Somatostatin analogues in the treatment of endocrine tumours of the gastrointestinal tract

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Abstract
Somatostatin and its long-acting analogues have been introduced for the treatment of endocrine tumours of the gastrointestinal tract as they have been shown to effectively control symptoms resulting from excessive hormone release in patients with carcinoid, Verner-Morrison and glucagonoma syndromes. This beneficial effect is due to the presence of somatostatin receptors in high densities on the majority of endocrine tumours. The symptomatic effect is less pronounced in insulinomas, since 30 – 50% of these tumours lack or express only a few somatostatin receptors. With respect to symptomatic control, somatostatin receptor subtypes 2 and 5 are the most important and the currently available long-acting analogues octreotide and lanreotide bind preferentially to these receptor subtypes. Long-term studies have shown that somatostatin analogues are safe and that the most important adverse advent is the development of gallstones. The antiproliferative potency of somatostatin and its analogues in vitro and in experimental tumour models prompted a number of studies in patients with metastatic endocrine tumours that are generally unresponsive to conventional chemotherapeutic protocols. Stabilisation of tumour growth lasting for months to a few years was the most favourable result, occurring in 30 – 70% of patients. However, definite proof of antiproliferative potency in man is still pending since placebo-controlled studies are not available. Radioligand therapy based on 111Indium, 90Yttrium and 177Lutetium coupled to somatostatin analogues via bifunctional chelators is currently under investigation with promising data concerning long-lasting control of symptoms and tumour growth from Phase I trials.