Identification of l-Tryptophan Derivatives with Potent and Selective Antagonist Activity at the NK₁ Receptor

As part of a program of screening the Merck sample collection, N-ethyl-l-tryptophan benzyl ester was identified as a weak antagonist at the substance P (NK₁) receptor. Structure-activity studies showed that the indole ring system could be replaced by 3,4-dichlorophenyl, α-or β-naphthyl, or benzthiophene with retention or only small loss of affinity. It was found that acylation of the tryptophan nitrogen gave compounds with higher affinity than N-ethyl or other basic amines. Optimization of substitution on the benzyl ester led to the identification of the 3,5-bis-(trifluoromethyl)benzyl ester of N-acetyl-l-tryptophan 26 as a potent and selective substance P receptor antagonist. Compound 26 blocked substance P induced dermal extravasation in vivo and was the most potent compound from this structurally novel class of antagonists which further adds to the diversity of small molecules that bind to the (NK₁) receptor.