Current status and prospects of new radionuclides and radiopharmaceuticals for cardiovascular nuclear medicine.

The rapid emergence of new imaging modalities like positron emission tomography (PET) and single photon emission computerized tomography (SPECT) and their advance into the clinical arena offered new opportunities for, but also stimulated research and development of new radiopharmaceuticals suitable for cardiac imaging. While tracers of myocardial blood flow remained in the center of interest, other trends heralded possibilities of studying more comprehensively cardiac physiology and pathophysiology as, for example, metabolism, the severity of tissue injury, neural activity and membrane function. N-13 ammonia and rubidium-82 became the primary tracers for evaluating and possibly quantifying regional myocardial blood flow with PET, while cationic Tc-99m isonitrile complexes have now reached a stage where high contrast images of the human heart are obtained on planar scintigraphy and SPECT. These radiopharmaceuticals hold considerable promise for routine clinical use. Tracers of metabolism, especially those labeled with positron emitting isotopes as for example, C-11 palmitate, F-18 2-deoxyglucose, are approaching the phase of clinical use and provide information on regional myocardial substrate metabolism and oxidative processes. Less successful and more limited were developments of single photon emitting tracers of metabolism which remained largely confined to radioiodinated fatty acid analogs. Exploration and characterization of the metabolic fate of the radiolabel in tissue and its relation to the externally observed signal have been truly impressive. Tested in humans primarily in western European countries, these tracers promise to yield metabolic information on a more limited scope. Most widely applied are iodohepta- and hexadecanoic acid and, more recently, the aromatic fatty acid analog, paraiodophenylpentadecanoic acid. Labeled monoclonal antibodies rapidly advanced to the point of clinical use. Accurate identification and sizing of acute myocardial infarction is now possible with Tc-99m or indium-111 labeled specific antimyosin antibody fragments. This success stimulated new research activities for use of labeled antibody techniques in other areas as for example, scintigraphic evaluation of formation and presence of vascular thrombi. While promising, these efforts have however remained in an early stage of development. The same holds true for single photon and positron emitting tracers that are suitable for assessing sympathetic neuron densities in myocardium as well as imaging of both cholinergic and adrenergic receptors. While still in their infancy, these latter developments hold considerable promise but await further development and clinical testing. It thus appears that while the mainstream of new radiopharmaceutical development has been in the areas of imaging of blood flow and tissue necrosis, more recent accomplishments with tracers of other aspects of cardiovascular physiology and pathophysiology promise to trespass traditional boundaries of nuclear cardiology techniques and offer the prospect for more comprehensive evaluation of the human heart in health and disease.