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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 Sep 01 [updated 2009 Oct 08].


Somatostatin (SST) is an inhibitor of the release of somatotropin, glucagon, insulin, gastrointestinal hormones, and other secretory proteins (1). SST is also known as somatotropin release-inhibiting factor (SRIF). SST is a cyclic polypeptide with two biologically active isoforms, SRIF-14 and SRIF-28, of 14 and 28 amino acids, respectively. SRIF has a short plasma half-life of <3 min (2). SST receptors (SSTRs) (G-protein–coupled) have been found on a variety of neuroendocrine tumors and cells of the immune system, and five individual subtypes (sst1–sst5) have been identified and subsequently cloned from animal and human tissues (3, 4). SST also inhibits cell proliferation and promotes apoptosis through binding to specific cell-surface SSTRs (5). 111In-Labeled diethylenetriaminepentaacetic acid-octreotide (111In-DTPA-OCT) is an SSTR analog that, over the last decade, has remained the most widely used radiopharmaceutical for the scintigraphic detection and staging of primary and metastatic neuroendocrine tumors bearing SSTRs with single-photon emission computed tomography (SPECT) (6). It has also shown promising results in peptide-receptor radionuclide therapy (7). OCT is a cyclic peptide with eight amino acids (D-Phe-cyclo(Cys-Phe-D-Trp-Lys-Thr-Cys)-Thr(OH)). 111In-DTPA-OCT binds with high affinity to SSTR subtypes 2 and 5 (sst2 and sst5) and to sst3 to a lesser degree, but it does not bind to sst1 or sst4 (8). Currently used targeting SSTR peptides mainly have affinity for sst2. D-Phe-cyclo(Cys-Tyr-D-Trp-Lys-Thr-Cys)-Thr(OH) ([Tyr3]-octreotate (Y3-TATE)) was found to be a selective sst2 agonist. For evaluation as a positron emission tomography (PET) imaging agent for sst2, 64Cu has been attached to Y3-TATE via bis(carboxymethyl)-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (CB-TE2A) to form 64Cu-CB-TE2A-Y3-TATE (9).

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