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Leung K1.


Molecular Imaging and Contrast Agent Database (MICAD) [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2004-2013.
2012 Jan 07 [updated 2012 Mar 22].

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National Center for Biotechnology Information, NLM, NIH, Bethesda, MD


A variety of 11C- and 18F-labeled amino acids have been studied for potential use in positron emission tomography (PET) oncology (1, 2). Most brain tumors show an increased uptake of amino acids compared with uptake in normal brain tissue (3). These amino acids are composed of naturally occurring amino acids, such as l-[11C]leucine, l-[11C]methionine (MET), and l-[11C]tyrosine, and of non-natural amino acids, such as [11C]aminoisobutyric acid, [11C]1-aminocyclopentane-1-carboxylic acid, and [11C]1-aminocyclobutane-1-carboxylic acid. 123I-Labeled amino acids are also used for imaging in oncology, although no radiolabeled amino acid is approved at present (1, 4, 5). More than twenty amino acid transporter systems have been identified (1). Most of the amino acids are taken up by tumor cells through an energy-independent l-type amino acid transporter system and the Na-dependent transporter system A, as well as through the Na+-dependent system B0 (6). The amino acids are retained in tumor cells due to their high metabolic activities, including incorporation into proteins, which are higher than the metabolic activities of most normal cells (1). Malignant transformation increases the use of amino acids for energy, protein synthesis, and cell division. Tumor cells have been found to have overexpressed transporter systems (7). l-[11C]MET, [18F]fluorotyrosine, l-[11C]leucine, and [18F]fluoro-α-methyl tyrosine have been widely used in the detection of tumors (2, 6), but they are not approved by the United States Food and Drug Administration. These radiolabels are moved into cells by various amino acid transporters and are incorporated into proteins, although only leucine is quantitatively incorporated into protein. The fraction of radiolabeled amino acid that is incorporated into proteins is usually small compared to the total amount taken up into the cell. Imaging techniques that use natural amino acids are based on amino acid transport and protein incorporation. Glutamine is utilized by proliferating cancer cells in glutaminolysis to provide energy and in synthesis of nucleotide and amino acid (8, 9). The Myc oncogene upregulates glutamine transporter (SLC1A5, neutral amino acid transporter) and glutaminase expression (10, 11). Glutaminase converts glutamine to glutamic acid, which is required for tumor growth (12). L-[5-11C]-Glutamine (L-[5-11C]-Gln) has been synthesized and evaluated as an amino acid PET tracer for the detection of tumors (13).

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