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vitamin c quercetin

Coronavirus Illness (COVID-19): Pathophysiology, Epidemiology, Medical Administration and Public Well being Response

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Viral Immunology


REVIEW article



It’s serendipitous (or maybe indicative of onerous work) that the Nobel prize winner Szent-Gyorgyi found each ascorbic acid (vitamin C) and the flavonoid quercetin (on the time labeled vitamin P) (1). Ascorbic acid is an important vitamin with identified antiviral properties (2) which is beneath investigation for its helpful results in the course of the stress response in sepsis and critically in poor health sufferers (3).

Vitamin C exerts its antiviral properties by supporting lymphocyte exercise, rising interferon-α manufacturing, modulating cytokines, decreasing irritation, bettering endothelial dysfunction, and restoring mitochondrial operate (4–6). There are additionally strategies that vitamin C could also be straight viricidal (7). These in vitro results, as we beforehand mentioned (2), represent a mirrored image of each the supra-physiological concentrations of ascorbate and the interplay between vitamin C and metal-containing tradition media—each of that are pro-oxidant, producing reactive oxygen species.

Quercetin (also referred to as 3,3′,4′5,7-pentahydroxyflavone) is a extensively distributed plant flavonoid, present in a number of greens, leaves, seeds, and grains, the place it’s conjugated with residual sugars to type quercetin glycosides (8). Research recommend that quercetin supplementation might promote antioxidant (9), anti-inflammatory, antiviral (10), and immunoprotective results (11). Quercetin has been studied in varied sorts and fashions of viral an infection because of its promising antiviral results in inhibiting polymerases (12), proteases (13), reverse transcriptase (14), suppressing DNA gyrase, and binding viral capsid proteins (15, 16).

On this assessment we collate the proof of the antiviral properties of quercetin, describe its biologic motion and pharmacokinetics profile, develop on our earlier assessment of vitamin C, talk about their synergistic actions, and suggest this experimental multi-drug strategy for the prevention and remedy of SARS-CoV-2/COVID-19 pandemic.


Chemistry of Quercetin

In crops, quercetin is produced from the phenylpropanoid pathway and is in the end derived from phenylalanine. It’s transformed to 4-coumaroyl-CoA, through phenylalanine ammonia-lysate, to cinnamate-4-hydroxylase and 4-coumaroyl-CoA-ligase. That is mixed with malonyl-CoA in a 1:3 ratio by 7,2′-dihydroxy-4′methoxyisoflavanol synthase to type tetrahydroxy chalcone. This in flip is transformed to naringenin and to eriodyctiol by way of flavonoid 3′-hydroxylase. Lastly, eriodyctiol is hydroxylated and transformed to quercetin (Determine 1) utilizing flavanol synthase (17).


Biology of Quercetin – “vitamin c quercetin”

Flavonoid compounds, similar to quercetin, have been initially studied for his or her organic exercise in affecting capillary wall resistance (19) and proceed to be investigated for his or her results on vascular rigidity (20). Dietary dietary supplements differ, however typically include the free type of quercetin—quercetin aglycone—beneath the FDA nationwide drug code numbers 65448-3085, 65448-3005 (21). As soon as consumed, quercetin passes predominantly unaltered into the massive gut (22). Quercetin acts as a free radical scavenger, donating two electrons through o-quinone/quinone methide (23); each in vitro and in vivo (24, 25) research implicate quercetin as a potent antioxidant. This antioxidant exercise can also be potentiated by vitamin C (26), as can be mentioned beneath. There may be additionally vital longstanding curiosity within the anti-inflammatory exercise of quercetin, because it has been steered to be a key mediator within the cardiovascular protecting component of the “Mediterranean” weight loss plan (27). This organic rationale is secondary to quercetin’s free radical scavenging capability, alongside various roles recognized in in vitro and in vivo fashions together with: inhibition of platelet aggregation (28), inhibition of lipid peroxidation (29), and its inhibitory results on pro-inflammatory mediators similar to lipoxygenase (30) and phospholipase A2 (31). This anti-inflammatory impact is primarily mediated by flavonoid exercise on arachidonic acid metabolism and the related leukotriene/prostaglandin pathways. Moreover, 3-methyl-quercetin, a quercetin metabolite, shows stimulatory results on nasal epithelial cell ciliary beat frequency, each in vitro and in vivo, when administered both alone or with absorption enhancer HP-β-CD (32). Quercetin additionally impacts the operate of a number of lipids, protein tyrosine, and serine/threonine kinases (33, 34), similar to phosphatidylinositol (PI)-3-kinase and inducible nitric oxide synthase (NOS2) (35, 36).

