Somatostatin receptors in gastroenteropancreatic neuroendocrine tumours

Five somatostatin receptor (sst) subtype genes, sst(1), sst(2), st(3), sst(4), and sst(5), have been cloned and characterised. The five sst subtypes all bind natural somatostatin-14 and somatostatin-28 with high affinity. Endocrine pancreatic and endocrine digestive tract tumours also express multiple sst subtypes, but sst(2) predominance is generally found. However, there is considerable variation in sst subtype expression between the different tumour types and among tumours of the same type. The predominant expression of sst(2) receptors on pancreatic endocrine or carcinoid tumours is essential for the control of hormonal hypersecretion by the octapeptide somatostatin analogues such as octreotide and lanreotide. Somatostatin and its octapeptide analogues are also able to inhibit proliferation of normal and tumour cells. The high density of sst(2) or sst(5) on pancreatic endocrine or carcinoid tumours further allows the use of radiolabelled somatostatin analogues for in vivo visualisation. The predominant expression of sst(2) receptors in these tumours and the efficiency of sst(2) receptors to undergo agonist-induced internalisation is also essential for the application of radiolabelled octapeptide somatostatin analogues. Currently, [In-111-DTPA(0)]octreotide, [Y-90-DOTA(0),Tyr(3)]octreotide, [Lu-177-DOTA(0)Tyr(3)]octreotate, [In-111-DOTA(0)]lanreotide and [Y-90-DOTA(0)]lanreotide can be used for this purpose.