Highly Potent Cyclic Disulfide Antagonists of Somatostatin

Abstract

The search for synthetic analogues of somatostatin (SRIF) which exhibit selective affinities for the five known receptor subtypes (sst1-5) has generated a large number of potent agonist analogues. Many of these agonists display good subtype selectivities and affinities for the subtypes 2, 3, and 5, with very few selective for sst1 or sst4. Until the recent report by Bass and co-workers (Mol. Pharmacol. 1996, 50, 709-715; erratum Mol. Pharmacol. 1997, 51, 170), no true antagonists of somatostatin had been discovered, let alone any displaying differential receptor subtype selectivity. In this present study, we further explore the effect of this putative L,5D6 antagonist motif on somatostatin octapeptide analogues with a cyclic hexapeptide core. The most potent antagonist found to date is H-Cpa-cyclo[DCys-Tyr-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 Ki of 26 nM. The highest hsst2 affinity analogue was H-Cpa-cyclo[DCys-Pal-DTrp-Lys-Tle-Cys]-Nal-NH2, PRL-2915 (15), with a Ki of 12 nM (IC50 = 1.8 nM). This analogue was also selective for hsst2 over hsst3 and hsst5 by factors of 8 and 40, respectively, and had no agonist activity when tested alone at concentrations up to 10 microM. Regression analysis of the binding affinities versus the observed antagonist potencies revealed high correlations for hsst2 (r = 0.65) and hsst3 (r = 0.52) with a less significant correlation to hsst5 (r = 0.40). This is quite different from the somatostatin agonist analogues which show a highly significant correlation to hsst2 (r > 0.9). Receptor-selective somatostatin antagonists should provide valuable tools for characterizing the many important physiological functions of this neuropeptide.

Intra-Islet Somatostatin Regulates Glucagon Release via Type 2 Somatostatin Receptors in Rats

“…However, in perfused pancreata at 5.5 mmol/l glucose, DC-41-33 increased the second phase of insulin release by 34.6% (P ϭ 0.4). Although this compound shows 20-fold higher affinity to SSTR2 (A-cells) than SSTR5 (B-cells) (25), it is possible that the antagonist enhanced glucagon as well as insulin release in experiments with higher basal glucose in the perfusate that facilitates SST release. Significant enhancements of insulin secretion occurred in most but not all of our batch incubations of isolated islets at 2 mol/l DC-41-33.…”

“…DC-41-33 is a highly specific SSTR2 antagonist (25). In CHO cells transfected with human SSTRs, this compound binds competitively to SSTR2 with a binding affinity (K i ) of 26 Ϯ 3.1 nmol/l and displays selectivity for human SSTR2 over SSTR5 by factor 20 (25).…”

“…In CHO cells transfected with human SSTRs, this compound binds competitively to SSTR2 with a binding affinity (K i ) of 26 Ϯ 3.1 nmol/l and displays selectivity for human SSTR2 over SSTR5 by factor 20 (25). In primary cultures of rat pituitary cells, growth hormone release factor-stimulated (34) and reverses urethane-induced SST-mediated inhibition of gastric acid secretion (35).…”

Gliclazide Directly Inhibits Arginine-Induced Glucagon Release

“…A potent and highly specific SSTR2 antagonist, DC-41-33, has been synthesized (8). Recently we used this antagonist to investigate whether paracrine interactions operate in pancreatic islets, and demonstrated that the arginine effect on glucagon release was markedly enhanced by DC-41-33 (K.C., D.H.C., S.E., submitted manuscript).…”

“…In rodents, somatostatin inhibits glucagon and insulin release via SSTR2 and SSTR5, respectively (6,7). A novel highly specific SSTR2 antagonist, DC-41-33, has recently been synthesized (8). When applied in both perifused isolated rat islets and isolated perfused rat pancreas, this compound enhances arginine-stimulated glucagon release nearly threefold, which implies that islet somatostatin exerts a pronounced paracrine effect on A cell secretion (K.C., D.H.C., S.E., submitted manuscript).…”