Small-molecule metabolites in SARS-CoV-2 treatment: a comprehensive review

Kinase inhibitors

So far, the in vitro cell-based investigations have suggested that the ABL kinase inhibitors suppress the replications of various other viruses at different phases of their lifecycles, including the Coxsackie, dengue, Ebola, and vaccinia viruses (Clark et al. 2016; Coyne and Bergelson 2006; García et al. 2012; Newsome et al. 2006; Reeves et al. 2011). In this line, when the Coxsackie virus adheres to the glycosylphosphatidylinositol (GPI)-anchored protein decay-accelerating factor (DAF) on the apical cell surface, the ABL kinase inhibitor is activated, which then induces the Rac-dependent actin reassembly, forming the virus-induced tight junctions (Coyne and Bergelson 2006). The ABL kinase inhibitors, imatinib and dasatinib, have been further found to reduce both the SARS-CoV and the MERS-CoV replications, while nilotinib can solely inhibit the SARS-CoV replication, in vitro (Dyall et al. 2014). The early phases of the life of the virus, as well as the viral replication, have also been inhibited by preventing the coronavirus virion fusion with the endosomal membrane, indicated by the mode of action of imatinib against the SARS-CoV and the MERS-CoV (Coleman et al. 2016; Sisk et al. 2018). To note, knocking down the ABL2, but not the ABL1, can greatly reduce the SARS-CoV and the MERS-CoV replication and entrance in vitro (Coleman et al. 2016). The fact that the SARS-CoV and the MERS-CoV need high doses, but not the toxic levels, of imatinib and dasatinib, may lie in the experimental factors, such as drug resistance in the cell lines, which boosts the virus propagation (Coleman et al. 2016; Dyall et al. 2014). Therefore, the optimal dosing requires in vivo testing to be effectively determined. Importantly, antiviral efficacy is cell-type-dependent in many cell-based experiments evaluating the effects of medicines on virus titer, and strain-dependent variability can be also observed in viruses. Imatinib, among 17 other Food and Drug Administration (FDA)-approved medicines, similar in terms of the half maximal inhibitory concentration (IC50) values for the SARS-CoV and the MERS-CoV, suppresses the SARS-CoV-2 in vitro, according to the recent preprints (BioRxiv 2020). Virus replication has been further linked to some SRC family of protein tyrosine kinases (SFKs), including those linked to the SARS-CoV-2 or other different viruses. At the μM range doses and the early initial lifecycle stages of the virus, the MERS-CoV is comparably shown to be suppressed by the ABL/SRC inhibitor saracatinib (Shin et al. 2018). It is implied that the SRC family proteins, LYN and FYN, are necessary for the MERS-CoV replication, since the suppression of their associated small interfering RNAs (siRNAs) has resulted in large reductions in the MERS-CoV titer (Shin et al. 2018). The FYN is also associated with the Coxsackie virus entry via the epithelial tight junctions (Coyne and Bergelson 2006). As well, saracatinib has been demonstrated to work in tandem with gemcitabine, a type of medicine that has an anti-MERS-CoV action (Shin et al. 2018). The SRC has been further proven to be critical for the dengue virus replication using the siRNA knockdown. The dengue infection has been additionally suppressed by dasatinib, which inhibits the virus replication complex to produce the infectious virus particles (Chu and Yang 2007; Kumar et al. 2016). Likewise, saracatinib and dasatinib have functioned in vitro against the dengue virus, targeting the RNA replication factor FYN (de Wispelaere et al. 2013). Through genetic knockdown, YES has shown to lower the West Nile virus titers by affecting the viral replication cycle, and then inhibiting the viral assembly and egress (Hirsch et al. 2005). At last, the importance of the c-terminal SRC kinase (Csk) has been indicated by the siRNA library screenings, aimed at finding the host components essential for the hepatitis C virus (HCV) and dengue replications.

