Coronaviruses: Volume 2 E-Book
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- Herausgeber: Bentham Science Publishers
- Kategorie: Fachliteratur
- Serie: Coronaviruses
- Sprache: Englisch
- Veröffentlichungsjahr: 2006
Scientific literature on SARS-COV-2 viruses and its variants (especially variants of concern such as the ‘Delta variant’) and important cellular targets is crucial to help researchers, virologists and clinicians around the globe to develop a new generation of safer and more effective vaccines, and other treatments to address COVID-19 disease. The accompanying damage to the many organs and tissues of SARS-Co-2-infected people also needs to be understood and researchers are using data to devise meaningful protocols for treating these symptoms. This second volume of Coronaviruses brings together more useful information about the prevention/vaccination, and chemotherapies for the potential treatment of coronavirus infections. The volume includes eight chapters: (1) Broad spectrum antivirals to combat COVID-19 The reality and challenges, (2) COVID-19: Preventive and protective control management strategies, (3) Plant-derived extracts and bioactive compounds against coronavirus progression: preventive effects, mechanistic aspects, and structures, (4) Gastroenteritis: symptoms and epidemiology of SARS-CoV-2, (5) The chronicles of coronavirus: A Chinese king who conquered the entire world, (6) Traditional medicine as a natural remedy in ARDS & COVID-19, (7) Molecular pathogenesis of human coronaviruses of the 21st century, (8) COVID-19, mental health and neuropathophysiology of pain related to temporomandibular disorder. The volume serves as a novel compilation of key data on SARS-CoV-2 and COVID-19 and represents a resource of the utmost value for all scholars studying SARS-CoV infections. It should also be of great interest to clinicians who may be facing an overwhelming number of individuals affected with COVID-19, with over 267 million global cases documented as of the first week of December 2021).
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PREFACE
In this still pandemic period of SARS-CoV-2 (and its variants, especially ‘Delta’ variant) infection that is responsible for Covid-19, the scientific literature on these viruses and related cellular targets is crucial to help the researchers/virologists and clinicians from all countries to develop a new generation of safer and more effective vaccines, as well as treatments to cure the more or less severe diseases and damages to the many organs and tissues of SARS-CoV- 2-infected people. This second book on coronaviruses mainly brings together some useful information regarding the prevention/vaccination, and chemotherapies to the potential treatment of coronavirus infections. The collection combines eight chapters titled: (1) Broad-spectrum antivirals to combat covid-19: The reality and challenges (chapter 1), (2) Covid-19: Preventive and protective control management strategies (chapter 2), (3) Plant-derived extracts and bioactive compounds against coronavirus progression: preventive effects, mechanistic aspects, and structures (chapter 3), (4) Gastroenteritis: symptoms and epidemiology of SARS-CoV-2 (chapter 4), (5) The chronicles of coronavirus: A Chinese king who conquered the entire world (chapter 5), (6) Traditional medicine as a natural remedy in ARDS & Covid-19 (chapter 6), (7) Molecular pathogenesis of human coronaviruses of 21st century (chapter 7), (8) COVID-19, mental health and neuropathophysiology of pain related to temporomandibular disorder (chapter 8).
Such a novel book compiling key data on SARS-CoV-2 and Covid-19 actually represents a tool of the utmost value for all researchers working on these research fields. It should also be of great interest to clinicians who are facing an overgrowing number of individuals with Covid-19. The data from 20th October 2021 give ca. 242 million cases of SARS-CoV-2 infection worldwide, with over 4.9 million deaths.
List of Contributors
Broad-Spectrum Antivirals to Combat COVID-19: the Reality and Challenges
Wafaa A. Hewedy1,*Abstract
Viral infections, which lack effective treatment, have posed an ongoing threat to human health. Most approved antiviral agents selectively target a single virus, providing a “one drug-one bug” solution. However, this approach has limited efficacy, particularly with emerging and re-emerging viruses with no specific, licensed antiviral drug or vaccine.
Since the outbreak of the COVID-19 pandemic, tremendous studies have focused on the effect of some (broad-spectrum) antiviral agents on this emerging virus. The concept of broad-spectrum antivirals refers to the group of drugs with the capability of combating more than one virus rather than “one drug-one bug” agents. This approach may offer a new horizon for the management of emerging viral threats.
Among BSAs, nucleotide and nucleoside analogs target enzymatic functions shared by some viruses, thus, inhibit their replication. An alternative approach of BSA agents is to target host factors commonly required by multiple viral pathogens, on which the viruses intimately rely. For example, anti-malarial agents (chloroquine and hydroxychloroquine) inhibit acidification of endosomes, an essential process for uncoating of some RNA viruses, kinase inhibitors impair intracellular viral trafficking, and statins attenuate replication of some enveloped viruses.
In this review, we will shed light on BSA agents with potential efficacy against SARS-CoV-2 infection. The time-consuming process of new drug development makes repositioning drugs, already approved for use in humans, the only solution to the epidemic of sudden infectious diseases as COVID-19.
