Peptide/benzodiazepine hybrids as ligands of CCKA and CCKB receptors

The (neuro)hormones gastrin and cholecystokinin (CCK) share a common C-term inal tetrapeptide amide sequence that has been recognized as the message po rtion while the N-terminal extensions are responsible for the CCKA and CCKB receptor subtype selectivity and avidity. 1,4-Benzodiazepine derivatives a re potent and selective antagonists of these receptors, and according to co mparative molecular field analysis, the structures of these nonpeptidic com pounds could well mimic the message sequence of the peptide agonists at lea st in terms of spatial array of the aromatic residues. Docking of a larger series of low molecular weight nonpeptide antagonists to a homology modelin g derived CCKB receptor structure revealed a consensus binding made that is further validated by data from site-directed mutagenesis studies of the re ceptors. Whether this putative binding pocket of the nonpeptide antagonists is identical to that of the message portion of the peptide agonists, or wh ether it is distinct and spatially separated, or overlapping, but with dist inct interaction sites, is still object of debate. Using a 1,4-benzodiazepi ne core amino-functionalized at the C3 position, related tryptophanyl deriv atives were synthesized as mimics of the tetrapeptide and subsequently exte nded N-terminally with gastrin and CCK address sequences. All hybrid constr ucts were recognized cis antagonists by the CCKA and CCKB receptors, but th eir address portions were uncapable of enhancing in significant manner sele ctivity and avidity. Consequently, the binding of the peptide/benzodiazepin e hybrids has to be dictated mainly by the benzodiazepine moiety, which app arently prevents optimal interactions of the address peptides with extracel lular receptor subdomains. These findings would strongly support the view o f distinct binding sites for the message portion of the peptide agonists an d the benzodiazepine-based nonpeptide antagonists. (C) 2001 John Wiley & So ns, Inc.