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Flavonoids as Antiviral Agents for Enterovirus A71 (EV-A71)

1. Introduction

Flavonoids are naturally occurring polyphenolic biomolecules widely found in plants and are responsible for a wide variety of biological functions. Pharmacological properties of flavonoids include antioxidant, anti-inflammatory, anticancer, antimicrobial and immunomodulatory functions. More than 6000 compounds have been identified as flavonoids based on their basic structures consisting of C6-C3-C6 and they are divided into several classes such as flavonols, flavanones, isoflavones, flavones, and anthocyanidins. Due to a wide range of biological activities being exhibited by flavonoids, they have become molecules of interest for natural drug discovery research. In this review, antiviral mechanisms of flavonoids, mainly against Enterovirus A-71 (EV-A71), will be discussed to provide evidence-based efficacy of antiviral properties.

2.1. Flavonoids against Non-Picornaviruses

Research has been carried out on plants that are being used in traditional medicines in different parts of the world. Many natural compounds of various chemical classes have been identified as potential therapeutic agents. Several flavonoids have been studied to evaluate their antiviral potential. For example, quercetin and quercitrin were first demonstrated by Cutting et al. (1949) to prophylactically inhibit rabies virus in mice. Moreover, quercetin was found to exhibit its antiviral activity against vesicular stomatitis virus (VSV) by activation of macrophages. Biologically active components of Elderberry extract dihydromyricetin and tetra-O-methyl quercetin were found to inhibit influenza A virus in vitro, most likely by binding to viral mannose-rich hemagglutinin domains which are important for the virus to enter the host cells. Mechanistic studies later showed that it was able to inhibit neuraminidase activity of influenza A virus by interaction with the subunit 2 of the hemagglutinin that resulted in virus entry inhibition. Quercitrin, a rhamnoside derivative of quercetin, was found to inhibit the initial stage of viral replication of the influenza A virus, evident by a decrease in mRNA synthesis of the virus. The study also concluded that quercitrin was unable to interact directly with virus particles. Quercetin was shown to inactivate the NS3 helicase and NS5 protease of hepatitis C virus (HCV). Quercetin also blocked viral binding and entry of herpes simplex virus (HSV)-1, HSV-2, and drug-resistant HSV-1 to Madin–Darby canine kidney NBL-2 (MDCK) cells. Another flavonoid derivative, 3-β-O-d-glucoside, was able to protect mice from Ebola virus infection when given prior to virus challenge. However, the mechanism by which this flavonoid inhibited viral entry is unclear.

In a similar manner, sulfated rutin was shown to block the entry of human immunodeficiency viruse (HIV)-1 without interactions with the host cell membrane. Cell fusion and entry assays revealed that sulfated rutin could drastically inhibit HIV-1 infection when cells were treated during the early adsorption phase. The probable mechanism proposed by investigators was the inhibition of HIV glycoprotein-mediated cell-cell fusion step. In the same study, it was identified that sulfated rutin could also inhibit HSV, the mechanism of inhibition is still unknown.

Apigenin was reported as an antiviral against HCV by host factor modulation. It caused a reduction in mature miRNA122 production that regulated HCV infection in vitro. Baicalin was found to interfere with the interaction between the HIV-1 envelope protein and host immune cells. This anti-HIV activity was achieved by inhibition of the HIV-1 envelope glycoprotein (gp120)-mediated fusion with T cells and monocytes expressing CD4/CXCR4 or CD4/CCR5. In addition, baicalin was also reported to inhibit dengue virus (DENV-2) by the virucidal mechanism. It blocked the attachment of DENV-2 to the Vero cells but did not show any activity during the virus entry stage. Baicalin was able to induce the production of interferon-gamma (IFN-ɣ) in natural killer (NK), CD4+ and CD8+ T cells during in vitro influenza A virus infection. It was suggested that baicalin was able to directly bind NS1–p85β (RNA binding domain), which was the main mediator to downregulate IFN-ɣ. The induction of IFN-ɣ triggered the immune system to activate the Janus kinase/signal transducer and activator of transcription protein 1 (JAK/STAT1) pathway, which led to the expression of IFN-ɣ-inducible genes. This expression of IFN-ɣ genes and signaling pathway facilitated the reduction of viral loads by immune modulation. Baicalein, the parent compound of baicalin, has also been reported as antiviral against Japanese encephalitis virus (JEV) and DENV-2. It is postulated that baicalein inhibited JEV by probable interaction with structural proteins of virus and this prevented entry of the virus into the cells. However, the antiviral mechanism against DENV-2 was identified as replication inhibition in silico by interaction with viral NS3/NS2B and NS5 proteins.

