Evolution of Tc-99m in diagnostic radiopharmaceuticals

The progress in diagnostic nuclear medicine over the years since the discovery of ^99mTc is indeed phenomenal. Over 80% of the radiopharmaceuticals currently being used make use of this short-lived, metastable radionuclide, which has reigned as the workhorse of diagnostic nuclear medicine. The preeminence of ^99mTc is attributable to its optimal nuclear properties of a short half-life and a gamma photon emission of 140 keV, which is suitable for high-efficiency detection and which results in low radiation exposure to the patient. ^99mTcO₄⁻, which is readily available as a column eluate from a ⁹⁹Mo/^99mTc generator, is reduced in the presence of chelating agents. The versatile chemistry of technetium emerging from the 8 possible oxidation states, along with a proper understanding of the structure-biologic activity relationship, has been exploited to yield a plethora of products meant for morphologic and functional imaging of different organs. This article reviews the evolution of ^99mTc dating back to its discovery, the development of ⁹⁹Mo/^99mTc generators, and the efforts to exploit the diverse chemistry of the element to explore a spectrum of compounds for diagnostic imaging, planar, and single photon emission computed tomography. A brief outline of the ^99mTc radiopharmaceuticals currently being used has been categorically presented according to the organs being imaged. Newer methods of labeling involving bifunctional chelating agents (which encompass the “3+1” ligand system, Tc(CO)₃⁺¹-containing chelates, hydrazinonicotinamide, water-soluble phosphines, and other Tc-carrying moieties) have added a new dimension for the preparation of novel technetium compounds. These developments in technetium chemistry have opened new avenues in the field of diagnostic imaging. These include fundamental aspects in the design and development of target-specific agents, including antibodies, peptides, steroids, and other small molecules that have specific receptor affinity.