Bivalent ligand approach on N-{2-[(3-methoxyphenyl)methylamino]ethyl}acetamide : synthesis, binding affinity and intrinsic activity for MT1 and MT2 melatonin receptors

We report the synthesis, binding properties and intrinsic activity at MT1 and MT2 melatonin receptors of new dimeric melatonin receptor ligands in which two units of the monomeric agonist N-{2-[(3-methoxyphenyl) methylamino]ethyl}acetamide (1) are linked together through different anchor points. Dimerization of compound 1 through the methoxy substituent leads to a substantial improvement in selectivity for the MT1 receptor, and to a partial agonist behavior. Compound 3a, with a trimethylene linker, was the most selective for the MT1 subtype (112-fold selectivity) and compound 3d, characterized by a hexamethylene spacer, had the highest MT1 binding affinity (pKiMT1 = 8.47) and 54-fold MT1-selectivity. Dimerization through the aniline nitrogen of 1 abolished MT1 selectivity, leading to compounds with either a full agonist or an antagonist behavior depending on the nature of the linker