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In a series of human corticotroph adenomas, we recently found predominant mRNA expression of somatostatin (SS) receptor subtype 5 (sst5). After 72 h, the multiligand SS analog SOM230, which has a very high sst5 binding affinity, but not Octreotide (OCT), significantly inhibited basal ACTH release. To further explore the role of sst5 in the regulation of ACTH release, we conducted additional studies with mouse AtT-20 cells. SOM230 showed a 7-fold higher ligand binding affinity and a 19-fold higher potency in stimulating guanosine 5'-O-(3-thiotriphosphate) binding in AtT-20 cell membranes compared with OCT. SOM230 potently suppressed CRH-induced ACTH release, which was not affected by 48-h dexamethasone (DEX) pretreatment. However, DEX attenuated the inhibitory effects of OCT on ACTH release, whereas it increased the inhibitory potency of BIM-23268, an sst5-specific analog, on ACTH release. Quantitative PCR analysis showed that DEX lowered sst(2A+2B) mRNA expression significantly after 24 and 48 h, whereas sst5 mRNA levels were not significantly affected by DEX treatment. Moreover, Scatchard analyses showed that DEX suppressed maximum binding capacity (B(max)) by 72% when 125I-Tyr3-labeled OCT was used as radioligand, whereas B(max) declined only by 17% when AtT-20 cells were treated with [125I-Tyr11]SS-14. These data suggest that the sst5 protein, compared with sst2, is more resistant to glucocorticoids. Finally, after SS analog preincubation, compared with OCT both SOM230 and BIM-23268 showed a significantly higher inhibitory effect on CRH-induced ACTH release. In conclusion, our data support the concept that the sst5 receptor might be a target for new therapeutic agents to treat Cushing's disease.

Functional gastroenteropancreatic tumors express all 5 somatostatin receptor subtypes (sst) in different quantities. Octreotide and lanreotide treat patients with these tumors by binding preferentially to sst₂ and, to a lesser extent, to sst₃ and sst₅ receptors, thereby controlling prominent symptoms caused by hormone hypersecretion (diarrhea and flushing). Although symptoms initially improve in most patients, a loss of response occurs in about 50% during continuous treatment. The functional activity at sst receptors of SOM230, a new multiligand somatostatin analog, has been described and compared with that of somatostatin (SRIF-14) and octreotide. These data show that SOM230 is a full agonist with nanomolar potency at sst_1,2,3 and sst₅ receptors. The multiligand activity profile of SOM230, together with its nondesensitizing inhibitory effect on growth hormone and insulin-like growth factor-I secretion in rats, may underlie its successful use in clinical trials and its potential for use in refractory patients with carcinoid tumors.

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