COVID-19 is a viral disease caused by a new severe acute respiratory syndrome (SARS-CoV-2), which has quickly resulted in a pandemic. As a great threat to global public health, the development of a treatment has become vital, and a rush to find a cure has mobilized researchers from all areas across the world. Synthetic drugs, such as hydroxychloroquine, have gained attention. However, the efficacy of repositioned drugs is still under evaluation, and besides, some severe side effects are a cause for concern. This emphasizes the urgency for treatment options, which can be both safe and effective. With this in mind, natural products could be an important resource in the development of COVID-19 treatment, as they have already contributed in the past to treatments against other viruses, such as HIV, MERS-CoV, and influenza. Natural products are described long term as bioactive substances and some phytochemical classes such as flavonoids, alkaloids, and peptides are known antiviral bioproducts, and have been virtually tested with success against COVID-19. However, important issues still need to be addressed as to their bioavailability and true efficacy in vivo. This review intends to systematically evaluate the natural metabolites that could potentially be used against this new disease looking at their natural sources, mechanism of action and previous pharmacological usages. The aim is to provide a starting point for this research area in order to speed up the establishment of anti-SARS-CoV-2 bioproducts.
Repurposed drugs against COVID-19
In early March 2020, the antimalarial hydroxychloroquine (HCQ) and chloroquine (CQ) displayed in vitro and in vivo activity against SARS-CoV-2 in small geospecific test groups.Hydroxychloroquine and chloroquine are synthetic derivates of quinine, an alkaloid extract from the bark of Remija and Cinchona (Rubiaceae) species, widely used as antimalarial drug. All the three chemical species are toxic to humans, causing a decrease of cardiac functions inducing arrhythmias or hypotension effects in single and continuous intake.In an adult man, poisoning by a single ingestion of quinine can occur in a range from 10 to 15 g, whilst with chloroquine and hydroxychloroquine this ranges from 2 to 5 g,emphasizing the danger of these compounds in preventive treatment with no proper evaluation of effective dose.
During nearly three months, several claims regarding the use of hydroxychloroquine and chloroquine were spread through public based on preliminary data of this drug repurposing as “the cure” for COVID-19. Those pretentions news induced several people to self-medication with no medical attention of cardiac functions.35 Although global population is anxious to scientific community provide a reliable treatment, the usage of HCQ and CQ in COVID-19 combat is still in preliminary test fase.Such assertive treatment request several safety and randomized clinical test with unbiased data obtained in trials within different ethnicities, which is being a hard task to accomplish in face of the supposed underreporting of COVID-19 in several countries.
Another drug drawing attention in COVID-19 treatment is the remdesivir, which act as a RNA polymerase inhibitor.The inhibition of RNA-dependent RNA polymerase is a strategy in virus treatment to develop high specific antivirus, reducing the damage to the host cells. RNA polymerase of viruses is responsible for its replication mechanism as its promotes a −1 ribosomal frameshifting that is fundamental to synthesize their structure protein.38 Remdesivir have successfully inhibit SARS-CoV-1 and MERS-CoV in primary human airway epithelial cell cultures, with EC50 of approximately 0.07 μM and 50% cytotoxic less than 10 μM.11
Whilst the attention hydroxychloroquine dragged in early pandemic scenario, other drugs also presented similar results to HCQ and CQ in silico efficacy in inhibit SARS-CoV-2. Among those drugs it can be emphasized the Nelfinavir, which presented a binding energy to SARS-CoV-2 close to the values described for chloroquine, −8.4 and −8.0 kcal mol−1, respectively.39–41 Despite repurposing drugs are an approach which aid to reduce the time necessary to control a pandemic scenario,41 reckless usage of preliminary data can become a public health issue.
Possible anti-COVID-19 natural products
Although the development of bioactive natural products against a specific disease, such as COVID-19, is faster than vaccine development, it is still an arduous task due to the diversity of natural metabolites, their chemical complexity and extraction. Virtual screening for bioactive compounds is a useful tool in natural products research in order to shortening the time spend in phytochemical screening of several natural products extracts. This approach is known as in silico analysis by molecular docking.
