Mapping the Melatonin Receptor. 5. Melatonin Agonists and Antagonists Derived from Tetrahydrocyclopent[b]indoles, Tetrahydrocarbazoles and Hexahydrocyclohept[b]indoles

Abstract

Tetrahydrocyclopent[b]indoles, tetrahydrocarbazoles, and hexahydrocyclohept[b]indoles have been prepared as melatonin analogues to investigate the nature of the binding site of the melatonin receptor. The affinity of analogues was compared in a radioligand binding assay using chicken brain membranes and agonist and antagonist potency measured in clonal Xenopus laevis melanophore cells. Comparison of the N-acyl-3-amino-6-methoxytetrahydrocarbazoles (2) with N-acyl-4-(aminomethyl)-6-methoxy-9-methyltetrahydrocarbazoles (9) showed that the latter have much higher binding affinities for the chicken brain receptor. Comparison of N-acyl-1-(aminomethyl)-7-methoxy-4-methyltetrahydrocyclopent[b]ind oles (10), 6-methoxytetrahydrocarbazoles (9), and N-acyl-10-(aminomethyl)-2-methoxy-5-methylhexahydrocyclohept[b]ind oles (11) showed that the tetrahydrocarbazoles had the highest binding affinity with the cyclohept[b]indoles and the cyclopent[b]indoles having rather lower affinities. All of these observations are in agreement with our postulated model of melatonin orientation at the binding pocket in which the 3-amidoethane side chain is in a conformation close to the 5-methoxyl group, as is shown in the X-ray crystallographic structure of 9m and in the energy-minimized computed structures. Separation of the enantiomers of members from each of these three systems was accomplished by chiral HPLC. It was found that in all cases the (-)-enantiomer had a higher binding affinity than the (+)-enantiomer. An X-ray crystallographic analysis of the two enantiomers of 9a showed that the (+)-enantiomer had the (R) absolute stereochemistry. Since the sign of the Cotton curves, determined from circular dichroism studies, was the same for all (+)-enantiomers, it is assumed that the absolute stereochemistry at these centers is identical. In the Xenopus melanophore assay, the tetrahydrocarbazoles 2 (R = H) were mainly weak antagonists, while those with R = OMe were agonists. The biological behavior of the tetrahydrocarbazoles 9 (R = H) depended on R1, some being agonists and some antagonists, whereas those with R = OMe were generally agonists. Variation of the R and R1 groups in compounds of type 9 produced both agonists and antagonists. The tetrahydrocylopentaindoles 10 had similar biological properties to the corresponding analogues of 9, but the hexahydrocycloheptaindoles 11 showed a much greater propensity to be antagonists. In all cases the (S)-enantiomers were found to be more potent agonists than the (R)-enantiomers.

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

“…The conformation melatonin adopts when interacting with its receptors has been debated. Evidence from conformationally constrained and stereoselective analogs Beresford et al, 1998;Davies et al, 1998;Leclerc et al, 1998;Jellimann et al, 2000) has provided information on the orientation likely to be adopted by the 5-methoxy and N-acetylaminoethyl groups when interacting with the receptor. Some of these ligands [(Ϫ)-and (ϩ)-AMMTC] have been used as tools in experiments designed to determine whether a biological response is really mediated by melatonin receptor activation (Ting et al, 1997).…”

Indole Melatonin Agonists and Antagonists Derived by Shifting the Melatonin Side Chain from the C-3 to the N-1 or to the C-2 Indole Position

“…MLT agonist compounds (2-15), and MLT partial agonist (24, 29-33, 38), antagonist compounds (27,28,(34)(35)(36)(37) or putative inverse agonists (25,26) were discovered by shifting the melatonin side chain from the C-3 to the N-1 or C-2 indole position, respectively.…”

“…These ligands bind MLT receptors with low to moderate affinity. An exception is represented by derivative 23 [26], in which the presence of a cycloheptane ring fused to the [b] face of the indole ring (restricting the flexibility of the side chain) is assumed to be important for antagonist activity. As a part of our research program directed towards the design of MLT antagonists, we decided to retain both the secondary amide and the methoxy substituent, but to modulate their relative distance by changing their position on the indole ring as well as the length of the ethylamido side chain ( fig.…”

“…Conversely, the derivative obtained by lengthening the side chain (38) retained a residual agonist effect. Moving the methoxyl group from position 4-to position 5-(28 vs. 24), 6-(25 vs. 24), 7-(26 vs. 24) or removing it altogether (27 vs. 24) is detrimental for the affinity of the ligand, but it changes the profile from that of a partial agonist (24,30) to that of an antagonist (27,28) or a putative inverse agonist (25,26).…”

Design of subtype selective melatonin receptor agonists and antagonists

“…, has allowed us to use these melatonin-responsive cells to investigate the structural features of the melatonin molecule necessary for binding to and activation of its receptor [3,[7][8][9][10]. Res-ponses to melatonin in melanophores can be measured by manual scoring of the distribution of melanosomes within the cell using the melanophore index [22], which, although subjective, is direct and simple and useful for studies on small numbers of cells.…”