Technetium-99m chelators in nuclear medicine. A review

Nuclear medicine is a branch of medical imaging that uses radioactive tracers to examine the function of body systems. The radionuclide used in about 90% of all examinations is 99Tcm, which is available from 99Mo/99Tcm generators at most nuclear medicine departments. In aqueous medium, technetium is chemically stable as pertechnetate, 99TcmO4-. Injected into the human body, pertechnetate will be absorbed by the thyroid gland because of the similarity to iodide in its radius and charge. To reach targets in the human body other than glandula thyreoidea, 99Tcm needs a carrier molecule, usually a chelating agent. Many chelators that form stable complexes with 99Tcm have affinities for certain tissues in the human body. Other chelators can be manipulated by pharmaceutical formation to be retained in certain body systems. In order to form bonds with technetium, the chelator must contain electron donors like nitrogen, oxygen and sulfur. Space between multiple electron donor atoms is required to allow several bonds to form with the central metal. The stability of the complex increases with increasing number of bonds. Today, chelators for the use with 99Tcm exist for a number of highly sensitive scintigraphic studies of the brain, heart, skeleton, kidneys, hepatobiliary system and lungs. This includes chelators such as dimercaptosuccinic acid, 1,2-ethylenediylbis-L-cysteine diethyl ester, methylenediphosphonate, hexamethylpropyleneamineoxime and hexakis(methoxy isobutyl isonitrile).