DISCOVERY OF A NOVEL CLASS OF NEUROMEDIN B-RECEPTOR ANTAGONISTS, SUBSTITUTED SOMATOSTATIN ANALOGS

Bombesin-related peptides have widespread activities in the central ne rvous system and peripheral tissues. Recent studies show two subtypes of receptors; a gastrin-releasing peptide (GRP) receptor subtype and a neuromedin B (NMB) receptor subtype exist. In contrast to the GRP rec eptor, no antagonists exist for the NMB receptor. In the present study we report that certain somatostatin (SS) octapeptide analogues functi on as selective NMB receptor antagonists. The most potent analogue, D- Nal-Cys-Tyr-D-Trp-Lys-Val-Cys-Nal-NH2, inhibited binding of I-125-[D-T yr(o)]NMB to NMB receptor-transfected 3T3 cells and C6 cells. This ana logue had 100-fold lower affinity for GRP receptors. Structure-functio n studies were performed by synthesizing 18 structurally related SS oc tapeptide analogues; each of these analogues, but not native SS-1 4 or SS-28, also inhibited binding to NMB receptors. The stereochemistry a t positions 1, 2, 7, and 8, the hydrophobicity and ring size of the su bstitution in positions 1, 3, and 4, and the basicity of the group in position 5 were all important in determining NMB receptor affinity. No SS octapeptide analogue increased [H-3]inositol phosphates in NMB rec eptor-transfected cells; however, each analogue inhibited NMB-stimulat ed increases. The most potent analogue, D-Nal-Cys-Tyr-D-Trp-Lys-Val-Cy s-Nal-NH2, caused a parallel rightward shift of the NMB dose-response curve, the Schild plot slope was not significantly different from unit y, and the affinity was 230 nm. SS octapeptide analogues also interact ed with SS receptors and mu-opioid receptors; however, there was no co rrelation between the affinities of the analogues for these receptors and their affinities for NMB receptors, demonstrating that these activ ities can be separated. The results demonstrate for the first time a c lass of antagonists with >100-fold selectivity for NMB versus GRP rece ptors. Because the structural requirements for determining NMB, SS, an d mu-opioid receptor activity differ, it is likely that highly selecti ve, specific, high affinity NMB receptor antagonists can now be develo ped that will be useful in defining the role of NMB in various physiol ogical processes.