Flavonoids, genistein and ginkgetin, were reported to inhibit assembly and release of HIV and influenza A virus, respectively. Genistein inhibited Vpu protein that is involved in the formation of ion channels in infected cells and thus controlled the release of HIV. Ginkgetin inhibited sialidase activity of influenza A virus and thus inhibited virus assembly and release. Fisetin possessed post-infection anti-CHIKV activity by the inhibition of NS proteins 1 and 3 in a dose-dependent manner. Moreover, it was observed that CHIKV E2 protein and its precursor pE2 were also downregulated during treatment with fisetin. Similarly, a decreased replication process of DENV-2 was observed when Vero cells were treated with fisetin after DENV-2 infection. It was postulated that this antiviral activity could be due to the direct binding of fisetin to the viral RNA, thus impeding polymerase activity. Kaempferol and its structural flavonol derivatives were studied to identify the suitable pharmacophore to design antiviral against the influenza A virus. This study revealed that most of the flavonols with structural similarity with kaempferol were able to noncompetitively inhibit neuraminidase enzyme. However, the exact interacting partner is still unknown. Moreover, kaempferol was also reported to inhibit 3a channels of coronavirus and blocked the release of virus progeny. Luteolin was found to interfere with the entry of influenza A virus and severe acute respiratory syndrome coronavirus (SARS-CoV) by interacting with hemagglutinins of influenza A virus and S2 protein of SARS CoV viruses, respectively.

Another group of flavonoids, viz. methoxyflavone, isoscutellarein, and 8-methoxy-isoscutellarein, were reported to inhibit early replication of influenza A virus by reduction of sialidase activity, inhibition of lysosomal fusion and RNA polymerase activity. Naringenin, a flavanone, also possessed antiviral activities against HCV, DENV-2, DENV-4 and CHIKV by replication inhibition at RNA and protein levels. Other flavonoid such as silymarin which has a broad-spectrum antiviral activity was able to block entry and fusion of HCV viral pseudo-particles (pp) when Huh-7.5 cells were pre-treated with silymarin. Moreover, it was shown to interfere with the replication process of DEN-V, CHIKV and influenza A virus.

Tea catechins were also reported to inhibit the influenza A virus by binding to the hemagglutinins and restricted virus adsorption, preventing the penetration of the virus into the cells. Tea catechins were studied in clinical trials by Matsumoto and co-workers (2011). They conducted a randomized controlled trial to study the effects of tea catechins in the prevention of influenza virus infections in healthcare workers in elderly homes. Tea catechins were effective in preventing the clinical incidence of influenza infections in the treatment group as compared to the placebo group. Epigallocatechin (EGC), a tea catechin flavonoid, was reported to inhibit the replication of influenza A and B viruses by in vitro acidification of the lysosomal and endosomal environment through clathrin-mediated endocytosis. Apart from having antiviral activity against the influenza virus, another flavonoid from tea catechins, epigallocatechin gallate (EGCG), was shown to directly bind to CD4+ T-cells. This binding blocked the binding of the HIV-1 envelope protein (gp120) and caused viral entry inhibition. EGCG was also demonstrated to bind to the glycoprotein B and glycoprotein D of HSV to render it non-infective. Moreover, EGCG was shown to have potent antiviral activity against CHIKV by blocking its entry, probably by competing for cellular co-receptors of target cells such as heparan sulfate or sialic acid. EGCG was also found to inhibit the replication of Epstein–Barr virus (EBV) by inhibition of Zta, Rta and EA-D genes. This inhibition interrupted the MEK/ERK1/2 and PI3-K/Akt signaling pathway of the lytic cycle EBV. EGCG also inhibited hepatitis B virus (HBV) replication by impairment of the production of pre-core mRNA and replicative intermediates of DNA. Zika virus was also inhibited by EGCG, as it was able to interact with the lipid envelope and blocked the entry of virus into cells. It also caused acidification in lysosomes to make an unfavorable environment for virus replication. The antiviral activities of several other flavonoids according to the mechanisms of inhibition against various viruses are summarized in Table 1. Structures of these flavonoids can be found in Supplementary material Table S1: Molecular structures of antiviral flavonoids.

