Natural products and their derivatives against coronavirus: A review of the non‐clinical and pre‐clinical data

Abstract

Several corona viral infections have created serious threats in the last couple of decades claiming the death of thousands of human beings. Recently, corona viral epidemic raised the issue of developing effective antiviral agents at the earliest to prevent further losses. Natural products have always played a crucial role in drug development process against various diseases, which resulted in screening of such agents to combat emergent mutants of corona virus. This review focuses on those natural compounds that showed promising results against corona viruses. Although inhibition of viral replication is often considered as a general mechanism for antiviral activity of most of the natural products, studies have shown that some natural products can interact with key viral proteins that are associated with virulence. In this context, some of the natural products have antiviral activity in the nanomolar concentration (e.g., lycorine, homoharringtonine, silvestrol, ouabain, tylophorine, and 7‐methoxycryptopleurine) and could be leads for further drug development on their own or as a template for drug design. In addition, a good number of natural products with anti‐corona virus activity are the major constituents of some common dietary supplements, which can be exploited to improve the immunity of the general population in certain epidemics.

1 INTRODUCTION

Coronaviruses (CoV) (Family: Coronaviridae) are enveloped viruses containing non‐segmented, positive‐stranded genomic RNA (Li, 2016). These viruses are pleomorphic particles with sizes ranging from 80 to 120 nm in diameter (Neuman et al., 2011). Their entire replication cycle takes place in the cytoplasm. Research findings indicated that the CoV envelope is involved in critical aspects of the viral life cycle, and that CoVs lacking CoV envelope make promising vaccine candidates (Schoeman & Fielding, 2019). CoVs are able to cause a number of diseases, including bronchitis, gastroenteritis, hepatitis, systemic diseases, and even death in birds, humans, and other animals (Chafekar & Fielding, 2018). Moreover, CoVs were found to be the causative agents of Middle East Respiratory Syndrome (MERS‐CoV) and Severe Acute Respiratory Syndrome (SARS‐CoV‐2). A novel coronavirus 2019 (nCoV‐19) has been newly identified in humans that caused thousands of death during January to March 2020.

Coronavirus could possibly infect animals as well as humans causing severe gastroenteritis and respiratory complications. Serologically, three identified strains of the virus have been reported to date. They are classified as per their genome sequence and the host range. Two strains HCoV‐229E and HCoV‐OC43 have been recognized in 1960 causing the well‐controlled common cold symptoms. The third life‐threating corona virus named SARS‐CoV can lead to lethal pneumonia. SARS‐CoV has been identified as the most lethal coronavirus till then as has been mentioned in an article published in February 2004 (van der Hoek et al., 2004). Researchers have also isolated another fourth viral strain named HCoV‐NL63 from a 6 months old child and identified its genomic sequence. Recently, a novel strain of lethal coronavirus struck China mainly in Wuhan provinces. It is a beta‐type coronavirus proposed with the name nCoV‐19 (SARS‐CoV‐2 by ICTV Coronaviridae Study Group). Infection began in 2nd of January 2020 by admission of around 40 Chinese patients to the hospital suffering from fever, fatigue, cough, and myalgia. All patients suffered lately from severe pneumonia and 30% were admitted to the intensive care unit (ICU) with severe acute respiratory syndrome, whereas 6 patients passed away (Huang et al., 2020). This novel strain of corona virus managed to spread over a wide geographic location within a very short period of time. As of February 28, 2020, the total number of confirmed nCoV‐19 infections worldwide is 83,652, and the number of deaths is over 3,000 (WHO, 2020).

Despite the failure in treating Ebola, the nucleoside inhibitor Gilead's NUC was effective in treating a nCoV‐19 patient in the United States, and thus suggested as a possible treatment option for this novel strain of corona virus. Unfortunately, this CoV is spreading with different RNA sequence and raised the question of its effectiveness against different CoV variants (Nguyen, Zhang, & Pandolfi, 2020). Another recommended line of treatment is remdesivir, an adenosine analogue, which was found to be effective in controlling Ebola virus as well as other RNA viruses. Preliminary data proved its effectiveness in controlling such emerged nCoV‐19. Similarly, chloroquine with its antimalarial, antiviral, as well as immunomodulatory effect has been found useful in inhibiting nCoV‐19 (Wang et al., 2020). Limited therapeutic options coupled with variable treatment outcomes render the nCoV‐19 a serious challenge for any country.

Natural products and their derivatives are used in folk medicine to treat numerous ailments including viral infections (Ganjhu et al., 2015). The scope of herbal medicines in the context of nutraceuticals market is vast (Williamson, Liu, & Izzo, 2020). Interestingly, the acceptability and, therefore, research on plant‐based drugs are growing on a daily basis. Along this line, Nigella sativa demonstrated its inhibitory activity against hepatitis C virus (Oyero et al., 2016). Some natural products have been found to exhibit their antiviral activity through the inhibition of viral replication (Moghadamtousi, Nikzad, Kadir, Abubakar, & Zandi, 2015; Oliveira et al., 2017). Apart from plant‐derived compounds (Jardim, Shimizu, Rahal, & Harris, 2018), several marine natural products (Wang, Zhang, Guan, & Wang, 2014) as well as biotechnologically produced compounds (Neumann & Neumann‐Staubitz, 2010) are also reported for their antiviral capacities against different viruses.

Nature provides a vast library of chemicals to explore and develop drugs for treatment of various ailments including viral diseases (Denaro et al., 2019). To date, a good number of herbal medicines or their constituents have shown potential antiviral activity (Lin, Hsu, & Lin, 2014). However, there is a lack of adequate research on the development of anti‐CoV agents from such natural products. Such agents are not only important to combat CoV, but also play an important role to prevent viral attack. On the basis of the preceding discussion, this review aims to sketch a current status of natural compounds (e.g., medicinal plants, microorganisms, and marine drugs) and/or their derivatives acting against different species of CoV.

2 REPLICATION OF CoVs

Coronaviruses (CoVs) enter into the host cell through interaction between the S protein of the virus species and the receptor of the host cell. Some species use the N‐terminus (e.g., SARS‐CoVs), while others use the C‐terminus of the S1 site of the receptor binding domains (RBD) (Cheng et al., 2004; Kubo, Yamada, & Taguchi, 1994). SARS and HCoV‐NL63 utilize angiotensin‐converting enzyme 2 (ACE2) as their receptor, whereas MERS‐CoV enters cells through CEA cell adhesion molecule 1 (CEACAM1) and dipeptidyl‐peptidase 4 (DPP4). Then, an acid‐dependent proteolytic cleavage of S protein occurs by a cathepsin, transmembrane protease, serine 2 (TMPRRS2), or another protease, followed by the fusion within the acidified endosomes or at the site of the plasma membrane in the host cell cytosol (Belouzard, Chu, & Whittaker, 2009). Cleavage at S2′ results in an antiparallel six‐helix bundle (Bosch, van der Zee, de Haan, & Rottier, 2003) and allows for the mixing of viral and cellular membranes that releases the viral genome into the cytosol.

