Melatonin facts: Lack of evidence that melatonin is a radical scavenger in living systems

Melatonin is a small natural compound, so called a neuro-hormone that is synthesized mainly in pineal gland in animals. Its main role is to master the clock of the body, under the surveillance of light. In other words, it transfers the information concerning night and day to the peripheral organs which, without it, could not "know" which part of the circadian rhythm the body is in. Besides its main circadian and circannual rhythms mastering, melatonin is reported to be a radical scavenger and/or an antioxidant. Because radical scavengers are chemical species able to neutralize highly reactive and toxic species such as reactive oxygen species, one would like to transfer this property to living system, despite impossibilities already largely reported in the literature. In the present commentary, we refresh the memory of the readers with this notion of radical scavenger, and review the possible evidence that melatonin could be an in vivo radical scavenger, while we only marginally discuss here the fact that melatonin is a molecular antioxidant, a feature that merits a review on its own. We conclude four things: (i) the evidence that melatonin is a scavenger in acellular systems is overwhelming and could not be doubted; (ii) the transposition of this property in living (animal) systems is (a) theoretically impossible and (b) not proven in any system reported in the literature where most of the time, the delay of the action of melatonin is over several hours, thus signing a probable induction of cellular enzymatic antioxidant defenses; (iii) this last fact needs a confirmation through the discovery of a nuclear factor-a key relay in induction processes-that binds melatonin and is activated by it and (iv) we also gather the very important description of the radical scavenging capacity of melatonin in acellular systems that is now proven and shared by many other double bond-bearing molecules. We finally discussed briefly on the reason-scientific or else-that led this description, and the consequences of this claim, in research, in physiology, in pathology, but most disturbingly in therapeutics where a vast amount of money, hope, and patient bien-être are at stake.

Radiosynthesis and In Vivo Evaluation of Four Positron Emission Tomography Tracer Candidates for Imaging of Melatonin Receptors

Melatonin is a neurohormone that modulates several physiological functions in mammals through the activation of melatonin receptor type 1 and 2 (MT₁ and MT₂). The melatonergic system is an emerging therapeutic target for new pharmacological interventions in the treatment of sleep and mood disorders; thus, imaging tools to further investigate its role in the brain are highly sought-after. We aimed to develop selective radiotracers for in vivo imaging of both MT₁ and MT₂ by positron emission tomography (PET). We identified four previously reported MT ligands with picomolar affinities to the target based on different scaffolds which were also amenable for radiolabeling with either carbon-11 or fluorine-18. [¹¹C]UCM765, [¹¹C]UCM1014, [¹⁸F]3-fluoroagomelatine ([¹⁸F]3FAGM), and [¹⁸F]fluoroacetamidoagomelatine ([¹⁸F]FAAGM) have been synthesized in high radiochemical purity and evaluated in wild-type rats. All four tracers showed moderate to high brain permeability in rats with maximum standardized uptake values (SUV_max of 2.53, 1.75, 3.25, and 4.47, respectively) achieved 1-2 min after tracer administration, followed by a rapid washout from the brain. Several melatonin ligands failed to block the binding of any of the PET tracer candidates, while in some cases, homologous blocking surprisingly resulted in increased brain retention. Two ¹⁸F-labeled agomelatine derivatives were brought forward to PET scans in non-human primates and autoradiography on human brain tissues. No specific binding has been detected in blocking studies. To further investigate pharmacokinetic properties of the putative tracers, microsomal stability, plasma protein binding, log D, and membrane bidirectional permeability assays have been conducted. Based on the results, we conclude that the fast first pass metabolism by the enzymes in liver microsomes is the likely reason of the failure of our PET tracer candidates. Nevertheless, we showed that PET imaging can serve as a valuable tool to investigate the brain permeability of new therapeutic compounds targeting the melatonergic system.