Extracellular signals from the growth hormones and cytokines are both mediated and amplified by the transmembrane protein Janus-kinase (JAK) inhibitors, which prevent the JAK activation, and have further proven beneficial for the treatment of inflammatory disorders (Bertsias 2020). The JAK1 and JAK2 are also primarily inhibited by both baricitinib and ruxolitinib (Bertsias 2020). The JAK inhibitors have been further shown to reduce the high doses of cytokines, and thereby inflammation, in the severely infected SARS-CoV-2 cases (Gaspari et al. 2020). Excessive cytokine release is thus assumed as a key factor in disease development in off-label situations, wherein such inhibitors have shown to be effective (La Rosée et al. 2020). Although some small studies (Zhang et al. 2020) have supported the concept that the JAK inhibitors can effectively counteract the high levels of the cytokine production in the SARS-CoV-2 infection, their impact on a larger population has yet to be examined.

The cyclin-dependent kinases (CDKs) also play a leading role in the control and advancement of the cell division cycle (Meijer 2000). Viruses, usually the ones replicated via altering the CDK signaling system, in turn regulate the cell division (Meijer 2000). In line with the new findings, the SARS-CoV-2 makes changes in the CDK activity, and causes cell cycle arrest through enhancing the CDK2 phosphorylation. The CoV infectious bronchitis virus and other RNA viruses have been further reported with the similar effects (Bouhaddou et al. 2020). The host cell division cycle arrest also ensures the appropriate repair of nucleotide and deoxyribonucleic acid (DNA), as well as the sufficient amount of the DNA-replication proteins for the virus (Bouhaddou et al. 2020). Moreover, CDKs have been discovered to be essential for the DNA and RNA virus replication (Schang 2004). The multiplication of cytomegalovirus (CMV), herpes simplex virus (HSV), varicella-zoster virus (VZV), and some other DNA viruses have been further indicated to be hindered by the CDK inhibitors (CDKIs) (Meijer 2000). The R-roscovitine (CYC202) and flavopiridol (as CDKIs) also inhibit the viral replication of the RNA viruses, like human immunodeficiency virus 1 (HIV-1) (Pumfery et al. 2006). Indeed, the CDKIs function by targeting the host cellular proteins rather than the viral ones, to prevent viral replications (Schang et al. 2002). Considering the lack of specificity in the CDKIs, such inhibitors have a broader antiviral effect than the traditional ones (Schang 2004).

The CDKIs may also have a favorable effect on the COVID-19 management; however, further research is needed to verify this speculation. The CGP-604747, as a CDKI, is thought to play a role in the treatment of the COVID-19-induced lung injury (He and Garmire 2020). The CGP-604747 had thus lowered the expression of the indicator genes for the lung injury in the lung tissues of the COVID-19 patients, according to the RNA sequencing (He and Garmire 2020). Dinaciclib had also induced the potent antiviral activity in two cell lines against the SARS-CoV-2 (Bouhaddou et al. 2020). Nonetheless, randomized control trials (RCTs) are still required to approve the promising therapeutic effects of the CDKIs, as previously established in small studies. Worryingly, the CDKI-based medicines may impose cytotoxic outcomes due to their deregulatory impacts on the cell cycle. However, in the human cancer treatment trials, low molecular weight (LMW) CDKIs, like flavopiridol and roscovitine, have thus far exhibited little harm (Schang 2004). Neutropenia, corrected QT (QTc) interval prolongation, and hepatotoxicity are also among the few known side effects of the breast cancer treatment with the CDK 4/6 inhibitors, including palbociclib, ribociclib, and abemaciclib (Thill and Schmidt 2018). As a result, RCTs are needed to assess the possible short-term complications of the CDKI-based treatments for the COVID-19 patients.

The FDA-approved kinase inhibitors have been further analyzed in terms of their ability to target the associated proteins in the SARS-CoV, the MERS-CoV, and the similar virus infections, such as the ABL proteins, AAK1, CSK, FYN, GAK, CDK9, KIT, LYN, CDK6, EGFR, SRC, RET, AXL, and YES, using the KINOME scan biochemical kinase profiling data from the Harvard Medical School Library of Integrated Network-based Cellular Signatures (LINCS) (Rouillard et al. 2016) (Figure 2).

Figure 2:

Kinase inhibitors repurposed as antiviral treatments for their respiratory advantage.