INTRODUCTION
In the past two decades, humanity has been exposed to several successive epidemics including the emergence of the severe acute respiratory syndrome (SARS-CoV-1) in 2003, Middle East respiratory syndrome (MERS) in 2013, Ebola virus disease in 2014, and currently, the Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) outbreak that represents an unprecedented public health challenge imposing great impact on nearly all countries around the world. The constant threat with the emergence of new strains of viruses has necessitated the search for novel effective therapeutic options.
Despite high species diversity, viruses share key elements that are essential for the design of therapeutic targets [1]. Although targeting specific viral factors via a “one drug, one bug” approach demonstrated measurable success in treating some viral infections such as influenza virus and hepatitis C virus, this approach is expensive, time-consuming, and more importantly, is associated with the rapid emergence of drug resistance [2]. Consequently, the concept of broad-spectrum antiviral drugs has been emphasized rather than developing drugs that are targeted to every specific virus. In this context, targeting common enzymes or proteins crucial in the life cycle of viruses or targeting host factors exploited by multiple viruses could provide broad-spectrum coverage for treating emerging viral infections [3, 4].
SARS-CoV-2, a member of the coronavirus family, has a very similar genome sequence identity with the SARS virus, and to a lesser extent with the MERS virus [5]. Moreover, the pathological features of those devastating virus infections are substantially similar. Hence, drugs that have been used previously to treat SARS or MERS may have the potential in treating patients with SARS-CoV-2 [6]. Coronaviruses (CoVs) undergo a distinct replication cycle, involving virion entry, RNA genome replication and transcription of viral mRNAs, protein translation, virion assembly, and packaging in the host cell, after which viral particles are released [7].
Coronaviruses specify two groups of (druggable) proteins; structural proteins and non-structural proteins. The structural proteins are functionally conserved among very closely related viruses. They include Spike (S), Membrane (M), Envelope (E), and Nucleocapsid (N) [8]. These proteins perform important functions in the viral life cycle: S is the main determinant of cell tropism, host range, and viral entry; E facilitates viral assembly and release, and has viroporin activity; M maintains the membrane structure of the virion, and N encapsidates the viral RNA genome [9]. Non-structural proteins are more conserved among CoVs and they are involved in essential functions of the viral lifecycle, such as 3C-like protease (3CLpro; nsp3), papain-like Protease (PLpro; nsp5), and RNA-dependent RNA polymerase (RdRp; nsp12) [10]. These proteins are critical to the viral life cycle and provide potential targets for drug therapies.
On the other hand, several host factors have been identified to regulate signaling proteins crucial for the replication of viruses. Targeting host cell factors provides a different strategy against viral infections especially for those for which no effective treatment exists yet. An advantage of this strategy is that host factors do not undergo the same mutation rate that is seen for genomes of viruses. Furthermore, it may provide additive and possibly synergistic effects in combination with other strategies being developed to combat emerging viral infections.
Here, we will review the antiviral drug with broad-spectrum activity and its relevance in the treatment of coronavirus as per available data of clinical studies.
DRUGS TARGETING ENTRY PROCESS
Coronaviruses go through a staged entry process involving virion binding, receptor-mediated endocytosis, intracellular trafficking of virions to endosomes, and protease-dependent cleavage of spike (S) protein to facilitate fusion of the virion membrane to the endosomal membrane and deposition of the nucleocapsid into the cytoplasm [11, 12].
The membrane fusion relies on the expression of fusogenic glycoproteins on infected cell surfaces. Both viral envelope proteins and host cellular proteins are crucial for this process, hence providing potential antiviral therapeutic targets.
Viral Envelope Proteins
The spike (S) glycoprotein, a key immunogenic CoV antigen, is essential for virus-host cell receptor interactions. It exists in trimeric forms, giving them their characteristic corona structures [13]. The S protein is embedded in viral envelopes and mediates a crucial role in the entry of viral particles into the host cell [14]. It includes two functionally distinct subunits; S1 and S2 subunits, which process by cellular proteases to remain activated. The S1 subunit of SARS and also SARS-CoV-2 contains a receptor-binding domain (RBD) that binds to angiotensin-converting enzyme 2 (ACE2) receptors to mediate viral entry [15, 7]. The S2 domain is responsible for the fusion of the viral envelope with the cell membrane through its putative fusion peptide region. It has been reported that the RBD domain of SARS CoV-2 Spike (S-new; Sn) has a higher binding affinity for the ACE2 receptor than that of SARS Spike (S-old; So), while the S2 proteins of these two viruses are nearly 90% identical [16, 17]. Recent studies owed the greater ability of SARS CoV-2 to spread from cell-to-cell, with avoidance of extracellular neutralizing antibodies, to the efficient expression and fusion of the SARS CoV-2 S glycoprotein in comparison to limited cell fusion caused by the SARS S glycoprotein [18].
