Octreotide is a synthetic analogue of somatostatin, which is a cyclic neuropeptide that is normally found in neuronal and endocrine cells (brain, peripheral nerves, pancreatic endocrine cells). The plasma half-life of natural somatostatin is 1-3 minutes. Indium-111 (111In)–labelled pentetreotide specifically binds to somatostatin receptors (specially to subtypes 2 and 5).
Other somatostatin analogs (eg, technetium 99m depreotide [99mTc depreotide], DTPA) are used in the imaging of pituitary tumors. The presence of somatostatin receptors in numerous pituitary and parasellar tumors allows visualization with radionucleotide-labelled somatostatin analogs in vivo. In the pituitary gland, prolactin– and adrenocorticotrophic hormone–secreting adenomas cannot be localized, but clinically nonfunctioning pituitary adenomas are visualized in 75% of cases with 111In-DTPA-octreotide.
A positive scan result in patients with growth hormone– and thyroid-stimulating-hormone–secreting pituitary tumors indicates a good suppressive effect of octreotide on hormone release by these tumors.
Octreotide is used for scintigraphic localization of primary and metastatic neuroendocrine tumors that bear somatostatin receptors.
Somatostatin receptors have been found in many neuroendocrine and several nonneuroendocrine cells. Capitalizing on this concept of somatostatin receptor positivity, somatostatin receptor scintigraphy has been developed to image tumors that arise from these cells.
The study of diagnostic approaches that address these biological characteristics of various tumors could open a whole new therapeutic vista.
Tumors with high expression of somatostatin receptors, which are normally detected with somatostatin receptor scintigraphy, include the following:
Gastroenteropancreatic neuroendocrine tumors (formerly classified into carcinoid, gastrinoma, glucagonoma, vasoactive intestinal polypeptide-secreting tumor, pancreatic polypeptide-secreting tumor, etc, or nonfunctioning gastroenteropancreatic tumors), more recently classified by the World Health Organization as low grade, intermediate grade, and high grade (G1, G2, G3, respectively)