Viral entry inhibitors

A number of proteases play critical roles in the SARS-CoV-2 infection and replication cycle in the human cells (Figure 3). The SARS-CoV-2, for example, is regulated by a vast array of host proteases during its binding and subsequent cell penetration (Millet and Whittaker 2015; Shang et al. 2020). The viral proteases, including the major protease (M^pro) and the PL^pro, accordingly can significantly contribute to the intracellular replication and maturation of the virus (Anand et al. 2003; Lin et al. 2020). Conducting mechanistic studies on proteases is therefore essential for the successful prevention or treatment of COVID-19, using newly designed antiviral medicines. While a considerable number of hurdles in the way of utilizing proteases as pharmaceutical targets are brought up by the wide diversity of proteases, such diversity can offer researchers multiple potential targets for developing new medicines (Figure 3).

Figure 3:

Host and viral proteases mediate SARS-CoV-2 infection and growth. Different host viral proteases, known to trigger some CoV S proteins, are depicted in the SARS-CoV-2 lifecycle. CoVs and other viruses also infect the cells by forming non-specific binds with heparan sulfate proteoglycans (HSPGs). Viral fusion can effectively take place at the plasma membrane, using the membrane proteases, like TMPRSS2 & 4, and perhaps furin. As well, endocytosis, which may only act as an auxiliary entry mechanism in the TMPRSS+ cells, bypasses the viral entrance, dependent on the cathepsin and pH levels. Viral activation may also involve extracellular proteases, such as elastin, typsin, and factor Xa protease. In the endoplasmic reticulum (ER), trans-Golgi network (TGN), and cytoplasm, the maturation of the viral proteins (viz., S, M, E, and N) is further mediated by the viral proteases, M^pro and PL^pro, which are then packaged into viral particles. In advance of the release of the mature viron, furin and perhaps the TMPRSS protease stimulate the S protein cleavage in the viron-associated ER and TGN systems. The unactivated virion (uncleaved, green), the semi-activated virion (S1/S2 cleaved, yellow), and the fully activated virion (S1/S2 and S1 cleaved, red) are thus the three models suggested for the SARS-CoV-2 particles. The ACE2⁺/furin⁺ and TMPRSS⁺ cells can also generate semiactive (yellow) and full-active (red) virions, thereby infecting the surrounding ACE2⁺ cells with or without the cell wall proteases. Spodoptera frugiperda furin (Sfurin) and cathepsin, for example, can stimulate the activation of the virus outside the cell, when secreted as the soluble proteases. This Figure with modifications taken from Tungadi et al. (2020).