2.2. Flavonoids against Non-EV-A71 Picornaviruses

Picornaviruses are non-enveloped RNA viruses belonging to the picornaviridae family. Their natural hosts are vertebrates such as reptiles, fish, amphibians, birds and mammals. Viral infections such as poliomyelitis, hepatitis A, common cold and hand, foot and mouth disease are caused by picornaviruses. Flavonoids, in particular, have shown promising results in the laboratory against picornaviruses. Some of these flavonoids are discussed.

Quercetin, a well-studied flavonoid, exhibited anti-rhinoviral (RV) activity by inhibition of the viral transcription and translation processes. Quercetin decreased endocytosis of RV and reduced phosphorylation of Akt (effector of phosphoinositol 3-kinase) when epithelial cells were pretreated with quercetin. In addition, it also reduced viral infection when added 6 h post-infection in vitro. Moreover, it also repressed interferon and interleukin-8 response, resulting in lower viral RNA and capsid protein production. Quercetin demonstrated similar effects in vivo with decreased viral loads and suppression of viral immune mediators. Quercetin was shown to protect mice when challenged with the quercetin-treated Mengo virus. It was interesting to note that there was no antiviral activity detected when quercetin was tested in L-929 cells. This observation led to the study of the role of interferon induction in antiviral activity. However, there was no significant detection of interferons in drug-treated mice. This resulted in speculating that the antiviral activity was achieved by macrophage activation instead of interferon induction. In the same study, quercetin was also found to inhibit encephalomyocarditis virus (EMCV) by macrophage activation. A methyl derivative of quercetin was shown to inhibit poliovirus by blocking the synthesis of viral genomic RNA and proteins. Similarly, trimethylquercetin was shown to inhibit the production of progeny coxsackievirus B4 (CV-B4) and protected the mice from death upon lethal challenge. In another independent study, dihydroquercetin isolated from Larix sibirica was found to mediate the in vivo CV-B4 infection by inhibition of ROS induced oxidative stress.

Similarly, luteolin and its derivatives have been studied against picornaviruses. For example, luteolin was found to inhibit the RNA synthesis of Coxsackievirus A16 (CV-A16). Moreover, it was also reported to inhibit poliovirus at replication stage. However, the mechanism of inhibition of poliovirus by luteolin is still unknown. In contrast to luteolin, its derivative eupafolin was shown to inhibit CV-A16 by inhibition of viral attachment. It was shown to reduce the IL-6 and RANTES which led to the inactivation of virus-induced upregulated the ERK1/2, c-Jun, and STAT3 signaling pathways.

In a similar manner, kaemferol and its derivatives were also identified to be potent inhibitors of picornaviruses. Methylkaempferol was reported to inhibit the late replication step of the poliovirus. The inhibition mechanism is attributed to the potential of this flavonoid to inhibit the positive-strand synthesis of viral RNA. Furthermore, Kaempferol-3-O-[2″,6″-di-O-Z-p-coumaroyl]-β-d-glucopyranoside and its derivatives were identified as replication inhibitors of Rhinovirus 1B (HRV-1B) and Coxsackievirus B3 (CV-B3) in vitro. The group of methoxy flavones were reported to inhibit poliovirus-1 by downregulation of signaling pathway and apoptosis. Furthermore, entry of rhinovirus was shown to be inhibited in silico by inhibition of the protein grid of virus. Another flavonoid sakuranetin was also identified as a potent inhibitor of rhinovirus-3 by its antioxidant properties. Several other flavonoids were reported to inhibit the activity of various picornaviruses. However, their exact molecular mechanisms of actions are still unknown. Examples of bioflavonoids from various chemical classes exhibiting antiviral activities against picornaviruses are summarized in Table 2. Structures of these flavonoids can be found in Supplementary material Table S1: Molecular structures of antiviral flavonoids.

3.1. Apigenin

Apigenin is a flavone that has been tested for its antiviral potential against Fuyang and BrCr strains of EV-A71. It was able to reduce infection at the inhibitory concentration (IC50) of 10.3 μM. Apigenin inhibited EV-A71 infection by suppressing viral internal ribosome entry site (IRES) activity. Apigenin was found to disrupt the viral RNA association with hnRNP A1 and A2 proteins.

