A new peptidic somatostatin agonist with high affinity to all five somatostatin receptors

All commercially available somatostatin analogs for clinical use have a preference for some but not all somatostatin receptor subtypes. We describe here the synthesis and evaluation in binding and cAMP assays with cell lines stably transfected with sst(1)-sst(5) of a new type of nonapeptide somatostatin analog with a reduced-sized and stabilized structure, Tyr(0)-(cyclo-D-Dab-Arg-Phe-Phe-D-Trp-Lys-Thr-Phe) (KE108). All five somatostatin receptors subtypes have an extremely high affinity for KE108, equivalent to SS-28 at sst(1) and two to four times higher than SS-28 at sst(2), sst(3), sst(4) and sst(5). Moreover, the compound has agonistic properties at all five subtypes, since it is able to inhibit the forskolin-stimulated cAMP production in sst(1)-sst(5) cells. It is stable for several hours in human serum. This analog may therefore represent a considerable improvement over commercially available somatostatin analogs as it will target all somatostatin receptor subtypes, a particular advantage for cancer-related applications, as human cancers can express concomitantly several somatostatin receptor subtypes.