The RBD interaction within the S1 region of the SARS-CoV-2 S glycoprotein and the human ACE2 receptor, latter of which also facilitating the SARS-CoV contact, can cause tissue tropism and enable the host cell adherence to the virus (Hoffmann et al. 2020; Wrapp and Wang 2020). The 20-fold greater binding affinity between the CoV-2 RBD and the ACE2 is similarly the cause of the greater infectivity of the SARS-CoV-2, compared to that of the SARS-CoV (Wrapp and Wang 2020). Following the receptor interaction, the transmembrane protease serine 2 (TMPRSS2) and/or the cathepsin B/L (CatB/L) cleave the S protein at the S1/S2 and S2 sites, allowing access to the host cellular cytoplasm (Hoffmann et al. 2020). The site S2 cleavage also results in the development of an antiparallel six-helix bundle (6-HB), which leads to the fusion process, and thereby the viral RNA uncoating and release into the cytoplasm (Bosch et al. 2003). The replicase gene is then translated into the viral RNA and polyproteins, pp1a and pp1ab, which produce non-structural proteins due to autocleavage (Nakagawa et al. 2016). For the replication of the viral genome, the replicase-transcriptase complex is formed by the non-structural proteins. In addition to the structural proteins (viz., S, E, M, and N), which form new virions at the final lifecycle stages of the virus, the genome also expresses the accessory proteins (Fehr and Perlman 2015), which induce viral pathogenicity, and suppress the host immune system (Zhao et al. 2012). Employing effector cells harboring the super folder green fluorescent protein (sGFP) fusion, the peptides generated from the HR2 regions of the SARS-CoV (Liu et al. 2004) and the MERS-CoV (Lu et al. 2014) have subsequently shown to suppress the intercellular fusion, mediated by the HCoV S. The IC50 value has been further estimated by the cell fusion assays to be around 0.6 µM for the MERS-CoV HR2P (SLTQINTTLLDLTYEMLSLQQVVKALNESYIDLKEL), while the HR2P-M1 mutant with two mutation and HR2P-M2 mutant with seven mutations, which have the potential of creating intramolecular salt-bridges, have proven more stable and water-soluble than their precursor HR2P peptide with a similar potency (Lu et al. 2014). In the cell fusion test, the CP-1 peptide (GINASVVNIQKEIDRLNEVAKNLNESLIDLQELGKYE) of the SARS-CoV HR2 domain has been further estimated to have an IC50 of 19 µM (Liu et al. 2004). In cell fusion investigations for the WIV1-, NL63-, OC43-, 229E-, Rs3367-, SARS-, MERS-, and SHC014-CoVs, the EK1 peptide displays wide-ranging suppression with the IC50 values of 0.19–0.62 µM (Xia and Yan 2019). The EK1 dose-dependently suppresses infection and the replication of the NL63-, 229E-, OC43- and MERS-CoVs when tested against live HCoV infections. Furthermore, EK1 given intranasally had protected the OC43 and MERS-CoV-infected mice, and boosted their survival rates (Xia and Yan 2019). Preliminary data have further shown that the EK1 peptide is effective against the SARS-CoV-2 S protein-mediated membrane fusion in a dose-dependent manner (Xia et al. 2020). The EK1 peptide has also shown to be dose-dependently effective against the PsV infection and membrane fusion induced by the SARS-CoV-2 S protein, according to the preliminary results.

As an anti-HIV protease inhibitor that induces apoptosis and necrosis, and inhibits the SARS-CoV replication, nelfinavir mesylate (or Viracept) has recently been described as an inhibitor of the SARS-CoV-2 S-mediated cell-cell fusion and multinucleated cell generation (Yamamoto et al. 2004). Since the virus can intercellularly transmit without entering into the extracellular areas or being exposed to neutralizing immune agents, the syncytia development is considered as a critical cytopathic event. Furthermore, an inflammatory response with the possible side effects on the host cells can be activated by the virus-induced cell fusion during the CoV infection. In immunofluorescence experiments, nelfinavir mesylate has been demonstrated by the immunofluorescence assessments to completely inhibit the cell-cell fusion mediated by the SARS-CoV-2 and the SARS-CoV S glycoproteins at the 10 μM dosage, without influencing the S cell surface expression. Although there is no experimental proof, nelfinavir mesylate may attach to the trimeric S, close to the fusogenic region, according to the computational research. The compound is thus suggested to have the potential for intervening the post-translational procedures or preventing the cellular proteases involved in the S-n or S-o fusion activation, based on the pleiotropic effect previously reported for nelfinavir mesylate on the ER stress; however, experimental findings are yet to be achieved to support these assumptions. The SARS-CoV-2 S protein priming, along with the SARS-CoV and other CoVs, has recently been demonstrated to be mediated by theTMPRSS2 (Glowacka et al. 2011; Hoffmann et al. 2020; Kawase et al. 2012; Matsuyama et al. 2010; Simmons et al. 2005). Also known as epitheliasin, the TMPRSS2 is a type II TTSP of 492 amino acids, located on the cell membrane, regulating intercellular and cell-matrix interactions. As of now, 17 members of the human TTSP family have been identified, sharing structural similarities. In the N-terminal intracellular domain, the stem region, which possess a binding site in the initial extracellular part for low-density lipoprotein (LDL) and calcium in a single scavenger receptor Cys-rich (SRCR) domain and an LDL receptor A motif, proceeds by the transmembrane domain and several phosphorylation sites. The catalytic triad His-Asp-Ser, which contains the Ser hydroxyl group, is further located in the C-terminal extracellular endoprotease domain. The Ser hydroxyl group also enhances the priming site nucleophilic attacks (Szabo and Bugge 2008). The prostate is the primary site of the TMPRSS2 expression, but it has been detected in the lungs, colon, and pancreas. The TMPRSS2 expression in the physiologically unknown regions of the upper airways, bronchi, and lungs, correspondingly suggests its vital role in the pneumotropic process of the SARS-CoV, the MERS-CoV, the SARS-CoV-2, the HCoV-NL63, and some other life-threatening viruses. In fact, the ACE2, an enzyme ubiquitously found in the body organs especially the lungs, mediates the CoV entrance into the host cells. Furthermore, despite the fact that the ACE2 is found in the pneumocytes, type I and II, the SARS-CoV has been shown to induce the type I pneumocyte infection at the early stages (Fukushi et al. 2005; Matsuyama et al. 2010). As well, the TMPRSS2 induces the CoV transmission and immunopathology, according to in vivo experiments (Iwata-Yoshikawa et al. 2019). As previously stated, the endosomal pathway mediates the SARS-CoV-2 entrance into the cells using CatB/L; therefore, blocking this pathway can potentially treat the SARS-CoV-2 infection as evidenced by prior research on the SARS-CoV-2 and similar CoVs (Hoffmann et al. 2020; Shirato et al. 2017, 2018; Simmons et al. 2005; Zhou et al. 2015). The CatB/L, a member of the papain structure clan, are human lysosomal cysteine proteases that are comprised of 11 cathepsin subclasses of L (L1), B, C, F, H, K, O, V (L2), X, S, and W. Unlike lysosomes (LYSs), which are responsible for the immune reactions against external agents and the recycling and degradation of biological agents, cathepsins are involved to a greater or lesser extent in some procedures, such as protein degradation, autophagy, cell death, and immune signaling, depending on the cell/tissue types (Reiser et al. 2010).

