Characterization of the interaction of N-acyl-L-tryptophan benzyl ester neurokinin antagonists with the human neurokinin-1 receptor

We have recently shown that a series of N-acyl-L-tryptophan benzyl esters are potent substance P antagonists (Macleod, A. M., Merchant, K. J., Cascieri, M. A., Sadowski, S., Ber, E., Swain, C. J., and Baker, R. (1993) J. Med Chem. 14, 2044-2045). We now report the detailed characterization of the interaction of N-acetyl-L-tryptophan-3,5-bistrifluoromethyl benzyl ester (L-732,138) with the human neurokinin-1 (NK-1) receptor. L-732,138 inhibits the binding of 125I-substance P to the cloned human NK1 receptor expressed in Chinese hamster ovary cells with an IC50 of 2.3 +/- 0.7 nM. In contrast, it has 200-fold lower affinity for the cloned rat NK-1 receptor and has > 1000-fold lower affinity for the human NK-2 and NK-3 receptors. L-732,138 acts as a competitive antagonist of substance P, as shown by functional Schild analysis of the inhibition of substance P-induced inositol phosphate synthesis, by kinetic analysis of the dissociation rate, and by thermodynamic analysis of the equilibrium binding of 125I-substance P to the NK-1 receptor. L-732,138 also competitively inhibits the binding of the quinuclidine amine antagonist, [125I]L-703,606, to the receptor. The compound has 230- and 10-fold reduced affinity for mutant NK-1 receptors in which histidine 265 or histidine 197, respectively, are replaced with alanine. We have previously shown that these residues play key roles in the binding of quinuclidine antagonists to the NK-1 receptor. These results suggest that the tryptophan and quinuclidine series of NK-1 antagonists bind to similar binding sites on the human NK-1 receptor.