Radioiodinated tracers for myocardial imaging

Recent advances in the efficient production of high purity radioiodine (¹²³I) and new efficient radiolabeling techniques have allowed the development of new classes of cardiovascular radiopharmaceuticals. These include ¹²³I-labeled fatty acids to assess myocardial metabolism, ¹²³I-metaiodobenzylguanidine (MIBG) for myocardial neuronal activity, labeled monoclonal antibodies for myocardial necrosis, and labeled lipoproteins for receptor concentration. ¹²³I-labeled fatty acids and MIBG are under clinical investigation with encouraging results. ¹²³I-and ¹¹¹In-labeled fragments of monoclonal antibodies to myosin have been used for imaging myocardial necrosis in humans. The development of radiotracers for imaging of cholinergic and adrenergic receptors is still in the experimental stage. Recent advances in imaging instrumentation and radiopharmaceuticals have resulted in cardiac imaging applications beyond blood pool ventriculography, perfusion, and infarctavid imaging. Developments of radioiodine (¹²³I)-labeled agents promise to play an important role in the assessment of myocardial metabolism, neuronal activity, and receptor concentration. The chemistry activity, and receptor concentration. The chemistry of iodine is well defined compared with that of ^99mTc; therefore, iodine isotopes are well suited for labeling biologically important molecules. Among the iodine isotopes, ¹²³I has nearly ideal nuclear properties for nuclear medical applications with a 13.3-hour half-life (T_1/2) and 159 keV gamma emission (83%). Dispite the nearly ideal chemical and nuclear properties of ¹²³I, the widespread application of ¹²³I-based radiopharmaceuticals in clinical practice has been limited by high production costs (¹²³I is produced in a cyclotron), relatively limited availability, and the presence of undesirable radionuclidic impurities (¹²⁴I, T_1/2=4.2 days; ¹²⁵I, T_1/2=60 days; ¹²⁶I, T_1/2=13.1 days). The relatively long T_1/2 and beta particle emission can substantially increase the higher radiation burden to the patient. High energy gamma rays (>600 KeV) from these impurities can degrade images obtained using low energy collimators. Recent developments in production techniques have greatly reduced the levels of the undesirable radio-nuclides in ¹²³I. Ready availability of pure ¹²³I at modest cost, in concentrations suitable for the radiolabeling of a variety of useful biomolecules, should enhance the clinical applications of ¹²³I-labeled compounds. Molecules labeled with ¹²³I that are useful in cardiac imaging studies are fatty acid analogs, monoclonal antibodies, receptor binding agents, and norepinephrine analogs. This article will discuss developments in radioiodine (¹²³I)-labeled radiotracers for mycordial imaging.