Nuclear factor erythroid 2-related factor 2 (Nrf-2) modulators

Known as a cap‘n’collar (CNC) basic-leucine-zipper transcription-factor family member, Nrf2 proposedly play a key role in the regulation of oxidative stress (OS) by upregulating glutathione and thioredoxin-antioxidant systems and some other anti-oxidant genes, along with the NAD (P) H quinine-oxide-reductase-1 (NQO1) and aldo-keto reductase (AKR). Moreover, the Nrf2 system suppresses inflammation, apoptosis, and oxidation (Al-Huseini et al. 2019; Al-Mudhaffer et al. 2019). The activating transcription factor (ATF) and cAMP-responsive element (CRE)-binding protein (CREB), Nrf-1, CREB/ATF AP-1 and -2, small MAFS, and nuclear factor/erythroid 2 (NFE-2) are also among other members of the CNC family. The Nrf2 is categorized with seven domains, namely the Nrf2-ECH-homology (Neh) domains of Neh-1 to Neh-7 (Figure 4) (Ahmed et al. 2017). The anti-protease secretion is thus boosted by the Nrf2 activators, such as the secretory leukocyte protease inhibitor (SLPI), which inhibits the serine protease activity. The Nrf2 activators can also bind to the promoter genes of the target cells, and then downregulate the TMPRSS2, thereby defending the target cells against viral infections (Schultz et al. 2014). As a result, these activators reduce the virus multiplication by impeding its entry into the respiratory ECs (Figure 4). In a 2005 study, Iizuka et al. had accordingly discovered that the SLPI had not been expressed in the Nrf2-knockout mice, causing the cells to be prone to inflammation by disrupting the protease/anti-protease balance, and that the SLPI gene had upregulated when the Nrf2 had been activated for inducing the anti-oxidant effect and preserving the balance between the proteases and anti-proteases (Iizuka et al. 2005). Likewise, Ling et al. (2012) had found that the gastrointestinal delivery of the epigallocatechin gallate (EGCG, as an Nrf2 activator), similar to that of seltamivir, had minimized the viral pneumonia entrance and replication in the lungs, thus increasing the survival rate.

