Melatonin multifaceted pharmacological actions on melatonin receptors converging to abrogate COVID-19.

Data indicate that controlling inflammatory responses to COVID-19 may be as important as antiviral therapies or could be an important adjunctive approach. Melatonin possesses anti-inflammation, antioxidation, and immune-enhancing features directly and/or indirectly through its own receptor signaling and is therefore well suited to reduce the severity of COVID-19. Studies have proposed that melatonin regulates COVID-19-associated proteins directly through regulation of molecules such as calmodulin (CALM) 1 and CALM 2, calreticulin (CalR), or myeloperoxidase (MPO) and/or indirectly through actions on GPCR (eg, MTNR1A, MTNR1B) and nuclear (eg, RORα, RORβ) melatonin receptor signaling. However, the exact mechanism(s) and doses by which melatonin reduces the severity of COVID-19 is still open for debate, warranting the need for further testing of melatonin in placebo-controlled randomized clinical trials for COVID-19.

INTRODUCTION

As of March 2021, globally more than 110 million people have been infected with SARS‐CoV‐2, the virus that causes COVID‐19 (see COVID‐19 Dashboard for real‐time cases: https://coronavirus.jhu.edu/map.html). Since the beginning of the pandemic, scientists have been repurposing molecules to treat, prevent, or reduce the severity of COVID‐19. Melatonin is one such molecule under investigation. , Melatonin, synthesized in several tissues including the retina and immune cells, is released into the circulation primarily from the pineal gland following a circadian rhythm. , , Melatonin at 0.1‐10 mg promotes the onset of sleep and synchronizes circadian rhythms and related physiological functions through the activation of G protein–coupled receptors (ie, the MT1 and MT2). , , , , , Melatonin also has anti‐inflammatory, antioxidant, analgesic, anti‐anxiety, and immune‐regulating properties. , Melatonin is safe, has low toxicity, is readily available and inexpensive, and is amenable for the treatment of a large number of people, making it an exceptional candidate for the treatment of COVID‐19.

MELATONIN'S EFFECTS ON COVID‐19

In an open‐label uncontrolled study, high‐dose melatonin (36‐72 mg/d), given as adjuvant therapy to 10 patients admitted with COVID‐19 pneumonia, induced clinical stabilization within 4‐5 days with no significant side effects except for sleepiness, which is expected as melatonin is known to promote the onset of sleep. Interestingly, a network‐based drug repurposing in silico modeling platform using existing drug‐target networks and the global map of the SARS‐CoV‐2 interactome identified COVID‐19–associated proteins targeted by melatonin directly through signaling molecules (eg, calmodulin [CALM] 1 and CALM 2; calreticulin [CalR], myeloperoxidase [MPO]) or indirectly through GPCR (eg, MTNR1A, MTNR1B) and/or nuclear (eg, RORα, RORβ) receptor signaling. , Similarly, Artigas et al (2020) using a systems biology and artificial intelligence–based approach identified melatonin and pirfenidone combination as modulators of COVID‐19 protein targets. Findings from this study suggest that melatonin through its receptor signaling pathways inhibits immunomodulatory molecules induced in the COVID‐19 cytokine storm. Further, melatonin usage was associated with a 28% reduction in infection with SARS‐CoV‐2 and a 53% reduction in infection in the black population. , Together, evidence suggests that melatonin is a unique molecule with multifaceted pharmacological actions, targeting directly or indirectly via its GPCR and/or nuclear receptors SARS‐CoV‐2 associate proteins. ,

