99mTc and 111In-Labeling of Small Biomolecules: Bifunctional Chelators and Related Coordination Chemistry

Radiolabeled small biomolecules provide a unique tool for target-specific delivery of radionuclides to the diseased tissues and have emerged as promising candidates in molecular imaging and radiotherapy of cancers. In general, a target-specific radiopharmaceutical can be divided into four parts: targeting biomolecule (BM), pharmacokinetic modifying (PKM) linker, bifunctional coupling or chelating agent (BFC), and radionuclide. The targeting biomolecule serves as a “carrier” for specific delivery of the radionuclide. PKM linkers are used to modify the radiotracer's pharmacokinetics. BFC is needed for radiolabeling of biomolecules with a metallic radionuclide. Different radiometals have significant difference in their coordination chemistry, and require BFCs with different donor atoms and chelator frameworks. Since the radiometal chelate can have a significant impact on biological properties of the target-specific radiopharmaceutical, its in vivo pharmacokinetics can be altered by modifying the coordination environment with various chelators or coligand. Among various SPECT radionuclides, 99mTc remains to be the most prominent radionuclide for the development of diagnostic radiopharmaceuticals because of its ideal nuclear properties and easy availability at low cost. 111In is also widely used in gamma scintigraphy (only second to 99mTc). In addition, 111In-labeld radiotracers are often used as the imaging surrogates for biodistribution and dosimetry determination of their corresponding 90Y analogs due to their similar coordination chemistry. This chapter will focus on the on the use of different BFCs for 99mTc and 111In-labeling of small biomolecules and the related coordination chemistry.