Mutagenesis at the human tachykinin NK2 receptor to define the binding site of a novel class of antagonists

The pharmacological profile of novel antagonists endowed with high affinity for the human tachykinin NK2 receptor is presented. MEN13918 (Ngamma{Nalpha[Nalpha(benzo[b]thiophen-2-yl)carbonyl]-1-aminocyclohexan-1-carboxy]-D-phenylalanyl}-3-cis-aminocyclohexan-1-carboxylic-acid-N-(1S,2R)-2-aminocyclohexyl)amide trifluoroacetate salt) and MEN14268 (Nalpha[Nalpha(benzo[b]thiophen-2-yl)carbonyl)-1-aminocyclopentane-1-carboxyl]-D-phenylalanine-N-[3(morpholin-4-yl)propyl]amide trifluoroacetate salt) were more potent in blocking neurokinin A (NKA, His-Lys-Thr-Asp-Ser-Phe-Val-Gly-Leu-Met-NH2) induced contraction in human, which induced greater contraction in human (pK(B) 9.1 and 8.3) than rat (pK(B) 6.8 and < 6) urinary bladder smooth muscle preparation in vitro. In agreement with functional data, in membrane preparations of CHO cells stably expressing the human NK2, receptors, both MEN13918 and MEN14268 potently inhibited the binding of agonist ([I-125]NKA, K-i 0.2 and 2.8 nM) and antagonist ([H-3]nepadutant, K-i 0.1 and 2.2 nM, [H-3]SR48968 K-i 0.4 and 6.9 nM) radioligands. Using site-directed mutagenesis and radioligands binding we identified six residues in the transmembrane (TM) helices that are critical determinants for the studied antagonists affinity. To visualize these experimental findings, we constructed a homology model based on the X-ray crystal structure of bovine rhodopsin and suggested a possible binding mode of these newly discovered antagonist ligands to the human tackykinin NK2 receptor. Both MEN13918 and MEN14268 bind amongst TM4 (Cys167Gly), TM5 (Tyr206Ala), TM6 (Tyr266Ala, Phe270Ala), and TM7 (Tyr289Phe, Tyr289Thr). MEN13918 and MEN14268 diverging binding profile at Y289 mutations in TM7 (Tyr289Phe, Tyr289Thr) suggests a relation of their different chemical moieties with this residue. Moreover, the different influence on binding of these two ligands by mutations located deep along the inner side of TM6 (Phe270Ala, Tyr266Ala, Trp263Ala) indicates a nonequivalent positioning, although occupying the same binding crevice. Furthermore, binding data indicate the Ile202Phe mutation, which mimics the wild-type rat NK2, receptor sequence, as a species selectivity determinant. In summary, data with mutant receptors describe, for these new tachykinin NK receptor antagonists, a binding site which is partially overlapping either with that of the cyclized peptide antagonist nepadutant (cyclo-{[Asn(beta-D-GlcNAc)-Asp-Trp-Phe-Dpr-Leu]cyclo(2beta-5beta)} or the nonpeptide antagonist SR48968 ((S)-N-methyl-N[4-(4-acetylamino-4-phenylpiperidino)-2-(3,4-dichlorophenyl)butyl]benzamide). (C) 2004 Elsevier B.V. All rights reserved.