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18F-Fluoroethyl triazole-βAG-[(d)-Phe1-c(Cys2-Tyr3-(d)-Trp4-Lys5-Thr6-Cys7)Thr8].


Shan L1.


Molecular Imaging and Contrast Agent Database (MICAD) [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2004-2013.
2012 Mar 07 [updated 2012 Mar 28].

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National Center for Biotechnology Information, NLM, NIH


The 18F-labeled fluoroethyl triazole (FET)-βAG-[(d)-Phe1-c(Cys2-Tyr3-(d)-Trp4-Lys5-Thr6-Cys7)Thr8] (TOCA or Tyr3-octreotide), abbreviated as 18F-FET-βAG-TOCA, is a Tyr3-octreotate analog that was synthesized for positron emission tomography of gastroenteropancreatic neuroendocrine tumors (GEP-NETs) by targeting somatostatin receptors (SSTR, mainly SSTR-2) (1, 2). GEP-NETs are a class of fairly rare tumors with secretion of various peptides and neuroamines. Although significant progress has been made in the understanding of the molecular events underlying these tumors, it is challenging for the early diagnosis, new target identification, and effective management of these tumors (3, 4). In 1989, Krenning et al. first introduced the use of 123I-labeled Tyr3-octreotide for in vivo imaging of SSTR-expressing GEP-NETs (5). However, this agent exhibited high nonspecific accumulation in the liver and intestine, which can obscure the detection of early-stage GEP-NETs. In 1993, Krenning et al. developed another 111In-labeled somatostatin analog, [111In-DTPA0]octreotide (6), and since then this agent has become the gold standard in nuclear imaging for patients with GEP-NETs (6, 7). An obvious disadvantage for [111In-DTPA0]octreotide is that imaging has to be performed 1–2 days after its injection to ensure sufficient contrast (half-life of 111In, 2.8 days). Recently, studies have been mainly focused on two aspects: searching for new analogs that would have a better affinity profile compared with [111In-DTPA0]octreotide, and developing effective methods for other radionuclide labeling (4, 7, 8). In general, positron-emitting radionuclide-labeled analogs share excellent imaging quality with better spatial resolution compared to the imaging quality of γ-emitting analogs. One representative agent is [68Ga-DOTA0,Tyr3]octreotide (68Ga-DOTATOC), which is the first 68Ga-labeled somatostatin analog that is studied in patients (9). 18F-Labeled analogs, such as the glycosylated analog Gluc-Lys([18F]FP)-TOCA, which is comparable with 68Ga-DOTATOC in in vivo imaging, have also been studied (10). However, the preparation of Gluc-Lys([18F]FP)-TOCA is time-consuming. Iddon et al. labeled a novel class of alkyne-linked Tyr3-octreotate analogs with a copper-catalyzed azide-alkyne cycloaddition reaction (CuAAC) to form a 1,4-substituted triazole using the reagent [18F]2-fluoroethyl azide (1). The 18F-labeling process is fast and efficient. With this labeling method, investigators have identified two lead alkyne-linked Tyr3-octreotate analogs, G-TOCA and βAG-TOCA (1). Both lead analogs were highly reactive in the CuAAC reaction, showing complete conversion to the [18F]2-fluoroethyl triazole-linked Tyr3-octreotate analogs FET-G-TOCA and FET-βAG-TOCA under mild conditions and with short synthesis times (5 min at 20°C). Leyton et al. further tested these 18F-labeled analogs for their receptor binding and in vivo tumor imaging (2). This chapter summarizes the data obtained with these analogs (1, 2).

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