A translation of the replicase gene occurs from the virion genomic RNA, which encodes two large open reading frames (ORFS), rep1a and rep1b that express two co‐terminal polyproteins, pp1a and pp1ab. For this, the CoV species utilizes a slippery sequence (5′‐UUUAAAC‐3′) and an RNA pseudoknot that cause ribosomal frameshifting from the rep1a reading frame into the rep1b ORF (Baranov et al., 2005). These polyproteins contain the nonstructural proteins (nsps) (Ziebuhr, Snijder, & Gorbalenya, 2000). CoVs encode either two or three proteases that cleave the replicase polyproteins. The papain‐like proteases (PLpro) encodes within nonstructural protein 3 (nsp3), while a serine‐type protease (Mpro) encodes within nsp5. In this context, SARS and MERS only express one PLpro (Mielech, Chen, Mesecar, & Baker, 2014). The nsps assemble into the replicase‐transcriptase complex (RTC) and create a suitable environment for viral RNA synthesis (Snijder et al., 2003). This process follows the translation and assembly of the viral replicase complexes (Sethna, Hofmann, & Brian, 1991).

CoVs have the ability to use both homologous and non‐homologous recombination (Keck et al., 1987; Lai et al., 1985) through the strand switching ability of the RNA‐dependent RNA polymerase (RdRp). Finally, the viral structural proteins, S, E, and M are translated and inserted into the endoplasmic reticulum (ER) (Krijnse‐Locker, Ericsson, Rottier, & Griffiths, 1994), and through viral genomes encapsidation by N protein form mature virions (de Haan & Rottier, 2005), whereas the M and E proteins produce the CoV envelopes (Bos, Luytjes, van der Meulen, Koerten, & Spaan, 1996). On the other hand, the N protein facilitates virus‐like particle (VLP) formation (Siu et al., 2008), and the M protein binds to the nucleocapsid and promotes the completion of virion assembly (Hurst et al., 2005). The virions are then transported to the cell surface in vesicles and released by exocytosis. The S protein, in some CoVs, does not get assembled into virions transits to the cell surface where it mediates cell–cell fusion between infected cells and the adjacent uninfected cells, leading to the formation of giant, multinucleated cells, which allow the virus to spread within an infected organism without being detected or neutralized by virus‐specific antibodies.

3 PATHOGENESIS OF CoVs

Previously, it was thought that CoVs only cause mild, self‐limiting respiratory infections in humans; however, SARS and MERS CoVs outbreak changed the thoughts (Woo et al., 2005). These CoVs are endemic in the human populations, causing 15–30% of respiratory tract infections each year. In this context, CoVs cause a more severe disease in the elderly than the neonates and adults with underlying illnesses, with a greater incidence of lower respiratory tract infection in these populations. On the other hand, HCoV‐NL63 is associated with acute laryngotracheitis (croup) (van der Hoek et al., 2005), whereas HCoV‐229E has minimal sequence divergence (Chibo & Birch, 2006). HCoV‐OC43 and nCoV‐19 show significant genetic variability (Chen et al., 2020; Peiris, Yuen, Osterhaus, & Stohr, 2003). In this regard, HCoV‐229E is evident to infect mice, while HCoV‐OC43 is capable of infecting mice and several ruminant species. Human CoVs have been reported to cause the development of multiple sclerosis (MS). In 2002–2003, SARS CoV was identified as the causative agent of the SARS outbreak in the Guangdong Province of China. Novel coronavirus disease 2019 (COVID‐19) has also been reported in December 2019 in Wuhan in the same country (Bogoch et al., 2020; Hui et al., 2020). The last has been defined as a pandemic outbreak, which already infected over 200,000 and caused more than 7,000 deaths worldwide. The nCoV‐19 has been found to attack all types of people, especially the elderly patients having diabetes, hypertension, cerebral infarction, chronic bronchitis, Parkinson's disease, chronic obstructive pulmonary disease, cardiovascular disease, and cancer (Deng & Peng, 2020; Guan et al., 2020; Huang et al., 2020).

To date, a number of animals have been speculated to be the reservoirs for SARS and MERS CoVs, but the animal reservoirs are yet to be confirmed (Kannan, Shaik Syed Ali, Sheeza, & Hemalatha, 2020). COVID‐19 outbreaks have been documented from 'wet markets' in South China from live wild animals. However, snakes as possible reservoir for nCoV‐19 are still controversial. In this respect, it has been reported that nCoV‐19 may be transmitted to humans via pangolin (Lam et al., 2020) or other wild animals (Lu et al., 2020; Zhang, Shen, Chen, & Lin, 2020) sold at the Huanan seafood market, where a number of animals and birds may be the reservoirs of this virus (Zhou et al., 2020). Along this line, phylogenetic analysis indicates that nCoV‐19 is similar to the CoVs circulating in Rhinolophus (horseshoe bats), with 98.7% nucleotide similarity to the partial RNA‐dependent RNA polymerase (RdRp) gene of the bat CoVs strain BtCoV/4991 (GenBank KP876546, 370 bp sequence of RdRp) and 87.9% nucleotide similarity to bat CoVs strain bat‐SL‐CoVZC45 and bat‐SL‐CoVZXC21 (Lai, Shih, Ko, Tang, & Hsueh, 2020). Betacoronaviruses are also evident in some veterinary species such as bovine, porcine, canine, equine, and camel (Malik et al., 2020).

Transmission of SARS‐CoVs was relatively inefficient, as they only spread through direct contact with infected individuals after the onset of illness, thus, the outbreak was largely contained within households and healthcare settings (Peiris et al., 2003). Generally, the SARS‐CoVs infect the lung epithelial cells in human and are also capable to enter into macrophages and dendritic cells (Spiegel, Schneider, Weber, Weidmann, & Hufert, 2006). Moreover, CoVs‐infected cells produce several pro‐inflammatory cytokines e.g., IL2, IL7, IL10, granulocyte‐colony stimulating factor (GCSF), interferon gamma‐induced protein 10 (IP10), monocyte chemoattractant protein 1 (MCP1), macrophage inflammatory protein (MIP)1α, and tumor necrosis factor alpha (TNFα) (Huang et al., 2020) that may contribute to inflammatory and immune suppressive diseases (Zhao, Zhao, Legge, & Perlman, 2011; Zhao, Zhao, & Perlman, 2010). In addition, SARS CoVs strains show varieties of clinical features in humans, including an age‐ and sex‐dependent increase in disease severity (Roberts et al., 2005). SARS CoVs are evident to use the same receptor as the human virus ACE2 receptor to establish infection.

On the other hand, the MERS CoVs were found to be the causative agents in a series of highly pathogenic respiratory tract infections in Saudi Arabia and other countries in the Middle East (Zaki, van Boheemen, Bestebroer, Osterhaus, & Fouchier, 2012) and showed a high mortality rate (∼50% in the early stages). MERS CoVs are highly related to two previously identified bat CoVs: HKU4 and HKU5 (van Boheemen et al., 2012). It is believed that the virus originated from bats, whereas camels have been considered as a neutral host for this virus replication (Eckerle et al., 2014). In both camels and human, MERS CoVs cases were reported in nearby proximities in Saudi Arabia (Azhar et al., 2014; Memish et al., 2014); this CoV utilizes dipeptidyl peptidase 4 (DPP4) as its receptor of certain species such as bats, humans, camels, rabbits, and horses to establish infection (Raj et al., 2013).

