Mapping the melatonin receptor. 2. synthesis and biological activity of indole derived melatonin analogues with restricted conformations of the C-3 amidoethane side chain

Mapping the Melatonin Receptor. 8. Selective MT2 Agonists derived from 5,6-dihydroindolo[2,1-a]isoquinolines and related systems

A series of substituted indolo[2,1- a ]isoquinolines and indolo[1,2- a ]benzoxazines have been prepared, as melatonin analogues, to investigate the nature of the binding site of the melatonin receptor. Agonist and antagonist potency of all the analogues was measured using the [35S]GTPγS binding assay protocol. The binding affinity of the analogues were measured by competition binding studies against the human MT1 (hMT1) and MT2 (hMT2) receptors stably transfected in Chinese Hamster Ovarian (CHO) cells, using 2-[ 125 I]-iodomelatonin, as a ligand.  N -Acetyl 2-(10-methoxy-5,6-dihydroindolo[2,1- a ]isoquinolin-12-yl)propyl-1-amine  (12a)  binds strongly to both the hMT1 and hMT2 receptors, and shows a preference for the hMT2, as does Its propanamido counterpart  12b . The introduction of two methyl groups into their side chain, analogues  15a  and 1 5b,  leads to antagonism, in the case of the former, and drastically diminishes its hMT1 binding; an analogous profile is seen for  15b , which, however, is a partial agonist.. Introduction of chlorine or methoxyl groups into ring 4 gives compounds, that are weakly binding, with a preference for MT2. Substitution of oxygen for carbon at position 5 gives the indolo[1,2- c ]benzoxazines  33 ,  36a  and  b , that bind strongly to the human receptors,  33 ,  36b  are potent agonists at the melatonin receptors, but do not discriminate between hMT1 and hMT2.

International Union of Basic and Clinical Pharmacology. LXXV. Nomenclature, classification, and pharmacology of G protein-coupled melatonin receptors

The hormone melatonin (5-methoxy-N-acetyltryptamine) is synthesized primarily in the pineal gland and retina, and in several peripheral tissues and organs. In the circulation, the concentration of melatonin follows a circadian rhythm, with high levels at night providing timing cues to target tissues endowed with melatonin receptors. Melatonin receptors receive and translate melatonin's message to influence daily and seasonal rhythms of physiology and behavior. The melatonin message is translated through activation of two G protein-coupled receptors, MT(1) and MT(2), that are potential therapeutic targets in disorders ranging from insomnia and circadian sleep disorders to depression, cardiovascular diseases, and cancer. This review summarizes the steps taken since melatonin's discovery by Aaron Lerner in 1958 to functionally characterize, clone, and localize receptors in mammalian tissues. The pharmacological and molecular properties of the receptors are described as well as current efforts to discover and develop ligands for treatment of a number of illnesses, including sleep disorders, depression, and cancer.

7-Azamelatonin: Efficient Synthetic Routes, Excited-State Double Proton Transfer Properties and Biomedical Implications

On the basis of a seven-step synthetic route, the total synthesis of 7-azamelatonin, an analogue of melatonin, has been achieved with an overall yield of approximately 9.2%. In aqueous solution, 7-azamelatonin exhibits a unique excited-state double proton transfer (ESDPT) property, resulting in dual emission bands (405 and 560 nm). The ESDPT property makes 7-azamelatonin superb as a potential molecular probe for future bioapplication and for pharmacological research.

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.

Stereocontrolled synthesis of anticancer β-lactams via the Staudinger reaction

Stereocontrolled synthesis of novel beta-lactams using polyaromatic imines following the Staudinger reaction has been accomplished. The effects of domestic microwave irradiation on this type of reaction have been investigated. Formation of trans-beta-lactams has been explained through isomerization of the enolates formed during the reaction of acid chloride (equivalent) with imines in the presence of triethylamine. A donor-acceptor complex pathway is believed to be involved in the formation of cis-beta-lactams. The effect of a peri hydrogen has been found to be significant in controlling the stereochemistry of the resulting beta-lactams. SAR has identified beta-lactams with anticancer activity. The presence of an acetoxy group has proven obligatory for their anticancer activity.

