Strategy, Progress, and Challenges of Drug Repurposing for Efficient Antiviral Discovery

Emerging or re-emerging viruses are still major threats to public health. Prophylactic vaccines represent the most effective way to prevent virus infection; however, antivirals are more promising for those viruses against which vaccines are not effective enough or contemporarily unavailable. Because of the slow pace of novel antiviral discovery, the high disuse rates, and the substantial cost, repurposing of the well-characterized therapeutics, either approved or under investigation, is becoming an attractive strategy to identify the new directions to treat virus infections. In this review, we described recent progress in identifying broad-spectrum antivirals through drug repurposing. We defined the two major categories of the repurposed antivirals, direct-acting repurposed antivirals (DARA) and host-targeting repurposed antivirals (HTRA). Under each category, we summarized repurposed antivirals with potential broad-spectrum activity against a variety of viruses and discussed the possible mechanisms of action. Finally, we proposed the potential investigative directions of drug repurposing.


INTRODUCTION
Since December 2019, a novel coronavirus disease 2019  has rapidly spread all over the globe to cause a pandemic (Li et al., 2020b;Zhu et al., 2020). The pneumonia causative agent was identified to be a new coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). As of April 6, 2021, more than 130 million cases have been confirmed globally, including approximately 2.85 million deaths. The still ongoing pandemic represents the most recent example of how emerging or re-emerging human or zoonotic viruses pose a threat to public health. These viruses include but not limited to Ebola virus (EBOV), Zika virus (ZIKV), West Nile virus (WNV), yellow fever virus (YFV), dengue virus (DENV), henipaviruses (Nipah, Hendra), SARS-CoV, Middle East respiratory syndrome (MERS-CoV), Lassa virus (LASV), Crimean-Congo hemorrhagic fever virus (CCHFV), Rift Valley fever virus (RVFV), chikungunya virus (CHIKV), human immunodeficiency virus (HIV) and influenza A virus (IAV). We listed six viral families in which a number of viruses have merged or remerged in recent years to have caused or potentially cause an epidemic or pandemic, including Coronaviridae, Filoviridae, Flaviviridae, Arenaviridae, Nairoviridae, and Orthomyxoviridae. The genome structure, important viruses, and key features regarding virus-host interactions are summarized in Table 1.
The emerging or remerging virus outbreak has emphasized the urgent need for preventative or treatment regimens. Vaccines are recognized as a preferred promising line of defense. However, vaccine development is a complex process and multiple challenges are involved in light of the fact that the pathogens that need to be confronted may display high genetic variability (e.g., HIV) or an identity hardly predicted in advance (e.g., SARS-CoV-2 or ZIKV). Thus, unprecedented demands have emerged on antivirals that can be rapidly available in clinical practices. In the absence of a vaccine available to use, hepatitis C virus (HCV) is supposed to be eliminated in the use of the direct-acting antivirals, which probably represents the first virus to be cured by antivirals. That strengthens the promising potential of antivirals in terms of virus treatment.
Drug repurposing (also called drug repositioning) is a strategy for identifying new uses for approved or investigational drugs that beyond the original indicative scope to facilitate antiviral development. Typically, antiviral discovery development is time and resource-consuming, which involves three major stages including drug discovery (3-6 years), preclinical studies in experimental animal models (about 3 years), clinical trials in humans from phase I to III (about 5 years). Finally, if a therapeutic succeeds to pass all the processes, it needs to get approved by the appropriate agency. It is estimated that only 5% of the candidate molecules are finally approved and up to 3 billion dollars are consumed. Given that the repurposed drugs have been proven to be safe in humans, drug repurposing likely can skip phase I and probably the phase II clinical trials. Thus, the attrition rate to be a novel antiviral is reduced, although the phase III trial is still needed. Remdesivir, an adenosine analog to inhibit EBOV RNA-dependent RNA polymerase (RdRp) (Tchesnokov et al., 2019), is the latest example. Although remdesvir did not show therapeutic activity against EBOV infection in a real-world phase III clinical trial (Nakkazi, 2018), remdesivir shows potent antiviral activity against SARS-CoV-2, SARS-CoV, and MERS-CoV in vitro or in vivo in preclinical animal models (de Wit et al., ∼3,500 2,500 approved molecules, plus about 1,000 investigational compounds Huang et al. (2011) ReFRAME compound library ∼12,000 Containing nearly all small molecules that have reached clinical development or undergone significant preclinical profiling, 38% of which are approved drugs 2020; Wang et al., 2020a). Two randomized phase III clinical trials indicate that patients who received remdesivir had a shorter time to recover (Spinner et al., 2020;Wang et al., 2020c), based upon which the U.S. Food and Drug Administration (FDA) has approved remdesivir for use in COVID-19 patients, less than 1 year after the outbreak of the pandemic. From the above example, drug repurposing could significantly facilitate antiviral development for emergency use. Given the urgent need for therapeutics for emerging or re-emerging viruses and a great number of approved or developmental therapeutics, drug repurposing represents a better way for antiviral discovery. In this review, we discussed the strategies of drug repurposing for antiviral development, summarized the promising drug candidates that have the antiviral potency with broadspectrum activity, and analyzed the possible caveats of this strategy of drug discovery.