Melatonin receptors as potential targets for the treatment of COVID‐19

The mechanism(s), that is, receptor vs. non–receptor‐mediated, by which melatonin may modulate the immune system response to SARS‐CoV‐2 is still open for debate. Both membrane (MT1 and MT2) and nuclear (RORα/RORβ) melatonin receptors identified in target immune tissues (eg, spleen, thymus) and cells (eg, monocytes, lymphocytes, macrophages) are known to modulate immune system responses , and MT1‐mediated mitochondrial dysfunction. Melatonin has been shown to suppress TLR9‐triggered pro‐inflammatory cytokine production in macrophages independent of melatonin receptors likely by inhibiting ERK1/2 and AKT activation and by downregulating iNOS via modulation of NF‐κB. Melatonin at 0.25‐1 mM has been shown to decrease mitochondrial dysfunction, oxidative stress, and cytokine response in human blood cells and respiratory bursts in mitochondria, , which would have a dramatic effect on its own receptor expression (ie, desensitization, internalization, supersensitization). Melatonin differentially regulates MT1 and MT2 melatonin receptor density and functional sensitivity depending on the cellular milieu, time of exposure, time of day, and concentration. Melatonin desensitizes and internalizes recombinant human and rodent MT2 melatonin receptors expressed in neuronal (eg, SCN2.2) or non‐neuronal (eg, CHO) cells following exposure to both physiological (3‐300 pM) and supraphysiological (10 nM) concentrations of melatonin in a time‐, concentration‐, and protein synthesis–dependent manner. Desensitization is reversible depending on exposure concentration and length of time. Melatonin is also known to increase MT1 and MT2 receptor expression and signaling through supersensization. Prolonged exposure (eg, 8‐16 hrs) of MT1 receptors (but not MT2) to physiological nocturnal concentrations (up to 300 pM) of melatonin increases signaling responses (eg, receptor density, forskolin‐stimulated cAMP and CREB phosphorylation) only upon withdrawal, promoting gene expression (eg, per1—pars tuberalis; insulin—pancreatic β‐cells). MT1 and MT2 mRNA and protein expression in the liver is pineal‐dependent and rhythmic with maximal levels at night. By contrast, the expression of endogenous mRNA, protein, and/or cytoplasmic ROR/RZR nuclear melatonin receptors is low during the night, rhythmic and pineal‐dependent in the liver. Interestingly, melatonin (40 and 200 mg/kg) significantly enhances membrane melatonin receptor expression, with no effect on the ROR/RZR nuclear receptors. Taken together, it is conceivable that the multifaceted pharmacological actions of melatonin on membrane MT1 and MT2 receptors, both by inhibiting and by supersensitizing signaling after exposures to either physiological or pharmacological doses of melatonin, modulate the receptor sensitivity and cellular milieu on target tissues. , , , We propose that at either low or high concentrations melatonin desensitizes MT2 receptors rendering them inactive, and supersensitizes MT1 receptors increasing signaling responses, thus generating opposite and complementary signaling. , , , , The proposed mechanism(s) for melatonin‐mediated signaling at MT1 and MT2 receptors may provide the model by which melatonin at physiological or pharmacological levels modulates a multiplicity of functions including chronobiological responses when given at specific periods of sensitivity. In conclusion, melatonin through plastic changes in melatonin receptors and associated proteins may optimally shape the cellular milieu to modulate the immune response and lessen the course and severity of COVID‐19.

CLINICAL TRIALS USING MELATONIN FOR THE TREATMENT OF COVID‐19

Currently, there are 9 clinical trial studies reported in Clinicaltrials.gov database (see https://www.clinicaltrials.gov/ for current studies), proposing melatonin (8) or a melatonin agonist (1) for COVID‐19 treatment in mild‐to‐moderate (4), or severe hospitalized (4), or as a prophylactic indication (1). With the exception of our two randomized, double‐blind, placebo‐controlled studies using melatonin in COVID‐19 outpatients at the University at Buffalo, all the clinical trials listed above are testing combination drug therapy (eg, estrogen, vitamin C, pentoxifylline). It is therefore imperative to design well‐controlled and powered clinical trials to test the hypothesis that melatonin is safe and efficacious to treat COVID‐19. In fact, our study, currently enrolling, assesses the safety and efficacy of melatonin (9, 30, and 90 mg/day in 3 divided doses) in mitigating the COVID‐19. Melatonin, if proven effective in this double‐blind randomized study, could be tested in children and the elderly, as well as under‐represented minorities that are disproportionately affected by COVID‐19, and provide an inexpensive therapy with minimal side effects.

CONFLICT OF INTEREST

The authors declare no conflict of interest.

AUTHOR CONTRIBUTIONS

JLR and MLD contributed equally to the writing and preparation of the manuscript.