Melatonin, melatonin receptors and melanophores: a moving story

Melatonin (5-methoxy N-acetyltryptamine) is a hormone synthesized and released from the pineal gland at night, which acts on specific high affinity G-protein coupled receptors to regulate various aspects of physiology and behaviour, including circadian and seasonal responses, and some retinal, cardiovascular and immunological functions. In amphibians, such as Xenopus laevis, another role of melatonin is in the control of skin coloration through an action on melanin-containing pigment granules (melanosomes) in melanophores. In these cells, very low concentrations of melatonin activate the Mel(1c) receptor subtype triggering movement of granules toward the cell centre thus lightening skin colour. Mel(1c) receptor activation reduces intracellular cAMP via a pertussis toxin-sensitive inhibitory G-protein (Gi), but how this and other intracellular signals regulate pigment movement is not yet fully understood. However, melanophores have proven an excellent model for the study of the molecular mechanisms which coordinate intracellular transport. Melanosome transport is reversible and involves both actin- (myosin V) and microtubule-dependent (kinesin II and dynein) motors. Melanosomes retain both kinesin and dynein during anterograde and retrograde transport, but the myosin V motor seems to be recruited to melanosomes during dispersion, where it assists kinesin II in dominating dynein thus driving net dispersion. Recent work suggests an important role for dynactin in coordinating the activity of the opposing microtubule motors. The melanophore pigment aggregation response has also played a vital role in the ongoing effort to devise specific melatonin receptor antagonists. Much of what has been learnt about the parts of the melatonin molecule required for receptor binding and activation has come from detailed structure-activity data using novel melatonin ligands. Work aiming to devise ligands specific for the distinct melatonin receptor subtypes stands poised to deliver selective agonists and antagonists which will be valuable tools in understanding the role of this enigmatic hormone in health and disease.

Synthesis of anticancer β-lactams: mechanism of action

Synthesis of the trans 1-N-chrysenyl and 1-N-phenanthrenyl 3-acetoxy-4-phenyl-2-azetidinones has been achieved. Microwave-assisted reaction has proved useful in the synthesis of these compounds. Cell growth inhibition study has indicated selective anticancer activity against two leukemia and colon carcinoma cell lines. A mechanistic correlation of their anticancer activity has been described. Striking G2 blockade that is clearly distinct in cell cycle analysis and demonstrated only in sensitive cell lines has been observed. They do not induce apoptosis in sensitive or resistant lines. They also do not inhibit topoisomerases. Ames test has shown they are nonmutagenic.

Synthesis of 5-substituted-2,3-dihydro-1,4-benzoxathiins: biological evaluation as melatonin receptors ligands

The synthesis of benzoxathiins bearing a retroamide function is described from 8-hydroxythiochroman, the key step involving the synthesis of the benzoxathiin ring through a sulfonium salt. These new melatonin analogues were evaluated on human receptors MT1 and MT2 and have a similar affinity to that of melatonin itself.

Synthesis of new arylalkoxy amido derivatives as melatoninergic ligands

Amido derivatives 10-18 of the corresponding oxyamines were synthesised as melatoninergic ligands by the reaction of hydroxyphtalimide with the halogeno derivatives or the corresponding alcohols using Mitsunobu reaction conditions. The affinity of the compounds for chicken brain melatonin receptors and recombinant human MT(1) and MT(2) receptors was evaluated using 2-[125I]-iodomelatonin as the radioligand. Overall, the introduction of an oxygen atom in the amido chain was not a favourable parameter as the compounds were less potent than the corresponding deoxy derivatives. However, nanomolar compounds were obtained with the arylethyloxy derivatives (13c (R'=nPr), chicken brain, hMT(1), hMT(2), K(i) values: 4.8, 3.86, 2.4 nM, respectively) and the 2,7-dimethoxynaphthalene derivatives (17c (R'=nPr), chicken brain, hMT(1), hMT(2), K(i) values: 0.04, 0.13, 0.1 nM, respectively). The functional activity of these compounds was evaluated by the aggregation of melanophores in Xenopus laevis tadpoles and the potency was related to the affinity of the molecules for melatonin receptors. The compounds were found to be full agonists and compound 17a was 20-fold more potent than melatonin in this bioassay.

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, pharmacological characterization and QSAR studies on 2-substituted indole melatonin receptor ligands

A number of 6-methoxy-1-(2-propionylaminoethyl)indoles, carrying properly selected substituents at the C-2 indole position, were prepared and tested as melatonin receptor ligands. Affinities and intrinsic activities for the human cloned mt1 and MT2 receptors were examined and compared with those of some 2-substituted melatonin derivatives recently described by us. A quantitative structure activity relationship (QSAR) study of the sixteen 2-substituted indole compounds, 5a-k, 1, 8-11, using partial least squares (PLS) and multiple regression analysis (MRA) revealed the existence of an optimal range of lipophilicity for the C2 indole substituent. There are also indications that planar, electron-withdrawing substituents contribute to the affinity by establishing additional interactions with the binding pocket. No mt1/MT2 subtype selectivity was observed, with the relevant exception of the 2-phenethyl derivative 5e, which exhibited the highest selectivity for the h-MT2 receptor among all the compounds tested (MT2/mt1 ratio of ca. 50). Conformational analysis and superposition of 5e to other reported selective MT2 ligands revealed structural and conformational similarities that might account for the MT2/mt1 selectivity of 5e.