Figure 4:

Structure of human Nrf-2 and its function. The Nrf2 activator inhibits the virus entrance and replication by over-expressing the anti-oxidant enzymes and the SLPI anti-protease protein, as well as down-expressing TMRPSS2.

Once the nuclear factor kappa B (NF-κB) interacts with the Nrf2 transcriptional-CREB-binding protein (CBP), the Nrf2 pathway induces ubiquitination and gene transcription to degrade the ikappaB kinase (IKK) and further limit the NF-κB activity. The activation of the Nrf2-dependent anti-oxidant genes further inhibits the production of inflammatory agents, including the TNF, IL-6, monocyte chemoattractant protein-1 (MCP-1), and macrophage inflammatory protein 2 (MIP-2) (Thimmulappa et al. 2006). The expression of the toll-like receptors (TLRs) is also regulated by the Nrf2, which downregulates the receptor production (Al-Mudhaffer et al. 2019). Li et al. had accordingly found that the TLR4 expression was higher in the Nrf2 knockout mice than in the wild type ones (Wang et al. 2018). Wang et al. (2018) had similarly conducted immunohistochemistry (IHC) to investigate the way the Nrf2 activator therapy had affected the number of the TLR-positive cells, in which the TLR-positive cells had decreased in number compared to those in the control group (Wang et al. 2018). As well, ticfedra, a fumarate ester of the dimethyl fumarate (DMF), has been recently approved by the FDA for treating multiple sclerosis (MS) and psoriasis (Fox 2012). The DMF binds to and activate the IKK at Cys-179, thereby inhibiting the NF-κB pathway and exerting its powerful anti-oxidant and anti-inflammatory effects. The NF-κB is therefore prevented from being released from its cytoplasmic complex, NF-κB-IB, which results in the suppression of the downstream pro-inflammatory signaling pathways (Gold et al. 2012) (Figure 5). Furthermore, it improves cellular response to OS by modulating the glutathione system (Dibbert et al. 2013). The Nrf2 activation and the NF-κB prevention also signify the molecular mechanism of action of the DMF, which exerts a vital anti-inflammatory effect. The Nrf2 further interacts with the Kelch-like ECH-associated protein 1 (Keap1) in the absence of the DMF, urging it to be ubiquitinated. Once the DMF disrupts the Keap1-Nrf2 complex at the Cys residues, the Nrf2 is released and translocated to the nucleus (Figure 5) (Gold et al. 2012).

Figure 5:

The mechanism(s) of action proposed for DMF are depicted in a simplified flow diagram.

Other SM metabolites

Nucleoside and non-nucleoside medicines are the two categories into which the ribonucleic acid (RNA)-dependent RNA polymerase (RdRp) inhibitors are divided. Drug resistance also limits the application of the non-nucleoside inhibitors, since they are unable to function on additional subtypes. The nucleoside medicines, on the other hand, can impose their direct effects on highly conserved active pockets while acting as the RdRp catalytic substrates. RDV (GS-5734), ribavirin/favipiravir, penciclovir, and ribavirin are currently the most effective nucleoside RdRp inhibitors for COVID-19 (Gordon and Tchesnokov 2020; Tchesnokov et al. 2019). RDV, known by some studies as Veklury, is further applied as a prodrug, and is converted into an active SM metabolite based on the metabolism inside the cell. This medicine is initially introduced into the body in the form of monophosphoramidate nucleoside (GS-5734) as a prodrug. After entering the cell, it finally turns into its active form, the active SM metabolite nucleoside triphosphate. This form is also called the RDV-triphosphate (GS-443902), whose mechanism is targeting the RNA polymerase SARS-CoV-2 (RdRp). In another expression, the GS-443902 acts as a substrate for the RdRp, competitively prevents the binding of the adenosine 5′-triphosphate (ATP) to the RdRp, and ultimately disrupts the viral RNA replication pathway (Malin et al. 2020). Meanwhile, the mechanism of the inhibition of the SARS-CoV-2 by favipiravir is still unknown. In general, favipiravir is used as a prodrug and is converted into the active form (favipiravir-ribofuranosyl-5′-triphosphate [RTP]) in the body. The active form inhibits the RNA replication by affecting the viral RdRp. In some studies, the stopping of the replication pathway by favipiravir has been attributed to the induction of the mutations in the virus genome. This theory has not been proven conclusively (Pavlova et al. 2021) (Figure 6).

