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Leung K.


Molecular Imaging and Contrast Agent Database (MICAD) [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2004-2013.
2009 Aug 20 [updated 2012 May 09].


Malignant melanoma is the most deadly form of skin cancer (1). Early and accurate diagnosis is necessary for surgery and successful treatment (2). The melanocortin (MC) system is a neuropeptide network of the skin, and it is involved in pigmentation regulation, cortisol production, and many other physiological processes (3). Most cutaneous cell types express MC receptors, pro-opiomelanocortin (POMC), and prohormone convertases, and they also release MCs. Melanotropin-stimulating hormones (α-, β-, and γ-MSH) are derived from POMC by the proteolytic action of prohormone convertases. There are five MC receptors (MC1R to MC5R), which belong to the G-protein−coupled receptor superfamily, and these receptors have been found to be overexpressed in melanoma cells (4, 5). α-MSH (Ac-Ser1-Tyr2-Ser3-Met4-Glu5-His6-Phe7-Arg8-Trp9-Gly10-Lys11-Pro12-Val13-NH2), produced by the brain and the pituitary gland, is a tridecapeptide (13 amino acids) and is the most potent melanotropic peptide (6) in the regulation of skin pigmentation via MC1R. Positron emission tomography (PET) imaging with [18F]fluoro-2-deoxy-2-d-glucose ([18F]FDG) in cancer has been approved by United States Food and Drug Administration with many on-going clinical trials. Although [18F]FDG is effective in the detection of melanoma, it is not melanoma-specific and some melanoma cells do not take up [18F]FDG (7, 8). Radiolabeled α-MSH peptide analogs have been shown to specifically bind to MC1R that is overexpressed on human and mouse melanoma cells (4, 5, 7, 9, 10). N-(2-Aminoethyl)-trans-1,2-diaminocyclohexane-N,N”,N”’-pentaacetic acid (CHX-A”) has been successfully coupled to α-MSH peptide analogs for radiolabeling with a variety of radionuclides (7, 11, 12). A rhenium-cyclized α-MSH analog, Re-[Cys3,4,10,D-Phe7,Arg11]α-MSH3-13 (ReCCMSH(Arg11)), was conjugated with CHX-A” (CHX-A”-ReCCMSH(Arg11)) and radiolabeled with 111In (111In-CHX-A”-ReCCMSH(Arg11)) as a molecular imaging agent for single-photon emission computed tomography (SPECT) to target melanoma (13). Rhenium-mediated cyclization exhibits significantly reduced nonspecific renal radioactivity accumulation, high tumor uptake and retention coupled with rapid clearance kinetics, compared with its free sulfhydryl and disulfide bond–containing counterparts (10). 111In is a gamma emitter with a physical half-life of 2.8 days.

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