Radioligand binding affinity and biological activity of the enantiomers of a chiral melatonin analogue

Mapping the Melatonin Receptor. 7. Subtype Selective Ligands Based on β-SubstitutedN-Acyl-5-methoxytryptamines and β-SubstitutedN-Acyl-5-methoxy-1-methyltryptamines

A series of beta-substituted and beta,beta-disubstituted N-acyl 5-methoxy-1-methyltryptamines and 5-methoxytryptamines have been prepared as melatonin analogues to investigate the nature of the binding site of the melatonin receptor. The affinity of analogues was determined in a radioligand binding assay using cloned human MT(1) and MT(2) receptor subtypes expressed in NIH 3T3 cells. Agonist and antagonist potency of all analogues was measured using the pigment aggregation response of a clonal line of Xenopus laevis melanophores. beta-Methylmelatonin (17a) and beta,beta-dimethylmelatonin (17b), though showing a slight decrease in binding at human receptors, show an increase in potency on Xenopus. N-Butanoyl 5-methoxy-1-methyl-beta,beta-trimethylenetryptamine (12c) is an antagonist at human MT(1) receptors but an agonist at MT(2), while N-butanoyl 5-methoxy-1-methyl-beta,beta-tetramethylenetryptamine (13c) is an antagonist at MT(1) but had no action at MT(2) and is one of the first examples of an MT(1) selective antagonist.

Reassessing the melatonin pharmacophore—Enantiomeric resolution, pharmacological activity, structure analysis, and molecular modeling of a constrained chiral melatonin analogue

3-(Acetylaminomethyl)-2-(ethoxycarbonyl)-6-methoxy-1,3,4,5-tetrahydrobenzo[cd]indole (2) is a rigid melatonin analogue that as a racemate displays about the same affinity and intrinsic activity of melatonin (1) in in vitro experiments. We report here the resolution of the racemate by preparative medium pressure liquid chromatography (MPLC) and the X-ray determination of the R absolute configuration of the (-)-enantiomer. The two enantiomers were separately tested as MT1 and MT2 ligands, and the (+)-(S)-2 showed a potency comparable to that of melatonin and about three orders of magnitude greater than that of its enantiomer. The information obtained by crystallographic analysis and NMR studies about the conformational preference for 2 and by the pharmacological characterization of (R)-2 and (S)-2 was employed in a molecular modeling study, aimed at reassessing the melatonin receptor pharmacophore model for agonist compounds. Chiral enantioselective agonists reported in the literature were also included in the study.

Recent Advances in Melatonin Receptor Ligands

Melatonin is a hormone exerting its multiple actions mainly through two G-protein-coupled receptors MT(1) and MT(2). Exploring the physiological role of each of these subtypes requires subtype selective MT(1) and MT(2) ligands. While several MT(2)-selective ligands were developed in the 1990s, no selective agonists and antagonists for the MT(1) subtype were described. The present article reviews mela toninergic ligands developed in the current millennium focusing on subtype selective agents and on drug candidates. Notable compounds are the MT(1)-selective agonists 35 and 134, MT(1)-selective antagonists 117 and 131, MT(2)-selective agonists 58, 70, 79, 97 and 125, MT(2)-selective antagonists 27, 73 and 119, and the highly potent non-selective agonist 120. The non-selective agonists agomelatine 2, and ramelteon 87 are drug candidates as antidepressive agent and for the treatment of insomnia and circadian rhythm disfunction, respectively.

Piracetam and TRH analogues antagonise inhibition by barbiturates, diazepam, melatonin and galanin of human erythrocyte D-glucose transport