4 METHODOLOGY

A search was conducted in the following databases: PubMed, Science Direct, MedLine, Scopus, Web of Science, Google Scholar, and Clinicaltrials.gov for published articles. The keyword 'coronavirus' was paired with 'natural products', 'marine drugs', 'medicinal plants', 'phytochemicals', 'alkaloids', 'glycosides', 'flavonoids', 'saponins', 'terpenes', 'monoterpenes', 'diterpenes', 'sesquiterpenes', 'triterpenes', 'terpenoids', 'tannins', 'saponins', 'phenols', 'polyphenols', 'microbial metabolite', 'herbal drugs', 'crude extracts', or 'synthetic derivatives of natural products' to obtain published records till February 2020. No language restriction was imposed. Obtained records in this study were included and excluded on the basis of the following criteria.

Data inclusion criteria included: (a) Studies involving crude extract, fraction or their preparation of plants, microorganisms, or marine origins acting against CoV, (b) Studies related to derivatives of natural products (e.g., isolated compounds) and/or chemicals or biochemicals acting against CoV, and (c) Studies with natural product inspired synthetic derivatives acting against CoV. Exclusion criteria included: (a) Data duplication and titles or contents that do not meet the inclusion criteria, (b) Reports on antiviral activities of natural products or their derivatives against other than CoV, and (c) Studies that involve synthetic (conventional) chemicals (not from natural origin).

5 FINDINGS WITH DISCUSSION

Literature search in the abovementioned databases yielded reports, which upon refining through inclusion and exclusion criteria reduced the list to 36 articles. This review deals with the outcome of these works and explores the future direction of natural product research in the fight against CoV.

Traditional herbs from diverse geographical locations and various habitats are considered as potential sources of new drugs for treatment of viral infections, including those caused by SARS‐CoV. Some of early reports on the activity against CoV include concanavalin A (conA), a phytagglutinin found in jack beans (Canavalia ensiformis). ConA was responsible for the transient inactivity of hemagglutinating encephalomyelitis CoV, possibly through binding with glycosylated membrane proteins that help virus in host cell recognition (Greig & Bouillant, 1977). However, inhibition of the viral attachment and replication requires a minimum concentration of 10 μg/mL of conA. In a similar fashion, cobra venom renders complete loss of virulence, while bromelain only reduced the viability of the CoV isolated from pigs. However, the therapeutic utility of conA, a legume lectin with 25 kDa of molecular weight, is restricted by its severe hepatotoxicity in test animals (Mitchell, Ramessar, & O'Keefe, 2017).

The outbreak of SARS‐CoV led to catastrophic events, as there was no specific treatment known at that time. This led to worldwide search for agents that can act against SARS‐CoV as a precautionary measure to prevent future threats. In this context, some Chinese herbs have long been known for their antiviral effects and thus were investigated for a possible role against SARS‐CoV. Out of 200 herbal extracts tested, Lycoris radiata, Artemisia annua, Pyrrosia lingua, and Lindera aggregata exerted anti‐SARS‐CoV effect with 50% effective concentration (EC50) in the range of 2.4–88.2 μg/mL (Li et al., 2005). Bioassay‐guided chromatographic separation of Lycoris radiate, the most active extract, resulted in the isolation of lycorine, which inhibited SARS‐CoV with an EC50 value of 15.7 nM. In addition, the high selectivity index (> 900) of lycorine in Vero E6 and HepG2 cell lines makes it a good candidate for further drug development. Research published by Lau and coworkers showed that the aqueous extract of Houttuynia cordata inhibits two key proteins of SARS‐CoV, namely chymotrypsin‐like protease (3CLpro) and RdRp (Lau et al., 2008). The extract also increased CD4+ and CD8+ cell count in test animals suggesting its immune‐stimulatory effect that can be an additional advantage on top of its role in slowing down viral replication.

Similarly, the aqueous leaf extract of another Traditional Chinese Medicinal herb, Toona sinensis, inhibited SARS‐CoV replication with EC50 values ranging from 30 to 40 μg/mL and SI values ranging from 12 to 17 (Chen et al., 2008). In addition, extracts of Rheum officinale, Polygonum multiflorum, emodin, and some other major constituents of these plants were tested and were found to inhibit the interaction of SARS‐CoV (S) protein and ACE2 with IC50 values ranging between 1 and 10 μg/mL for extracts, and 200 μM for emodin (Ho, Wu, Chen, Li, & Hsiang, 2007). Note to mention here that the spike protein (S) of SARS‐CoV uses ACE2 as the functional receptor to infect host cells (Li, 2016).

Out of 200 extracts of more than 50 Chinese medicinal herbs tested, six plant extracts (Gentiana scabra, Dioscorea batatas, Cassia tora, Taxillus chinensis, Cibotium barometz) exhibited inhibitory activity against SARS‐CoV 3CLpro. In this regard, the methanol fractions of C. barometz and D. batatas showed significant inhibition of SARS‐CoV 3CLpro activity with IC50 values of 39 and 44 μg/mL, respectively (Wen et al., 2011). Similarly, Anthemis hyalina, Nigella sativa, and Citrus sinensis extracts decreased the replication of virus when HeLa‐CEACAM1a (HeLa‐epithelial carcinoembryonic antigen‐related cell adhesion molecule 1a) cell was infected with MHV‐A59 (mouse hepatitis virus–A59) CoV, with A. hyalina being the most active among the three plants tested (Ulasli et al., 2014). Although the TRP gene expression was lowered after the treatment with these extracts, an increase in the intracellular calcium level made it inconclusive to correlate this effect with the decreased viral replication.

Recent findings indicated that three alkaloids, tetrandrine, fangchinoline, and cepharanthine, significantly inhibit early stage viral‐induced cell death in HCoV‐OC43‐infected MRC‐5 human lung cells with IC50 values 0.33, 1.01, and 0.83 μM, respectively (Kim et al., 2019). Inspired by the antiviral activity of several natural products, 221 phytochemicals were tested against SARS‐CoV; ten diterpenes, two sesquiterpenes, two triterpenes, five lignans, and curcumin showed inhibitory activity in the range of 3–10 μM (Wen et al., 2007). Furthermore, the diterpenoid, 8b‐hydroxyabieta‐9(11),13‐dien‐12‐one and a lignin, savinin were found to inhibit SARS‐CoV 3CLpro activity with a SI > 667. On the other hand, betulinic acid (SI 180) and savinin were competitive inhibitors of SARS‐CoV 3CLpro with Ki values of 8.2 and 9.1 μM, respectively. Early work by Koehn and colleagues demonstrated that halituna, a diterpene aldehyde isolated from the marine alga Halimeda tuna, exhibits antiviral effect against murine coronavirus A59 (Koehn, Gunasekera, Neil, & Cross, 1991). Moreover, tanshinones (e.g., tanshinone I, rosmariquinone) derived from Salvia miltiorrhiza inhibited SARS‐CoV 3CLpro and PLpro infection and replication at 1–1000 μM (Park et al., 2012). Out of the seven compounds tested, most prominent IC50 values observed against 3CLpro and PLpro were for tanshinone I (38.7 and 8.8 μM, respectively) and dihydrotanshinone I (14.4 and 4.9 μM, respectively).

