Analysis of Structure−Activity Relationships for MT₂ Selective Antagonists by Melatonin MT₁ and MT₂ Receptor Models

Abstract

Three-dimensional homology models of human MT(1) and MT(2) melatonin receptors were built with the aim to investigate the structure-activity relationships (SARs) of MT(2) selective antagonists. A common interaction pattern was proposed for a series of structurally different MT(2) selective antagonists, which were positioned within the binding site by docking and simulated annealing. The proposed antagonist binding mode to the MT(2) receptor is characterized by the accommodation of the out-of-plane substituents in a hydrophobic pocket, which resulted as being fundamental for the explanation of the antagonist behavior and the MT(2) receptor selectivity. Moreover, to assess the ability of the MT(2) receptor model to reproduce the SARs of MT(2) antagonists, three new derivatives of the MT(2) selective antagonist N-[1-(4-chloro-benzyl)-4-methoxy-1H-indol-2-ylmethyl]-propionamide (7) were synthesized and tested for their receptor affinity and intrinsic activity. These compounds were docked into the MT(2) receptor model and were submitted to molecular dynamics studies, providing results in qualitative agreement with the experimental data. These results confirm the importance of the out-of-plane group in receptor binding and selectivity and provide a partial validation of the proposed G protein-coupled receptor model.

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

“…In fact, poor selectivity had been suggested by the similar fitting (RMSD of the pharmacophore elements) obtained for the two enantiomers superposed with the reference antagonist 4P-PDOT. [16] The two enantiomers can be docked into the putative binding site of our previously developed 3D model of the MT 2 receptor, [17] both fulfilling the putative requirements for receptor binding and antagonist behavior. These requirements call for the formation of a hydrogen bond between the amide oxygen atom of the antagonist and the hydroxy group of Tyr 183 from the second extracellular loop, resulting in a T-shaped interaction between the aromatic portion of the ligand and the NH group of His 208 in transmembrane helix 5, and for a lipophilic group fitting into a hydrophobic pocket, the occupation of which is related to MT 2 selectivity and decreased intrinsic activity.…”

“…These requirements call for the formation of a hydrogen bond between the amide oxygen atom of the antagonist and the hydroxy group of Tyr 183 from the second extracellular loop, resulting in a T-shaped interaction between the aromatic portion of the ligand and the NH group of His 208 in transmembrane helix 5, and for a lipophilic group fitting into a hydrophobic pocket, the occupation of which is related to MT 2 selectivity and decreased intrinsic activity. [17] The skewed arrangement of the tricyclic structure of 5 allows the accommodation of the benzene ring within the hydrophobic cavity for both enantiomers (pose A, Figure 3 A). With respect to this hypothetical binding mode, the similar MT 2 affinities obtained for www.chemmedchem.org 5, 11, and 12 seriously challenge the previous hypothesis on the importance of the T-shaped interaction between the aromatic portion of the antagonist, mimicking the indole nucleus of MLT, and His 208, as the electron-donor methoxy substituent should lead to a more stabilized interaction and to a higher binding affinity than the electron-withdrawing acetyl group.…”

“…This pocket could also be occupied by one of the benzene rings of the dihydrodibenzocycloheptene nucleus. [17] The structural exploration of the tricyclic alkylamides was rather limited, and the compounds were tested as racemates, thus providing no information on their enantiomeric selectivity at the MT 1 and MT 2 receptors. [16] Herein we further explore the structure-activity relationships for this class of MLT receptor ligands, having synthesized a series of 10,11-dihydro-5H-dibenzoA C H T U N G T R E N N U N G [a,d]cycloheptene analogues of 5 modified either at the amide side chain or at position 8 (Table 1).…”

“…[17] Ligand geometry was optimized using the MMFF94s force field, [38] implemented in Macromodel 8.6, [39] to a gradient of 0.01 kJ A C H T U N G T R E N N U N G (mol )…”

MT2 selective melatonin receptor antagonists: design and structure-activity relationships

“…19 In particular, we were looking for an explanation at the receptor level of the role of the out-of-plane substituent in receptor selectivity and intrinsic activity. During the building process relevant modifications were applied to the rhodopsin template, in particular to the region of transmembrane (TM) 5, to allow the accommodation of the bulky MT 2 antagonists in the too narrow space available within the TM helices.…”