N1-linked melatonin dimers as bivalent ligands targeting dimeric melatonin receptors

A novel series of dimeric melatonin analogues obtained by connecting two melatonin molecules through N1 with spacers of 15–24 atoms was synthesized and characterized in 2-[125-I]-iodomelatonin binding and bioluminescence resonance energy transfer (BRET) experiments at MT1 and MT2 receptors. Compounds 4 (16 atoms spacer) and 13 (24 atoms spacer) are among the ligands inducing the maximal BRET at MT2-homodimers as well as at both types of MT1/MT2 heterodimers. Notably, ligand-induced BRET changes observed for compounds linked through spacers of 22–24 atoms could be attributed to ligand-induced conformational changes between the two protomers of MT1 and MT2 homo- and heterodimers providing evidence for the binding of both pharmacophores of dimeric melatonin analogues to the two protomers of receptor dimers.