Nelfinavir
Nelfinavir mesylate (Viracept, formally AG1343) was approved by the FDA in1997 as a potent inhibitor of the HIV-1 protease [19]. It is commonly prescribed in combination with other antiretroviral medications as part of the highly active antiretroviral (HAART) regimen [20]. Being a member of protease inhibitors, nelfinavir prevents maturation of the viral particles through inhibition of the HIV aspartyl protease (retropepsin), which is the viral enzyme responsible for cleavage of the viral polyprotein into several essential enzymes (RT, protease, and integrase) and several structural proteins [21].
In addition to its antiretroviral activity, nelfinavir has been shown to block replication of other viruses such as vesicular stomatitis virus and influenza virus by blocking the viral envelope fusion at the endosomes [22]. On the other hand, nelfinavir is found to block human herpes virus replication at late stages of virus maturation with no effect on the herpes simplex serine protease [23, 24].
Regarding COVs, nelfinavir was found to strongly inhibit replication of SARS-CoV, most probably through its effect on the post-entry step of SARS-CoV replication [25]. Recently, it was confirmed by computational model data that nelfinavir can bind SARS-CoV-2 main protease (Mpro) at the Glu166 position inhibiting viral entry into the cell [26]. Nelfinavir was suggested to bind inside the S trimer structure and directly inhibit S-n- and S-o-mediated membrane fusion. These results stimulate further investigations on the potential of nelfinavir mesylate to inhibit virus spread at early times of infection [25, 26].
Generally, nelfinavir is a well-tolerated drug with mild diarrhea as the most common side effect. Similar to other protease inhibitors, dyslipidemia, insulin resistance, and diabetes are major concerns. Nelfinavir is not recommended for patients with moderate to severe hepatic impairment, and should be given with great caution with major CYP3A4 substrates and/or inducers.
Arbidol
Arbidol (umifenovir) is an antiviral compound approved in Russia and China for prophylaxis and treatment of human influenza A and B infections. Next, arbidol was shown to be active against a diverse array of DNA/RNA, enveloped/non-enveloped viruses such as Zika virus, respiratory syncytial virus, adenovirus, Coxsackie B5, parainfluenza, Ebola and hepatitis B and C viruses, and SARS-CoV [27-29].
The broad-spectrum antiviral activity of arbidol suggests that it acts on common critical step(s) of virus-cell interactions. Specifically, it has been shown that arbidol inhibits the membrane fusion between virus particles and plasma membranes or the membranes of endosomes [30].
Recently, Wang et al. [31] evaluated six currently available anti-influenza drugs (arbidol, baloxavir, laninamivir, oseltamivir, peramivir, and zanamivir) against SARS-CoV-2 on Vero E6 (ATCC-1586) cells. Among them, only arbidol efficiently inhibited SARS-CoV-2 infection. The data revealed that arbidol impeded both viral entry and post-entry stages.
The therapeutic efficiency of arbidol (200 mg, 3 times a day for 4–8 days) was evaluated in relatively mild COVID-19 patients from a shelter hospital in China [28]. Data showed that arbidol could accelerate fever recovery, viral clearance in respiratory specimens, and decrease the duration of hospital stay especially if given to male patients, at an early stage of infection. A multicenter randomized phase 4 trial was initiated in China to evaluate the efficacy and safety of arbidol in COVID-19 patients (Table 1).
Host Cell Proteins
Several cellular proteins are utilized by many viruses as an essential part of their life cycle. Targeting those proteins is a widely accepted approach to inhibit the replication of many virus species whilst viral escape by mutation is less likely.
Regarding COVs, cellular proteases play critical roles in processing viral S glycoproteins at the cell surface during viral entry. They facilitate a critical step in the virus infectivity where they promote the cleavage of S1-S2 subunits to expose S2 for fusion to the cell membrane. The most important proteases include cell surface transmembrane serine proteases (TMPRSS), furin, cathepsins, trypsin, and factor Xa [32, 33].
TMPRSS2 Inhibitors
The host cell surface transmembrane serine protease type 2 (TMPRSS2) is a serine protease located on the host cell membrane that facilitates entry of highly pathogenic human coronaviruses (SARS-CoV and MERS-CoV) into the host cell [34]. It has been implicated in priming S2′ cleavage as well as ACE2 cleavage leading to initiation of the membrane fusion and subsequent cellular invasion [32, 35]. Moreover, cleavage of S protein by TMPRSS2 is preferred for entry of human coronaviruses than endosomal cathepsins [36]. Considering the central role of TMPRSS2 in activating S protein, drugs with inhibitory activity of TMPRSS2 have been investigated as a potential therapy against SARS-CoV-2.
