Methods to radiolabel somatostatin analogs with [ 18 F]fluoride: current status, challenges, and progress in clinical ap
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Methods to radiolabel somatostatin analogs with [18F]fluoride: current status, challenges, and progress in clinical applications David J. Pérez1 · Miguel A. Ávila‑Rodríguez1 Received: 12 June 2020 / Accepted: 29 September 2020 © Akadémiai Kiadó, Budapest, Hungary 2020
Abstract There is an ongoing interest of having a somatostatin analog radiotracer labeled with 18F for PET imaging of neuroendocrine tumors. Nonetheless, there is none yet approved for clinical use. 18F is the ideal radionuclide for PET imaging due to its high β+ decay, suitable half-life and ease of production. As with other peptides, the labeling of SSTA with 18F comes along with its complications. In this review, we discuss the methods to label SSTA with 18F highlighting the chemistry as well as an overview in the progress on preclinical and clinical applications of 18F-labeled SSTA tracers. Keywords Somatostatin analogs · Neuroendocrine tumors · Radiolabeling · 18F · PET-imaging
Introduction Neuroendocrine tumors (NET) are a type of carcinoids that develop from the neuroendocrine cells –those that form the neuroendocrine system and receive signals from the nervous system (as neurons), as well as biosynthesize and release hormones – which are mainly found and distributed in lungs, bowel tissue and pancreas known as the gastro-entero-pancreatic (GEP) tract [1–4]. The neuroendocrine system comprises the endocrine glands (pituitary, parathyroid, and adrenal), the pancreatic tissues, and scattered endocrine cells through the digestive and respiratory tracts [1, 2]. Complete, extensive, and excellent literature reviews and book chapters related to NET have been published [1, 3–5]. NET have a distinctive feature: 90% of them express high levels of “proteins” known as the somatostatin receptors, which are a target for both diagnosis and treatment of NET [2, 6]. Somatostatin (SST) is a natural occurring neuropeptide that has two active forms: SST-14 and SST28 (formed by 14 or 28 residues, respectively) produced by * David J. Pérez [email protected] * Miguel A. Ávila‑Rodríguez [email protected] 1
Unidad Radiofarmacia‑Ciclotrón, División de Investigación, Facultad de Medicina, Universidad Nacional Autónoma de México, 04510 Mexico City, México
SST-producing cells located throughout the gastrointestinal tract (GIT), endocrine pancreas and the nervous system (Fig. 1a) [7–10]. SST exerts many complex biological functions related to the control of hormone secretion, intestinal motility, vascular contractility, and cell proliferation, which are mediated through the interaction with its receptors [7, 8, 10]. SST also displays cell anti-proliferative activity by different mechanisms [7], as well as an indirect anti-proliferative activity by inhibiting growth factors and tumor angiogenesis [6, 7]; nonetheless, it is prone to enzyme degradation, thus has a short lifespan (1–2 min) [8, 11]. Somatostatin receptors (SSTR) are G-protein-coupled membrane glycoproteins, and five different subtypes (1–5) have been identified [11]. As aforementioned, high pro
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