A new melatonin receptor ligand with mt1‐agonist and MT2‐antagonist properties

Abstract

It has been difficult, so far, to obtain melatonin analogs possessing high selectivity for the respective melatonin receptors, mt1 and MT2. In the present work, we report the synthesis and pharmacological characterization of a new compound N-¿2-[5-(2-hydroxyethoxy)-1H-indol-3-yl)] ethyl¿ acetamide or 5-hydroxyethoxy-N-acetyltryptamine (5-HEAT). To assess the activity of the compound, the following tests were performed: affinity determination for the high- and low-affinity receptor states (2-[125I]iodomelatonin binding), potency and intrinsic activity in inducing G protein activation ([35S]GTPgammaS binding assay). 5-HEAT showed little selectivity for the mt1 receptor, with pKi values of 7.77 for mt1 and 7.12 for the MT2 receptors, respectively. 5-HEAT was able to differentiate between the high- and the low-affinity receptor states in the mt1 but not in the MT2 receptor. 5-HEAT induced a high level of G protein activation when acting through the mt1 receptor, with a relative intrinsic activity of 0.92. On the contrary, it elicited only minimal MT2 receptor-mediated G protein activation, with a relative intrinsic activity of 0.16, and was also able to inhibit the melatonin-induced MT2 receptor-mediated G protein activation, with a pKB value of 7.4. In conclusion, it appears that 5-HEAT possesses very different efficacies at the two melatonin receptors, behaving as a full melatonin receptor agonist at the mt1 and as an antagonist/weak partial agonist at the MT2 receptor. Therefore, it is a promising ligand for use in functional studies aimed at distinguishing between the effects mediated by the different melatonin receptors in the human.

Synthesis, Enantiomeric Resolution, and Structure–Activity Relationship Study of a Series of 10,11‐Dihydro‐5H‐Dibenzo[a,d]cycloheptene MT2 Receptor Antagonists

“…[29] This is consistent with pose B, accommodating position 8 within the small lipophilic pocked lined by Trp 264, and providing circumstantial evidence of a SAR different from that known for 5-indole substituents. The poor MT 2 receptor affinity of compound 10 probably reflects the steric limitation of the lipophilic pocket.…”

“…On indole derivatives, the 5-hydroxyethyloxy group modulates intrinsic activity at the MT 1 and MT 2 subtypes differently, with 5-hydroxyethyloxy-N-acetyltryptamine (5-HEAT) being an MT 1 agonist and an MT 2 antagonist. [29] Hydrophilic substituents lead to a significant decrease in binding affinity, which is likely due to the preference for lipophilic groups. Compound 9 behaves as an antagonist, with no difference in intrinsic activity between receptor subtypes.…”

Towards the Development of Mixed MT1‐Agonist/MT2‐Antagonist Melatonin Receptor Ligands

“…The organic layer was washed with brine, dried (Na 2 SO 4 ), and evaporated under reduced pressure to give a crude residue, which was purified by flash chromatography (EtOAc/ The title compound was prepared according to published procedures. [30] Methyl-[1-benzyl-3-(2-acetylaminoethyl)-1H-indol-5-yl]oxyacetate (6): A solution of 3 c (0.29 g, 1 mmol) in dry DMF (3 mL) was added to a suspension of sodium hydride (60 % in mineral oil, 0.052 g, 1.3 mmol) in dry DMF (3 mL) at 0 8C. After stirring for 30 min, benzyl bromide (0.13 mL, 1.1 mmol) was added to the reaction mixture, and stirring was continued for 20 h at room temperature.…”

“…N-Acetylserotonin 2, the common starting material for most of the target compounds, was prepared by hydrogenolysis of N-[2-(5-benzyloxy-1H-indol-3-yl)ethyl]acetamide 1 [32] over 10 % Pd-C (Scheme 1). Compounds 3 a, 3 b, and 3 c [30] were prepared by O-alkylation of N-acetylserotonin 2 with methyl acrylate, chloroacetonitrile, or methyl chloroacetate re- spectively, in the presence of sodium hydride as a base. The methyl ester derivatives 3 a and 3 c were converted into the corresponding alcohols 4 a and 4 c (5-HEAT) by reduction with LiAlH 4 .…”

“…We previously reported the discovery and binding characteristics of a novel MLT analogue, 5-hydroxyethoxy-N-acetyltryptamine (5-HEAT, Figure 1), a ligand of human MT 1 and MT 2 melatonin receptors [30] which was obtained by replacing the 5-methoxy substituent of MLT with a 2-hydroxyethoxy group. Although this ligand binds to h-MT 1 and h-MT 2 receptors with lower affinity than MLT, it presents an interesting pharmacological profile.…”