Nearly 45 days after COVID-19 become pandemic, natural metabolites of different chemical classes presented promising data on virtual molecular docking. Despite the distinct molecular structure, several chemical classes, such as flavanones, flavonols, alkaloids, fatty acids, quinones, terpenes and steroids presented similar binding energy or docking score to repurposed drugs (e.g.: remdesivir and chloroquine) with proteins involved in COVID-19 replication, including ACE2, 3CLpro and TMPRSS2. Most of docking evaluation were focused in inhibitors of ACE2 as a possible consequence of the first finding regarding COVID-19 replication and the implication of this enzyme in the formation of the risk group
ACE2 inhibitors
Since ACE2 has been indicated as the major receptor of SARS-CoV-2 viruses in humans, attention has been given to understanding its regulation as a form to treat this virus.
As a part of the renin-angiotensin system, the main function of ACE2 is to convert angiotensin II, a strong vasoconstrictor, to angiotensin (structural forms I, III, IV, V, VI and VII), a vasodilator that contributes in the maintenance and reduction of the blood pressure by counter-regulating ACE. Despite it being an analogue of ACE, their similarity is only approximately 42%.An issue regarding ACE2 and coronaviruses infections is that most of the chronic treatment of hypertension and diabetes involves the use of ACE inhibitors (ACEIn).47 These substances are also known to cause the expression of ACE2 to be upregulated, putting the patient in the risk group of COVID-19.4,5 In fact, most of the COVID-19 confirmed patients that presented severe or fatal forms of the infection had comorbidities, hypertension or diabetes in particular.4,8 Meanwhile, commonly used ACEIn present in hypertensive medicines, such as perindopril, enalapril, and losartan, do not cause any inhibition of ACE2.ACE2 upregulation by ACEIn is currently attributed to its partial capacity to cleave angiotensin I. As angiotensin I concentration increases due to the ACE inhibition, ACE2 mRNA is enhanced to compensate.
Several natural products present ACE inhibition activity and are extensively used in ethnobotanics, and in some cases are deeply rooted in the human diet.The application of bioproducts, like ACE inhibitors, is widespread, primarily because the synthetic substances, such as enalapril, were developed using a natural metabolite as a scaffold. This demonstrates their reliability as a new medicine sources; they present less side effects than synthetic drugs; and, in some cases, even natural extracts can present lower IC50 values.
There are at least 300 plants that have ACE inhibitors activity, including some well-know medicinal and food species, such as cinnamon (Cinnamomum zeylanicum Blume or Cinnamomum verum J. Presl.), pepper (Capsicum spp.), olive (Olea europaea L.), hawthorn (Crataegus pinnatifida Bunge), black nightshade (Solanum nigrum L.), passion fruit (Passiflora edulis Sims) and grape (Vitis vinifera L).ACE inhibitors from natural products belong to several phytochemical classes, including flavonoids, xanthones, alkaloids, peptides, terpenes, and tannins. Some compounds have presented both AceIn and ACE2 inhibitor activity, for example, phenolic compounds like myricetin and quercetin glycosylated derivates.The ability to inhibit both, ACE and ACE2, is caused by their closely related active sites, which are distinct mainly in terms of the smaller intramolecular size of ACE2 sites.
Briefly, ACE2In natural metabolites are of the same chemical classes of ACEIn and can be readily obtained in medium polarity extracts of angiosperms species such as roots and barks.
ACE and ACE2 inhibitors present amphiphilic molecular structures usually with an aromatic moiety, in order to enable their interaction with the protein, a similar pattern to the observed with SARS-CoV-1 inhibitors.Pharmacophere analysis of suggested natural metabolites capable to inhibit SARS-CoV-2 through ACE2 inhibitions usually have structural features in accordance with Lipinski's rule with molecular weight lower than 500 Da, less than 5 hydrogen bond donors and a log P under 5. The number of hydrogen bond acceptors were more variable among suggested phytochemicals against COVID-19, ranging until 20.For instance, ACEIn peptides have as first residue an aromatic amino acid, and the third is a hydrophobic one.The sequence trending necessary for peptides to be ACEIn was extensively exploited by Daskaya-Dikmen et al. (2017).