Synthesis and SAR Studies of Isoquinoline and Tetrahydroisoquinoline Derivatives as Melatonin Receptor Ligands

In our constant search for new successors of agomelatine, we report herein a new series of compounds resulting from bioisosteric modulation of the naphthalene ring. The isoquinoline and tetrahydroisoquinoline derivatives were synthesized and pharmacologically evaluated. This isosteric replacement of the naphthalene group of agomelatine has led to potent agonist and partial agonist compounds with nanomolar melatonergic binding affinities. Overall, the presence of a nitrogen atom was accompanied with a decrease in the binding affinity toward both MT₁ and MT₂ and the loss of 5HT_2C response, especially for tetrahydroisoquinoline in comparison with the parent compound. Interestingly, due to the presence of this nitrogen atom, a notable improvement in the pharmacokinetic properties was observed for all compounds.

Quinazoline and phthalazine derivatives as novel melatonin receptor ligands analogues of agomelatine

For further development of successors of Agomelatine through modulation of its pharmacokinetic properties, we report herein the design, synthesis and pharmacological results of a new family of melatonin receptor ligands. Issued from the introduction of quinazoline and phthalazine scaffolds carrying an ethyl amide lateral chain and a methoxy group as bioisosteric ligands analogues of previously developed Agomelatine. The biological activity of the prepared analogues was compared with that of Agomelatine. Quinazoline and phthalazine rings proved to be a versatile scaffold for easy feasible MT₁ and MT₂ ligands. Potent agonists with sub-micromolar binding affinity were obtained. However, the presence of two nitrogen atoms resulted in compounds with lower affinity for both MT₁ and MT₂, in comparison with the parent compound, balanced by the exhibition of good pharmacokinetic properties.

(Hetero)aryl substituted thiazol-2,4-yl scaffold as human carbonic anhydrase I, II, VII and XIV activators

Using histamine as lead molecule, a library of (hetero)aryl substituted thiazol-2,4-yl derivatives incorporating pyridine as proton shuttling moiety were obtained and investigated as activators of human carbonic anhydrase (CA, EC 4.2.1.1) isoforms I, II, VII and XIV. Some derivatives displayed good activating and selectivity profiles. This study provides an interesting opportunity to study the thiazole scaffold for the design of CA activators (CAAs), possibly acting on the central nervous system and targeting pathologies involving memory and learning impairments.

Carbazole scaffolds in cancer therapy: a review from 2012 to 2018

For over half a century, the carbazole skeleton has been the key structural motif of many biologically active compounds including natural and synthetic products. Carbazoles have taken an important part in all the existing anti-cancer drugs because of their discovery from a large variety of organisms, including bacteria, fungi, plants, and animals. In this article, we specifically explored the literature from 2012 to 2018 on the anti-tumour activities reported to carbazole derivatives and we have critically collected the most significant data. The most described carbazole anti-tumour agents were classified according to their structure, starting from the tricyclic–carbazole motif to fused tetra-, penta-, hexa- and heptacyclic carbazoles. To date, three derivatives are available on the market and approved in cancer therapy.

Publisher Correction: Structural basis of ligand recognition at the human MT1 melatonin receptor

Change history: In this Letter, the rotation signs around 90°, 135° and 15° were missing and in the HTML, Extended Data Tables 2 and 3 were the wrong tables; these errors have been corrected online.

New phenylaniline derivatives as modulators of amyloid protein precursor metabolism

The chloroquinoline scaffold is characteristic of anti-malarial drugs such as chloroquine (CQ) or amodiaquine (AQ). These drugs are also described for their potential effectiveness against prion disease, HCV, EBV, Ebola virus, cancer, Parkinson or Alzheimer diseases. Amyloid precursor protein (APP) metabolism is deregulated in Alzheimer's disease. Indeed, CQ modifies amyloid precursor protein (APP) metabolism by precluding the release of amyloid-beta peptides (Aβ), which accumulate in the brain of Alzheimer patients to form the so-called amyloid plaques. We showed that AQ and analogs have similar effects although having a higher cytotoxicity. Herein, two new series of compounds were synthesized by replacing 7-chloroquinolin-4-amine moiety of AQ by 2-aminomethylaniline and 2-aminomethylphenyle moieties. Their structure activity relationship was based on their ability to modulate APP metabolism, Aβ release, and their cytotoxicity similarly to CQ. Two compounds 15a, 16a showed interesting and potent effect on the redirection of APP metabolism toward a decrease of Aβ peptide release (in the same range compared to AQ), and a 3-10-fold increased stability of APP carboxy terminal fragments (CTFα and AICD) without obvious cellular toxicity at 100 µM.

