Synthesis and in vitro activities of NK-1 antagonists derived from L-tryptophan

A study of structure-activity relationships of a series of L-tryptophan derivative NK-1 antagonist was performed using 3,5-bis(trifluoromethyl)benzyl ester of N-acetyl-L-Tryptophan (IV) as a starting point. The ester moiety was replaced with several amidic functions while the N-acetyl group (Ac-) was retained (compounds 1-8) or changed into a benzyloxycarbonyl group (Z-) (compounds 9-16). The compounds were tested on guinea pig ileum longitudinal muscle, rat colon muscolaris mucosae, and rat everted portal vein, representative of tachykinin NK-1, NK-2 and NK-3 receptors, respectively. Both, Ac- and Z-series showed generally moderate antagonist activity on tachykinin NK-1 receptors with respect to the reference drug IV. The most potent term was compound 2 (Ac-Trp-N(CH3)CH(CH3)Ph with S-configuration at the C-terminus) which exhibited pA2 values of 7.0, 4.2 and 4.4 on NK-1, NK-2 and NK-3 sites, respectively.