Peptide/benzodiazepine hybrids as ligands of CCK(A) and CCK(B) receptors

The (neuro)hormones gastrin and cholecystokinin (CCK) share a common C-terminal tetrapeptide amide sequence that has been recognized as the message portion while the N-terminal extensions are responsible for the CCK(A) and CCK(B) receptor subtype selectivity and avidity. 1,4-Benzodiazepine derivatives are potent and selective antagonists of these receptors, and according to comparative molecular field analysis, the structures of these nonpeptidic compounds could well mimic the message sequence of the peptide agonists at least in terms of spatial array of the aromatic residues. Docking of a larger series of low molecular weight nonpeptide antagonists to a homology modeling derived CCK(B) receptor structure revealed a consensus binding mode that is further validated by data from site-directed mutagenesis studies of the receptors. Whether this putative binding pocket of the nonpeptide antagonists is identical to that of the message portion of the peptide agonists, or whether it is distinct and spatially separated, or overlapping, but with distinct interaction sites, is still object of debate. Using a 1,4-benzodiazepine core amino-functionalized at the C3 position, related tryptophanyl derivatives were synthesized as mimics of the tetrapeptide and subsequently extended N-terminally with gastrin and CCK address sequences. All hybrid constructs were recognized as antagonists by the CCK(A) and CCK(B) receptors, but their address portions were incapable of enhancing in significant manner selectivity and avidity. Consequently, the binding of the peptide/benzodiazepine hybrids has to be dictated mainly by the benzodiazepine moiety, which apparently prevents optimal interactions of the address peptides with extracellular receptor subdomains. These findings would strongly support the view of distinct binding sites for the message portion of the peptide agonists and the benzodiazepine-based nonpeptide antagonists.