High-valent technetium chemistry-new opportunities for radiopharmaceutical developments

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

Hydroxamamide (Ham) is a thiol-free chelating agent that forms technetium-99m (^99mTc)-complexes with a metal-to-ligand ratio of 1:2 under moderate reaction conditions. Therefore, Ham-based chelating agents will produce ^99mTc-labeled compounds with a bivalent targeting scaffold. For their universal usage, we developed a novel Ham-based bifunctional chelating agent, “Ham-Mal”, with a maleimide group that can easily conjugate with a thiol group, for to preparing ^99mTc-labeled bivalent ligand probes. Ham-Mal was synthesized by a four-step reaction, and then reacted with cysteine or c(RGDfC) to produce Ham-Cys or Ham-RGD. These precursors were reacted with ^99mTcO₄^- for 10 min under room temperature to obtain ^99mTc-(Ham-Cys)₂ and ^99mTc -(Ham-RGD)₂. The cellular uptake level of ^99mTc-(Ham-RGD)₂ by U87MG (high Integrin ɑ_vβ₃ expression) cells was significantly higher than that by PC3 (low Integrin ɑ_vβ₃ expression) cells at 60 min after the incubation, and the uptake was significantly suppressed by pre-treatment for 15 min with excess c(RGDfK) peptide. In the in vivo study with U87MG/PC3 dual xenografted BALB/c-nu mice, the radioactivity of U87MG tumor tissue was significantly higher than that of PC3 tumor tissue at 360 min after the administration of ^99mTc-(Ham-RGD)₂. These results suggest Ham-Mal may have potential as a bifunctional chelating agent for ^99mTc-labeled bivalent ligand probes.

Technetium Complexes and Radiopharmaceuticals with Scorpionate Ligands

Scorpionate ligands have played a crucial role in the development of technetium chemistry and, recently, they have also fueled important advancements in the discovery of novel diagnostic imaging agents based on the γ-emitting radionuclide technetium-99m. The purpose of this short review is to provide an illustration of the most general and relevant results in this field, however without being concerned with the details of the analytical features of the various compounds. Thus, emphasis will be given to the description of the general features of technetium complexes with scorpionate ligands including coordination modes, structural properties and an elementary bonding description. Similarly, the most relevant examples of technetium-99m radiopharmaceuticals derived from scorpionate ligands and their potential interest for nuclear imaging will be summarized.

Technetium-99m radiochemistry for pharmaceutical applications

Technetium-99m (^99m Tc) is a widely used radionuclide, and the development of ^99m Tc imaging agents continues to be in demand. This overview discusses basic principles of ^99m Tc radiopharmaceutical preparation and design and focuses on the ^99m Tc radiochemistry relevant to its pharmaceutical applications. The ^99m Tc complexes are described based on the most typical examples in each category, keeping up with the state-of-the-art in the field. In addition, the main current strategies to develop targeted ^99m Tc radiopharmaceuticals are summarized.