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Leung K.


Molecular Imaging and Contrast Agent Database (MICAD) [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2004-2013.
2010 Dec 18 [updated 2011 Mar 09].


One of the characteristics of tumor cells is their unchecked proliferation. It is important to measure the proliferation rate of cancer lesions to help differentiate benign tumors from malignant tumors and to characterize malignant tumors among normal tissues. 2-[18F]Fluoro-2-deoxy-d-glucose ([18F]FDG) has been approved for cancer positron emission tomography (PET) imaging by the United States Food and Drug Administration. However, enhanced uptake of FDG as measurement of glycolysis occurs in inflammatory cells and lesions as well as in necrotic cells (1, 2). Thymidine (TdR) and TdR analogs are the standard markers for DNA synthesis, and [11C]TdR has been used in PET to measure tumor growth rates in situ. Because of the short half-life of 11C and the extensive metabolism of [11C]TdR in the blood (3), 3'-deoxy-3'-[18F]fluorothymidine (FLT) was developed for PET imaging. FLT is an analog of TdR and is phosphorylated by TdR kinase-1 (TK-1), an enzyme expressed during the DNA synthesis phase (S-phase) of the cell cycle (4). On the other hand, RNA synthesis occurs in all phases of the cell cycle except the M phase, even in slow-growing solid tumors. Tracers targeted to RNA synthesis could be used to visualize tumors with low TK-1 expression. Phosphorylation of deoxyribonucleosides and their nucleoside analogs is carried out with deoxycytidine kinase (DCK) (5). DCK is ubiquitously expressed in normal tissues but is highly expressed in hematolymphoid tissues (spleen, thymus, and lymph nodes) and epithelial cells (6). Lymphoid cells and rapidly proliferating cells prefer the salvage pathway over the de novo pathway for DNA synthesis. 1-(2'-Deoxy-2'-fluoroarabinofuranosyl)cytosine (FAC) was found to be retained in proliferating T cells. FAC is taken up by cells and rapidly phosphorylated by DCK and trapped inside the cells. In this chapter, 1-(2'-deoxy-2'-[18F]fluoroarabinofuranosyl)cytosine ([18F]FAC) is being developed as a PET probe for imaging of epithelium, lymphoid tissues, and immune activation (6-8).

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