Melatonin receptors and ligands

The goal of the article is to provide a clearer understanding of how melatonin and its related analogs interact with melatonin receptors with the hope of developing important tools and agents of significant clinical and scientific importance. The review provides a compilation of the currently published melatonergic ligands and their relative affinities for melatonin receptors and discusses the importance of developing reversible, high-affinity, and subtype selective melatonin receptor antagonists. In addition, the review discusses the utility of developing high-affinity charged melatonergic ligands and irreversible ligands. Finally, the review discusses some of the problems associated with the current models used to study receptor pharmacology and function. As the availability of tools increases in the melatonin receptor field, a great body of knowledge is also gained about the structure of the melatonin receptor and the role that specific melatonin receptor subtypes have in physiologic processes. Further design, synthesis, and application of melatonergic ligands will lead us to a clearer understanding of the role that melatonin and its receptors play in humans.

Substituted oxygenated heterocycles and thio-analogues: synthesis and biological evaluation as melatonin ligands

A new series of substituted oxygenated heterocycles and thio-analogues were synthesized and evaluated as melatonin receptor ligands. The replacement of the indolic moiety of melatonin by heterocyclic skeleton such as 1,4-benzodioxin, 2,3-dihydro-1,4-benzodioxin, chroman, 2,3-dihydro-1,4-benzoxathiin, thiochroman, carrying the amidic chain on the aromatic ring, leads to compounds showing a weak affinity for melatonin receptors, except for the compounds 1cb and 1hb.

Synthesis of benzocycloalkane derivatives as new conformationally restricted ligands for melatonin receptors

Benzocycloalkane derivatives 1-4 were synthesized as new conformationally restricted melatoninergic ligands. They were prepared by the reaction of the ketones 5 with diethylcyanophosphonate and the reduction of the corresponding cyano compounds or by the Wittig reaction and Curtius degradation to obtain the amines 8. The 1-Cyanobenzocyclobutane derivative was obtained by the benzyne cyclisation reaction. The amines 8 were acylated with acetyl, propionyl or butyryl groups. The affinity of the compounds for chicken brain melatonin receptors was evaluated using 2-[125I]-iodomelatonin as the radioligand. The indanyl (2b,c), tetralin (3a-c) and benzocycloheptane (4c) derivatives were potent compounds with nanomolar affinity and an important enantioselectivity of the receptor was observed with the (+) enantiomers 2b and 3b.

Tetrahydronaphthalenic derivatives as new agonist and antagonist ligands for melatonin receptors

Tetrahydronaphthalenic ligands were synthesized and evaluated as melatonin receptor ligands. Biological studies show that the aromaticity of the ring bearing the side chain is not essential for affinity and activity and that replacement of the methoxy group with the bioisostere ethyl which does not offer the possibility of H-bonding, leads to antagonist or forskoline potentiating properties.

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.

The high affinity melatonin binding site probed with conformationally restricted ligands--II. Homology modeling of the receptor

We present the first 3-D model of the melatonin receptor based on the recently published amino acid sequence of the cloned melatonin receptor. The seven trans membrane helices were positioned using the helices found in the structure of Bacterio Rhodopsine. From the results of an indirect modeling study with six melatonergic agents, an alignment of these compounds was found directing towards common interaction points. These points are suggested to be the two serines in helix three and the histidine in helix five, forming hydrogen bonds with the amide function and the methoxy-oxygen in melatonin, respectively. The ligands were docked into these binding sites and the receptor-ligand complexes were energy minimized. Considering the position of the active and inactive ligands in the receptor and their respective occupied volumes, the structure-activity relationships are rationalized by the suggested model. This model can be of use as a pharmacological test model in molecular biological studies and as a basis to develop compounds being active as synchronizing circadian agents.

