryl isocyanide ligands, which have greater π-accepting ability compared with alkyl ones, provided ^99mTc-labeled hexavalent probes in high radiochemical yields (>95%) even at low ligand concentration (50 μM). We applied this finding to the synthesis of a ^99mTc-labeled hexavalent RGD probe, targeting integrin α_vβ₃. This ^99mTc-labeled probe was prepared in a 5 min reaction at ligand concentration of 50 μM, and exhibited high tumor localization in vivo without post-labeling purification.

CONCLUSION

The present findings indicate that aryl isocyanide ligands would be a useful precursor to a variety of ^99mTc-labeled hexavalent targeting probes for molecular imaging of saturable systems.

ADVANCES IN KNOWLEDGE

Aryl isocyanide is a better precursor than alkyl isocyanide for preparing ^99mTc-labeled hexavalent targeting probe.

IMPLICATION FOR PATIENT CARE

This work provides a straightforward method to prepare molecular imaging agents of high target uptake, which would facilitate nuclear medicine imaging in clinical settings.

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

The accumulation of ^99mTc-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 ^99mTc-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 [^99mTc]Tc-[(Pen-Hx-c(RGDfK))₂ that possessed much higher integrin α_vβ₃ 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 ^99mTc-Pen₂-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 ^99mTc-labeled compound was generated at low ligand concentration. However, Pen-Hx-sss-c(RGDfK) provided the objective bivalent ^99mTc-labeled probe in high radiochemical yields at a concentration similar to that of Pen-Hx-c(RGDfK). [^99mTc]Tc-[Pen-Hx-sss-c(RGDfK)]₂ also possessed stability and integrin α_vβ₃ binding affinity similar to those of [^99mTc]Tc-[Pen-Hx-c(RGDfK)]₂. As a result, [^99mTc]Tc-[Pen-Hx-sss-c(RGDfK)]₂ exhibited tumor and abdominal radioactivity levels similar to and significantly lower than those of [^99mTc]Tc-[Pen-Hx-c(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 ^99mTc-labeled bivalent probe without impairing the targeting ability. Thus, the [Pen-Hx-(PK modifier)-(targeting device)] would constitute a basic formulation for preparing the ^99mTc-Pen₂-based bivalent probes for imaging saturable targets of the body.

The synthesis of a 99mTc-labeled tetravalent targeting probe upon isonitrile coordination to 99mTcI for enhanced target uptake in saturable systems

The difference in 2-proton's acidity between L_β and L_G led to dramatically different results of their reactions with [^99mTc][Tc(CO)₃(OH₂)₃]⁺.