STRATEGIES TO DEVELOP REPURPOSED ANTIVIRALS
A typical drug repurposing strategy comprises four steps (Figure 1), including the identification of a candidate therapeutic for the new indication as an antiviral; antiviral efficiency confirmation and/or mechanistic analysis in preclinical animal models; antiviral efficacy evaluation in clinical trials (phase I may be not prerequisite if sufficient safety data has already been obtained as parts of the original indication); and approval of the novel indication by government agencies such as the FDA, the European Medicines Agency (EMA), Ministry of Health, Labor and Welfare (MHLW) of Japan, and National Medical Products Administration (NMPA) of China.

APPROACHES FOR ANTIVIRAL REPURPOSING
The identification of the right drug for the new indication is crucial. The major approaches involve high throughput in silico or in vitro screening. The in silico screening is commonly used for the identification of a compound that binds to the given target, commonly a virally encoded protein, such as RNA-dependent RNA polymerase (Patel and Kukol, 2017). The in vitro screening involves the high throughput antiviral screening, leading to the subsequent validation for the most potent candidates. These candidates can target host proteins or viral proteins (Kouznetsova et al., 2014;Chopra et al., 2016;Xu et al., 2016;Li et al., 2017c). For either approach, compound libraries, in particular those with approved molecules, are needed ( Table 2). These include the Drugbank library, NIH Clinical Compound (NCC) Collection (van Cleef et al., 2013), the Prestwick Chemical Library (Ulferts et al., 2016), the Library of Pharmacologically Active Compounds (LOPAC) (Hu et al., 2014), a library of approved drugs that were assembled by the NIH Chemical Genomics Centre (NCGC) called the NCGC Pharmaceutical Collection (NPC) (Huang et al., 2011), and the ReFRAME (Repurposing, Focused Rescue, and Accelerated Medchem) Library (Janes et al., 2018). Recently, the LOPAC and ReFRAME drug libraries were successfully used for the discovery of the SARS-CoV-2 antiviral candidates (Riva et al., 2020).

CATEGORIES OF REPURPOSED ANTIVIRALS
Based on the origin and feature of the repurposed antiviral targets, two major categories are divided: direct-acting repurposed antiviral (DARA) and host-targeting repurposed antiviral (HTRA) repurposing. The representative antivirals with repurposed potentials are summarized in Figure 2.

Direct-Acting Repurposed Antiviral (DARA)
A large majority of antivirals approved by the FDA are directacting antivirals (DAA) other than host-targeting agents (HTA) (Chaudhuri et al., 2018). DARAs contain antiviral activity relying on structural similarity or identical enzymatic activity of virally encoded targets, particularly viral polymerase, protease, reverse transcriptase, or viral proteins with ion channel activity. Below we reported the advances in repurposed antivirals targeting the two important viral enzymes, RdRp and protease.

RdRp Inhibitors
Remdesivir (GS-5734) Remdesivir was an investigational compound in the class of nucleotide analogs, which was originally developed to treat Filoviridae members EBOV or Marburg infection and rapidly pushed through clinical trials due to the EBOV epidemic in West Africa from 2013 through 2016. However, in August 2019, remdesivir was announced to be less effective than the other two monoclonal antibody regimens (Mulangu et al., 2019 (Warren et al., 2016;Lo et al., 2017;Sheahan et al., 2017a;Choy et al., 2020) (Table 3). The parent nucleoside of remdesivir, GS-441524 (1′-cyano substituted adenine nucleoside analog or Nuc) also shows a broad-spectrum but less effective antiviral activity against infections of coronaviruses like MERS-CoV and feline coronavirus (Warren et al., 2016;Pedersen et al., 2019).
Remdesivir shows prophylactic or therapeutic potency when administrated in SARS-CoV-infected mice, in which reduced viral load in lung and improved clinical symptoms and respiratory function was observed (Sheahan et al., 2017a). A similar prophylactic or therapeutic potency of remdesivir against MERS-CoV was seen in macaques or mouse models (de Wit et al., 2020;Sheahan et al., 2020). Remdesivir efficiently inhibits SARS-CoV-2 replication in vitro , and was used as a compassionate use in the first COVID-19 case in the United States (Holshue et al., 2020) before large-scale clinical studies (NCT04280705; NCT04292899; NCT04292730; NCT04257656) were launched. One large-scale study in which hospitalized COVID-19 patients were given remdesivir for 10 days showed significantly shortened time to recovery (Beigel et al., 2020). Another study indicated that remdesivir treatment in moderate COVID-19 patients for 5 days led to symptom improvement significantly higher than the standard care group (Spinner et al., 2020). Contrarily, a smaller scale study only found remdesivir resulted in a marginally but numerically faster time to clinical improvement (Wang et al., 2020c). Based upon these clinical studies, the full and conditional use of remdesivir in hospitalized COVID-19 patients was approved by FDA in October 2020. Although World Health Organization (WHO) recommends against it, based on the interim result of the WHO Solidarity Trial.
Mechanistically, remdesivir exerts the antiviral activity through competing with ATP that is supposed to incorporate into viral RdRp for RNA replication. It results in delayed EBOV and MERS-CoV RNA chain termination at the fifth and third position, respectively after the initiation site (Warren et al., 2016;Tchesnokov et al., 2019;Gordon et al., 2020).

