Peptide Receptor Radionuclide Therapy with 177Lu-octreotate: clinical aspects

Somatostatin is a neuropeptide with a variety of functions and is produced in several tissues. The isoform somatostatin-14 (SS-14) is more biologically active than SS-28. Its main physiological function is to inhibit secretion of hormones such as growth hormone or thyroid stimulating hormone in the pituitary, insulin or glucagon in the pancreas and of exocrine gut secretion. The effects of somatostatin are mediated by binding to the somatostatin receptor (sst), which are G-protein-coupled receptors. Five sst subtypes have been identified (sst1 - sst5) which all have different roles. The naturally occurring SS-14 and SS-28 have high affinity for all these subtypes. Several tumours (Table 1), amongst others gastroenteropancreatic tumours (GEPNET), have overexpression of sst on the cell surface. This makes sst a possible target for therapy with somatostatin analogues to try and inhibit the secretion of bioactive substances by these tumours and possibly to slow down tumour growth. However, the biological half-life of SS-14 and SS-28 is short, and this makes it impossible to use them for therapeutic goals. With the invention of the cyclic octapeptide octreotide, half-life increased significantly and octreotide then could be used in treatment of patients with sst-positive tumours.

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