1 Nootropic drugs increase glucose uptake into anaesthetised brain and into Alzheimer's diseased brain. Thyrotropin-releasing hormone, TRH, which has a chemical structure similar to nootropics increases cerebellar uptake of glucose in murine rolling ataxia. This paper shows that nootropic drugs like piracetam (2-oxo 1 pyrrolidine acetamide) and levetiracetam and neuropeptides like TRH antagonise the inhibition of glucose transport by barbiturates, diazepam, melatonin and endogenous neuropeptide galanin in human erythrocytes in vitro. 2 The potencies of nootropic drugs in opposing scopolamine-induced memory loss correlate with their potencies in antagonising pentobarbital inhibition of erythrocyte glucose transport in vitro (P<0.01). Less potent nootropics, D-levetiracetam and D-pyroglutamate, have higher antagonist Ki's against pentobarbital inhibition of glucose transport than more potent L-stereoisomers (P<0.001). 3 Piracetam and TRH have no direct effects on net glucose transport, but competitively antagonise hypnotic drug inhibition of glucose transport. Other nootropics, like aniracetam and levetiracetam, while antagonising pentobarbital action, also inhibit glucose transport. Analeptics like bemigride and methamphetamine are more potent inhibitors of glucose transport than antagonists of hypnotic action on glucose transport. 4 There are similarities between amino-acid sequences in human glucose transport protein isoform 1 (GLUT1) and the benzodiazepine-binding domains of GABAA (gamma amino butyric acid) receptor subunits. Mapped on a 3D template of GLUT1, these homologies suggest that the site of diazepam and piracetam interaction is a pocket outside the central hydrophilic pore region. 5 Nootropic pyrrolidone antagonism of hypnotic drug inhibition of glucose transport in vitro may be an analogue of TRH antagonism of galanin-induced narcosis.

Melatonin stimulates cell proliferation in zebrafish embryo and accelerates its development

All vertebrates show a dramatic circadian rhythm in circulating melatonin with high levels at night and very low levels during daytime. In adults, melatonin is thought to synchronize other circadian rhythms and regulate seasonal rhythms in photoperiodic animals by acting on specific G-protein coupled receptors. The role of melatonin in development is unknown, even though melatonin receptors appear to be more highly expressed in developing embryos and neonates than in adults. In this study on zebrafish embryos, we describe a role for melatonin in increasing cell proliferation and accelerating development. We propose that melatonin has a role in extending the safe limit of proliferation rate at night to allow more rapid development when potentially damaging ultraviolet light is absent.

Design, synthesis and biological evaluation of novel beta-substituted indol-3-yl ethylamido melatoninergic analogues

A series of new melatonin analogues have been synthesized. Interestingly, two of the new compounds, 11c and 11e, which did not show any appreciable affinity for the melatonin receptor, were found to be potent inhibitors of lipid peroxidation in rat liver microsomes. Analogue 11c, in particular, is a better antioxidant than melatonin.

Synthesis of phenalene and acenaphthene derivatives as new conformationally restricted ligands for melatonin receptors

Conformationally restricted phenalene and acenaphthene derivatives 5 were synthesized from phenalen-1-one and acenaphthen-1-one derivatives using the Horner-Emmons reaction. The amines were prepared through the corresponding isocyanates by the Curtius reaction on the acids or by the reduction of the nitriles. Amido derivatives (R(3) = Me, Et, n-Pr, c-Pr) were prepared by acylation of the amines with the appropriate anhydrides or acid chlorides or by the reductive acylation of the nitriles. The affinities of the compounds for melatonin binding sites were evaluated in vitro in binding assays using chicken brain melatonin and the human mt(1) and MT(2) receptors expressed in HEK-293 cells. The functionality of the compounds was determined by the potency to lighten the skin of Xenopus laevis tadpoles. Highly potent compounds were obtained. The data highlighted the role of the methoxy group located in the ortho position to the ethylamido chain as compounds with picomolar affinities such as 14c were obtained (chicken brain, hmt(1), hMT(2) K(i) values = 0.02, 0.008, 0.069 nM, respectively). Compound 14c was equipotent to the corresponding dimethoxy derivative 15c (chicken brain, hmt(1), hMT(2) K(i) values = 0.07, 0.016, 0.1 nM, respectively). On the other hand, the restricted conformation of the amido chain did not influence selectivity for the cloned hmt(1) and hMT(2) receptors. These compounds were also potent agonists of melanophore aggregation in X. laevis. 15a,c were several hundred fold more potent than melatonin (EC(50) = 0.025, 0.004 nM, respectively). Conformational studies indicated that the minimum energy folded conformation of the ethylamido chain could constitute the putative active form in the receptor site in agreement with previous results.