Moreover, apigenin was found to inhibit cellular apoptosis by caspase-3 cleavage, which is considered as a prime process to release viral progeny. It is also a known scavenger of reactive oxygen species (ROS) and was found to reduce infection-induced ROS generation in EV-A71 damaged cells. Furthermore, it decreased the cytokine levels involved in infection. Apigenin was confirmed to inhibit the activation of the c-Jun N-terminal kinase (JNK) pathway required for the replication process and release of progeny EV-A71. Another downstream p38 MAP kinase signaling pathway was found to be partially inhibited by apigenin. All these inhibitory mechanisms were suggested to suppress viral infection.

In a recent study by Dai et al. (2019), apigenin exhibited potent antiviral activity against EV-A71 genotype C4 strain in vitro and in vivo. It was able to reduce the cytopathic effect to 50% at the concentration of 24.74 µM in 293S cells. Moreover, apigenin significantly reduced viral RNA and protein synthesis in vitro. When evaluated for its potential to protect newborn BALB/c mice from intracranial lethal challenge with EV-A71, apigenin was found to protect mice at the dose of 50 mg/Kg with 88.89% survival rate. Significant improvement in clinical scores and body mass were also observed in the treatment group as compared to the control group.

3.2. Baicalin

Baicalin is a flavone reported to inhibit the BrCr-Tr strain of EV-A71 in post-infection assays with an IC50 of 4.96 μg/mL. Baicalin was shown to inhibit 75% viral infection at 8 h post-infection. Inhibition by baicalin at early stages of infection was due to a decrease in mRNA and protein levels of 3D polymerase. Moreover, it was also demonstrated that baicalin reduced the expressions of FasL and caspase-3, hence inhibiting EV-A71 induced apoptosis in Rhabdomyosarcoma cells (RD). Additionally, it was identified to inhibit proinflammatory cytokines by the reduction in NF-κB p65 expression.

3.3. Chrysin and Its Derivative

Chrysin is a flavone widely found in leaves, fruits, and vegetables. Chrysin and its ester (diisopropyl chrysin-7-yl phosphate) were evaluated against the clinical isolate EV-A71 Shzh-98 and were found to inhibit the viral replication process by blocking the activity of the 3C protein. Chrysin ester was found to be the more potent inhibitor of 3C protein than the parent chrysin. The activity was predicted in silico by simulation and was confirmed by in vitro protease inhibition assays. These flavonoids were able to decrease the production of the viral capsids, infectious virions and RNA of EV-A71.

3.4. Fisetin

Fisetin, a flavonol, was reported to possess antiviral activity against the clinical isolate EV-A71 CMUH01 with an IC50 of 84.5 μM. Fisetin was able to inhibit the enzymatic activity of recombinant 3C protease of the virus in a cell-based assay. To further confirm its activity in vitro, antiviral assays were performed in RD cells. Fisetin was found to block the cleavage activity of 3C and thereby inhibited the viral replication.

3.5. Formononetin

The isoflavone formononetin was tested against several isolates of EV-A71 including BrCr, H, JS-52 and Shzh 98. Inflammatory pathways such as ERK, p38, JNK, MAPK are usually upregulated in EV-A71 infections and help in viral reproduction. Formononetin was shown to act on the MAPK pathway and reduced the activation of the downstream regulator of infection-induced inflammation mediators, prostaglandin E2 (PEG2) and cyclooxygenase 2 (COX2). This, in turn, reduced the viral infection. Moreover, it also suppressed the signaling cascade of the ERK, p38 and JNK pathways to bring down the infection.

In another study, formononetin was found to inhibit EV-A71 strains G082, SH12-036 and SH12-276. Formononetin was able to inhibit attachment and entry of EV-A71 in Vero cells. To further evaluate the exact target, the virus was passaged with increasing concentrations of formononetin and sequenced after the 10th passage. A common mutation causing K58T in VP4 was observed and this mutation was able to confer viral resistance towards formononetin. This resistance was further confirmed by the absence of K58T mutation in the wild type virus. The mutant virus was evaluated for attachment in Vero cells and the K58T mutation was found to inhibit viral attachment. It was hypothesized that formononetin could possibly cause an overall conformational change in the viral capsid.

Formononetin was also evaluated for its antiviral potential against EV-A71 subgenotype C4 strain in 239S cells. It exhibited antiviral activity with an IC50 of 12.5 µM and significantly reduced viral replication and protein synthesis. It also protected newborn mice with a survival rate of 75% and significantly improved clinical score and animal body weight at a dose of 10 mg/Kg.

