SSTR2 and neuroendocrine neoplasm: While somatostatin can activate five somatostatin receptor subtypes (SSTR1-SSTR5) [107], it has been shown that cyclic octapeptide octreotide selectively binds to the somatostatin subtype-2 (SSTR2) receptor (and, to a lesser extent, SSTR5), which is predominantly expressed in neuroendocrine tumours.