Figure 6:

Mechanism of RDV and ribavirin/favipiravir in the SARS-CoV-2 treatment.

Gilead Sciences, Inc. also produced RDV (GS-5734), a new nucleotide analog antiviral prodrug, as a therapy for the infection caused by the Ebola virus and the Marburg virus (Agostini et al. 2018; Wise 2020). Given its anti-SARS-CoV-2 capacity in vitro, RDV, a negative regulator of the viral RNA-dependent RNA polymerase with previously approved antiviral action against the SARS-CoV-1 and the MERS-CoV in vitro, has been recently introduced as a viable therapy option for COVID-19 (Agostini et al. 2018; Brown et al. 2019; Sheahan et al. 2017, 2020; Wang et al. 2020). In this respect, RDV could effectively prevent the lung damage and viral infection in non-human primate samples once administered 12 h after the MERS-CoV injection (de Wit et al. 2013; de Wit et al. 2020).

Toyama Chemical Co., Ltd. further developed favipiravir by chemically modifying a pyrazine analogue in a pyrazine carboxamide derivative (6-fluoro-3-hydroxy-2-pyrazinecarboxamide) and screening a chemical library for its inhibitory action against the flu virus (Furuta et al. 2017). Fujifilm and MediVector additionally collaborated on the international development of favipiravir (Joshi et al. 2021), as a T-705 prodrug with 157.1 g/mol molecular weight. In 2014, it successfully helped to manage the unprecedented or reappearing pandemic influenza virus infections, thereby receiving medical approval in Japan (Hayden and Shindo 2019; Shiraki and Daikoku 2020). Already been approved for the treatment of a new Chinese influenza in February 2020, favipiravir is currently being examined on the Chinese population regarding its possible therapeutic effects on COVID-19 (Li and De Clercq 2020).

A wide array of influenza viruses, such as the avian flu viruses of A(H1N1)pdm09, A(H5N1), and A(H7N9) have been successfully treated with favipiravir. As well, arenaviruses, phleboviruses, hantaviruses, Western equine encephalitis virus, noroviruses, flaviviruses, and the Ebola virus are among the RNA viruses that may be inhibited by this prodrug (Furuta et al. 2013).

Favipiravir has further exhibited an EC50 value of 67 M in Vero E6 cells, indicating its significant anti-SARS-CoV-2 action in these cells. Moreover, this medicine has proven highly efficient in protecting mice from EBOV (Wang et al. 2020). The Chinese State Medicine Administration also authorized favipiravir as the China’s first anti-COVID-19 medicine in March 2020, citing its strong effectiveness and low rate of adverse effects in a clinical trial. In a prospective, controlled, randomized, open-label human trial held at multiple centers (ChiCTR2000030254), the higher serum uric acid levels were the most frequently seen favipiravir-linked side effect (Chen et al. 2020).

Ribavirin is also a guanosine derivative that prevents the RNA and DNA viruses from replications. Nevertheless, ribavirin not only interferes with polymerases, but also disrupts the RNA capping that inhibits the RNA breakdown using natural guanosine. Furthermore, ribavirin directly suppresses inosine monophosphate dehydrogenase, essential for guanine precursor transformation to guanosine, thereby inhibiting natural guanosine formation and promoting viral RNA instability (Graci and Cameron 2006). Little research on the CoV treatment during the Chinese and North American outbreaks of the SARS-CoV, as well as the MERS-CoV outbreaks in the Middle East and Asia have supported the clinical advantages of the ribavirin application; nonetheless, new studies are warranted to be done on the potential action of ribavirin against the novel CoV 2019 (nCOV2019). The first ribavirin application for treating the nCoV-related diseases in clinics is linked to its administration against the SARS-CoV, since the SARS-CoV symptoms are highly similar to those of acute respiratory syndrome, for which ribavirin has been typically prescribed (Lee et al. 2003; Peiris et al. 2003).