Antagonists of the adenosine A2A receptor based on a 2-arylbenzoxazole scaffold: Investigation of the C5- and C7-positions to enhance affinity

We have recently reported a series of 2-furoyl-benzoxazoles as potential A_2A adenosine receptor (A_2AR) antagonists. Two hits were identified with interesting pharmacokinetic properties but were find to bind the hA_2AR receptor in the micromolar-range. Herein, in order to enhance affinity toward the hA_2AR, we explored the C5- and C7-position of hits 1 and 2 based on docking studies. These modifications led to compounds with nanomolar-range affinity (e.g. 6a, Ki = 40 nM) and high antagonist activity (e.g. 6a, IC₅₀ = 70.6 nM). Selected compounds also exhibited interesting in vitro DMPK (Drug Metabolism and Pharmacokinetics) properties including high solubility and low cytotoxicity. Therefore, the benzoxazole ring appears as a highly effective scaffold for the design of new A_2A antagonists.

Design, synthesis and evaluation of 2-aryl benzoxazoles as promising hit for the A2A receptor

The development of adenosine A2A receptor antagonists has received much interest in recent years for the treatment of neurodegenerative diseases. Based on docking studies, a new series of 2-arylbenzoxazoles has been identified as potential A2AR antagonists. Structure-affinity relationship was investigated in position 2, 5 and 6 of the benzoxazole heterocycle leading to compounds with a micromolar affinity towards the A2A receptor. Compound F1, with an affinity of 1 μm, presented good absorption, distribution, metabolism and excretion properties with an excellent aqueous solubility (184 μm) without being cytotoxic at 100 μm. This compound, along with low-molecular weight compound D1 (Ki = 10 μm), can be easily modulated and thus considered as relevant starting points for further hit-to-lead optimisation.

Synthesis, chiral resolution, absolute configuration assignment and pharmacological evaluation of a series of melatoninergic ligands

(−)-R-Enantiomers of agomelatine analogues were more potent at serotonin 5-HT_2C receptors than (+)-S-enantiomers, and showed antidepressant-like properties in a tail suspension test.

New radioligands for describing the molecular pharmacology of MT1 and MT2 melatonin receptors

Melatonin receptors have been studied for several decades. The low expression of the receptors in tissues led the scientific community to find a substitute for the natural hormone melatonin, the agonist 2-[125I]-iodomelatonin. Using the agonist, several hundreds of studies were conducted, including the discovery of agonists and antagonists for the receptors and minute details about their molecular behavior. Recently, we attempted to expand the panel of radioligands available for studying the melatonin receptors by using the newly discovered compounds SD6, DIV880, and S70254. These compounds were characterized for their affinities to the hMT1 and hMT2 recombinant receptors and their functionality in the classical GTPS system. SD6 is a full agonist, equilibrated between the receptor isoforms, whereas S70254 and DIV880 are only partial MT2 agonists, with Ki in the low nanomolar range while they have no affinity to MT1 receptors. These new tools will hopefully allow for additions to the current body of information on the native localization of the receptor isoforms in tissues.

Design and synthesis of naphthalenic derivatives as new ligands at the melatonin binding site MT3

Naphthalenic analogs of MCA-NAT (5-methoxycarbonylamino-N-acetyltryptamine) have been synthesized and evaluated as melatonin receptor ligands. Introduction of a methoxycarbonylamino substituent at the C-7 position of the naphthalenic nucleus yields MT3 selective ligands. This selectivity can be modulated with suitable variations of the C-7 position and the acyl group on the C-1 side chain. We identified new series of compounds with affinity for the MT3 binding site in the nanomolar range, and singled out a selective ligand, (N-[2-(7-methylsulfamoyl-naphth-1-yl)ethyl]acetamide (17), with a Ki of 4.9 nM and selectivity of 1024 and 2040 versus MT1 and MT2 receptors respectively.

