key: cord-278440-vti6xp9v
authors: Paraiso, Ines L; Revel, Johana S; Stevens, Jan F
title: Potential use of polyphenols in the battle against COVID-19
date: 2020-09-09
journal: Curr Opin Food Sci
DOI: 10.1016/j.cofs.2020.08.004
sha: 
doc_id: 278440
cord_uid: vti6xp9v

The coronavirus disease 2019 (COVID-19) is a public health emergency of international concern. The rising number of cases of this highly transmissible infection have stressed the urgent need to find a potent drug. Although repurposing of known drugs currently provides an accelerated route to approval, there is no satisfactory treatment. Polyphenols, a major class of bioactive compounds in nature, are known for their antiviral activity and pleiotropic effects. The aim of this review is to assess the effects of polyphenols on COVID-19 drug targets as well as to provide a perspective on the possibility to use polyphenols in the development of natural approaches against this viral disease.

The worldwide outbreak of highly transmissible fatal pneumonia referred to as J o u r n a l P r e -p r o o f 4 Therapies against coronavirus can be categorized into two groups: drugs targeting the virus and drugs acting on human cells or the immune system. The key SARS-CoV-2 targets comprise three non-structural proteins (3CL pro , PL pro and RdRp) and a structural protein (S protein), which are responsible for replication, transcription and host cell recognition [7] .

However, therapies such as vaccines and monoclonal antibodies may lose their efficiency if the virus mutates and changes its antigenicity. Therefore, drugs targeting host-cell viral receptors (ACE2) and improving the immune response have strong potential. Polyphenols have a broad antiviral activity against a diverse group of viruses such as influenza A virus (H1N1), hepatitis B and C viruses (HBV/HCV), herpes simplex virus 1 (HSV-1), human immunodeficiency virus (HIV) and Epstein-Barr virus (EBV) [8] . The present mini-review aims to report in silico and in vitro evidence of the potential of polyphenols as anti-SARS-CoV-2 agents. Putative mechanisms of action by which these natural compounds exert their potential activity against SARS-CoV-2 are presented in Figure 1 . We also summarize research approaches that may accelerate the discovery of anti-SARS-CoV-2 polyphenols. We have reviewed literature spanning from 2000 to 2020 and 53 % of the cited references were published in the last two years.

The S protein is a large membrane glycoprotein that belongs to a group of class I viral fusion glycoproteins that also includes HIV glycoprotein 160 (Env), influenza haemagglutinin (HA) and Ebola virus glycoprotein [9] . The peripheral amino (S1) subunit can independently bind cellular receptors while the carboxy (S2) terminus is embedded into the viral envelope and is required to mediate fusion of viral and cellular membranes [10] . In coronaviruses, the S protein is the sole viral membrane protein responsible for cell entry. 

ACE2 is a type I transmembrane metallocarboxypeptidase found in many tissues such as the lungs, heart, blood vessels, kidneys, liver and epithelial cells [22] . ACE2 is a pivotal enzyme in the physiological renin-angiotensin system, as it hydrolyzes vasoconstricting angiotensin II to generate vasodilating angiotensin (1-7) [23]. Being SARS-CoV-2's point of entry into the host cells, ACE2 has gained attention as a potential drug target. Screening for ligands of ACE2 with a binding affinity strong enough to inhibit virus entry has unveiled polyphenols as promising candidates. A molecular docking study using a computational model of the SARS-CoV-2 spike protein interacting with human ACE2 receptor found that eriodictyol, a flavanone found in yerba santa (Eriodictyon californicum) had one of the greatest binding affinity for the human ACE2 showed that flavonoids curcumin and catechin establish hydrogen bonds, carbon-hydrogen bonds and π-σ interactions with ACE2, resulting in binding affinities of -7.8 Kcal/mol and -8.9

Kcal/mol respectively [25] . Although in silico experiments predict promising results, more in vitro and in vivo studies are needed to evaluate whether polyphenols binding to ACE2 impacts viral entry.

Growing evidence suggests that controlling ACE2 expression might help modulate COVID-19 symptoms. In fact, SARS-CoV infection was found to downregulate ACE2 receptor Therefore, polyphenols might (i) reduce SARS-CoV-2 viral infection by binding to the ACE2 receptor, preventing the viral entry, and (ii) modulate the severity of lung injury associated with COVID-19 by regulating ACE2 expression. However, it is important to note that, given ACE2 pivotal role in physiopathological processes, targeting the enzyme still needs careful evaluation to ensure the benefit-risk balance is favorable. [38, 39] . SARS-CoV-2 polyproteins are processed by a main protease, 3CL pro (also known as M pro ), and by papain-like proteases, PL pro [39] . These proteases are involved in the replication and transcription of the SARS-CoV-2, especially 3CL pro , which plays a vital role in polyprotein processing and virus maturation [4, 7] . Hence, 3CL pro is one of SARS-CoV-2 best characterized drug targets, and studies have shown that development of antiviral agents targeting 3CL pro could provide an effective first line of defense against coronaviruses infections [39] [40] [41] [42] . Natural compounds inhibitors of SARS-CoV proteases include diarylheptanoids [43, 44] , terpenoids [7,45], cinnamic amides [46] , flavonoids [47] [48] [49] [50] and coumarins [47] .