Camostatmesylate (CM) is a synthetic serine protease inhibitor that was developed in the 1980s as a standard inhibitor of TMPRSS2. It has been approved since 1985 in Japan for the treatment of chronic pancreatitis with an acceptable safety profile [37]. Kawase et al. [34] proved that camostat was able to partially block SARS-CoV infection by human coronavirus NL63 (HCoV-NL63) in HeLa cells expressing ACE2 and TMPRSS2. More recently, Hoffmann et al. [33] showed that camostat mesylate can reduce the probability of SARS-CoV-2 penetration in cell experiments in vitro through inhibition of the proteolytic activity of TMPRSS2. Interestingly, the expression of TMPRSS2 seems to be androgen-dependent which may partially explain why the incidence and severity of COVID-19 and other TMPRSS2-dependent viral infections are higher in men than in women [38].
Based on preclinical data, German guidelines mentioned the compassionate use of camostat as a treatment option for COVID-19 [39], and around 14 clinical trials were registered to delineate the efficacy of camostat (alone or in combination) in COVID-19 patients. A randomized, phase II/III multi-center, prospective, open-label, community-based clinical trial enrolled 389 non-hospitalized COVID-19 patients aims to determine if camostat can reduce the clinical progression of COVID-19 and therefore the need for hospital admission and supplemental oxygen. Another phase III trial aims to determine the therapeutic effect and tolerance of Camostat mesylate, compared to placebo in adult patients with ambulatory COVID-19 disease but presenting with risk factors of severe COVID-19.
Furin Inhibitors
Furin is a member of proprotein convertases that directly and specifically cleave viral envelope glycoproteins of a broad range of viruses. It is distributed in various organs with little difference in expression level. Several peptidic and non-peptidic furin inhibitors had been tested to block the infection with enveloped viruses such as HIV, avian influenza viruses, hepatitis B virus, flaviviruses, and coronaviruses [40-42].
The newly emerged SARS-CoV-2 S was demonstrated to harbor a furin cleavage site at the S1/S2 boundary that affects the viral life cycle and pathogenicity [41]. More recently, Wu et al. [43] speculated that the high infectivity of SARS-CoV-2 may be attributed to the redundant furin cleavage site in its Spike protein. Further, they tested various compounds that inhibit furin enzyme activity as drug candidates for the treatment of COVID-19 and found that diminazene, an anti-parasitic drug, has the strongest inhibitory activity with an IC50 of 5.42 ± 0.11 μM. These results might open a new avenue for the treatment of COVID-19. Further in vitro and in vivo experiments are needed to verify both efficiency and safety of these agents.
DRUGS TARGETING VIRAL REPLICATION
The RdRp is one of the key targets for antiviral drug development. Since RdRp is highly conserved at the amino acid level in the active site among different positive-sense RNA viruses, including coronaviruses, drugs that target RdRp are supposed to have broad-spectrum activity against a wide range of CoVs and other viruses [44]. This RNA polymerase is highly error-prone [45], and therefore has the ability to accept modified nucleotide analogs as substrates. Nucleotide and nucleoside analogs that inhibit polymerases comprise an important group of antiviral agents [44, 46]. Nucleoside analogs have been initially developed for the treatment of cancer via targeting cellular DNA/RNA polymerases. The therapeutic applications of nucleoside analogs have been expanded by the observation that they target RNA-dependent RNA polymerases (RdRp), structurally conserved enzymes that play a key role in the replication of a broad range of viruses. Currently, nucleoside analogues represent an important class of antiviral agents that have proven efficacious against many serious and life-threatening viruses [47]. They have been proposed as a treatment for COVID-19 on the basis of in vitro activity, preclinical studies, and observational studies.
Nucleoside analogs are transported into the cells and phosphorylated by the consecutive action of viral or cellular kinases, eventually generating nucleotide triphosphates. The antiviral activity of nucleoside analogs is based on their direct action on viral polymerization where they directly incorporated into the growing viral genome during polymerization, resulting in the termination of chain reaction or the accumulation of mutations [48]. Other mechanisms have been also proposed for the antiviral effect of nucleoside analogs. Since viral replication is highly dependent on the availability of host nucleotides, it has been reported that nucleoside analogs interfere with host nucleos(t)ide synthesis pathways, causing depletion or imbalance of (d)NTP pools [49, 50].
Depending on their mechanism of action, nucleoside analogs fall into the following three classes: (pseudo-) obligate chain terminators (directly block the progression of the polymerase as a result of their lack of the reactive 3'-hydroxyl group, e.g. zidovudine, azidothymidine), delayed chain terminators (block transcription despite still possessing the 3′-hydroxyl group), or mutagenic nucleosides (target the viral reliance on an RdRp to catalyze the replication of the RNA genome from the original RNA template) [51].
1) Ribavirin
Ribavirin (1-b-D-ribofuranosyl-1H-1,2,4-triazole-3-carboxamide) is a water-soluble guanosine nucleoside analog that was synthesized in 1970. It possessed an antiviral activity against both DNA and RNA viruses including RSV infection, influenza, and parainfluenzaviruses, and hepatitis viruses. Additionally, several studies demonstrated that SARS-CoV, MERS-CoV, and HCoV-OC43 were sensitive to ribavirin in vitro [52, 54].
