CHARACTERIZATION OF THE INTERACTION OF DIACYLPIPERAZINE ANTAGONISTS WITH THE HUMAN NEUROKININ-1 RECEPTOR - IDENTIFICATION OF A COMMON BINDING-SITE FOR STRUCTURALLY DISSIMILAR ANTAGONISTS

We recently described a novel series of diacylpiperazine antagonists o f the human neurokinin (NK)-1 receptor. The diacylpiperazine compounds are structurally dissimilar from previously described NK-1 antagonist s. L-161,664 laminocarbonyl)-4-(N',N'-di-n-pentylaminocarbonyl) pipera zine-2-diethylaminopropylcarboxamide] inhibits I-125- substance P bind ing to the human NK-1 receptor with an IC50 of 43 +/- 21 nM but has 50 -fold and 200-fold lower affinity for the human NK-2 and NK-3 receptor s, respectively. L-161,664 inhibits substance P-stimulated inositol mo nophosphate accumulation in Chinese hamster ovary cells expressing the human NK-1 receptor by increasing the EC(50) for substance P but not its maximal effect. The compound decreases the apparent affinity of th e NK-1 receptor for I-125-substance P and does not alter the rate of d issociation of I-125-substance P from the receptor. These data indicat e that L-161,664 is a potent and selective competitive antagonist of t he human NK-1 receptor. L-161,664 has reduced affinity for mutants of the NK-1 receptor in which alanine has replaced Gln-165 in transmembra ne helix 4, His-197 in helix 5, His-265 in helix 6, or Tyr-287 in heli x 7. Similarly, a novel series of acyclic 2-benzhydryl-2-aminoethyl et hers that we have recently shown to be competitive NK-1 receptor antag onists have reduced affinity for the Q165A, H197A, and H265A mutant re ceptors. These residues have been shown to be important for binding of quinuclidine, tryptophan benzyl ester, and perhydroisoindole antagoni sts to the receptor. Analysis of the interaction of structural analogs of L-161,664 with the Q165A mutant receptor suggests that this residu e interacts with the 2-diethylaminopropylcarboxamide side chain of L-1 61,664. Thus, even though the diacylpiperazine antagonists are structu rally dissimilar from other classes of antagonists described to date, these data suggest that a common antagonist binding site that accomoda tes much structural diversity is present in the human NK-1 receptor. F urthermore, these data, combined with those obtained from medicinal ch emistry approaches, suggest a minimum pharmacophore map for the intera ction of these diverse ligands with the NK-1 binding site.