3.6. Hydroxyflavone and Its Derivatives

The hydroxyflavones are synthetic molecules, the core structure of which is also a backbone of flavonols. Wang et al. (2014) predicted that 7-hydroxyflavone and its phosphate ester diisopropyl-flavon7-yl phosphate were able to inhibit the activity of 3C protease of EV-A71 in silico. They further evaluated in vitro potential of these flavonoids as antivirals against Shzh-98 isolate of EV-A71. It was found that 7-hydroxyflavone and diisopropyl-flavon7-yl phosphate both inhibited virus-induced cytopathic effect in RD cells with an IC50 of 23.45 and 13.63 μM, respectively. In addition, both flavonoids significantly reduced viral protein synthesis.

3.7. Kaempferol

The flavonol kaempferol was reported to inhibit the EV-A71 clinical isolate Chuh-050530-5. Kaempferol treated cells were observed to reduce virus yield by 6 log units 24 hpi. Kaempferol was able to exert its action through induction of trans-factors such as the far upstream element-binding proteins (FUBP) and hnRPs. These trans-factors were identified to bind the highly conserved 5‘ UTR region of the virus genome and attenuated the viral infection. Moreover, it also disrupted the activity of IRES by an overall change in the composition of the trans-factors. This resulted in reduced viral infection by inhibition of translation and replication.

Kaempferol was found to inhibit EV-A71 subgenotype C4 strain with an IC50 of 52.75 μM. It significantly reduced RNA copy number and protein synthesis 16 hpi. Newborn BALB/c mice were also protected from lethal challenge with the virus with 50 mg/Kg of kaempferol. The clinical score and body weight were found to be similar to that of the solvent treated group (negative control).

3.8. Luteolin and Its Derivatives

Luteolin is one of the examples of flavonoids that acted at an early step of the EV-A71 (subgenotype C4b) replication cycle by post-attachment inhibition of the virus. The IC50 was found to be 10 μM. Moreover, when luteolin was tested against the prototype BrCr strain and the Fuyang0805 isolate of EV-A71, it was able to prevent virus-induced ROS generation, cellular apoptosis and inflammatory cytokine production such as IL-6 and CCL5. In another independent study, Dai et al. (2019) showed that luteolin exhibited similar inhibition patterns with an IC50 of 13.5 μM. It was identified to reduce viral replication and protein synthesis. Moreover, in the murine model, it showed 91.67% protection of newborn mice at a dose of 10 mg/Kg.

Eupafolin, a methoxy derivative of luteolin, was also found to inhibit EV-A71 with an IC50 of 1.39 µM in an independent study. It was shown to inhibit IL-6 and chemokine RANTES which were upregulated during the in vitro viral infection. This inhibition resulted in the down-regulation of ERK, activated protein-1 (AP-1) and STAT3 mediated inflammatory signaling pathways.

3.9. Penduletin

Penduletin is a methylated flavonol abundantly found in the leaves of Laggera pterodonta. The compound was tested for its antiviral activity against EV-A71 strain GZ-08-02. The flavonoid was a potent inhibitor of EV-A71 with an IC50 of 0.17 and 0.20 µM in Vero and RD cells, respectively. Mechanistic studies revealed that penduletin specifically inhibited the production of infectious RNA viral progeny. Moreover, it was also able to reduce VP1 expression in post-infection assays, suggesting its ability to halt protein synthesis during viral replication process.

Penduletin also exhibited potent in vitro and in vivo antiviral activity against EV-A71 subgenotype C4 strain. The IC50 was found to be 0.63 µM in 293 S cells and 66.67% newborn BALB/c mice survived the lethal challenge with EV-A71 when mice were treated with 5 mg/Kg dose of penduletin.

3.10. Peracetate Pulicarine

Peracetate pulicarine is one of the less-studied flavonoids against viruses. It is a methoxyphenyl acetate, a derivative of a flavonoid which was found to possess anti-EV-A71 activity in a recent screening of a flavonoid library. The antiviral activity of pulicarine was associated with inhibition of the 3D polymerase expression in EV-A71 replication process.

3.11. Prunin

Prunin is a flavanone glycoside recently found to be potent antiviral against enteroviruses A and B including clinical isolates and strains of H, B5 and C4 genotypes of EV-A71. Prunin was able to suppress the activity of IRES and disrupted protein and RNA synthesis with an IC50 of 115.3 nM. It was further evaluated in the murine model using newborn BALB/C mice. Prunin was able to protect from the lethal challenge of EV-A71 (strain 41) with a 100% survival rate at the doses of 3 mg/Kg and 10 mg/Kg per mice. Apart from suppression of IRES activity in vitro, prunin successfully protected mice from the infiltration of immune cells and inflammation of tissues. Moreover, prunin was also able to reduce viral antigen in hindlimb muscles of mice.

