Novel chiral-diazepines function as specific,selective receptor agonists with variable coupling and species variability in human,mouse and rat BRS-3 receptor cells

Bombesin receptor subtype-3 (BRS-3) is an orphan G-protein coupled receptor which is classified in the bombesin receptor (BnR) family with which it shares high homology. It is present widely in the central nervous system and peripheral tissues and primarily receptor-knockout studies suggest it is involved in metabolic-glucose-insulin homeostasis, feeding and other CNS behaviors, gastrointestinal motility and cancer growth. However, the role of BRS-3 physiologically or in pathologic disorders has been not well defined because the natural ligand is unknown. Until recently, no selective agonists/antagonists were available; however, recently synthetic high-affinity agonists, chiral-diazepines nonpeptide-analogs (3F, 9D, 9F, 9G) with low CNS penetrance, were described, but are not well-categorized pharmacologically or in different labarotory species. The present study characterizes the affinities, potencies, selectivities of the chiral-diazepine BRS-3 agonists in human and rodents (mice,rat). In human BRS-3 receptors, the relative affinities of the chiral-diazepines was 9G > 9D > 9F > 3F; each was selective for BRS-3. For stimulating PLC activity, in h-BRS-3 each of the four chiral diazepine analogs was fully efficacious and their relative potencies were: 9G (EC₅₀: 9 nM) > 9D (EC₅₀: 9.4 nM) > 9F (EC₅₀: 39 nM) > 3F (EC₅₀: 48 nM). None of the four chiral diazepine analogs activated r,m,h-GRPR/NMBR. The nonpeptide agonists showed marked differences from each other and a peptide agonist in receptor-coupling-stiochiometry and in affinities/potencies in different species. These results demonstrate that chiral diazepine analogs (9G, 9D, 9F, 3F) have high/affinity/potency for the BRS-3 receptor in human and rodent cells, but different coupling-relationships and species differences from a peptide agonist.