Old and new inhibitors of quinone reductase 2

Quinone reductase 2 is a cytosolic enzyme which catalyses the reduction of quinones, such as menadione and coenzymes Q. Despite a relatively close sequence-based resemblance to NAD(P)H:quinone oxidoreductase 1 (QR1), it has many different features. QR2 is the third melatonin binding site (MT3). It is inhibited in the micromolar range by melatonin, and does not accept conventional phosphorylated nicotinamides as hydride donors. QR2 has a powerful capacity to activate quinones leading to unexpected toxicity situations. In the present paper, we report the characterization of three QR2 modulators: melatonin, resveratrol and S29434. The latter compound inhibits QR2 activity with an IC(50) in the low nanomolar range. The potency of the modulators ranged as follows, from the least to the most potent: melatonin<resveratrol<S29434. These molecular tools might permit to explore and better understand the relationship existing between QR2 catalytic activity and the various pathological situations in which QR2 has a key role.

Rexinoid bexarotene modulates triglyceride but not cholesterol metabolism via gene-specific permissivity of the RXR/LXR heterodimer in the liver

OBJECTIVE

Bexarotene (Targretin) is a clinically used antitumoral agent which exerts its action through binding to and activation of the retinoid-X-receptor (RXR). The most frequent side-effect of bexarotene administration is an increase in plasma triglycerides, an independent risk factor of cardiovascular disease. The molecular mechanism behind this hypertriglyceridemia remains poorly understood.

METHODS AND RESULTS

Using wild-type and LXR alpha/beta-deficient mice, we show here that bexarotene induces hypertriglyceridemia and activates hepatic LXR-target genes of lipogenesis in an LXR-dependent manner, hence exerting a permissive effect on RXR/LXR heterodimers. Interestingly, RNA analysis and Chromatin Immunoprecipitation assays performed in the liver reveal that the in vivo permissive effect of bexarotene on the RXR/LXR heterodimer is restricted to lipogenic genes without modulation of genes controlling cholesterol homeostasis.

CONCLUSIONS

These findings demonstrate that the hypertriglyceridemic action of bexarotene occurs via the RXR/LXR heterodimer and show that RXR heterodimers can act with a selective permissivity on target genes of specific metabolic pathways in the liver.

Lipid-lowering properties of 6-benzoyl-2(3H)-benzothiazolone and structurally related compounds

Fifteen compounds derived from the 2(3H)-benzothiazolone template with an acyl side-chain in position-6 were evaluated for their lipid-lowering action in mice. Among these compounds, 6-benzoyl-2(3H)-benzothiazolone was found to be the most potent one both in mice models receiving a hypercholesterolemic diet (for 15 days) or a standard diet (for 21 days). 6-Benzoyl-2(3H)-benzothiazolone compares favorably with fenofibrate, the standard drug, both in terms of HDL-C/Chol (High Density Lipoprotein-Cholesterol/Total Cholesterol) ratio and absence of liver hepatomegaly.

Quantification of the water/lipid affinity of melatonin and a pinoline derivative in lipid models

This study assessed the location of melatonin (N-acetyl-5-methoxytryptamine) and of a pinoline derivative (GWC22) [6-ethyl-1-(3-methoxyphenyl)-2-propyl-1,2,3,4-tetrahydro-beta-carboline], when present in lipid assemblies such as linoleate micelles, phosphatidylcholine liposomes or low density lipoproteins (LDL). The efficiency of radical scavenging by these compounds is highly dependent on their partitioning between the lipidic and aqueous phases. We determined the proportion of melatonin or GWC22 in the aqueous and lipid phases of each system (concentrations of the antioxidants ranging between 3 x 10(-5) and 10(-4) m) by assaying melatonin or GWC22 by HPLC/UV detection, or by fluorescence for melatonin in micelles. Our results show that melatonin and GWC22 were preferentially located in the aqueous phase of micelles (68.4% and 59.0%, respectively), whereas only 30.5% of melatonin and 39.0% of GWC22 were found in the lipid phase. By contrast, in phosphatidylcholine liposomes, both compounds were essentially present in the lipid phase (73.5% for melatonin and 79.1% for GWC22, versus 25.9% and 19.5% in the aqueous phase, respectively). In the case of LDL, 99.9% of the melatonin added was found in the methanol/water extracting phase containing phospholipids, unesterified cholesterol and apolipoprotein B100. The partitioning of melatonin and GWC22 in linoleate micelles gave new insights on the marked protective effect of GWC22 towards radiation-induced lipid peroxidation and allowed us to determine more accurately the lower limit values of the reaction rate constants of the two molecules studied with lipid peroxyl radicals, i.e. k(LOO.+melatonin)) >or= 9.0 x 10(4)m(-1)s(-1) and k(LOO.+GWC22) >or= 3.5 x 10(5)m(-1)s(-1).

