Side Chain Substituted Indolealkyl Amides as Potent Melatonin Receptor Ligands

Abstract

Melatonin (N‐acetyl‐5‐methoxytryptamine) modulates circadian rhythms and sleep‐wake cycles, primarily via activation of the MT1 and MT2 melatonin receptors which exhibit distinct pharmacological profiles. The MT1/MT2 melatonin receptor ligand, luzindole (LUZ), delays re‐entrainment of running wheel activity following an advance of dark onset via the MT1, and exerts antidepressant‐like effects via the MT2 melatonin receptor. Our goal is to design and synthesize novel analogues with at least 50‐fold selectivity for either MT1 or MT2 and a pharmacological profile on each receptor compatible with that of LUZ (ei., antagonist/inverse agonists at MT1; antagonist/partial agonists at MT2) to potentially mimic distinct receptor‐mediated behaviors. Our strategy is to assess structure‐activity relationships of indolealkyl amides to identify optimal functional groups that yield a more defined efficacy. A series of b‐methyl, b,b‐dimethyl and a‐methyl C3‐side chain functionalized indolealkyl amides, incorporating C2‐Me, C2‐Ph or C2‐H substituents were prepared. Four C5‐non‐methoxylated derivatives of melatonin, but with its side chain translocated from C3 to C2, were also synthesized. The compounds were tested in vitro for competition with 2‐[125I]‐iodomelatonin (100 pM) binding at hMT1 and hMT2 receptors stably expressed in CHO cells to assess binding affinities (Ki) as well as apparent intrinsic efficacy in the presence of GTP (100 μM). The apparent efficacy of LUZ in GTP‐shift binding assays with 2‐[125I]‐iodomelatonin shows an antagonist/inverse agonist profile at MT1 (KiGTP/Control: 0.20 ± 0.08, n=3), and an antagonist/partial agonist profile at MT2 (KiGTP/Control: 1.5 ± 0.4, n=4). The high affinity of the various analogues was dependent on the presence of b‐ and α‐substituents, and the nature of the C2‐functionality. A C2‐Ph analogue, named ATBT‐23 exhibited high affinity and selectivity for the hMT2 receptor. Indeed, ATBT‐23 showed 190‐fold selectivity for the MT2 receptor (MT1 Ki: 2,794 nM, MT2 Ki: 15 nM, KihMT1/KihMT2 Ratio: 190, n=4). This selectivity is distinct from that of LUZ which demonstrates approximately 26‐fold affinity for the MT2 receptor. However, GTP shift binding assays revealed apparent affinity ratios compatible with ATBT‐23 being a melatonin receptor antagonist/weak inverse agonist at hMT1 and an antagonist/partial agonist at hMT2 receptors, similar to non‐selective LUZ. Brain penetration determined by ex vivo binding as well as the in vivo behavioral profile for the MT2 selective analogue ATBT‐23 on circadian re‐entrainment and depressive‐like behaviors in C3H/HeN mice will be reported and compared with the behavioral profile of LUZ and other melatonin receptor type‐selective analogues. ATBT‐23 and newly synthesized analogues could signify a novel class of melatonin receptor ligands with higher affinity and efficacy yet similar pharmacology to LUZ, for the treatment of circadian sleep and depressive disorders.Support or Funding InformationSupported by the Jacobs School of Medicine and Biomedical Sciences to MLD.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.