The exact mechanism of action of ribavirin is still debated. Once transported into the host cell, ribavirin is actively phosphorylated to RBV 5′-monophosphate (a rate-limiting step) and 5′-triphosphate by cellular kinases [55]. The triphosphate form is thought to confer most of the antiviral activity [56] predominantly via inhibition of cellular inosine5′-monophosphate dehydrogenase (IMPDH) activity, a key enzyme of the purine biosynthesis pathway, which results in depletion of intracellular guanosine-5’-triphosphate (GTP). Ribavirin is reported to be a mutagenic nucleoside where it inhibits viral replication after mispairing with the template base resulting in lethal mutagenesis of the RNA genome [57]. It also interferes with mRNA capping that relies on natural guanosine to prevent RNA degradation [58]. Moreover, ribavirin causes upregulation of IFN-stimulating genes, thereby potentiating the effect of IFN-α in combination treatment [59].
Ribavirin can be given orally (with an absolute bioavailability of 40% to 50% on account of the first-pass metabolism) or as an aerosolized inhalation for the treatment of RSV in infants [60]. It has an extensive volume of distribution due to its distribution into cellular compartments [61]. It is known to be concentrated into eukaryotic cells by two distinct sets of nucleoside transporters. Single nucleotide polymorphisms in these transporters may dictate its effectiveness and also its toxicity [62].
The major dose-limiting toxicity observed with ribavirin is hemolytic anemia [63]. The mechanism is reported to be due to the lack of cellular phosphatases in RBCs with an accumulation of phosphorylated forms of ribavirin whereas other nucleated cells actively dephosphorylate ribavirin triphosphate by 5′-nucleotidase and alkaline phosphatase allowing ribavirin to be exported back into the extracellular compartment. Accumulation of ribavirin triphosphate depletes intracellular ATP concentrations and damages the oxidative stress defense mechanism, resulting in damage of the cell membrane and lysis of the erythrocyte [63]. There are significant teratogenic and/or embryocidal effects in animal species exposed to ribavirin. Consequently, ribavirin is contraindicated during pregnancy and extreme care must be taken to avoid pregnancy in both males and females during therapy and for 6 months after completion of treatment.
Based on broad-spectrum antiviral activity, ribavirin has a well-established history of usage in emergency clinical management plans for emerging coronaviruses. Ribavirin has been given as part of treatment regimens during the SARS-CoVand MERS-CoVoutbreaks. However, following about 2 decades of challenges in the evaluation of ribavirin activity in patients with SARS-CoV or MERS-CoV outbreaks, no conclusive results of efficacy could be established in such conditions.
Regarding COVID-19, Elfiky et al. [64] targeted the RNA-dependent RNA polymerase (RdRp) of the newly emerged coronavirus using different anti-polymerase drugs including ribavirin. They found that ribavirin, among others, showed promising results where they were able to bind to the new coronavirus strain RdRp tightly, and they encouraged the use of a combined antiviral therapy against COVID-19. However, the challenges in the evaluation of ribavirin efficacy during SARS and MERS outbreaks led to a summary evaluation of its utility as controversial in the treatment of COVID-19 patients. A recent meta-analysis of case studies reported that ribavirin has only limited efficacy for the treatment of patients with highly pathogenic coronavirus respiratory infections if it is administered early upon presentation with pneumonia and before sepsis or organ system failure [65, 66].
One of the reasons that may explain this discrepancy is the difference between the in vitro and in vivo doses, where the former exceeded ribavirin concentrations attainable by typical human regimens. Higher doses of ribavirin were associated with significant toxicity, such as hemolytic anemia, and correlated with a longer length of hospital stay [67, 68]. Another reason is the notion that the coronavirus encodes RNA replication proofreading machinery that can partially resist one mechanism of action of ribavirin nucleoside analogs, decreasing the efficacy of ribavirin than expected [69].
Several clinical trials have been registered to address the role of ribavirin (mostly in combination with other drugs) in COVID-19 patients. An open-label, non-randomized clinical trial was registered to evaluate the safety and efficacy of inhaled ribavirin (Virazole®) in hospitalized COVID-19 adult patients with significant respiratory distress (PaO2/FiO2 ratio <300 mmHg) (Table 1). Nevertheless, more studies are needed to critically evaluate the role of ribavirin, alone or in combinations, for the treatment of nCoV outbreaks and characterize the primary sources of the controversy.
