The putative somatostatin antagonist, cyclo-(7-aminoheptanoyl-Phe-D-Trp-Lys-Thr[BZL]), may act as potent antiproliferative agonist.

The cyclic somatostatin (SST) analogue, cyclo-(7-aminoheptanoyl-Phe-D-Trp-Lys-Thr[BZL]) (cSSTA), has been widely used as somatostatin antagonist. In the human neuroblastoma cell line SH-SY5Y the cyclopeptide acts as a somatostatin receptor agonist. Similar to SST, cSSTA inhibits cell proliferation, activates the protein tyrosine phosphatase SHP-2, and stimulates the activity of mitogen-activated protein kinase. These results suggest that in SH-SY5Y neuroblastoma cells somatostatin receptors may exist which exhibit altered antagonist binding properties.