3.12. Quercetin and Its Derivatives

Quercetin has been found to possess broad-spectrum antiviral activity by several mechanisms. When tested against EV-A71, quercetin was able to target the post-attachment stage of EV-A71 infection in vitro by disrupting viral RNA. In another study, quercetin was predicted to interact with catalytic residues of the 3C protease and could inhibit the activity of viral the 3C protease in silico. To further confirm the findings, quercetin was evaluated for its antiviral activity using the SK-EV006/Malaysia/97 isolate of EV-A71. The IC50 of 8.8 μM was observed in Vero cells and 12.1 μM in RD cells. Quercetin was also investigated for its anti-apoptotic potential against virus-induced cell death. Quercetin was able to prevent the viral spread induced by apoptosis, thus making it a dual action antiviral agent.

Rutin is another glycoside flavonol derivative of quercetin. It was identified as antiviral against EV-A71 using the recombinant 3C protease screening assay. To confirm its activity in vitro, rutin was tested against EV-A71 strain CMUH01 (B5). Rutin was able to inhibit the replication stage of infection with an IC50 of 110 μM. In another study, rutin was found to reduce the infectivity of the C4 subgenotype EV-A71 with an IC50 of 200 μM. The anticipated mechanism was recognized as the suppression of the MEK1-ERK signaling pathway. Direct inhibition of the 3C protease and interference with inflammatory pathways were assumed to contribute to the reduction of virus titres.

Quercetagetin, a synthetic derivative of quercetin, was identified to inhibit the RNA replication machinery and viral translation. The flavonoid was found to reduce the activity of the 3D polymerase in vitro. Similarly, chrysosplenetin, a methylated derivative of quercetagetin, was identified to potently inhibit EV-A71 by reducing the production of viral RNA and proteins.

Quercetin and its methoxy derivative isorhamnetin and chrysosplenetin were evaluated for their in vitro and in vivo potential as antivirals against EV-A71 (subgenotype C4). Quercetin and chrysosplenetin displayed potent antiviral activities in vitro with an IC50 of 1.2 µM and 0.68 µM, respectively while isorhamnetin showed inhibition with an IC50 of 60.7 µM. All three flavonoids reduced RNA and protein synthesis in vitro. In the same study, these flavonoids were evaluated to identify their protective effect in mice from lethal challenge of EV-A71. Isorhamnetin was found to confer the best protection at the dose of 10 mg/Kg with 100% survival while quercetin could only protect 50% of the animals using the same dose. Despite being the most potent antiviral amongst the three flavonoids when tested in vitro, chrysosplenetin was the least effective in vivo. Only 30% of the animals survived after treatment with 5 mg/Kg of chrysosplenetin. All three flavonoids were able to improve the clinical score and bodyweight of the treated animals. It should be taken into consideration that in this study, the route of virus inoculation was intracranial while flavonoids were administered by intraperitoneal (IP) route. This difference of injection routes could result in varying efficacy of flavonoids to cross the blood-brain barrier and potentially protect mice from virus. In vivo antiviral activity of different flavonoids against EV-A71 has been summarized in Table 3.

3.13. Flavonoids Isolated from Scutellaria Baicalensis Georgi

Flavone derivatives such as mosloflavones, norwogonin and oroxylin A are abundant in Scutellaria baicalensis Georgi. These flavones were isolated as pure compounds and were reported to inhibit early stages of EV-A71 replication with an IC50 of 37.72, 31.83 and 14.19 µg/mL, respectively. All three flavonoids were able to limit replication process, leading to inhibition of viral protein (VP2) synthesis. However, oroxylin was found to possess superior activity as compared to mosloflavones and norwogonin. Furthermore, these flavonoids were evaluated for their potential to inhibit viral attachment and entry. None of these flavonoids were found to be involved in the inhibition of attachment or entry of the virus in Vero cells.

3.14. Flavonoids with Unknown Mechanisms of Action against EV-A71

There are more examples of flavonoids that were reported to inhibit EV-A71. However, the mechanisms by which they modulated the infectivity of EV-A71 are still unknown. Examples include nobiletin, morin hydrate, myricetin, taxifolin, diosmetin, dihydromyricetin, hesperidin, thio flavones and galangin.