Tannic acid, 3‐isotheaflavin‐3‐gallate, and theaflavin‐3,3′‐digallate, three phenolic compounds from black tea exerted inhibitory effects on SARS‐CoV 3CLpro with IC50 values of 3, 7, and 9.5 μM, respectively (Chen et al., 2005). On the other hand, phenolic compounds from Isatis indigotica displayed an inhibitory effect on SARS‐CoV 3CLpro with IC50 values of 217, 752, 8.3, 365, and 1,210 μM for sinigrin, indigo, aloe emodin, hesperetin, and β‐sitosterol, respectively (Lin et al., 2005). Similarly, published work revealed that flavones and biflavones isolated from Torreya nucifera, also exhibit inhibitory effects on SARS‐CoV 3CLpro (Ryu et al., 2010). Furthermore, IC50 values of amentoflavone, apigenin, luteolin, and quercetin were 8.3, 280.8, 20.2, and 23.8 μM, respectively.

A paper published by Yu and coworkers showed that myricetin and scutellarein exert SARS‐CoV 3CLpro inhibitory effect at 0.01–10 μM (Yu et al., 2012). Furthermore, broussochalcone B, broussochalcone A, 4‐hydroxyisolonchocarpin, papyriflavonol A, 3′‐(3‐methylbut‐2‐enyl)‐3′,4,7‐trihydroxyflavane, kazinol A, kazinol B, broussoflavan A, kazinol F, and kazinol J isolated from Broussonetia papyrifera inhibited both SARS‐CoV 3CLpro and PLpro, where papyriflavonol A displayed the highest inhibition against PLpro with an IC50 value 3.7 μM (Park et al., 2017). Earlier work by other researchers revealed that four saikosaponins, namely saikosaponin A, B2, C, and D (5–25 μM/L) exhibit activity against human CoV‐229E, with EC50 values of 8.6, 1.7, 19.9, and 13.2 μM, respectively; saikosaponin B2 inhibited viral attachment and penetration stages (Cheng, Ng, Chiang, & Lin, 2006).

Hygromycin B derived from Streptomyces hygroscopicus reduced the replication of mouse hepatitis virus MHV‐A59 and necrotic liver foci in a dose‐dependent manner (Macintyre, Curry, Wong, & Anderson, 1991). In addition, actinomycin D, an antibiotic of the bacterium Streptomyces parvulus inhibited CoV attachment and penetration stages at 5–25 μM with an EC50 value of 0.02 μM (Cheng et al., 2006). Similarly, ginsenoside Rb1 (Gynosaponin C), one of the bioactive ginsenosides (class of steroid glycosides and triterpene saponins) obtained from Panax ginseng exhibited antiviral activity at 100 μM (Wu et al., 2004). Among the NIH clinical collection of 727 compounds tested for antiviral activity against both murine and human corona virus, the alkaloid macetaxine (homoharringtonine) was the most potent with an IC50 of ~11 nM (Cao, Forrest, & Zhang, 2015). On the other hand, tylophorine and 7‐methoxycryptopleurine, two alkaloids isolated from Tylophora indica inhibited N and S protein activity as well as viral replication of enteropathogenic coronavirus transmissible gastroenteritis virus (Yang et al., 2010). These compounds displayed remarkable antiviral activity with IC50 values of 0.018 and < 0.005 μM, respectively. Cepharanthine also inhibited SARS‐CoV protease enzyme at 0.5–10 μg/mL (Zhang et al., 2005). On the other hand, recent work by Shen et al. (2019) indicated that berbamine inhibits HCoV‐NL63 with an IC50 value 1.48 μM (Shen et al., 2019).

Recent findings showed that lycorine, emetine, and mycophenolate mofetil were seen to act against HCoV‐OC43, HCoV‐NL63, MERS‐CoV, and MHV‐A59 at 0–5 μM (Shen et al., 2019). In addition, lycorine and emetine inhibited cell division and inhibited RNA, DNA, and protein synthesis, respectively, whereas mycophenolate mofetil exerted an immune suppressing effect on the CoV species. On the other hand, eckol, 7‐phloroeckol, phlorofucofuroeckoln, and dieckol isolated from Ecklonia cava blocked the binding of virus to porcine epidemic cells at 1–200 μM with IC50 values of 22.5, 18.6, 12.2, and 14.6 μM, respectively (Kwon et al., 2013).

The natural compounds, procyanidin A2, procyanidin B1, and cinnamtannin B1, isolated from Cinnamomi cortex inhibited SARS‐CoV infection at 0–500 μM (Zhuang et al., 2009). On the other hand, tetra‐O‐galloyl‐beta‐D‐glucose, luteolin, and tetra‐O‐galloyl‐beta‐D‐glucose blocked the host cell entry of SARS‐CoV at 0–10−3 mol/L (Yi et al., 2004). In another study, bavachinin, neobavaisoflavone, isobavachalcone, 4'‐O‐methylbavachalcone, psoralidin, and corylifol isolated from Psoralea corylifolia inhibited papain‐like protease of SARS‐CoV (Kim et al., 2014). Interestingly, psoralidin exhibited a strong protease inhibitory effect on SARS‐CoV with an IC50 value 4.2 μM, whereas emodin, rhein, and chrysin inhibited interaction of SARS‐CoV (S) protein and ACE2 at 0–400 μM (Ho et al., 2007). Listed in Table 1 are crude extracts and Table 2 are isolated compounds that display activity against CoV.

TABLE 1. List of natural products inhibiting CoV

Extracts or preparations	Test system	Test dose/concentration	Proposed mechanism	IC50 or EC50 value	References
Lycoris radiata	SARS‐CoV	10−1–10−4 mg/mL	Undefined.	2.4 ± 0.2 μg/mL	Li et al., 2005
Artemisia annua	SARS‐CoV	10−1–10−4 mg/mL	Undefined.	34.5 ± 2.6 μg/mL	Li et al., 2005
Pyrrosia lingua	SARS‐CoV	10−1–10−4 mg/mL	Undefined.	43.2 ± 14.1 μg/mL	Li et al., 2005
Lindera aggregata	SARS‐CoV	10−1–10−4 mg/mL	Undefined.	88.2 ± 7.7 μg/mL	Li et al., 2005
Isatis indigotica	SARS‐CoV	1–500 μg/mL	3CL protease inhibition.	—	Lin et al., 2005
Extract (Rheum officinale Baill., Polygonum multiflorum Thunb.)	SARS‐CoV spike (S) protein.	0–100 μg/mL	Inhibits the interaction of SARS‐CoV S protein and ACE2.	1 to 10 μg/mL	Ho et al., 2007
Houttuynia cordata aq. Extract	SARS‐CoV	0–400 μg/mL	3CL protease and viral polymerase inhibition.	—	Lau et al., 2008
Herbal extracts (Gentiana scabra, Dioscorea batatas, Cassia tora, Taxillus chinensis, Cibotium barometz)	SARS‐CoV	25–200 μg/mL	3CL protease inhibition.	39 μg/mL and 44 μg/mL (two extracts of Cibotium barometz)	Wen et al., 2011
Anthemis hyalina, Nigella sativa, and Citrus sinensis extracts	Coronavirus‐infected HeLa‐epithelial carcinoembryonic antigen‐related cell adhesion molecule 1a cells inoculated with MHV‐A59 (mouse hepatitis virus–A59)	1/50 and 1/100 dilution of ethanolic extract (100 g/200 mL)	Increased IL‐8 level. Significantly changed the expression of TRPA1, TRPC4, TRPM6, TRPM7, TRPM8, and TRPV4 genes.	‐	Ulasli et al., 2014