The high affinity melationin binding site probed with conformationally restricted ligand--I. Pharmacophore and minireceptor models

The affinities of enantiomers of conformationally restricted melatonin analogues for the ML-1 and ML-2 putative melatonin receptor subtypes are reported. Most ligands exhibited reversed stereoselectivity when competing with 125I 2-iodomelatonin binding to chicken retinal (ML-1) and hamster brain (ML-2) membranes, further supporting the biochemical and pharmacological differences reported for these two sites. Based on the data for the ML-1 site and thorough conformational analyses of several ligands, two pharmacophore models were derived using the program APOLLO. The pharmacophoric elements included were putative receptor points from the amide NH, the amide CO, and the methoxy-O, together with the normal through the phenyl ring. The large drop in ML-1 affinity observed for 4-methoxy-2-acetamido-indan (6a) could not be explained from either of these models. Minireceptors were subsequently built around the two pharmacophores using Yak. Analysis of the resulting ligand-minireceptor interactions offered an explanation for the low affinity of 6a and allowed one of the pharmacophore models to be selected for use in future drug design.

Structure-activity relationships for substrates and inhibitors of pineal 5-hydroxytryptamine-N-acetyltransferase: preliminary studies

Tryptamine, (1-naphthyl)ethylamine and phenethylamine derivatives were tested as substrates of ovine pineal serotonin-N-acetyl transferase (5-HT-NAT), a key enzyme involved in the synthesis of melatonin. Almost all of the indole derivatives possessed affinity similar to that of tryptamine (Km = 0.05 mM), while the substituted naphthalene and phenyl derivatives were less potent. However, the Km values seem be influenced by the steric hindrance and polar properties of the substituent. Vmax values for the naphthyl and phenyl derivatives were generally 10-20-fold higher than those of the indole derivatives and no clear structure-activity relationship was observed. Melatonin and several bioisosteric derivatives were shown to be inhibitors of 5-HT-N-acetyltransferase. Preliminary data suggested that over the 5-50-microM concentration range, melatonin was a competitive inhibitor (IC50 = 10 microM) with a concentration-dependent inhibitory effect on its own synthesis in the pineal gland. However, the bioisosteric naphthalene derivatives were characterized instead as mixed inhibitors. (1-Napthyl)ethylacetamido, a putative melatoninergic antagonist, was also shown to be an inhibitor of 5-HT-N-acetyltransferase (IC50 = 8 microM) and is a promising tool for the regulation of melatonin synthesis and the understanding of its role.

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

Melatonin, a hormone secreted by the pineal gland, can act on the central circadian oscillator in the suprachiasmatic nucleus of the hypothalamus. It has been proposed that melatonin or its analogues may be useful in restoring disturbed circadian rhythms in jet-lag, shift-work and some blind subjects, and as sleep-promoting agents. In the present study, the (-)- and (+)-enantiomers of N-acetyl-4-aminomethyl-6-methoxy-9-methyl-1,2,3,4-tetrahydrocarbazole (AMMTC) were separated and tested. The affinity of the enantiomers at the specific 2-[125I]iodomelatonin binding site in chick brain membranes was compared in competition assays, and their biological activity in a specific melatonin receptor bioassay, aggregation of pigment granules in Xenopus laevis melanophores. The (-)-enantiomer of AMMTC was 130-fold and 230-fold more potent than the (+)-enantiomer in competition radioligand binding assays and melanophores, respectively. Both enantiomers are melatonin receptor agonists; (-)-AMMTC is slightly more potent than melatonin itself. As the tetrahydrocarbazole nucleus holds the C-3 amido side-chain of AMMTC in a restricted conformation, the analogues will be useful in modelling the melatonin receptor binding site.

Structural requirements at the melatonin receptor

1. High affinity, specific binding sites for the pineal hormone, melatonin (5-methoxy N-acetyltryptamine) can be detected in chick brain membranes by use of the radiolabelled agonist, 2-[125I]-iodomelatonin (2-[125I]-aMT). 2. The affinity of a number of analogues of melatonin at the 2-[125I]-aMT binding site was determined and compared with an analysis of their electronic structure and significant quantitative relationships obtained. 3. The best correlations indicated that binding affinity was correlated with delta E, the difference between the frontier orbital energies, and QNH, the electron density in the highest occupied molecular orbital of the side-chain nitrogen atom. 4. These findings suggest that ligand binding may involve hydrogen bonding between the 5-methoxy and amide moieties of melatonin and complementary amino acid residues, and charge transfer interactions between the indole ring of melatonin and an aromatic amino acid in the receptor binding site. 5. A molecular model of a putative binding site is proposed based on the predicted amino acid sequence of the cloned Xenopus laevis melanophore melatonin receptor and the quantitative structure-affinity relationships observed in the present study.