Radionuclides such as F, C, I, Cu, etc., are often used to label molecular imaging probes (MIP) for positron emission tomography (PET) of patients to detect and diagnose a variety of diseases, including cancer (1). However, some limitations (to mention a few) for the production and application of these MIPs are a short half-life (e.g., 110 min and 12 min for F and C, respectively), production of low quality images (e.g., with I), high radiation doses (e.g., Ga and Br), and accumulation in the liver or kidneys due to metabolic degradation (e.g., Cu) (1). In addition, MIPs labeled with nuclides that have a short half-life cannot be used in cases where long circulation times are necessary to obtain an optimal target/background signal. The limitations of various nuclides evaluated for the production and application of different PET MIPs are discussed in detail by Nayak and Brechbiel (1). Some investigators are particularly interested in the use of Cu to produce PET imaging probes because Cu is suitable as a source of the signal, it is relatively easily obtained, and its nuclear properties make it an excellent therapeutic radionuclide ( = 12.7 h; : 17.4%; max = 656 keV; : 39%; max = 573 keV) (2). Linking Cu to MIPs requires the use of a bifunctional chelator (BFC) that connects the radionuclide to the targeted probe. Therefore, it is important to have an appropriate BFC to create a Cu-labeled radiopharmaceutical that remains stable under conditions and generates suitable results during PET imaging. Several types of chelating agents have been used for the Cu-labeling of imaging probes, but some of the limitations for using these radiochemicals in humans include poor yields due to complicated procedures for the synthesis, harsh conditions used to label the compound that can degrade a probe, instability, lack of sufficient biological data, and accumulation in non-targeted organs that results in the masking of small lesions during PET imaging (3). In an effort to create a BFC that could be synthesized and labeled easily with Cu, a sarcophagine-based chelator, 4-((8-amino-3,6,10,13,16,19-hexaazabicyclo [6.6.6] icosane-1-ylamino)methyl)benzoic acid (AmBaSar), was produced by Cai et al. (2). A characteristic feature of this BFC is that it can rapidly form a coordination complex with metal ions within a cage-like structure that does not dissociate under physiological conditions. o further simplify the synthesis of AmBaSar, the investigators devised a four-step procedure to label the chelator with Cu ([Cu]-AmBaSar) (3). [Cu]-AmBaSar was then evaluated for its stability, microPET imaging characteristics, and biodistribution in normal mice (3). In addition, the stability, imaging, and biodistribution properties of [Cu]-AmBaSar were compared with those of Cu-labeled 1,4,7,10-tetraazacyclododecane-,','','''-tetraacetic acid ([Cu]-DOTA). In an earlier study AmBaSar was linked to Arg-Gly-Asp (RGD), a cyclic peptide that specifically binds to the αβ integrin receptor, and labeled it with Cu to obtain [Cu]-AmBaSar-RGD (2). Subsequently the labeled RGD complex was shown to be suitable for the detection of tumors expressing αβ integrin receptors in mice. Studies performed with [Cu]-AmBaSar-RGD are described in a separate chapter of MICAD (www.micad.nih.gov) (4).
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