Inhibition of 3CL pro was shown in silico and in vitro with epigallocatechin gallate (IC50 = 73 µM), gallocatechin gallate (IC50 = 47 µM) and quercetin (IC50 = 73 µM) [50, 51] . Structureactivity relationship analysis of seven polyphenols revealed that flavonoids and isoflavonoids lacking an OH group at 5'-position of the B ring decreased 3CL pro inhibitory activity [50] .

Screening by molecular docking of 33 molecules including natural products, antivirals, antifungals and antiprotozoal agents revealed that rutin (a citrus flavonoid) could bind to the active site of the SARS-CoV-2 3CL pro (PDB: 6Y84) with the highest affinity among the molecules screened [44] . Other citrus flavonoids such as tangeretin and naringenin and polyphenols from Curcuma spp. were also reported to bind strongly to SARS-CoV-2 3CL pro substrate binding domain, while interacting with the S protein and ACE2 in silico, predicting stronger antiviral potential of these polyphenols compared to lopinavir and nafamostat [8] .

Several polyphenols were also found to have a synergistic effect on 3CL pro and PL pro . In cell-free and cell-based assays, chalcones isolated from Angelica keiskei exhibited competitive inhibition of the SARS-CoV serine protease 3CL pro , whereas noncompetitive inhibition was observed with the SARS-CoV cysteine protease PL pro [47] . Dietary flavonoids such as kaempferol and isoliquiritigenin, as well as polyphenols from Broussonetia papyrifera also synergistically inhibited 3CL pro and PL pro in vitro [49] .

J o u r n a l P r e -p r o o f been proposed as an adjunct to COVID-19 antiviral therapy, since it combines anti-inflammatory properties and antiviral activity against SARS-CoV-2 [55] . Similarly, polyphenols, whose immunomodulatory properties are well documented [59] [60] [61] , could have a beneficial effect against SARS-CoV-2-induced cytokine storm. A non-extensive list of polyphenols that reduced pro-inflammatory cytokines in vitro and in vivo includes curcumin, resveratrol, epigallocatechin gallate, emodin, naringenin, apigenin and kaempferol [61, 62] . In our own research, oral treatment of high-fat fed mice with the hop flavonoid xanthohumol lowered plasma IL-6 levels by about 80% compared to control mice [63] . [65] . It was also reported that a nebulized formula of quercetin and N-acetylcysteine greatly alleviated SARS-CoV-2 respiratory symptoms in a patient treated with hydroxychloroquine and antibiotics [66] . This confirms the importance of further clinical studies to evaluate the potential of polyphenol-based nutraceuticals as adjuvant or main therapy for COVID-19.

High-throughput screening approaches can accelerate the in vitro discovery of lead candidates, the limitation being the availability of polyphenol libraries. Screening of polyphenolrich plant extracts is an alternative, widely used approach, but it has the disadvantage that extracts contain a multitude of natural products with inherent problems of not being able to readily identify active principles and the potential for pharmacological antagonism. These problems can be overcome by combining classical bioassay-guided fractionation with machine learning approaches to reveal the identity of bioactive natural products in extracts without the J o u r n a l P r e -p r o o f need for purification to homogeneity. In our opinion, the latter combination approach holds promise to accelerate discovery because many antiviral in vitro assays can be performed without handling live viruses and because identification of polyphenols (and other natural products) has become easier over the past several years thanks to advances in plant metabolomics and the ever growing natural product databases such as Phenol-Explorer [67] , KnapSack [68] , and the Global Natural Product Social Networking (GNPS) database [69] .

COVID-19 is a new disease with significant morbidity and mortality for which there is no satisfactory treatment available as of August 2020. The foregoing review of the literature demonstrates that polyphenols have not yet been widely considered and systematically investigated for potential antiviral effects against SARS-CoV-2. This area of research is at the proverbial infancy stage and certainly has the potential to deliver valuable antiviral therapeutics or anti-inflammatory agents in reducing SARS-CoV-2 morbidity and mortality. Many naturally occurring polyphenols are inexpensive to produce and have low risk for development of toxicity, making these compounds good candidates for preventive treatment to decrease viral infectivity and to dampen the risk of a virus-induced inflammatory storm. At the molecular level, polyphenols hold promise as inhibitors of viral proteases involved in viral replication due to their general affinity to proteins via hydrogen bonding and their low risk of toxic effects. The same may hold true for binding of polyphenols to S protein, although pre-clinical and clinical studies are required to strengthen existing evidence. Another point that should be taken into account is the proper formulation for these polyphenol-based nutraceuticals. To counteract low bioavailability concerns and increase concentrations of active polyphenols in the respiratory tract, the primary site of infection, using aerosol delivery systems, such as nebulizers and inhalers should be considered [66, 70] .

The authors declare no conflict of interest. •This study demonstrates that SARS-CoV-2 uses ACE2 as receptor for host-cell entry and the S protein needs the serine protease TMPRSS2 for priming. 

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This study was supported by National Institutes of Health grant # R01AT010271, the Linus Pauling Institute, and the Oregon State University College of Pharmacy.J o u r n a l P r e -p r o o f