2) Remdesivir
Remdesivir was developed by Gilead Sciences as part of an antiviral development effort to identify therapeutic agents for treating RNA-based viruses that maintained global pandemic potential. It interacts with the viral RdRp to induce delayed chain termination [70]. The prodrug, Remdesivir GS-5734, is metabolized within cells into an alanine metabolite (GS-704277), that is further processed to liberate the nucleoside monophosphate. The nucleoside monophosphate is highly polar and trapped within the cell, and further phosphorylated by host cell kinases into the nucleoside triphosphate analog that can be used as a substrate by the viral RdRp enzyme and mis-integrated into the viral RNA leading to inhibition of virus replication [71, 72]. Similar to other nucleoside analogs, remdesivir is a poor substrate for human polymerases [73]. In cell-based assays, remdesivir was found to selectively inhibit viral RdRp rather than human RNA Polymerase (RNAP) II and human mitochondrial RNAP [74].
Remdesivir showed an in vitro antiviral activity against various Filoviridae, Pneumoviridae, and Paramyxoviridae [75, 76]. Further in vitro and in vivo studies reported that remdesivir exhibited potential pan-coronavirus antiviral activity against HCoV-OC43 and HCoV-229E (causative agents of the common cold) [70], the highly pathogenic CoVs (MERS-CoV, SARS-CoV), and potentially emergent BatCoVs, validating its activity against coronaviruses [9] [77]. Furthermore, remdesivir was used against the Ebola virus epidemic in 2014 and displayed potential activity [74]. Accordingly, remdesivir was evaluated for the management of the COVID-19 outbreak.
A double-blind, randomized, placebo-controlled trial showed that patients with severe COVID-19 treated with a 10-day course of remdesivir had a significantly shorter time to recovery than those receiving placebo (11 days vs 15 days) [78]. Another randomized, open-label trial assigned patients with confirmed SARS-CoV-2 infection (relative hypoxia or requiring oxygen support but not requiring ventilatory support) in a 1:1 ratio to receive intravenous remdesivir for either 5 days or 10 days. All patients received 200 mg of remdesivir on day 1 and 100 mg once daily on subsequent days. Results showed that patients with severe COVID-19 had a clinical improvement of 2 points or more on the ordinal scale by day 14 with no significant difference between a 5-day course and a 10-day course of remdesivir [79]. Based on these results, the US Food and Drug Administration prompted to grant Emergency Use Authorization (EUA) of remdesivir for severe COVID-19 patients of 12 years of age or older with pneumonia who require supplemental oxygen [80].
On the other hand, the World Health Organization's (WHO's) giant Solidarity trial, one of the largest international randomized trials for COVID-19 treatments, showed a different outcome [81]. The study enrolled almost 12000 patients (2750 patients on remdesivir) and was conducted in 30 countries. It compared remdesivir and three other repurposed drugs (hydroxychloroquine, lopinavir/ritonavir, and interferon) to the standard of care, and evaluated the effect of drugs on 3 important outcomes in COVID-19 patients: mortality, need for assisted ventilation, and duration of hospital stay. This trial showed that remdesi-
vir had little or no effect on overall mortality, initiation of ventilation, or duration of hospital stay when compared to standard of care.
Another randomized, open-label trial [82] included 584 patients with moderate COVID-19 studied the efficacy of 5 or 10 days of remdesivir treatment compared with standard care on the clinical status on day 11 after initiation of treatment (Table 1). They reported that remdesivir did not have a statistically significant difference in clinical status compared with standard care at 11 days after initiation of treatment. They also reported that patients on 5 days course had significantly higher odds of having a better clinical status but with uncertain clinical importance.
The discrepant results of remdesivir in COVID-19 could be attributed to several factors including variation in study design, the genetic backgrounds of patients from different countries, and the presence of different strains of SARS-CoV2. It is worthy to note that data obtained from in vitro and in vivo animal studies suggested that remdesivir is optimally inhibiting early coronavirus life cycle [83]. Thus, remdesivir might require initiation for viral prophylaxis or immediately following viral inoculation before the peak viral replication. Practically, it is challenging to initiate such early antiviral therapy except in selected cases.
Another issue that should be evaluated is the safety aspect of remdesivir in the treatment of COVID-19 patients. Common side effects noted with remdesivir involve nausea, diarrhea or constipation, anemia, pyrexia, hyperglycemia, and increased alanine aminotransferase level. However, severe side effects were also reported, including multiple organ dysfunction, septic shock, kidney injury. Thus, the use of remdesivir should be administrated only under medical supervision [70].
3) Sofosbuvir
Sofosbuvir is a nucleoside analog clinically approved for the treatment of diverse genotypes of HCV [84]. It is also capable of suppressing other families of positive-strand RNA viruses (Flaviviridae and Togaviridae) that share a similar replication mechanism requiring an RNA-dependent RNA polymerase (RdRp). Sofosbuvir is also found to tightly bind RdRp of the newly emerged coronavirus [64]. Chien M, et al. [85] demonstrated that the triphosphate form of Sofosbuvir acts as a chain terminator for the SARS-CoV as well as SARS-CoV-2 with permanent blockade of further polymerase extension.