The first in silico screening for anti-SARS-CoV-2 natural metabolites was within traditional chinese herbs, such as species of the Citrus genus.The virtual molecular docking of Chinese herb metabolites with the ACE2 against SARS-CoV-2 suggested 11 natural products capable of inhibiting it.The natural metabolites suggested as possible bioactive substances against within Chinese medicinal botanic species includes baicalin (baicalein-7-O-glucuronide), scutellarin (scutellarein-7-glucuronide), hesperetin, nicotianamine, glycyrrhizin, naringin, naringenin, hesperidin, neohesperidin, and nobiletin.Similar to ACEin natural ACE2 inhibitors screened to combat SARS-CoV-2 are classified as alkaloids, flavonols, flavanones, terpenes, limonoids, lignans, terpenoids, tannins, phenolic acids and fatty acids.Disregarding the distinct software's and molecular docking models, the class with major representatives and better affinity results are within flavonoids
TMPRSS2 inhibitors
TMPRSS2 inhibitions search is the lowest within the major replication proteins despite molecular docking also indicate it as a strategy in COVID-19 treatment.TMPRSS2 is already known for its involvement in the inoculation and replication of influenza virus, cancer, and the SARS-CoV-1.TMPRSS2 natural inhibitors includes flavonoids, terpenes and peptides. For instance, the flavonoids baicalein and baicalin, which have already been reported as a down-regulators of the TMPRSS-2 expression,were also indicated on in silico studies against COVID-19.It is worth mentioning that baicalein was proposed by molecular docking studies to also be an ACE2 inhibitor.Indeed, it is ideal to the candidate metabolite be applied to interact with different binding sites of the virus, in order to increase its possible bioactivity in vivo.
Baicalin and baicalein are good examples of the potential of virtual molecular screening in the search for anti-COVID-19 natural metabolites. After its promising data within molecular docking,enriched fractions with both compounds were tested in vitro presenting antiviral efficacy similar to those obtained by repurposed drug. In vitro tests were performed by fluorescence resonance energy transfer protease assay and with Vero E6 cells contaminated with COVID-19. Within both assay, baicalein had the most promising results with IC50, EC50 and selectivity index (SI) to the SARS-CoV-2 3CLpro of 0.94 μM, 1.69 μM and 118, respectively, while baicalin presented the values 6.41 μM, 10.27 μM and 19, respectively. For instance, chloroquine EC50 was 1.13 μM, with a SI of 88.This data confirms that in silico experiments, such as those obtained for baicalein,can give promising insights on possible anti-COVID-19 natural metabolites. The major natural source of baicalein are Scutellaria and Oroxylum genera, mainly in the roots of S. baicalensis and the seeds of Oroxylum indicum.Both flavonoids have therapeutic properties as neuroprotective, antioxidant, anti-inflammatory, renal protector and anticancer.In addition, these flavonoids also presented remarkable activity as inhibitors of viruses, such as the Zika virus.
In addition to already know human TMPRSS2 inhibitors, Rahman et al. (2020) demonstrated by in silico studies that iridoids, diterpenes and lignans are promising anti-SARS-CoV-2 through TMPRSS2 interaction. The inhibition of TMPRSS2 requires similar structural features as those previously described for ACE2 inhibitors, presence of hydroxy moieties for hydrogen binding and presence of aromatic rings.
Rahma and coworkers suggested 12 natural metabolites with binding energy with TPMRSS2 ranging from −11.06 to −14.69 kcal mol−1. The natural metabolite with greater inhibition potential was the geniposide, an iridoid found in Gardenia genus (Rubiaceae) and endemic in Central America and China.Such values are higher than any molecular docking focused on ACE2 inhibition. Natural source of the 12 metabolites suggested as anti-COVID-19 by Rahma and coworkers included marine soft corals (Formosan gorgonian and Alcyoniidae family), free-floating algae from Sargassum genus, mushrooms of Paxillus genus and several species of angiosperms, such as magnolia-vine (Shisandra sphenanthera) green tea (Camellia sinensis) and the branched asphodel (Asphodelus ramosus).
