Highly potent cyclic disulfide antagonists of somatostatin

The search for synthetic analogues of somatostatin (SRIF) which exhibit sel ective affinities for the five known receptor subtypes (sst(1-5)) has gener ated a large number of potent agonist analogues. Many of these agonists dis play good subtype selectivities and affinities for the subtypes 2, 3, and 5 , with very few selective for sst(1) or sst(4). Until the recent report by Bass and co-workers (Mol. Pharmacol. 1996, 50, 709-715; erratum Mel. Pharma col. 1997, 51, 170), no true antagonists of somatostatin had been discovere d, let alone any displaying differential receptor subtype selectivity. In t his present study, we further explore the effect of this putative L,D-5(6) antagonist motif on somatostatin octapeptide analogues with a cyclic hexape ptide core. The most potent antagonist found to date is H-Cpa-cyclo[DCys-Ty r-DTrp-Lys-Thr-Cys]-Nal-NH2, PRL-2970 (21), which has an IC50 Of 1.1 nM in a rat pituitary growth hormone in vitro antagonist assay versus SRIF (1 nM) . This analogue bound to cloned human somatostatin subtype 2 receptors with a K-i of 26 nM. The highest hsst(2) affinity analogue was H-Cpa-cyclo[DCys -Pal-DTrp-Lys-Tle-Cys]-Nal-NH2, PRL-2915 (15), with a K-i of 12 nM (IC50 = 1.8 nM). This analogue was also selective for hsst(2) over hsst(3) and hsst (5) by factors of 8 and 40, respectively, and had no agonist activity when tested alone at concentrations up to 10 mu M. Regression analysis of the bi nding affinities versus the observed antagonist potencies revealed high cor relations for hsst(2) (r = 0.65) and hsst(3) (r = 0.52) with a less signifi cant correlation to hsst5 (r = 0.40). This is quite different from the soma tostatin agonist analogues which show a highly significant correlation to h sst(2) (r > 0.9). Receptor-selective somatostatin antagonists should provid e valuable tools for characterizing the many important physiological functi ons of this neuropeptide.