Novel 1,3-dicarbonyl compounds having 2(3H)-benzazolonic heterocycles as PPARγ agonists

A series of 1,3-dicarbonyl compounds having 2(3H)-benzazolonic heterocycles has been synthesized and tested for PPARgamma agonist activity. SAR were developed and revealed that 6-acyl-2(3H)-benzothiazolone derivatives with 1,3-dicarbonyl group were the most potent. IP administration of compound 22 exhibited comparable levels of glucose and triglyceride correction to PO administration of rosiglitazone in the ob/ob mouse studies.

The RXR agonist bexarotene improves cholesterol homeostasis and inhibits atherosclerosis progression in a mouse model of mixed dyslipidemia

OBJECTIVE

The activity of the antitumoral agent bexarotene (Targretin, Bexarotene) depends on its binding to the nuclear retinoid-X receptor (RXR) and subsequent transcriptional regulation of target genes. Through RXR activation, bexarotene may modulate numerous metabolic pathways involved in atherosclerosis. Here, we investigated the effect of bexarotene on atherosclerosis progression in a dyslipidemic murine model, the human apolipoprotein E2 knockin mouse, that develops essentially macrophage-laden lesions.

METHODS AND RESULTS

Atherosclerotic lesions together with different metabolic pathways involved in atherosclerosis were investigated in mice treated or not with bexarotene. Bexarotene protects from atherosclerosis development in mice, at least in part by improving the circulating cholesterol distribution profile likely via a marked decrease of dietary cholesterol absorption caused by modulation of intestinal expression of genes recently identified as major players in this process, Niemann-Pick-C1-Like1 (NPC1L1) and CD13. This atheroprotection appears despite a strong hypertriglyceridemia. Moreover, bexarotene treatment only modestly modulates inflammatory gene expression in the vascular wall, but markedly enhanced the capacity of macrophages to efflux cellular lipids.

CONCLUSIONS

These data provide evidence of a favorable pharmacological effect of bexarotene on atherosclerosis despite the induction of hypertriglyceridemia, likely via a beneficial action on intestinal absorption and macrophage efflux.

Characterization of the melatoninergic MT3 binding site on the NRH:quinone oxidoreductase 2 enzyme

Melatonin acts through a series of molecular targets: the G-protein coupled receptors, MT1 and MT2, and a third binding site, MT3, recently identified as the enzyme NRH:quinone oxydoreductase 2 (QR2). The relationship between the multiple physiological functions of melatonin and this enzyme remains unclear. Because of the relationship of QR2 with the redox status of cells, these studies could bring the first tools for a molecular rationale of the antioxidant effects of melatonin. In the present paper, we used a QR2-stably expressing cell line and hamster kidneys to compare the 2-[125I]-iodomelatonin and 2-[125I]-iodo-5-methoxycarbonylamino-N-acetyltryptamine binding data, and to characterize the MT3 binding site. We designed and tested compounds from two distinct chemicals series in a displacement assay of the two MT3 ligands, 2-[125I]-iodomelatonin and 2-[125I]-iodo-5-methoxycarbonylamino-N-acetyltryptamine from their cloned target. We also tested their ability to inhibit QR2 catalytic activity. These compounds were separated into two classes: those that bind within the catalytic site (and being inhibitors) and those that bind outside it (and therefore not being inhibitors). Compounds range from potent ligands (K(i) = 1 nM) to potent inhibitors (14 nM), and include one compound [NMDPEF: N-[2-(2-methoxy-6H-dipyrido[2,3-a:3,2-e]pyrrolizin-11-yl)ethyl]-2-furamide] active on both parameters in the low nanomolar range. To dissect the physio-pathological pathways in which QR2, MT3 and melatonin meet, one needs more compounds binding to MT3 and/or inhibitors of QR2 enzymatic activity. The compounds described in the present paper are new tools for such a task.

