M. Orbuch et al., DISCOVERY OF A NOVEL CLASS OF NEUROMEDIN B-RECEPTOR ANTAGONISTS, SUBSTITUTED SOMATOSTATIN ANALOGS, Molecular pharmacology, 44(4), 1993, pp. 841-850
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.