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Molecular superposition models guided the design of novel melatonin receptor ligands characterized by a 2-acylaminomethyltetrahydroquinoline scaffold. Starting from the structure of N-anilinoethylamide ligands, the flexible chain was conformationally constrained to reproduce the bioactive conformation of melatonin. Structure–activity relationships were investigated, focusing on the substituent at the nitrogen atom, the position of the methoxy group, and the replacement of the amide side chain by urea and thiourea groups. The compounds were tested for binding affinity and intrinsic activity at human MT₁ and MT₂ receptors. Structural optimization resulted in N-[(1-benzyl-1,2,3,4-tetrahydro-5-methoxyquinolin-2-yl)methyl]propionamide (UCM1014), with picomolar MT₂ binding affinity (K_i = 0.001 nM), more than 10000-fold selectivity over the MT₁ receptor, and a full agonist profile (GTPγS test), being the most potent …