Single-photon-emitting radiopharmaceuticals for diagnostic applications

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 99mTcbisphosphonates (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.