Mapping the melatonin receptor. 6. Melatonin agonists and antagonists derived from 6H-isoindolo[2,1-a]indoles, 5,6-dihydroindolo[2,1-a]isoquinolines, and 6,7-dihydro-5H-benzo[c]azepino[2,1-a]indoles

6H-Isoindolo[2,1-a]indoles (5, 7, 10, 13), 5,6-dihydroindolo[2, 1-a]isoquinolines (20, 21), and 6,7-dihydro-5H-benzo[c]azepino[2, 1-a]indoles (23, 25, 27, 30) have been prepared as melatonin analogues to investigate the nature of the binding site of the melatonin receptor. The affinity of analogues was determined in a radioligand binding assay using cloned human mt(1) and MT(2) receptor subtypes expressed in NIH 3T3 cells. Agonist and antagonist potency was measured using the pigment aggregation response of a clonal line of Xenopus laevis melanophores. The 2-methoxyisoindolo[2, 1-a]indoles (7a-d) showed much higher binding affinities than the parent isoindoles (5a-e), and whereas 7a-c were agonists in the functional assay, 7d and 5a-e were antagonists. The 2-ethoxyisoindolo[2,1-a]indoles (10a-d) showed reduced binding affinities compared to their methoxy analogues, while the 5-chloro derivative 13 showed a considerable reduction in binding affinity and potency compared to 7a. The 10-methoxy-5,6-dihydroindolo[2, 1-a]isoquinolines (21a-c) had higher binding affinities than the corresponding parent indoloisoquinolines (20a-c) in the human receptor subtypes, and the parent compounds were antagonists whereas the 10-methoxy derivatives were agonists in the functional assay. The N-cyclobutanecarbonyl derivatives of both the parent (20d) and 10-methoxyl (21d) series had similar binding affinities and were both antagonists with similar potencies. The 11-methoxy-6, 7-5H-benzo[c]azepino[2,1-a]indoles (25a-d) had higher binding affinities than the corresponding parent compounds (23a-d) at the MT(2) receptor but similar affinities at the mt(1) site; all of the compounds were antagonists in the functional assay. Changing 11-methoxy for 11-ethoxy decreased the binding affinity slightly, and this was more evident at the MT(2) receptor. All of the derivatives investigated had either the same or a greater affinity for the human MT(2) receptor compared to the mt(1) receptor (range 1:1-1:132). This suggests that the mt(1) and MT(2) receptor pockets differ in their ability to accommodate alkyl groups in the indole nitrogen region of the melatonin molecule. Two compounds (7c and 25c) were tested in functional assays on recombinant mt(1) and MT(2) melatonin receptors. Compound 7c is a potent agonist with some selectivity (44-fold) for the MT(2) receptor, while 25c is an MT(2)-preferring antagonist. Increasing the carbon chain length between N-1 of indole and the 2-phenyl group from n = 1 through n = 3 leads to a fairly regular decrease in the binding affinity, but, remarkably, when n = 3, it converts the methoxy compounds from melatonin agonists to antagonists. The Xenopus melatonin receptor thus cannot accommodate an N-n-alkyl chain attached to a 2-phenyl substituent with n > 2 in the required orientation to induce or stabilize the active receptor conformation.

Synthesis of beta-substituted naphth-1-yl ethylamido derivatives as new melatoninergic agonists

Naphthalene melatoninergic ligands with alkyl groups (Me, Et, Pr, Bz) in the beta position of the ethylamido chain were synthesised. The affinity of the compounds for chicken brain melatonin receptors was evaluated using 2-[125I]-iodomelatonin as the radioligand. An increase in the affinity was observed with the beta-methyl derivatives and the greatest increase was seen with the (-) enantiomers. The introduction of a 2- or 7-MeO group on the naphthalene ring and the lengthening (Et, Pr) of the alkylamido chain gave potent compounds such as (-)1h (Ki = 24 pM). The functional activity of these compounds was evaluated by the aggregation of melanophores in Xenopus laevis tadpoles. The potency to produce lightening of the skin of Xenopus laevis was related to the affinities values of the molecules at melatonin chicken brain receptors. The most potent ligands were found to be full agonists and compound 1h was 25 fold more potent than melatonin in this bioassay.