3CLpro inhibitors
The inhibition of 3CLpro, the main protein of SARS-CoV-2, has received more attention within researchers as it could prevent virus inoculation on the host.Although the 3CLpro is a enzyme specific to the virus, the one within SARS-CoV-2 has a large structural similarity with the one present in SARS-CoV-1 (96.08%). Gurung et al. (2020) in silico analysis demonstrated that the terpenoids bonducellpin D and caesalmin B and the flavonoid 5,7-dimethoxyflavanone-4′-O-β-d-glucopyranoside have binding affinity with 3CLpro of SARS-CoV-1, SARS-CoV-2 and MERS-CoV ranging from −8 to −11 kcal mol−1, an outstanding value compared to repurposed drugs. Despite the terpenoids and flavonoids described by Gurung and coworkers are common of Chinese herbs (Caesalpinia minax) and an European mistletoe (Viscum album), the simultaneous inhibition of different strains of coronaviruses is promising to develop a chemotherapy against the virus family.
Khaerunnisa et al. (2020) evaluated, by molecular docking, natural metabolites capable of inhibiting the 3Cpro of SARS-CoV-2, emphasizing the prominent results of kaempferol, quercetin, luteolin-7-glucoside, demethoxycurcumin, naringenin, apigenin-7-glucoside, oleuropein, catechin, curcumin, and epigallocatechin. The anti-SARS-CoV-2 activity of these flavonoids is advantageous as they can be easily found and are well-distributed in angiosperm botanical families, including Lauraceae, Lamiaceae, Apiaceae, and Leguminosae.
In addition to the promising results observed to flavonoids, volatile terpenoids are also specialized metabolites that present some very interesting preliminary results that indicate a possible use of these substances. In this case, the already existing supply-chains from industries that produce essential oils increase the sustainability of this kind of prospection. The mono and sesquiterpenes geraniol, linalool, (E)-β-farnesene and (E)-nerolidol presented in silico inhibition of 3CLpro with binding energy of −24.71, −24.05, −27.56 and −26.44 kcal mol−1, respectively. These compounds can be found in several plants species with ancient and very well-known uses as foods, medicinal and aromatics, such as lemon balm (Melissa officinalis), lemongrass (Cymbopogon citratus), lavender (Lavandula angustifolia), geranium (Pelargonium graveolens), basil (Ocimum basilicum), mandarin (Citrus reshni), cinnamon (Cinnamomum zeylanicum), chamomile (Matricaria recutita), ginger (Zingiber officinale) and copaiba (Copaifera sp.).
As TMPRSS2 inhibitors, molecular docking for 3CLpro inhibitors also suggest algae as a potential source of anti-COVID-19 metabolites. Gentile et al. (2020) molecular docking evaluation of a marine drug metabolite database evidence that algae polyphenols, known as phlorotannins, and quercetin derivates can be applied in chemotherapy development against SARS-CoV-2. These compounds had already been isolated from Sargassum genus species.
RNA polymerase inhibitors
A new course in anti-SARS-CoV-2 chemotherapy development is through an extremely specific mechanisms to inhibit viruses replication, the RNA polymerase inhibitors. Metabolites with this property are expected to be less toxic then ACE2 or TMPRSS2 inhibitors, which binds to the host cell. Although RNA polymerase inhibitors are less toxic, their exploitation applied to coronaviruses treatment is still rare. Bibliography survey only suggests two substances which inhibits RNA translation of coronaviruses, the remdesivir and a synthetic 1,4-diazepane derivate (IC50 of 0.45 μM). Molecular docking also suggested four commercial drugs which efficacy still need to be proven.Despite its potential in virus treatment, natural inhibitors of SARS-CoV-2 RNA polymerase screened so far in essential oils had docking values lower than the commercial. As natural inhibitors of Dengue and Chikungunya viruses RNA polymerase had already been described within natural extracts,it is expected that there are also natural metabolites able to inhibit SARS-CoV-2 RNA translation still undiscover.
Conclusions
In the face of this great global challenge, we are striving for a COVID-19 treatment that can be quickly produced and easily distributed. Natural products could provide an answer to this dilemma, as they often have low toxicity and are used in the pharmaceutical industry for their bioactivity, including antiviral. The similarity between SARS-CoV-1 and COVID-19 also brings light to the development of new drugs or even vaccine. Great hope has emerged from the possible anti-SARS-CoV-2 activity of flavonols, flavanones, and flavanols and the fact that these metabolites have a large occurrence within angiosperm plants. As most of the present research is theoretical or does not present analytical validation, a long path is still ahead in terms of biological analysis and optimized extraction and production. The systematic evaluation presented here intend to reinforce this research effort
Reference & source information: https://pubs.rsc.org/
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