TABLE 2. List of isolated compounds or their derivatives that inhibit CoV

Compounds (biological source)	Test system	Dose/concentration	Proposed mechanism	IC50 or EC50 value	References
Aloe emodin (Isatis indigotica)	SARS‐CoV	1–100 μg/mL	3CL protease inhibition.	8.3 μM	Lin et al., 2005
Amentoflavone (Torreya nucifera)	SARS‐CoV	1–1,000 μM	3CL protease inhibition.	8.3 μM	Ryu et al., 2010
Apigenin (Torreya nucifera)	SARS‐CoV	1–1,000 μM	3CL protease inhibition.	280.8 μM	Ryu et al., 2010
Bavachinin (Psoralea corylifolia)	SARS‐CoV	1–150 μM	Inhibitors of papain‐like protease (PLpro).	38.4 ± 2.4 μM	Kim et al., 2014
Berbamine	HCoV‐NL63	0–20 μM	Undefined.	1.48 μM	Shen et al., 2019
Beta‐sitosterol (Isatis indigotica)	SARS‐CoV	1–100 μg/mL	3CL protease inhibition.	1.210 μM	Lin et al., 2005
Betulonic acid	SARS‐CoV	0–10 μM	Inhibition of replication.	0.63 μM	Wen et al., 2007
Betulinic acid	SARS‐CoV	8–80 μM	Inhibition of 3CL protease.	10 μM	Wen et al., 2007
Betulonic acid	SARS‐CoV	8–80 μM	Inhibition of 3CL protease.	>100 μM	Wen et al., 2007
Broussochalcone A (Broussonetia papyrifera)	3‐chymotrypsin‐like and papain‐like coronavirus cysteine proteases	0–200 μM	Protease inhibition.	30.2 to 233.3 μM	Park et al., 2017
Broussochalcone B (Broussonetia papyrifera)	3‐chymotrypsin‐like and papain‐like coronavirus cysteine proteases	0–200 μM	Protease inhibition.	30.2 to 233.3 μM	Park et al., 2017
Broussoflavan A (Broussonetia papyrifera)	3‐chymotrypsin‐like and papain‐like coronavirus cysteine proteases	0–200 μM	Protease inhibition.	30.2 to 233.3 μM	Park et al., 2017
(−)‐Catechin gallate and (−)‐Gallocatechin gallate	SARS‐CoV	0.001–1 μg/mL	Inhibition of nanoparticle‐based RNA oligonucleotide.	0.05 μg/mL	Roh, 2012
Cepharanthine	SARS‐CoV	0.5–10 μg/mL	Protease inhibition	9.5 μg/mL	Zhang et al., 2005
Cepharanthine	HCoV‐OC43‐infected MRC‐5 human lung cells	2–20 μM	Undefined.	0.83 ± 0.07 μM	Kim et al., 2019
Cinanserin (1 dpi) (Houttuynia cordata)	Murine CoV	500–15.63 μg/mL	Undefined.	31.25 μg/mL	Chiow et al., 2016
Cinanserin (2 dpi) (Houttuynia cordata)	Murine CoV	15.63–500 μg/mL	Undefined.	62.50 μg/mL	Chiow et al., 2016
Cinnamtannin B1 (Cinnamomi cortex)	SARS‐CoV	0–500 μM	Inhibition of pseudovirus infection.	32.9 ± 3.9 μM	Zhuang et al., 2009
Chrysin (5,7‐dihydroxyflavone)	SARS‐CoV	0–400 μM	Inhibited interaction of SARS‐CoV (S) protein and ACE2.	200 μM	Ho et al., 2007
Concanavalin A	—	—	Lose the hemagglutination properties of the virus envelope and caused a transient interference with infectivity.	‐	Greig & Bouillant, 1977
Corylifol (Psoralea corylifolia)	SARS‐CoV	1–150 μM	Inhibitors of papain‐like protease (PLpro).	32.3 ± 3.2 μM	Kim et al., 2014
Curcumin	SARS‐CoV	8–80 μM	Inhibition of 3CL protease.	40 μM	Wen et al., 2007
Dieckol (Ecklonia cava)	Porcine epidemic diarrhea CoV	1–200 μM	Inhibition of viral replication.	14.6 ± 1.3 μM	Kwon et al., 2013
Diplacone (Paulownia tomentosa)	SARS‐CoV	0–100 μM	Inhibition of papain‐like protease.	10.4 ± 0.16 μM	Cho et al., 2013
3β,12‐diacetoxyabieta‐6,8,11,13‐tetraene	SARS‐CoV	0–10 μM	Inhibition of replication.	1.57 μM	Wen et al., 2007
1‐(4,5‐Dihydroxy‐3‐hydroxymethylcyclopenten‐2‐enyl)‐1H‐1,2,4‐triazole‐3‐carboxylic acid amide	SARS‐CoV	—	Undefined.	21 μM	Cho et al., 2006
1‐(4,5‐Dihydroxy‐3‐hydroxymethylcyclopenten‐2‐enyl)‐1Himidazole‐4‐carboxylic acid amide	SARS‐CoV	—	Undefined.	>100 μM	Cho et al., 2006
1‐(4,5‐Dihydroxy‐3‐hydroxymethylcyclopenten‐2‐enyl)‐1H‐1,2,3triazole‐4‐carboxylic acid amide	SARS‐CoV	—	Undefined.	47 μM	Cho et al., 2006
Eckol (Ecklonia cava)	Porcine epidemic diarrhea CoV	1–200 μM	Blockage of the binding of virus to cells.	22.5 ± 2.2 μM	Kwon et al., 2013
Emetine	HCoV‐OC43, HCoV‐NL63, MERS‐CoV, and MHV‐A59	0–5 μM	Inhibited RNA, DNA and protein synthesis.	0.30, 1.43, 0.34, and 0.12 μM, respectively.	Shen et al., 2019
Emodin (1,3,8‐trihydroxy‐6‐methylanthraquinone)	SARS‐CoV	0–400 μM	Inhibited interaction of SARS‐CoV (S) protein and ACE2.	200 μM	Ho et al., 2007
Fangchinoline	HCoV‐OC43‐infected MRC‐5 human lung cells	2–20 μM	Undefined.	1.01 ± 0.07 μM	Kim et al., 2019
Ferruginol	SARS‐CoV	0–10 μM	Inhibition of replication.	1.39 μM	Wen et al., 2007
6‐geranyl‐4′,5,7‐trihydroxy‐3′,5′‐dimethoxyflavanone (Paulownia tomentosa)	SARS‐CoV	0–100 μM	Inhibition of papain‐like protease.	13.9 ± 0.18 μM	Cho et al., 2013
Halituna (Halimeda tuna)	Murine coronavirus A59	—	Undefined.	‐	Koehn et al., 1991
Hesperetin (Isatis indigotica)	SARS‐CoV	1–100 μg/mL	3CL protease inhibition.	365 μM	Lin et al., 2005
Hexachlorophene	Murine CoV (MHV‐2aFLS).	0–10 μM	Undefined.	1.2 μM	Cao et al., 2015
Hinokinin	SARS‐CoV	8–80 μM	Inhibition of 3CL protease.	>100 μM	Wen et al., 2007
Homoharringtonine	Murine CoV (MHV‐2aFLS).	0–70 nM	Undefined.	12 nM	Cao et al., 2015
4‐Hydroxyisolonchocarpin (Broussonetia papyrifera)	3‐chymotrypsin‐like and papain‐like coronavirus cysteine proteases	0–200 μM	Protease inhibition.	30.2 to 233.3 μM	Park et al., 2017
Hygromycin B (Streptomyces hygroscopicus)	Mouse hepatitis virus (MHV‐A59)	0–1 μM/kg	Reduced virus replication and necrotic liver foci.	‐	Macintyre et al., 1991
8β‐hydroxyabieta‐9(11),13‐dien‐12‐one	SARS‐CoV	0–10 μM	Inhibition of replication.	1.47 μM	Wen et al., 2007
Indigo (Isatis indigotica)	SARS‐CoV	1–100 μg/mL	3CL protease inhibition.	752 μM	Lin et al., 2005
Isobavachalcone (Psoralea corylifolia)	SARS‐CoV	1–150 μM	Inhibitors of papain‐like protease (PLpro).	18.3 ± 1.1 μM	Kim et al., 2014
3‐Isotheaflavin‐3‐gallate (black tea)	SARS‐CoV	4–20 μM	Inhibition of 3C‐like protease (3CLPro).	7 μM	Chen et al., 2005
Juglanin	SARS‐CoV	10–40 μM	Blocks the 3a channel.	2.3 μM	Schwarz et al., 2014
Kazinol A (Broussonetia papyrifera)	3‐chymotrypsin‐like and papain‐like coronavirus cysteine proteases	0–200 μM	Protease inhibition.	30.2 to 233.3 μM	Park et al., 2017
Kazinol B (Broussonetia papyrifera)	3‐chymotrypsin‐like and papain‐like coronavirus cysteine proteases	0–200 μM	Protease inhibition.	30.2 to 233.3 μM	Park et al., 2017
Kazinol F (Broussonetia papyrifera)	3‐chymotrypsin‐like and papain‐like coronavirus cysteine proteases	0–200 μM	Protease inhibition.	30.2 to 233.3 μM	Park et al., 2017
Kazinol J (Broussonetia papyrifera)	3‐chymotrypsin‐like and papain‐like coronavirus cysteine proteases	0–200 μM	Protease inhibition.	30.2 to 233.3 μM	Park et al., 2017
Luteolin	SARS‐CoV	0–10−3 mol/L	Blocking the viral entry.	10.6 μM	Yi et al., 2004