Increased Susceptibility of Low-Density Lipoprotein to Ex Vivo Oxidation in Mice Transgenic for Human Apolipoprotein B Treated with 1 Melatonin-Related Compound Is Not Associated with Atherosclerosis Progression

Considerable evidence supports the hypothesis that LDL oxidation has an important role in atherosclerosis. It has been demonstrated that the feeding of hypercholesterolemic mice on an atherogenic diet supplemented with melatonin highly increases the surface of atherosclerotic lesions in aorta and the sensitivity of atherogenic lipoprotein to ex vivo oxidation even though high melatonin doses inhibit lipoprotein oxidation in vitro. A melatonin-related compound (DTBHB: N-[2-(5-methoxy-1H-indol-3-yl)ethyl]-3,5-di-tert-butyl-4-hydroxybenzamide) has been reported to strongly inhibit lipid peroxidation in vitro. In the present study, DTBHB treatment considerably increased the sensitivity of atherogenic lipoproteins to ex vivo oxidation but did not modify atherosclerotic lesion development in mice. Moreover, DTBHB treatment did not induce detectable lipidic alteration. These data confirm that the capacity of molecules to inhibit atherogenic lipoprotein oxidation in vitro offers no prediction of their capacity to inhibit in vivo atherosclerosis development.

Antioxidant activity of melatonin and a pinoline derivative on linoleate model system

This study aimed at investigating the in vitro protective effects of GWC22, a novel pinoline derivative [6-ethyl-1-(3-methoxyphenyl)-2-propyl-1,2,3,4-tetrahydro-beta-carboline] chlorhydrate, against radiation-induced oxidation of linoleate initiated by hydroxyl radicals ((*)OH). Using linoleate micelles (10(-2) m) as lipid model, two indexes of peroxidation have been measured, i.e. conjugated dienes and hydroperoxides. Similar determinations were performed with melatonin in order to compare the protective effects of the two compounds. It was observed that, the higher the concentration of GWC22 (or melatonin) (3 x 10(-5) to 10(-4) m), the stronger the antioxidant ability. In these in vitro assays, GWC22 showed a better antioxidant effect than melatonin for a given antioxidant concentration. A reaction scheme has been proposed to explain the inhibitory effect of an antioxidant via the propagating steps of the lipid peroxidation. Indeed, we have suggested that melatonin and GWC22 may compete with the fatty acid to scavenge lipid peroxyl radicals (LOO(*)). We have estimated a lower limit for the LOO(*) rate constant for GWC22 (>/=1.4 x 10(5)/m/s) and for melatonin (>/=2.8 x 10(4)/m/s) assuming that the k-value of the propagating step in linoleate (LOO(*) + linoleate) was 1.4 x 10(3)/m/s. The difference of reactivity between melatonin and GWC22 in this model system is assumed to be related to their relative lipophilicity.

Purification of the recombinant human serotonin N-acetyltransferase (EC 2.3.1.87): further characterization of and comparison with AANAT from other species

Melatonin is synthesized by a series of enzymes, the penultimate one, serotonin N-acetyltransferase, catalyzing the limiting reaction. In the present study, we compared the recombinant serotonin N-acetyltransferases from rat, ovine, and human. The human protein is particularly difficult to purify because it interacts strongly with a putative chaperone protein from bacteria whereas the rat and sheep enzymes, which interact less strongly with this protein, have been purified close to homogeneity. We identified the contaminating protein as GroEL, the bacterial equivalent of Hsp60. We present numerous catalytic activities (substrate and cosubstrate specificities as well as inhibitor specificities) measured on the three species enzymes from which we deduced that the presence of the chaperone might partly explain the differences between the various species enzyme characteristics, beside the inter-species ones resulting from sequence differences. Despite several trials reported in the literature, a purification to homogeneity of the human (recombinant) enzyme has never been described. We present a new purification method, by using an original denaturation/renaturation process in which the enzyme is immobilized on an affinity chromatography column. The enzyme is then eluted in an active and pure form (i.e., absence of chaperone). The up-scaled system permitted us to perform the necessary experiments for the measurement of more accurate affinities of human serotonin N-acetyltransferase towards its main natural substrates, showing that only the activity of the enzyme towards phenylethylamine was modified.