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Helpful Results of Vitamin C and Quercetin in Viral Infections

There’s a super quantity of literature supporting the antiviral properties of quercetin, in each in vitro and in vivo experiments. Quercetin inhibits a number of respiratory viruses in cultured cells (16, 37). It inhibits the cytopathic results provoked by many serotypes of rhinovirus, echovirus (kind 7, 11, 12, and 19), coxsackievirus (A21 and B1), and poliovirus (kind 1 Sabin) at a minimal inhibitory focus of 0.03 to 0.5 μg/ml in Hela or WI-38 cells (38). Quercetin additionally considerably reduces plaque formation by RNA and DNA viruses [Respiratory Syncytial Virus (RSV), Polio type 1, parainfluenza type 3, and Herpes Simplex Virus-1(HSV-1)] displaying anti-infective and anti-replicative properties (39). It inhibits the replication of cytomegalovirus (CMV) inoculated HeLa cells at a half inhibitory focus (IC50) of three.2 ± 0.8 μM and with a selectivity index (SI) of twenty-two (40). Dengue virus kind 2 (DENV-2) replication in Vero cells is inhibited by quercetin at an IC50 of 35.7 μg/mL, inflicting a DENV-2 RNA discount of 67%. That is attributed to quercetin’s potential to both block virus entry or inhibit viral replication enzymes similar to viral polymerases (41).