4. Structure–Activity Relationship among Potential Antiviral Flavonoids against EV-A71

Flavonoid classes differ from each other by various substitutions particularly phenolic or hydroxyl (OH) groups on its flavan nucleus. Flavonoids that have successfully accounted for antiviral activity against EV-A71 mostly belong to the flavonoid sub-classes flavonols and flavones. Flavones are dual fused aromatic rings (A and C) with an extension of a benzene ring (B) at position 2 of ring C (Figure 3). The skeleton “ring A fused with ring C” has previously reported to possess antiviral activity against the dengue virus NS2B-NS3 complex. Moreover, hydroxylation on ring A, particularly on positions 5 and 7, was found to be favorable for antiviral activity against the influenza A virus. Based on previous findings, it can be expected that the same structure could attribute towards antiviral activity against EV-A71.

It can be observed that the anti-EV-A71 flavonoids with methylation have drastically improved the antiviral activity such as in the case of chrysosplenetin, eupafolin, penduletin and pruning (Table 4). Moreover, presence of methoxy substitution at the position 7 of ring A, along with hydroxyl or methoxy group at position 6 rendered higher activity with lowest IC50 values (chrysosplenetin, eupafolin, penduletin) amongst all evaluated flavonoids. Prunin, a glucoside of naringenin, having large glucose moiety at position 7 instead of methoxy substitution, interestingly showed potent antiviral activity as well. From these examples, it can be speculated that positions 6 and 7 of ring A require bulky group substitutions in order to possess potent activity against EV-A71. Possible interactions between different substitutions and EV-A71 could be studied in silico to determine the nature of bonds involved in antiviral activity. It must be noted the antiviral activity of flavonoids was evaluated against specific EV-A71 genotype/subgenotype. It is unknown if there is a broad spectrum of antiviral activities against other EV-A71 genotypes/subgenotypes.

5. Limitations of Flavonoids as Antivirals against EV-A71

Flavonoids are abundant in fruits and vegetables and therefore are considered as safe and non-toxic for human consumption. However, the bioavailability of these flavonoids in their crude form is very poor. Use of isolated, purified or chemically synthesized small molecular weight flavonoids has increased prospects to reach the target. To further enhance the stability, solubility and improved systemic distribution, a nano-drug delivery system can be employed. Despite possessing the potential to treat EV-A71 in vitro and in mice, flavonoids have failed to reach clinical trials. Some of the possible reasons could include infancy of in vivo studies on flavonoids against EV-A71 as there have been only two reports which were published in 2019, claiming absolute protection of mice by prunin and isorhamnetin. Moreover, lack of long-term toxicity data on purified flavonoids contributes challenges to further progress these potential molecules to clinical trials. Another important factor could be the development of resistance by mutant viruses against these flavonoids. Like any other drug candidate, purified flavonoids require regulatory approval from the Food and Drug Administration (FDA) before it can be marketed as antivirals. Detailed pharmacokinetic, pharmacodynamics and long-term toxicology studies on these flavonoids are necessary before they can be offered as a therapeutic option.

6. Conclusions

Flavonoids such as penduletin, eupafolin, baicalin, luteolin, quercetin and chrysosplenetin have been shown to be antiviral agents against EV-A71 based on their IC50 values (<10 µM). These flavonoids were also found to be safe and non-cytotoxic to human cells as indicated by CC50 values. Flavonoids such as prunin could confer 100% protection of neonatal mice while luteolin showed 91.67% protection against lethal challenges with EV-A71. However, flavonoids with higher IC50 values can also serve as a potential antiviral against EV-A71. For instance, kaempferol and isorhamnetin exhibited higher IC50 in vitro but both conferred significant protection of mice from viral challenge. The flavonoids that have not yet been tested in vivo could be further evaluated to identify their effectiveness.

Flavonoids, with their ability to target various stages of viral infection, are becoming a more focused topic to explore their potential as antivirals in the current era. Apart from their classical antioxidant properties, some flavonoids have been shown to inhibit viruses at the molecular level both in vitro and in vivo. In light of the current finding of the in vivo efficacy of some flavonoids against EV-A71, it can be concluded that flavonoids have great potential to be developed as therapeutic candidates against EV-A71. However, to make these flavonoids available as antivirals, a more in depth understanding of their pharmacological properties and clinical outcomes is warranted.