Peptide receptors in gut endocrine tumours.

A great number of gut endocrine tumours show high expression of receptors for neuropeptides, such as SRIF and VIP. The expression of ssts is essential for the control of hormonal hypersecretion and tumour growth by octapeptide somatostatin analogues. Five different sst subtypes, named sst1-5, have been cloned and characterized. The therapeutic efficacy of the octapeptide analogues is determined by the expression of sst2 (sst3) and sst5 on the tumour. In general, there is a predominant expression of sst1 and sst2 mRNA in gut endocrine tumours. In vivo sst scintigraphy, after injection of [111In]pentetreotide, provides a useful tool for the diagnostic work-up of patients with these tumours. This technique can be used for the localization of the primary tumour(s), for the determination of the extent of metastatic spread and for the selection of potential candidates for therapy with (radiolabelled) octapeptide analogues. Differentiated gut endocrine tumours also show a high expression of VIP-Rs. However, undifferentiated tumours show VIP-R expression to a smaller degree. In vivo scintigraphy with 123I-labelled VIP is a sensitive technique for the in vivo identification of gut endocrine tumours and their metastases. The functional role of the tumoral VIP-Rs is still unclear and at present there are no known therapeutic applications for VIP-R agonists or antagonists in humans.