In vivo research point out that mice inoculated with meningoencephalitis virus are shielded from deadly an infection by quercetin (30 or 40 mg/Kg BID, po, for 4 days) in a dose dependent method (42). These helpful results are abolished if the compound is run for <3 days, once per day or via subcutaneous injection. This may suggest that the antiviral effects may be dependent on a minimum inhibitory concentration or from some form of metabolic drug conversion (42). Quercetin treatment also displayed a beneficial effect in immunocompetent mice infected with Mengo virus, where it lessened the severity of organ damage (43). Athletes supplemented with quercetin are protected from stress-induced susceptibility to upper respiratory tract infection (44)—which was not related to immunomodulation (45, 46). Vitamin C is an essential nutrient involved in a diverse array of immune functions; its supplementation has demonstrated beneficial effects in different types of viral infections. Reduced levels of ascorbate have been found in patients with viral infections (47), sepsis (48), sepsis-related ARDS (49), and other critical illness (50). During infection, vitamin C is necessary for neutrophil killing (51), is concentrated within macrophages (52), is responsible of T cell maturation (53), and promotes phagocytosis and apoptosis of spent neutrophils (4). It is not surprising, therefore, that viral infections, depending on their severity, are associated with an increased metabolism and reduced circulating ascorbate. Vitamin C has improved survival in different murine models of lethal infection. Mice infected with Venezuelan encephalitis virus and treated with vitamin C (50 mg/kg) exhibit half the mortality of controls with associated reductions in viral titers, lipid peroxidation products, and NO content (54). Mice incapable of synthetizing vitamin C (L-Gulono-gamma-lactone oxidase nulls) were infected with influenza; mice not receiving supplemental vitamin C exhibited greater lung pathology scores despite no differences in viral titers (55). In restraint-stressed mice with H1N1 viral-induced pneumonia, vitamin C reduced mortality dose-dependently (100% vs. 80% vs. 50% at 0, 125, and 250 mg/kg/day) and reduced capillary-alveolar structural damage (56). Mice inoculated with Rabies+ mouse brain cells and treated with daily 100 mg/kg IM vitamin C exhibited nearly half the mortality of controls (57). The only human study of vitamin C has been in USSR soldiers with severe viral infection indicated vitamin C supplementation (300 mg/day) protected from influenza-associated pneumonia and was associated with shorter hospital stays (58). Vitamin C administration (i.v. 5 g/day twice/week) in patients with herpes zoster exhibited a lower incidence of postherpetic neuralgia (31.1% vs. 57.1%) and at study end (week 16) there was a lower pain score in the treatment group (0.64+/−0.9 vs. 1.98 +/−0.7) (59). Vitamin C administered at 1 g BID to 133 patients, reduced the risk (OR 0.25) of herpes simplex keratitis (HSK) recurrence (60), in accordance with previous studies indicating reduced ascorbate availability in the eye (61). It is noteworthy that a growing number of case reports of virus-related acute respiratory distress syndromes (ARDS) indicate successful treatment with intravenous high doses of Vitamin C (62, 63). Co-administration of quercetin (12.5 mg/kg/week) and vitamin C and B3 in a murine model of exercise-induced susceptibility to influenza H1N1 prolonged time-to-death (median time to death: placebo 9.0 ± 0.33 vs. quercetin 16.5 ± 1.2) and improved survival (mortality: placebo 74% vs. quercetin 52%) when compared to mice receiving only vitamins B3 and C (64). An older, small clinical trial identified the combination of flavonoids and ascorbic acid (1:1 ratio) as beneficial for respiratory infection (200 mg TID) (65). Inhibiting Virus Entry Cell entry is a crucial step during viral infection and has been studied as a potential target of antiviral treatments (66–68). In an in vitro model of H1N1 and H3N2 influenza infection of MDCK cells, quercetin demonstrated reduced cytopathic effect 48 h post-infection (69). This effect was observed when quercetin was administered during viral entry (0–2 h), was maximal with quercetin pretreatment, and was dependent on quercetin's ability to bind hemagglutinin proteins (HA). Specifically, quercetin bound (dose-dependently) the HA subunit responsible for membrane fusion during virus entry and virus-mediated hemolysis (69). In vitro, quercetin pre-treatment (10 μM) inhibited Rhinovirus (RV) virulence, entry, and replication into BEAS-2B cells via multiple mechanisms: it impeded RV endocytosis though misdirecting EEA1 localization -an early endosomal marker- and inhibiting AKT phosphorylation with subsequent 3-fold viral load reduction at 24 h, lowering negative-strand RNA and modulating interferon (IFN) and IL-8 expression (70). These results were confirmed in vivo, with an estimated lower plasmatic concentration of quercetin (nM) (similarly to other studies (71–73)) during which quercetin reduced RV-RNA at 1 day post-infection, modulated KC, MIP-2, TNF-a, and MCP-2, decreased virus-induced airway hyper-responsiveness, and modulated IFNs (IFN-α and IFN-λ2) (70). Interfering With DNA and RNA Polymerases The in vitro antiviral effects of quercetin on herpesviruses (HSV-1, 2) and adenoviruses (ADV-3,−8,−11) suggest inhibition of early stage viral replication in a dose dependent manner (for HSV-1 100% inhibition at 60 mg/L) (16, 74), as well as inhibition of viral DNA and RNA polymerase (12, 75, 76). In human embryonic kidney cells (HEK), inoculated with polio, 3-methyl-quercetin disrupted plaque formation while quercetin itself demonstrated these effects when administered together with vitamin C (77). In fact, Vitamin C (either D- or L-ascorbate but not dehydroascorbate), prevented quercetin spontaneous degradation suggesting necessary co-administration with ascorbate to exert its antiviral effect. The beneficial effects of 3-methyl-quercetin (10 μM) were exerted primarily when the compound was administered 1–2 h post-poliovirus infection in Hela cells, inhibiting viral proteins and RNA synthesis in a dose dependent manner (78). In fact, 3-methyl-quercetin was identified as a molecule able to bind essential proteins required during the transcription from minus-strand RNA into positive polarity RNAs, thus interfering with cytoplasmic viral RNA replication (79). In an in vivo study, a quercetin metabolite (4′,5-diacetyl-3,Y,7-trimethyl-quercetin), administered orally BID for 4 days protected mice against lethal infection by Coxsackie virus, promoting survival in a dose-response scale: 10, 20, and 40 mg/kg increased survival by 30, 40, and 50%, respectively (38). These beneficial effects were ascribed to a complete inhibition of virus replication when the compound was added within 2 h after virus absorption and related to the blockade of the RNA polymerase complex, as demonstrated in vitro (38). Inhibition of Reverse Transcriptase Quercetin has been investigated in vitro as an antiviral agent for HIV due to its ability to inhibit crucial enzymes: reverse transcriptase (RT), integrase (IN), and protease (PR) (80). Quercetin significantly reduces HIV viral replication (81) and, when added to peripheral blood mononuclear cells (PBMNc) infected with HIV and compared to HIV infected controls, quercetin reduced the levels of p24, Long Terminal Repeat (LTR) gene expression, and viral infectivity together with an inhibition of TNF-α and upregulation of IL-13 (11). Quercetin has also been shown to inhibit non-HIV RT activity in vitro, including avian myeloblastosis reverse transcriptase (AMV-RT), Rous-associated virus-2 (RAV-2-RT), and Maloney murine leukemia virus (MMLV-RT). Quercetin displayed a dose-dependent inhibitory action: at 50 μM, 23% inhibition of both AMV-RT and RAV-2-RT, and at 10 μM inhibition of mammalian MMLV-RT of almost 60% were reached (14). HIV-RT was inhibited completely at 2 μg/ml quercetin in a partially-competitive mode (76). These antiviral effects of quercetin are believed to be related to the five hydroxyl groups on 3, 3′, 4′, 5, and 7 as the inhibitory activity is lower for baicalein, quercetagetin, or luteolin which lack these groups (75). Interestingly, Harakeh et al. studied the dose-dependent effect of ascorbic acid (0–150 μg/ml) on HIV-infected T-lymphocytes in vitro and reported that >99% reverse transcriptase and practically >90% p24 antigen suppression and a 93% inhibition of syncytia formation, a marker that correlates with viral infectivity and cytopathic results (82).

