A recent systematic review discusses naturally occurring flavonoids as a promising antiviral alternative against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Moreover, this review reports the need for further evaluation of flavonoids such as flavonols quercetin, myricetin, and their derivatives, the flavones baicalin and baicalein, the flavan-3-ol epigallocatechin gallate (EGCG), and as well as tannic acid for their in vivo use and in clinical studies.

Study: Promising Antiviral Activities of Natural Flavonoids against SARS-CoV-2 Targets: Systematic Review. Image Credit: DIVA.photo / Shutterstock.com

Introduction

In the past two decades, human coronaviruses have caused the SARS-CoV-1 epidemic in 2003, the Middle East respiratory syndrome coronavirus (MERS-CoV) epidemic in 2012, and the recently ongoing coronavirus disease 2019 (COVID-19) pandemic, caused by the SARS-CoV-2. All of these viruses belong to the beta-coronavirus genus, along with the human coronaviruses HCoV-OC43 and HCoV-HKU1, which cause the common cold.

SARS-CoV-2 alone has caused enormous global suffering that has been accompanied by lockdowns, national closures, morbidity, and mortality. To date, over 247 million confirmed cases of COVID-19 have been reported, including over 5 million deaths.

The assault of zoonotic viruses, unequal distribution of vaccines, as well as the emergence of new SARS-CoV-2 strains has supported global efforts to find potential inhibitors of key viral processes.

The development of both antiviral drugs and efficient therapeutic strategies is a long and tedious process, with most of these agents being single-target drugs designed against a unique viral enzyme. Thus, natural substances are often considered to be attractive alternative therapeutic solutions, as these compounds are also the primary sources of antimicrobial and antiviral drugs.

What are flavonoids?

Flavonoids are natural substances that have been found to act on essential coronavirus enzymes as drug targets by complementary approaches through both in silico virtual screenings and in vitro experiments.

Flavonoids are hydroxylated phenolic phyto-molecules that belong to secondary plant metabolites from fruits, vegetables, roots, and other plant products like tea and wine. The different subclasses of flavonoids include anthocyanins, chalcones, dihydrochalcones, dihydroflavonols, flavan-3-ols, flavanones, flavones, flavonols, flavanonols, and isoflavonoids.

Flavonoids defend plant cells against pathogens, insects, and other stressful environments. Functionally, flavonoids are found to have antimicrobial, antioxidant, anti-inflammatory, anti-mutagenic, anti-cancer, and antiviral properties.

These compounds are safe, have high bioavailability, lack systemic toxicity, and intercept multiple pathways with their functional groups to interact with different targets. Interestingly, about 194 countries participate in national-level policy for herbal medicines, making the use of flavonoids as potential and easy candidates against epidemics and pandemics.

In the current study, the reviewers highlight that the antiviral activity of flavonoids demonstrated in previous studies underlines the importance of exploring these natural products against SARS-CoV-2. To better understand the role of flavonoids as antivirals, the reviewers first explained the biology and pathogenesis of SARS-CoV-2, as well as current therapeutic strategies for the treatment of COVID-19 infection.

Flavonoids have emerged as a safe alternative therapeutic strategy against different targets for blocking the coronavirus life cycle at different stages of viral infection. In this context, the reviewers presented several possible targets in the virus and infected host that can be developed to inhibit the viral pathogenesis of SARS-CoV-2.

Antiviral properties of flavonoids against coronaviruses

The flavonoids kaempferol, chrysin, and quercetin act against both human and bovine coronaviruses (BCV). Theaflflavin inhibits BCV, whereas quercetin 7-rhamnoside inhibits non-respiratory coronavirus porcine epidemic diarrhea virus (PEDV).

Multiple polyphenols, especially chalcone Broussochalcone B, effectively inhibit MERS-CoV cysteine proteases including 3CLpro and papain-like protease (PLpro). Herbacetin, isobavachalcone, quercetin 3-β-d-glucoside, and helichrysetin also presented anti-MERS-CoV 3CLpro activities.

Taken together, more than 69 flavonoids were identified with inhibitory activities against specific SARS-CoV-2 targets; thus, the reviewers presented a summary of these flavonoids, segregated based on subclass and target of inhibition. The most promising SARS-CoV-2 targets for these flavonoids included the 3CLpro, followed closely by disrupting the viral spike (S) protein-angiotensin-converting enzyme (ACE2) receptor, as well as PLpro, which is a non-structural protein involved in proteolytic cleavage.

Specifically, the authors of this study tabulated the antiviral activities of flavonols and flavanonols that are reported against SARS-CoV-2 3CLpro using in vitro methodologies segregated according to class. Among the flavanols, the most promising one is epigallocatechin gallate (EGCG) against 3CLpro activity.

Flavones like baicalien, baicalin, and scetullarein, Wogonin and Wogonoside, and oroxylin, as well as flavanones including naringenin, and isoflavones like puerarin, Daidzein, and genistin exhibited activities against the protease, 3CLpro.

Combining a mixture of flavonoids or encapsulating the active compound to increase uptake of the otherwise hydrophilic flavonoids, are some of the different methods that have been adopted in different studies with established results of significant viral inhibition.

Flavonoids are also used against the crucial interaction of the viral life cycle, where the viral S protein binds with the human ACE2 receptor during the viral entry phase. The reviewers tabulated a range of flavonoids with antiviral activities mediated through the inhibition of this interaction, some of which include isorhamnetin, quercetin, rutin, tamarixetin, (±)-eriodictyol, pinocembrin, and epicatechin (EC).

The reviewers also described the antiviral activities of flavonoids against other less-investigated SARS-CoV-2 targets in various cell-based methodologies. These modes of action included inhibiting SARS-CoV-2 endoribonuclease nsP15, reducing the ribonucleic acid (RNA) levels of RNA-dependent RNA polymerase (RdRp), membrane protein gene, and nucleocapsid protein gene, inhibiting the replication of SARS-CoV-2 by blocking mitochondrial OXPHOS, as well as targeting the endo-lysosomal two-pore channel 2 (TPC2).

Structure-activity relationships (SARs) of flavonoids

The authors of the current study also discussed the structure-activity relationships of the subclasses of flavonoids including flavanols, flavanonols, flavones, flavanones, flavanols, isoflavones, with the SARS-CoV-2 3CLpro, PLpro, S protein, and hACE2 receptor interaction. Overall, the authors reported that the presence of hydroxyl groups on all rings of the flavonoids increases their respective activities.

Specifically, myricetin was found to be the most effective flavonoid against 3CLpro, which is the most promising target. Myricetin has six hydroxyl groups spread over its three phenolic rings. The authors also investigated the effects of substitutions, polarity, and electronegativity of groups, and their effective activities.

Conclusion

The review discussed here presents natural flavonoids as a promising alternative based on findings that were collected from previously published studies. Moreover, the researchers focused on data from in vitro studies on the effects of flavonoids on the SARS-CoV-2 targets like 3CLpro, papain-like protease (PLpro), the S protein–ACE2 interaction, helicase, and the nucleocapsid (N) protein.

Taken together, the reviewers reported quercetin and myricetin derivatives like baicalein, baicalin, and tannic acid as the most promising flavonoids against the SARS-CoV-2 targets.

Written by

Dr. Ramya Dwivedi

Ramya has a Ph.D. in Biotechnology from the National Chemical Laboratories (CSIR-NCL), in Pune. Her work consisted of functionalizing nanoparticles with different molecules of biological interest, studying the reaction system and establishing useful applications.