Melatonergic properties of the (+)- and (-)-enantiomers of N-(4-methoxy-2,3-dihydro-1H-phenalen-2-yl)amide derivatives

N-(4-Methoxy-2,3-dihydro-1H-phenalen-2-yl)amide derivatives, conformationally restricted ligands for melatonin receptors, were synthesized by an alternative synthetic method from the corresponding 1,8-naphthalic anhydride which was transformed into the phenalenecarboxylic acid 7. A Curtius reaction on 7 gave the amino compound which was acylated to give compounds 4a-c. The (+)- and (-)-4a-c enantiomers were separated by semipreparative chiral HPLC. Compounds were evaluated for their affinity for chicken brain melatonin receptors in binding assays using 2-[125I]iodomelatonin and for their potency to lighten the skin of Xenopus laevis tadpoles. The butyramido derivative 4c was the most potent ligand (Ki = 1.7 nM). No enantioselectivity was observed with the enantiomers which were equipotent to the racemic mixture. In contrast to the reference compounds, melatonin, agomelatine (S 20098), and N-[2-(2, 7-dimethoxynaphth-1-yl)ethyl]acetamide, which were very potent at lightening the skin of X. laevis tadpoles, compounds 4a-c were inactive or weakly active (EC50 > 1 microM). In this bioassay, compound 4a was characterized as a putative antagonist of melatonin receptors.

Mapping the melatonin receptor. 5. Melatonin agonists and antagonists derived from tetrahydrocyclopent[b]indoles, tetrahydrocarbazoles and hexahydrocyclohept[b]indoles

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.

Studies on the vasoconstrictor action of melatonin and putative melatonin receptor ligands in the tail artery of juvenile Wistar rats

1. In this study we compared the vasoconstrictor activity of melatonin in rat isolated tail artery using two different recording systems, the Halpern pressure myograph and the Halpern-Mulvany wire myograph, with the view to determining a reliable method for obtaining pharmacological data on vascular melatonin receptors. In addition, we characterized the melatonin receptor in this preparation, using analogues of melatonin, and examined the activity of various naphthalenic derivatives with biological activity in non-vascular models of melatonin receptors. 2. Using the Halpern pressure myograph, cumulative addition of melatonin (0.1 nM to 1 microM) produced direct vasoconstriction (19.3+/-6.4% reduction in lumen diameter, n=5) in five of 11 pressurized segments, with pEC50 of 9.14+/-0.17. Similarly, non-cumulative application of melatonin caused vasoconstriction (19.7+/-4.6% reduction in lumen diameter, n=7) in seven of 20 preparations examined with pEC50 of 8.74+/-0.26. The selective alpha2-adrenoceptor agonist, UK-14304 (5-bromo-6-[2-imidazolin-2-ylamino]-quinoxaline bitartrate), produced vasoconstriction in all 'melatonin-insensitive' preparations. 3. Melatonin (0.1 nM to 1 microM) failed to elicit isometric contractions of tail artery segments in the Halpern wire myograph, but produced concentration-dependent potentiation of electrically-evoked, isometric contractions (maximum effect of 150-200% enhancement) when applied either noncumulatively (seven of seven preparations) or cumulatively (four of seven preparations). The pEC50 value of melatonin (non-cumulative) was 8.50+/-0.10 (n=7) which was not different from that obtained in the pressure myograph. All further experiments were conducted using a non-cumulative protocol against electrically-evoked, isometric contractions. 4. Based on the pEC50 values for the melatonin analogues examined, the pharmacological profile for the enhancement of electrically-evoked contractions was 2-iodomelatonin > 6-chloromelatonin > or = (-)-AMMTC > or = S21634 > or = melatonin > or = S20098 > S20242 > or = S20304 > 6-hydroxymelatonin > S20932 > (+)-AMMTC > N-acetyl-5-HT. Our data suggests the vascular receptor belongs to the MEL1-like subtype. All the indole-based analogues of melatonin, 2-iodomelatonin, (-)-AMMTC, (+)-AMMTC, S20932, 6-chloromelatonin, 6-hydroxymelatonin and N-acetyl-5-HT, behaved as full agonists. All the naphthalenic derivatives examined, S21634, S20098, S20242 and S20304 behaved as partial agonists relative to melatonin. 5. The naphthalenic-based antagonists, S20928 and S20929, did not modify electrically-evoked, isometric contractions of the tail artery, but produced a parallel, rightward displacement of the melatonin concentration-response curve. Based upon the effect of 1 microM S20928 and S20929, the estimated pK(B) values for these antagonists were 7.18+/-0.25 (n=4) and 7.17+/-0.25 (n=5), respectively. 6. We demonstrated that enhancement of electrically-evoked, isometric contractions of the rat isolated tail artery (using the Halpern-Mulvany wire myograph) is a simple and reproducible model for assessing the activity of putative agonists, partial agonists and antagonists at vascular melatonin receptors. Pharmacological characterization of the receptor suggests the presence of a MEL1-like subtype.

