Single-Photon Emitting Radiopharmaceuticals for Diagnostic Applications

Abstract

Radiopharmaceuticals contain a radionuclide and an agent to direct the radionuclide to a receptor, antigen, ionic pump, or other sites of interest. Some radiopharmaceuticals are simple, such as the ionic form of the radionuclide, while most radiopharmaceuticals have a complex chemical structure where the radionuclide provides a signal, indicating the site of localization of the carrier molecule.

Common single-photon radiopharmaceuticals used for oncological diagnosis include the agents labeled with ^99mTc such as ^99mTc-bisphosphonates (that accumulate at sites of bone mineral deposition), ^99mTc-labeled colloids (that are used for lymphoscintigraphy and for imaging of the liver and spleen), ^99mTc-hexakis-methoxy-isobutyl-isonitrile, and ^99mTc-tetrofosmin (initially employed for myocardial perfusion imaging, and also used for localization of parathyroid adenomas and for identification of other malignant tumors).

The most commonly used radiopharmaceuticals labeled with radioiodine (¹²³I or ¹³¹I) include iodide itself (for localization of thyroid tissue) and the catecholamine analog metaiodobenzylguanidine (MIBG, for localizing pheochromocytoma and neuroblastoma). Thallium-201 chloride (²⁰¹Tl) is used for myocardial perfusion imaging as well as tumor perfusion imaging, while ¹¹¹In-pentetreotide detects overexpression of somatostatin receptors, especially in neuroendocrine tumors and in lesions arising from the neural crest, such as carcinoid, paragangliomas, and medullary thyroid carcinomas. ¹¹¹In-capromab pendetide is a murine monoclonal antibody recognizing a transmembrane glycoprotein expressed by poorly differentiated and metastatic prostate adenocarcinomas. ⁶⁷Ga-citrate receptors are overexpressed on membranes of both tumor and inflammatory cells.