SARS-CoV and SARS-CoV-2 genomes could maintain the viral genome integrity via an exonuclease-based proofreader that removes mismatched nucleotides [86]. However, it has been reported that sofosbuvir-terminated RNA resists this exonuclease-based proofreader to a substantially higher extent than RNA terminated by remdesivir [69]. Hence, sofosbuvir attains a substantial level of resistance to excision by an exonuclease and can serve as an efficient terminator of the polymerase.
Recently, Jácome et al. recommended sofosbuvir as a possible antiviral for COVID-19 based on structural studies and bioinformatic analysis [87]. Similarly, Buonaguro et al. reported that sofosbuvir might be an optimal nucleotide analog to be repurposed for COVID-19 treatment [88]. Other studies have additionally demonstrated the ability of sofosbuvir to inhibit SARS-CoV-2 replication in lung and brain cells [44].
Considering the safety profile of sofosbuvir [89], its excellent oral bioavailability and high intracellular stability [46], clinical trials were designed to evaluate the effect of sofosbuvir for the treatment of COVID-19. Sofosbuvir was administered in combination with daclatasvir that was known to enhance RNA polymerase during viral RNA genome replication [90, 91].
Up till now, there are 9 studies registered on clinicaltrials.gov to assess the utility of sofosbuvir in the management of COVID-19 (3 completed, 12 recruiting). Most trials enrolled patients with a mild infection while excluding severe ICU patients. However, an open-label, randomized, controlled trial enrolled patients with an established diagnosis of COVID-19 in Egyptian quarantine hospitals at any clinical stage of the disease is ongoing. They evaluate the efficacy of the fixed combination of Sofosbuvir/Ledipasvir (400 mg and 90 mg, orally, daily for 14 days), or nitazoxanide (500 mg, orally four times per day for 14 days) as compared to standard of care treatment regimen alone. Another randomized, open-label trial will examine the efficacy of sofosbuvir/daclatasvir (fixed-dose combination) for COVID-19 prophylaxis in healthcare workers at high risk of exposure to SARS-CoV-2.
4) Favipiravir
Favipiravir (T-705) is a broad-spectrum synthetic antiviral prodrug developed by Toyama Chemical Co Ltd [92]. It was used against SARS-CoV-2 in Wuhan at the epicenter of the pandemic. Favipiravir is a purine nucleic acid analog that is activated intracellularly to favipira virribofuranosyl-5B-triphosphate (favipiravir-RTP) that selectively inhibits the RNA-dependent RNA polymerase of RNA viruses. Favipiravir is incorporated into the nascent viral RNA strand by error-prone viral RdRp, leading to the prevention of further extension (chain termination) and induction of lethal viral mutagenesis [93].
In 2014, favipiravir was approved in Japan for the management of emerging pandemic influenza infections (3200 mg on day 1, followed by 1200 mg for 4 days). It was found to be active against a broad range of influenza viruses, including A(H1N1) pdm09, A(H5N1), and the recently emerged A(H7N9) avian virus [94, 93].
During the Ebola virus outbreak in 2014, favipiravir was one of the drugs short-listed for trials by the WHO owing to its good safety profile and strong antiviral effect activity against Ebola in vitro and in vivo animal model, when administered early in the course of infection [95-97]. The administered dose was 6000 mg on day 1 and a maintenance dose of 2400 mg/day which was higher than what is approved in Japan for complicated influenza. It was shown to have some effect in patients with medium to high viremia but not in those with more severe viremia.
In addition to its activity against influenza and Ebola viruses, favipiravir showed a broad range of antiviral activity against many other RNA viruses, including arenaviruses, phleboviruses, hantaviruses, flaviviruses, enteroviruses, respiratory syncytial virus, and noroviruses [93, 98]. For SARS-CoV-2, data showed that high concentrations of favipiravir were required to effectively reduce the SARS-CoV-2 infection in Vero E6 cells, and further in vivo studies are required to evaluate this antiviral nucleoside [99].
In China, a prospective randomized controlled, open-label multicenter trial involving adult patients with COVID-19 randomly assigned patients to receive conventional therapy plus favipiravir (1600 mg twice, in the first day followed by 600mg twice, for 10 days) or arbidol 200mg thrice daily (registered with Chictr.org.cn; ChiCTR2000030254). They reported that favipiravir was associated with shorter latencies to relieve both pyrexia and cough, nevertheless, no significant difference between the two groups was noticed [100]. Another open-labeled nonrandomized study compared the effect of favipiravir versus lopinavir/ritonavir for the treatment of COVID-19 (registered on the Chinese Clinical Trial Registry, ID: ChiCTR2000029600). Both groups additionally received interferon-alpha by aerosol inhalation (5 million units twice daily). The favipiravir arm showed better therapeutic responses on COVID-19 in terms of disease progression and viral clearance compared with the control arm [101]. However, those were nonrandomized double-blinded studies, so that a potential selection bias may have confounded the results.
