Somatostatin receptor subtype expression in human tumors

The presence of functional SSR in tumors has several clinical implications which include the possibility a) to control hormonal hypersecretion and related symptomatology by treatment with SS-analogs, b) to detect SSR positive tumors and their metastases by in vivo SSR scintigraphy, and c) to carry out SSR-targeted radiotherapy using radiolabeled SS-analogs. The majority of SSR positive tumors show a differential expression of somatostatin receptor subtypes, sst(2) receptors being the most frequently expressed SSR subtype. The predominant expression of sst(2) receptors forms the basis for the successful application of sst(2) preferring agonists in the treatment of patients with GH-secreting pituitary adenomas, as well as in patients with carcinoid or islet cell tumors. Sst(2) and sst(5) receptors appear to be differentially involved in the regulation of normal and tumoral pituitary hormone secretion. Additionally, sst(2) receptors are involved in the receptor-mediated internalisation of sst(2) preferring radiolabeled SS-analogs. The predominant expression of sst(2) receptors in neuroendocrine tumors probably determines the successful application of radiolabeled SS-analogs for the detection of primary tumors and their metastases by SSR scintigraphy. In conclusion, the efficacy of treatment with SS-analogs, the visualisation of SSR-positive tumors, as well as the possibility to carry out SSR-targeted radiotherapy, may very well depend upon the density and subtype of SSR that is expressed by the tumors. Therefore, the characterisation of SSR subtypes in human tumors may have important clinical consequences.