Conformationally restrained melatonin analogues: synthesis, binding affinity for the melatonin receptor, evaluation of the biological activity, and molecular modeling study

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.

Analogues of diverse structure are unable to differentiate native melatonin receptors in the chicken retina, sheep pars tuberalis and Xenopus melanophores

1. The pineal hormone melatonin exerts its biological effects through specific, high affinity G-protein coupled receptors. Recently, three melatonin receptor subtypes (Mel1a, Mel1b and Mel1c) have been cloned. Neither the cloned subtypes, nor the native receptors have yet been compared in a detailed pharmacological analysis. 2. The present study examined the structure-activity relationships of a series of 21 melatonin analogues, by comparing their potency on the pigment aggregation response in Xenopus laevis melanophores with their affinity in radioligand binding competition studies in chicken retina and sheep pars tuberalis (PT), two tissues in which melatonin is known to mediate a biological response. 3. All but four of the analogues were full melatonin receptor agonists producing a concentration-related redistribution of pigment granules in cultured Xenopus melanophores. The remaining analogues produced little pigment aggregation at 10 microM. 4. Saturation studies with 2-[125I]-iodomelatonin identified a single binding site in the chicken retina and sheep PT membranes, with a KD of 36.6 +/- 2.8 and 37.3 +/- 4.3 pM, and a maximal number of binding sites (Bmax) of 16.6 +/- 0.5, and 40.1 +/- 1.7 fmol mg-1 protein, respectively. 5. Comparison of the potency/affinity of the analogues for the binding sites gave a highly significant correlation in each case, retina/melanophore, r = 0.97 (P < 0.001, n = 17), PT/melanophore, r = 0.97 (P < 0.001, n = 17) and PT/retina, r = 0.98 (P < 0.001, n = 21). 6. Despite their large range in affinity and structural diversity these melatonin agonists were unable to distinguish between melatonin receptors in the chicken retina, sheep pars tuberalis and Xenopus melanophores.

Mapping the melatonin receptor. 4. Comparison of the binding affinities of a series of substituted phenylalkyl amides

A series of 2-, 3-, and 4-substituted phenylalkyl amides were prepared as potential melatonin analogs in order to investigate the nature of the binding site of the melatonin receptor in chicken brain. The length of the alkyl chain was systematically varied from n = 1 to 4, and methoxyl substituents were incorporated into the phenyl ring at the 2-, 3-, and 4-positions. The maximum binding affinity was found to occur when n = 3 and when the methoxyl substituent was in the 3-position, the direct analog of the carbon framework of melatonin in which the 1,2-atoms of the indole ring have been removed. Whereas there was only a relatively small decrease in binding affinity for the corresponding 2-methoxy derivatives, 4-methoxyl substitution led to a large decrease in binding affinity, suggesting that the binding sites for the side chain and methoxyl group could not now be occupied at the same time. As in the indole analogs of melatonin, replacement of the methyl group of the amide by a longer alkyl chain led to an increase in binding affinity for ethyl and propyl with a subsequent decrease in binding affinity for butyl chains. Thus N-propanoyl-3-(3-methoxyphenyl)propanamine (6f) has a binding affinity of 5.6 nM, a remarkably high affinity for so simple a compound. Substitution of halogen for 3-methoxyl in the propanamide series gave a series of compounds with lower, but still substantial, binding affinities, the 3-chloro derivative 7e showing the highest affinity, 113 nM. In the case of the 3-fluoro propanamides, a maximum in the binding affinity was not observed in the series synthesized, and these derivatives will merit further exploration. These results demonstrate the utility of simple, readily modified phenylalkylamines as a "framework" for studying the effect of changes in the nature and position of substituents on the melatonin receptor binding affinity.