Around 40 studies were registered on clinicaltrials.gov to assess the utility of favipiravir, either alone or in combination with other drugs, in the management of COVID-19. However, a definite opinion about the rationale of the drug, duration of treatment, and dosages has not been justified yet. Nevertheless, the drug received approval for emergency use in Italy, and other countries and was included in their treatment protocol.
An open-label randomized multicenter comparative Phase III study is ongoing in 5 medical facilities in Russia. The objective of the study is to assess the efficacy and safety of favipiravir compared with the Standard of Care (SOC) in hospitalized patients with moderate COVID-19 pneumonia. Patients receive either Favipiravir 1600 mg twice a day on Day 1 followed by 600 mg twice daily on Days 2-14, or SOC. Another open-label randomized controlled, multicenter study of favipiravir in hospitalized subjects with COVID-19 was conducted in the USA. The dose regimen will be 1800 mg favipiravir BID plus SOC or SOC alone on Day 1 followed by 1000 mg BID favipiravir (800 mg BID for subjects with Child-Pugh A liver impairment) plus SOC or SOC for the next 13 days (Table 1).
Favipiravir has excellent bioavailability and exhibits both dose-dependent and time-dependent pharmacokinetics possibly due to saturation and/or auto-inhibition of the main enzymatic pathway. It is metabolized in the liver primarily by aldehyde oxidase and partially by xanthine oxidase to an inactive oxidative metabolite [92]. Favipiravir can boost its own concentration by dose- and time-dependent self-inhibition of aldehyde oxidase.
The most frequent adverse events of favipiravir reported during the development of influenza treatment, such as mild to moderate diarrhea, asymptomatic increase of blood uric acid and transaminases, and decrease of neutrophil count. It is contraindicated in pregnant and suspected pregnant women due to its teratogenic potential in animal studies [102, 103].
PROTEASE INHIBITORS
Proteinases play essential roles in the conversion of viral polyproteins into functional proteins. Specifically, the 3CLpro and PLpro of human coronavirus play an essential role during the viral lifecycle by processing viral polyproteins into functional nsps, providing a specific target for antiviral drug discovery [104, 105]. They also possess other essential functions including immune antagonism, double-membrane vesicle organization, scaffolding for replication complex formation, nucleic acid binding, helicase activity, and viral RNA proofreading. Computational studies were used to screen potent inhibitors of the SARS-CoV main proteinase [106]. A recent study found that lopinavir, oseltamivir, and ritonavir are potential candidates as SARS-CoV-2 protease inhibitors [107].
Lopinavir/Ritonavir
Lopinavir/ritonavir combination is used in the treatment of human immunodeficiency virus (HIV). Lopinavir is an HIV-1 protease inhibitor, which is combined with ritonavir to enhance its plasma concentration through inhibition of cytochrome P450 (CYP3A)-mediated metabolism of lopinavir [108]. Lopinavir is also found to inhibit the SARS-CoV nonstructural protein 3CLpro / main protease [109], which is critical for virus replication and appears to be highly conserved in SARS-CoV-2. Lopinavir also showed in vitro activity against MERS-CoV [110] and ameliorated the outcome of MERS-CoV infection in a model of Marmoset [111]. In 2004, an open-label study suggested that the addition of lopinavir-ritonavir (400 mg and 100 mg, respectively) to ribavirin reduced the complications and viral load among patients with SARS as compared to ribavirin alone [112].
Early during the COVID-19 pandemic, lopinavir/ritonavir combination was assumed as a treatment option for COVID-19 based on previous clinical studies conducted on patients with SARS. An open-label, prospective, randomized trial in adults with COVID-19 reported a positive outcome of the triple combination of lopinavir/ritonavir (400 mg and 100 mg, respectively, every 12 h), ribavirin (400 mg every 12 h), and interferon beta-1b compared to lopinavir/ritonavir alone [113]. Another open-label, randomized trial involving 199 hospitalized adult patients with confirmed SARS-CoV-2 infection found no significant benefit or reduction of viral RNA loads with lopinavir/ritonavir treatment (400 mg and 100 mg, respectively) beyond standard care alone [114]. However, the trial did not exclude a probable benefit of lopinavir/ritonavir administration if it is given at an early stage of illness. Another recent randomized controlled trial indicated that lopinavir/ritonavir (400 mg and 100 mg, respectively, for 10 days) was not associated with reductions in 28-day mortality, duration of hospital stay, or risk of progressing to invasive mechanical ventilation or death in patients admitted to hospital with COVID-19 [112]. In the solidarity trial, WHO has halted the lopinavir/ritonavir monotherapy and the lopinavir/ritonavir plus interferon-beta combination groups because the interim results addressed that lopinavir/ritonavir does not improve clinical outcomes for patients admitted to hospital with COVID-19 [112]. Additional trials may help to confirm or exclude the possibility of a treatment benefit of lopinavir/ritonavir.