Luteolin	HIV‐luc/SARS pseudo type virus	0–10−3 mol/L	Blocking the viral entry.	9.02 μM	Yi et al., 2004
Luteolin (Torreya nucifera)	SARS‐CoV	1–1,000 μM	3CL protease inhibition.	20.2 μM	Ryu et al., 2010
Lycorine (Lycoris radiata)	SARS‐CoV	10−1–10−4 mg/mL	Undefined.	15.7 ± 1.2 nM	Li et al., 2005
Lycorine	HCoV‐OC43, HCoV‐NL63, MERS‐CoV, and MHV‐A59	0–5 μM	Inhibited cell division	0.15, 0.47, 1.63, and 0.31 μM, respectively.	Shen et al., 2019
Mimulone (Paulownia tomentosa)	SARS‐CoV	0–100 μM	Inhibition of papain‐like protease.	14.4 ± 0.27 μM	Cho et al., 2013
Mycophenolatemofeti	HCoV‐OC43, HCoV‐NL63, MERS‐CoV, and MHV‐A59	0–5 μM	Immune suppression.	1.58, 0.23, 1.54, and 0.27 μM, respectively.	Shen et al., 2019
Myricetin	SARS‐CoV	0.01–10 μM	3CL protease inhibition.	‐	Yu et al., 2012
3′‐O‐methyldiplacol (Paulownia tomentosa)	SARS‐CoV	0–100 μM	Inhibition of papain‐like protease.	9.5 ± 0.10 μM	Cho et al., 2013
4′‐O‐methyldiplacol (Paulownia tomentosa)	SARS‐CoV	0–100 μM	Inhibition of papain‐like protease.	9.2 ± 0.13 μM	Cho et al., 2013
3′‐O‐methyldiplacone (Paulownia tomentosa)	SARS‐CoV	0–100 μM	Inhibition of papain‐like protease.	13.2 ± 0.14 μM	Cho et al., 2013
4′‐O‐methyldiplacone (Paulownia tomentosa)	SARS‐CoV	0–100 μM	Inhibition of papain‐like protease.	12.7 ± 0.19 μM	Cho et al., 2013
4'‐O‐methylbavachalcone (Psoralea corylifolia)	SARS‐CoV	1–150 μM	Inhibitors of papain‐like protease (PLpro).	10.1 ± 1.2 μM	Kim et al., 2014
7‐Methoxycryptopleurine (Tylophora indica)	CoV‐infected swine testicular cells.	—	Inhibition of viral replication	20 ± 1 nM	Yang et al., 2010
3′‐(3‐methylbut‐2‐enyl)‐3′,4,7‐trihydroxyflavane (Broussonetia papyrifera)	3‐chymotrypsin‐like and papain‐like coronavirus cysteine proteases	0–200 μM	Protease inhibition.	‐	Park et al., 2017
7‐Methoxycryptopleurine (Tylophora indica)	SARS‐CoV	—	Protease inhibition	<0.005 μM	Yang et al., 2010
Neobavaisoflavone (Psoralea corylifolia)	SARS‐CoV	1–150 μM	Inhibitors of papain‐like protease (PLpro).	18.3 ± 1.1 μM	Kim et al., 2014
Niclosamide	SARS‐CoV	8–80 μM	Inhibition of 3CL protease.	40 μM	Wen et al., 2007
Ouabain	Transmissible gastroenteritis CoV	0–3,000 nM	Diminished both the viral titers and viral yields. Reduction of the number of viral RNA copies.	143 ± 13 nM	Yang et al., 2018
Papyriflavonol A (Broussonetia papyrifera)	3‐chymotrypsin‐like and papain‐like coronavirus cysteine proteases	0–200 μM	Protease inhibition.	3.7 μM	Park et al., 2017
Phenazopyridine	HCoV‐OC43, HCoV‐NL63, MERS‐CoV, and MHV‐A59	0–5 μM	Undefined.	1.90, 2.02, 1.93, and 0.77 μM, respectively.	Shen et al., 2019
Phlorofucofuroeckoln (Ecklonia cava)	Porcine epidemic diarrhea CoV	1–200 μM	Inhibition of viral replication.	12.2 ± 2.8 μM	Kwon et al., 2013
Procyanidin A2 (Cinnamomi cortex)	SARS‐CoV	0–500 μM	Inhibition of pseudovirus infection.	29.9 ± 3.3 μM	Zhuang et al., 2009
Procyanidin B1 (Cinnamomi cortex)	SARS‐CoV	0–500 μM	Inhibition of pseudovirus infection.	41.3 ± 3.4 μM	Zhuang et al., 2009
7‐Phloroeckol (Ecklonia cava)	Porcine epidemic diarrhea CoV	1–200 μM	Blockage of the binding of virus to cells.	18.6 ± 2.3 μM	Kwon et al., 2013
Psoralidin (Psoralea corylifolia)	SARS‐CoV	1–150 μM	Inhibitors of papain‐like protease (PLpro).	4.2 ± 1.0 μM	Kim et al., 2014
Quercetin (Torreya nucifera)	SARS‐CoV	1–1,000 μM	3CL protease inhibition.	23.8 μM	Ryu et al., 2010
Quercetin (Houttuynia cordata)	Murine CoV	500–15.63 μg/mL	Undefined.	125.00 μg/mL	Chiow et al., 2016
Quercetrin (Houttuynia cordata)	Murine CoV	500–15.63 μg/mL	Undefined.	‐	Chiow et al., 2016
Rosmariquinone (Salvia miltiorrhiza)	SARS‐CoV	1–1,000 μM	Inhibition of SARS‐CoV viral infection and replication.	88.0 ± 1.1 μM	Park et al., 2012
Rhein (1,8‐dihydroxy‐3‐carboxyl‐9,10‐anthraquinone)	SARS‐CoV	0–400 μM	Inhibited interaction of SARS‐CoV (S) protein and ACE2.	200 μM	Ho et al., 2007
Rutin (Houttuynia cordata)	Murine CoV	500–15.63 μg/mL	Undefined.	‐	Chiow et al., 2016
Saikosaponins A	HCoV‐22E9	5–25 μM/L	Undefined.	8.6 ± 0.3 μM/L	Cheng et al., 2006
Saikosaponins B2	HCoV‐22E9	5–25 μM/L	Inhibited viral attachment and penetration stages.	1.7 ± 0.1 μM/L	Cheng et al., 2006
Saikosaponins C	HCoV‐22E9	5–25 μM/L	Undefined.	19.9 ± 0.1 μM/L	Cheng et al., 2006
Saikosaponins D	HCoV‐22E9	5–25 μM/L	Undefined.	13.2 ± 0.3 μM/L	Cheng et al., 2006
Savinin	SARS‐CoV	8–80 μM	Inhibition of 3CL protease.	25 μM	Wen et al., 2007
Scutellarein	SARS‐CoV	0.01–10 μM	3CL protease inhibition.	0.86 ± 0.48 μM	Yu et al., 2012
Silvestrol	HCoV‐229E	0.6–2 μM	Inhibition of cap‐dependent viral mRNA translation.	40 nM	Muller et al., 2018
Sinigrin (Isatis indigotica)	SARS‐CoV	1–100 μg/mL	3CL protease inhibition.	217 μM	Lin et al., 2005
Tannic acid (black tea)	SARS‐CoV	4–20 μM	Inhibition of 3C‐like protease (3CLPro).	3 μM	Chen et al., 2005
Tanshinone I (Salvia miltiorrhiza)	SARS‐CoV	1–1,000 μM	Inhibition of SARS‐CoV viral infection and replication.	0.7 μM	Park et al., 2012
Tetrandrine	HCoV‐OC43‐infected MRC‐5 human lung cells	2–20 μM	Undefined.	0.33 ± 0.03 μM	Kim et al., 2019
Tetra‐O‐galloyl‐beta‐D‐glucose (TGG)	SARS‐CoV	0–10−3 mol/L	Blocking the viral entry.	4.5 μM	Yi et al., 2004
Tetra‐O‐galloyl‐beta‐D‐glucose (TGG)	HIV‐luc/SARS pseudo type virus	0–10−3 Mol/L	Blocking the viral entry.	2.86 μM	Yi et al., 2004
Theaflavin‐3,3′‐digallate (black tea)	SARS‐CoV	4–20 μM	Inhibition of 3C‐like protease (3CLPro).	9.5 μM	Chen et al., 2005
Tomentin A (Paulownia tomentosa)	SARS‐CoV	0–100 μM	Inhibition of papain‐like protease.	6.2 ± 0.04 μM	Cho et al., 2013
Tomentin B (Paulownia tomentosa)	SARS‐CoV	0–100 μM	Inhibition of papain‐like protease.	6.1 ± 0.02 μM	Cho et al., 2013
Tomentin C (Paulownia tomentosa)	SARS‐CoV	0–100 μM	Inhibition of papain‐like protease.	11.6 ± 0.13 μM	Cho et al., 2013
Tomentin D (Paulownia tomentosa)	SARS‐CoV	0–100 μM	Inhibition of papain‐like protease.	12.5 ± 0.22 μM	Cho et al., 2013
Tomentin E (Paulownia tomentosa)	SARS‐CoV	0–100 μM	Inhibition of papain‐like protease.	5.0 ± 0.06 μM	Cho et al., 2013
Tylophorine (Tylophora indica)	SARS‐CoV	—	Protease inhibition	0.018 μM	Yang et al., 2010
Tylophorine (Tylophora indica)	CoV‐infected swine testicular cells.	—	Inhibition of viral replication	58 ± 4 nM	Yang et al., 2010
Tylophorine	CoV	0–1,000 nM	Targeting viral RNA replication and cellular JAK2 mediated dominant NF‐κB activation.	‐	Yang et al., 2017

