Use of radioactive iodine-labeled fatty acids for myocardial imaging

Under normal metabolic conditions, free fatty acids are the major energy source for the myocardium. During myocardial ischemia, cardiac metabolism is altered so that fatty acid clearance is decreased. Thus, iodine-radiolabeled fatty acids have attracted interest as cardiac imaging agents to localize ischemia and infarction in patients with coronary heart disease. ¹²³I-hexadecanoic acid and ¹²³I-heptadecanoic acid have been extensively studied in animals and humans. A major limitation of these compounds has been rapid deiodination by betaoxidation and subsequent development of significant blood pool activity. Consequently, alternative free fatty acids have been developed, and ¹²³I-phenylpentadecanoic acid (IPPA) has been shown to have excellent imaging characteristics. IPPA has a myocardial half-life of 70 minutes and does not release free iodine into the blood pool following betaoxidation. IPPA has been demonstrated to be useful for single photon emission computer tomography (SPECT) and planar imaging. Normal, ischemic, and infarcted myocardium can be identified by IPPA imaging. Normal myocardium demonstrates uniform uptake and clearance of IPPA. Ischemic myocardium demonstrates decreased clearance of IPPA with diminished or normal uptake, whereas infarcted myocardium demonstrates decreased IPPA uptake. In this manuscript, we review the background, technical developments, and clinical utility of iodine-labeled free fatty acids as myocardial perfusion-metabolic imaging agents. Representative illustrations of myocardial imaging with IPPA are also shown.