Design of novel neurokinin 1 receptor antagonists based on conformationally constrained aromatic amino acids and discovery of a potent chimeric opioid agonist-neurokinin 1 receptor antagonist

A screening of conformationally constrained aromatic amino acids as base cores for the preparation of new NK1 receptor antagonists resulted in the discovery of three new NK1 receptor antagonists, 19 [Ac-Aba-Gly-NH-3′, 5′-(CF₃)₂-Bn], 20 [Ac-Aba-Gly-NMe-3′,5′- (CF₃)₂-Bn], and 23 [Ac-Tic-NMe-3′,5′-(CF ₃)₂-Bn], which were able to counteract the agonist effect of substance P, the endogenous ligand of NK1R. The most active NK1 antagonist of the series, 20 [Ac-Aba-Gly-NMe-3′,5′-(CF₃) ₂-Bn], was then used in the design of a novel, potent chimeric opioid agonist-NK1 receptor antagonist, 35 [Dmt-d-Arg-Aba-Gly-NMe-3′,5′- (CF₃)₂-Bn], which combines the N terminus of the established Dmt¹-DALDA agonist opioid pharmacophore (H-Dmt-d-Arg-Phe-Lys-NH₂) and 20, the NK1R ligand. The opioid component of the chimeric compound 35, that is, Dmt-d-Arg-Aba-Gly-NH₂ (36), also proved to be an extremely potent and balanced μ and δ opioid receptor agonist with subnanomolar binding and in vitro functional activity.