Research by Schwarz et al. (2014) revealed that juglanin blocks the 3a channel of SARS‐CoV with an IC50 value of 2.3 μM. In addition, a number of compounds, such as tomentin A, B, C, D, and E, 3′‐O‐methyldiplacol, 4′‐O‐methyldiplacol, 3′‐O‐methyldiplacone, 4′‐O‐methyldiplacone, mimulone, diplacone, and 6‐geranyl‐4′,5,7‐trihydroxy‐3′,5′‐dimethoxyflavanone isolated from Paulownia tomentosa were found to inhibit PLpro of SARS‐CoV at 0–100 μM (Cho et al., 2013). In a similar fashion, (−)‐catechin gallate and (−)‐gallocatechin gallate at 0.001–1 μg/mL inhibited nanoparticle‐based RNA oligonucleotide of SARS‐CoV (Roh, 2012). On the other hand, quercetin, quercetrin, rutin, cinanserin (1 and 2 dpi) isolated from Houttuynia cordata were found to act against murine CoV at 15.63–500 μg/mL (Chiow, Phoon, Putti, Tan, & Chow, 2016).

Sivestrol, a natural compound isolated from the fruits and twigs of Aglaia foveolata, has been recognized to exhibit very potent in vitro cytotoxic activity against several human cancer cell lines (Kim et al., 2007). In addition, this compound at 0.6–2 μM inhibited cap‐dependent viral mRNA translation of HCoV‐229E with an IC50 of 40 nM (Müller et al., 2018). Similarly, ferruginol, a natural phenol that contains a terpenoid substructure, isolated from the needles of the redwood Sequoia sempervirens, exhibited antitumor properties against human colon, breast, and lung cancers (Son, Oh, Choi, Han, & Kwon, 2005). Furthermore, ferruginol, 8β‐hydroxyabieta‐9(11),13‐dien‐12‐one, 3β,12‐diacetoxyabieta‐6,8,11,13‐tetraene, betulonic acid, betulinic acid, hinokinin, savinin, and curcumin significantly inhibited SARS‐CoV replication at 0–80 μM (Wen et al., 2007). On the other hand, ouabain diminished both the viral titers and viral yields and reduced the number of viral RNA copies at 0–3000 nM (Yang, Chang, Lee, Hsu, & Lee, 2018). In a similar fashion, the laboratory derivatives, 1‐(4,5‐dihydroxy‐3‐hydroxymethylcyclopenten‐2‐enyl)‐1H‐1,2,4‐triazole‐3‐carboxylic acid amide, 1‐(4,5‐dihydroxy‐3‐hydroxymethylcyclopenten‐2‐enyl)‐1Himidazole‐4‐carboxylic acid amide, and 1‐(4,5‐dihydroxy‐3‐hydroxymethylcyclopenten‐2‐enyl)‐1H‐1,2,3‐triazole‐4‐carboxylic acid amide displayed antiviral activity against SARS‐CoV (Cho, Bernard, Sidwell, Kern, & Chu, 2006).