Inhibition of Proteases

Quercetin is a potent HIV protease inhibitor in vitro, with an IC50 of 58.8 μM (83). Hepatitis C virus (HCV) NS3 serine protease catalytic exercise was straight inhibited by quercetin remedy in a dose dependent method (95% NS3 inhibition at 1.25 mg/ml); on this research quercetin blocked virus RNA manufacturing and impeded virus replication by 70% at 72 h with out affecting cell viability (13).

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Blocking Virus Meeting

Quercetin remedy inhibits HCV replication (84). This impact is attributed to its potential to modulate Warmth Shock Protein expression (HSPs), thus impeding the essential binding between warmth shock issue and components (HSF-HSE) vital for the stress-induced transcription of stress genes (85, 86). Quercetin lowered HSP70 and HSP40, thereby impeding the formation of Non-Structural protein 5A complexes (NS5A-HSP70 and NS5A-HSP40) vital for HCV genome replication equipment by way of the interior ribosome entry website (IRES). Regardless of unaltered HCV titer, the manufacturing of infectious particles was decreased, curiously extra by quercetin remedy than by HSP knockdown, displaying a dose-dependent relationship: at 0.5 μM quercetin lowered viral manufacturing by 29%, at 5 μM by 90%, and at 50 μM by practically 100% (84).

Immunomodulatory Properties

Quercetin stimulates T-helper cells to provide (Th-1)-derived Interferon-γ (IFN- γ) and downregulates Th2-derived IL-4 when added to cultured blood peripheral mononuclear cells (11). Immunonutrition research in mice with supplementary polyphenols, together with quercetin, confirmed enhanced NK cell lytic exercise, neutrophil chemotaxis, and lymphocyte proliferation (87, 88).

Human foreskin fibroblast (HFF) and endothelial cells (EC) pretreated with 2-phospho-ascorbate (ASC-2P) resisted CMV an infection; they displayed a discount in rapid and late antigens and viral yield was inhibited 50–100-fold in ECs and 100–1,000-fold in HFF (89). This impact was not depending on a sustained ASC-2P presence however was abolished if the ASC2-P was added after the virus an infection, indicating an immunomodulatory impact, quite than straight antiviral. Animal fashions with gulo (–/–) mice inadequate in vitamin C, when contaminated with 20 hemagglutination models (HAU) of H3N2 influenza exhibited worse outcomes than wild kind and Gulo (–/–) adequate in vitamin C (90). Gulo (–/–) confirmed a discount in IFN-α/β whereas displaying larger ranges of IL-1α, TNF-α, and IL-1B. When Gulo (–/–) mice obtained supplemental Vitamin C, these cytokine expression profiles have been misplaced.

