Technetium coupled to biologically active moieties

Many ^99mTc pharmaceuticals were designed for the measurement of organ function, based on regional blood flows, ion transport, and cellular rentention. Organ specificity is governed by molecular characteristics (e.g., size, shape, charge) and physiological factors.
Primarily, these radiotracers are coordination complexes of technetium leaving either a positive or a negative charge; neutral, lipophilic complexes pass the blood-brain barrier. Organ function is related to regional perfusion (e.g., brain, heart). Hepatocyte function is measured by the excretion of iminodiacetic acid (IDA) derivatives into bile, simulating the active transport of bilirubin. Increased osteogenic activity correlates with increased regional uptake of ^99mTc-diphosphonate complexes in bone structures, delineating tumor and metastatic growth. The functional state of the kidneys as measured by active tubular secretion requires a negatively charged complex with a carboxylate anion.
^99mTc pharmaceuticals based on coordination complexes with functionalized ligands are also known as "Tc essentials"; those concerning labeled particles and macromolecules are called "Tc-tagged" radiopharmaceuticals. A variety of chelating agents have been developed for complex formation with certain oxidation states of technetium, providing the structural requirements for uptake and rentention (Schwochau 1000). Examples of Tc essentials are shown in Fig. 2.3.1.
The outstanding interest in the development of novel ^99mTc pharmaceuticals is documented in recent reviews (Hom and Katzenellenbogen 1997; Johannsen and Pietzsch 2002a; Jurisson and Lydon 1999)