Tylophorine and 7‐methoxycryptopleurine isolated from Tylophora indica were evident to inhibit viral replication in CoV‐infected swine testicular cells (Yang et al., 2010). In this study, 7‐methoxycryptopleurine (IC50: 20 nM) was more effective than the tylophorine (IC50: 58 nM). In another study, tylophorine was also found to target viral RNA replication and cellular JAK2‐mediated dominant NF‐κB activation in CoV at 0–1000 nM (Yang et al., 2017). Chemical structures of some important natural products acting against CoV are shown in Figure 1.

[Image Omitted. See PDF.]

6 LIMITATIONS: THE 'CONS'

In this review, the 'Pros' of certain natural products in the fight against the Corona virus different strains have been emphasized; therefore, the 'Cons' should be mentioned as well. It is well‐known that natural extracts are multicomplex metabolites, which are mainly beneficial for their producers, and it is a real challenge to specify the effective one(s) for humans. Another 'Cons' is dealing with biological reproducibility of natural extracts or defined compounds. Entitling a manuscript dealing with a phytopharmacological study as a 'piece of art' should fulfill important guidelines, which are recently mentioned in a paper by Michael and his coworkers (Michael et al., 2020). In this article, the authors summarized the best practice of an ethnopharmacological study based on editors' consensus statements of scientific journals interested in natural products such as Phytotherapy research, Fitoterapia, Journal of Ethnopharmacology, Phytomedicine, and Planta medica. Since most applied hypotheses of any herbal pharmacological study are based on a traditional use 'folk medicine' in different cultures, yet there should be criteria followed. The guidelines are from a chemistry and pharmacology point of views. Chemistry: Identifying authenticity of plant source and its correct taxonomy and its major components being biomarkers. Drugability of herbal extracts 'solubility and stability'. Pharmacology: Implementation of in‐silico or in vitro studies prior to in vivo ones are crucial. A true hypothesis for an evidence‐based experiment with the selection of correct positive and negative controls is necessary for appropriate evaluation of results for the reproducibility and significance of achieving results. Ethical prospective toward animals should be followed as listed in WHO guidelines (Erhirhie, Ekene, & Ajaghaku, 2014). Submitting pharmacological studies with negative results should be considered as well as the positive ones.

An important concept should be implemented 'standardization of an herbal extract' as previously mentioned being a complex of natural components. Numerous reviews and previous studies mentioned diversity of standardization techniques of herbal extracts using different chromatographic tools 'GC–MS, LC–MS/MS', and NMR to ensure the reproducibility of quality and so the biological efficacy. Several researches tentatively identified major constituents in different plant extracts either crude or in preparations (Farag & Wessjohann, 2012), whereas WHO set rules for herbal standardization as a quality control measures for safety, efficacy, and toxicity (Shulammithi, Sharanya, Tejaswini, & Kiranmai, 2016). The rise of standardization could be referred to as multiple of factors that could alter the constituent's quality or quantity in an herbal extract, and thus the results of a biological study. Factors listed previously viz. Genetic and ecological conditions, seasonal variations, genotype, ecotype, chemotype factors, harvest timing, drying and storage, environmental conditions, and manufacturing processes (Kumari & Kotecha, 2016).

Scientifically, implementation of in silico methods 'computer‐based programs' counting on the chemical structure of the molecule as 'docking studies' as well as in vitro tests could be time saving for irrelevant in vivo models. It is worth mentioning that positive results of in silico and in vitro tests should be encountered as primary step, which should be followed by in vivo models. As has previously been mentioned, the in vivo ethno pharmacological studies should always follow specific guidelines to be reproducible and relevant. Still there are limiting factors of using each study 'in‐silico, in vitro, and in vivo'. In a previous work (Benfenati, Gini, Hoffmann, & Luttik, 2010), the authors worked on the problems and prospects of those studies. A 'QIVIVE' term: Quantitative in vitro to in vivo extrapolation means switching from in vitro results to an in vivo dose model through a link 'living cell'. A recent article published by Hartung (Hartung, 2018) listed tips how to do a QIVIVE from working cell models. Another rate limiting factor is the high micromolar concentrations of antiviral natural compounds, which is difficult to formulate in proper dosages. Although Yang, Ha, and Oh (2016) screened the efficacy of isolated natural triterpenes against porcine epidemic diarrhea virus (PEDV), family Coronaviridae, the in vitro tests revealed their effectivness against PEDV in comparison to 6‐azauridine drug, where the recorded EC50 was with a submicromolar range.

Herbal extracts could be refered to as 'functional foods' or 'nutraceuticals': a term rises from both nutrional and pharmaceutical point of view. This is due to the fact that food and herbal constituents are encountered as medicine for the treatment of different ailments with probability of occurance of side effcets. Folk medicine is a well known throughout centuries in variant cultures viz. Egyptian Pharonic, Chinese, and Indian, thus it is important for those nutraceuticals to be adminstered properly. Clinical trials on herbal extracts or naturally isolated ingredients should be well implemented following the in vivo models to ensure the efficacy and safety of a natural product. In a previous article by Williams and colleagues (Williamson et al., 2020), authors highlighted the high sales of newly added herbal products to U.S. markets encompassing their clinical profiling, an idea which should be followed for every plant derived extract or isolated entity. Along this line, an intersting review on approches for treatments and prevention of respiratory viral attacks by a variety of antiviral drugs, including a sector of natural products was recently published (Papadopoulos et al., 2017). Authors of this review stated that about 80% of 46 ethnoextracts have been officially registered. Similarly, the Chinese Academy of Medical Sciences (CAMS) tested more than 10,000 botanicals for their curing efficacy since many natural extracts or ingredients could boost the immune system facing the viral attacks as in Echnicea species.

7 CONCLUSIONS

Development of antiviral drugs is a challenge as enzymes do not act like a typical living cell. Thus, some of the antiviral drugs can only prevent virus replication or inhibit further infection. In this respect, this review has highlighted the significance of some natural products to block the virulence of CoV through their inhibitory action against viral proteins including 3CLpro, PLpro, S, and ACE2, in addition to the inhibitory effect against viral replication or virulence. The crude extract from Lycoris radiata and a combination extract obtained from Rheum officinale Baill. and Polygonum multiflorum Thunb. were among the ones that exert strong effects against SARS‐CoV (IC50 value range: 1 to 10 μg/mL). In addition, compounds such as lycorine, homoharringtonine, silvestrol, ouabain, tylophorine, and 7‐methoxycryptopleurine showed strong inhibitory effects against different species of CoV with IC50 values ranging from 12 to 143 nM. In summary, we have shown through documented research that natural products and medicinal plants offer preventive and therapeutic options against viral infections. These natural compounds can be an important complementary medicine in the fight against viruses, owing to their natural origin, safety, and low cost compared to synthetic drugs. Thus natural products hold a great promise for drug development against CoV and require greater attention to the agents that have already been shown to exhibit potent activity against various strains of CoV.

CONFLICT OF INTEREST

The authors declare no potential conflict of interest.

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