Sufferers with acute Epstein-Barr an infection (EBV) handled with excessive doses of intravenous vitamin C (7.5–50 g) displayed decrease EBV-IgG (ranges, whereas EBV VCA IgM antibody ranges have been negatively correlated to rising plasma ascorbate focus (91). Sufferers with HTLV-1-associated myelopathy/tropical spastic paraparesis HAM/TSP have been all efficiently handled with 35–40 mg/kg oral vitamin C for 3–5 days regardless of no modifications in serum HTLV-1 or CSF HTLV-1 antibody titer, indicating an immunomodulative impact (92). Of those sufferers, 4 underwent a vitamin C on-off research which demonstrated a “positive dose response relationship with neurological symptoms.” A separate potential trial into a various variety of therapies indicated that vitamin C improved motor incapacity grades in HAM/TSP in 20% of sufferers (93). Excessive dose ascorbic acid was then proven to show antiproliferative (95% lower in lymphoproliferation) and immunomodulatory results (through discount of TNF-α, IFN-γ, IL-6, and p19) in peripheral blood mononuclear cells (PBMCs) extracted from HAM+ sufferers and T helper cell traces.

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Vitamin C administration has been associated to enhanced interferon manufacturing and was studied for its potential use for the prevention of vaccine failure. Rabies vaccination, when supplemented with 2 g oral vitamin C for every of the three injections provoked, at 24 h, elevated serum IFN-α ranges, indicating that “vitamin C is an effective stimulator of interferon production” (94). Mice on an advert libitum weight loss plan containing vitamin C elevated induction of interferon (62–145%) relying on the viral titer of inoculation (95), and L-ascorbate added to stimulated mouse cell traces will increase interferon synthesis (96). Low ranges of vitamin C, the truth is, have been associated to inadequate phosphorylation of sign transducers and activation of transcription (STATs), which characterize an important signaling strategy of IFNs (97). Particularly, T cells of mice poor in vitamin C show defects in STAT3 phosphorylation (90).


Deal with SARS-CoV-2

Quercetin has been investigated for its potential antiviral impact on a number of members of the Coronaviridae household and, as talked about by Ling Yi and colleagues, “quercetin offers great promise as a potential drug in the clinical treatment of SARS” (98). SARS-Coronavirus, described in 2003 (99), is a single-stranded RNA virus of ~29,700 nucleotides, which makes use of ribosome websites to encode two replicase glycoproteins, PP1a and PP1b, that mediate viral replication (99, 100). As soon as these precursor glycoproteins are synthesized, 3C-like protease (3CLpro) performs a crucial function within the lytic launch of its replicates (101). Quercetin-3β-galactoside binds SARS-Cov 3CL protease and inhibits its proteolytic exercise with an IC50 of 42.79 ± 4.95 μM (102). This inhibitory motion on 3CLpro depends on the hydroxyl group of quercetin which, as proven by way of molecular modeling and Q189A mutation, acknowledges Gln189 as an important website on 3CLpro chargeable for the binding of quercetin (102). Quercetin was additionally recognized as a compound in a position to block SARS-Coronavirus entry into Vero E6 cells with a half-effective focus (EC50) of 83.4 μM and with low cytotoxicity (CC50 3.32 mM) (98).

SARS-CoV-2, the virus chargeable for the 2020 COVID-19 pandemic (103), belongs to the genus Betacoronavirus and subgenus Sarbecovirus and, because of its comparable receptor-binding area, it’s assumed, equally to SARS-CoV, to contaminate kind II pneumocytes getting into through the angiotensin-converting enzyme II receptor (104). SARS-Cov-2 protease 3CL maintains the identical Gln189 website (105) of SARS-Cov 3CLpro, which beforehand was recognized because the binding website for the hydroxyl teams of quercetin and its derivates (102).

Curiously, an in vitro research of ascorbic acid remedy on chick-embryo ciliated tracheal organ cells (CETO) promoted resistance to Coronavirus an infection however didn’t present any impact on orthomyxovirus or paramyxovirus (106).

Regardless of the breadth and depth of anti-viral in vitro and in vivo research into the immunomodulatory results of quercetin and vitamin C administration, additional research are completely vital to verify quercetin inhibitory actions on SARS-Cov-2 virus entry, RNA polymerase, and on different vital viral life-cycle enzymes.


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