Conformationally Restrained Melatonin Analogues:  Synthesis, Binding Affinity for the Melatonin Receptor, Evaluation of the Biological Activity, and Molecular Modeling Study

Abstract

The design, synthesis, and biological profile of several indole melatonin analogues with a conformationally restricted C3 amidoethane side chain are presented. Examination of the accessible conformations of the melatonin side chain led us to explore some of its fully or partially restricted analogues, 2-12, the binding affinity values of which were utilized to gain further insight on the melatonin binding site. Two pharmacophoric models have been devised for melatonin and the active compounds by conformational analysis and superimposition performed using the DISCO program. In these models, the melatonin side chain can adopt a gauche/anti conformation out of the indole plane. Another contribution of this study regards the observation of a possible binding point interaction around the C2 position of the indole, as suggested by the remarkably increased binding affinity observed in the C2-substituted analogues 6 and 9 and especially in the more rigid analogue 5. The biological activity and the efficacy of the new compounds were tested by measuring the inhibition of the forskolin-stimulated cAMP accumulation and the GTP gamma S index. Both analyses demonstrated that all of the compounds were full agonists with the exception of 4 and 9, which showed a slight reduction in efficacy and would seem to be partial agonists.

Structure-Affinity Relationships of Indole-Based Melatonin Analogs

“…The majority of the other conformationally restricted analogs showed modest affinity for the melatonin Spadoni/Mor/Tarzia receptor. Furthermore, the remarkably increased binding affinity observed in all the restricted 2-substituted analogs 27, 32, 34 (compared to that of the corresponding unsubstituted compounds 28,31,33), and in particular in the more rigid 2-bromo derivative 34, demonstrates that a further binding point interaction is the cause of the increased affinity.…”

“…To this end, we performed binding assays of a variety of MLT analogs in which the conformational freedom of the 3-ethylamido side-chain is limited by inclusion in different rigid structures (27-37, fig. 1) [28]. Those compounds exhibited a broad range of binding affinities: from K i values as low as 0.39 nM to more than 10,000 nM ( fig.…”

“…When the three-dimensional structure of the target molecule is unknown (as in the case of melatonin receptors), the development of highaffinity, conformationally constrained compounds represents an efficient approach to explore the requirements governing drug-receptor interactions. In parentheses, K i (nM ) values determined in quail brain membranes (27-37) [28], or taken from literature (38 [29], 39 [33], and 40 [34]). …”

International Union of Basic and Clinical Pharmacology. LXXV. Nomenclature, Classification, and Pharmacology of G Protein-Coupled Melatonin Receptors

“…One effect of the 2-position substituent seems likely to be to "push" the N-acetylaminoethyl side chain into the preferred conformation for interaction with the receptor. In addition, there is evidence that substituents in the C-2 position can interact directly with the receptor, leading to increased ligand affinity (Mathé-Allainmat et al, 1996;Spadoni et al, 1997). In vivo melatonin is degraded rapidly, primarily in the liver by 6-hydroxylation followed by conjugation and excretion in the urine.…”

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

“…8,9,10,11 Starting from a series of conformationally constrained compounds, we proposed a pharmacophore model for agonists and a bioactive conformation for the natural ligand. 12 A 3D-QSAR CoMFA analysis allowed the rationalization of the structure-activity relationships for more than one hundred agonists. 13 To investigate the effect of different mutual arrangements of the pharmacophoric elements on binding affinity and intrinsic activity, the structure of melatonin was modified by shifting the acylaminoalkyl side chain at position 2 of the indole ring and by inserting different substituents on the indole nitrogen.…”