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Leung K.


Molecular Imaging and Contrast Agent Database (MICAD) [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2004-2013.
2007 Sep 10 [updated 2007 Oct 22].


In a variety of solid tumors, hypoxia was found to lead to tumor progression and the resistance of tumors to chemotherapy and radiotherapy (1-3). Tumor oxygenation is heterogeneously distributed within human tumors (4). Hypoxia in malignant tumors is thought to be a major factor limiting the efficacy of chemotherapy and radiotherapy. It would be beneficial to assess tumor oxygenation before and after therapy to provide an evaluation of tumor response to treatment and an insight into new therapeutic treatments (5). Tumor oxygenation is measured invasively using computerized polarographic oxygen-sensitive electrodes, which is regarded as the gold standard (6). Functional and non-invasive imaging of intra-tumoral hypoxia has been demonstrated to be feasible for the measurement of tumor oxygenation (7). This has led to the search and development of hypoxia-targeted, non-invasive markers of tumor hypoxia. Chapman proposed the use of 2-nitroimidazoles for hypoxia imaging (8). 2-Nitroimidazole compounds are postulated to undergo reduction in hypoxic condition, forming highly reactive oxygen radicals that subsequently bind covalently to macromolecules inside the cells (9). [18F]Fluoromisonidazole ([18F]FMISO) is the most widely used positron emission tomography (PET) tracer for imaging tumor hypoxia (7). However, it has slow clearance kinetics and a high lipophilicity, resulting in substantially high background in PET scan. Novel 2-nitroimidazoles, such as [18F]FETA, [18F]FETNIM, 4-Br[18F]FPN, [18F]EF1, and [18F]EF5, are currently being investigated as potential markers of tumor hypoxia [PubMed]. 2-(2-Nitroimidazol-1H-yl)-N-(3,3,3-trifluoropropyl)acetamide (EF3) is a 3-trifluorinated analog of EF1 (2-(2-nitro-1H-imidazol-1-yl)-N-(3-fluoropropyl)acetamide). EF3 binding to hypoxic tumor cells was shown to be dependent on oxygen and less dependent on the intracellular level of reductase system (10, 11). [18F]EF3 is being evaluated as a PET probe for detection of tumor hypoxia.

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