The role of iodine-124 positron emission tomography in molecular imaging

Radioactive iodine has been, in various forms, the mainstay of Nuclear Medicine. Iodine-123 is the most widely used iodine isotope for single photon imaging. Iodine-125 continues to be used in diverse applications from in vitro radioassay to in vivo estimation of various pathophysiologic correlates. Iodine-131 (131 I), useful for imaging as well as therapy, has contributed more than any other radionuclide to the growth and sustenance of Nuclear Medicine. Positron emission tomography (PET) is an indispensable tool in current clinical practice, spurred by the rapid and increasing availability of radiopharmaceuticals for in vivo imaging. Most clinical PET imaging utilizes fluorine-18; there is a need for positron emitters with longer half-lives, suitable for imaging larger molecules of interest. Iodine-124 (I-124) has approximately 23 % positron emission; its 4-day half-life lends itself to sequential imaging, and its dosimetry is comparable to iodine-131. Iodine can be easily attached to a variety of molecules without alteration of physico-chemical or biologic properties. PET with I-124-labeled molecules enables longitudinal in vivo assessment of their distribution; such pharmacokinetic and biodistribution information has considerable utility in oncologic and non-oncologic applications. We will provide a clinical perspective on the physical and chemical characteristics of I-124, as the iodide, as well as radiolabeled to a wide variety of molecules of interest. Radioiodination is accomplished based on the chemical and biologic nature of the ligand to be studied, and issues of relevance will be highlighted. We will conclude by describing the current and potential future clinical applications of I-124-based tracers used for molecular imaging in diagnosis and therapy.