Ribavirin (RBV)
RBV is on the WHO's list of essential medicines, it is licensed to treat RSV infection (Committee on Infectious Diseases, 1993), or HCV infection in combination with interferon (IFN)-α or directacting antivirals (AASLD-IDSA HCV Guidance Panel, 2018). RBV is also effective against other hepatotropic viruses including HBV(Galban- Garcia et al., 2000) and HEV (Kamar et al., 2014;Kamar et al., 2019) in clinical studies, although no convincing activity against HBV was obtained in cell culture systems (Isorce et al., 2016). Ribavirin was clinically used to treat a variety of viral hemorrhagic fevers, including Lassa fever (McCormick et al., 1986), Crimean-Congo hemorrhagic fever (Fisher-Hoch et al., 1995), and Hantavirus infection (Ogg et al., 2013) alone or in combination with favipiravir, even though RBV might be effective only at early stages (Johnson et al., 2018;Eberhardt et al., 2019).
The clinical use of RBV as a supplement to other agents like corticosteroid for SARS-CoV treatment was documented in China and Canada (Peiris et al., 2003), while RBV had an EC50 of 81.9 μM in vitro (Saijo et al., 2005). RBV is also effective to inhibit MERS-CoV with an EC50 ranging from 66.9 μM (16.33 μg/ml) to 169.7 μM (41.45 μg/ml) in vitro (Falzarano et al., 2013a). RBV alongside IFN-α was reported to reduce the hospital mortality rate from 70 to 29% in hospitalized MERS patients at 14 days after admission (Omrani et al., 2014). RBV also shows antiviral activity against SARS-CoV-2 in vitro with an EC50 of 109.5 μM , while another report did not find the favorable effect of RBV (Choy et al., 2020). The higher EC50 of RBV against either MERS-CoV or SARS-CoV-2 may be due to the reduced RBV uptake (Ibarra and Pfeiffer, 2009) or inabilities to accumulate sufficient amounts of phosphorylated RBV metabolites required for the effective RBV antiviral actions (Shah et al., 2010). As of early January 2021, at least seven clinical trials (phase I NCT04335123; phase II NCT04494399; phase II NCT04563208; phase II NCT04605588; NCT04664010; phase II NCT04276688; phase II/III NCT04402203) have been launched to investigate the efficacy of RBV alone or in combination with other agents for COVID-19 treatment.
Mechanistically, at least two types of antiviral machinery may be involved. Upon the uptake into cells, RBV is metabolized to form a purine RNA nucleotide-like form, which interferes with viral RNA polymerases, leading to hypermutation of RNA that reduces the viability and is lethal to RNA viruses. RBV is also an inhibitor of inosine-5′-monophosphate dehydrogenase (IMPDH), which is essential for the de novo synthesis of guanosine-5′-monophosphate (GMP). RBV structure may interfere with the RNA capping process that relies on natural guanosine to prevent RNA degradation. Upon the inhibition of IMPDH, RBV can lower the intracellular pool of GTP, and DNA virus replication is then inhibited. Alternatively, RBV has the potential to reduce cell death, affect type 1 cytokine production or inflammatory response, which may help combat HBV, HCV, or LASV infection (Tam et al., 1999;Oestereich et al., 2016).
A preclinical study in EBOV-infected macaques shows a higher plasma favipiravir concentrations greater than 70-80 μg/ml or 446-509 μM were associated with reduced viral loads and extended survival rate (Guedj et al., 2018). A similar study in IFNAR−/− mice also shows a high dose [300 mg/ (kg/d)] of favipiravir enhances EBOV clearance and prevents a lethal outcome (Oestereich et al., 2014a). Clinically, favipiravir showed good tolerance in EBOV patients but no strong antiviral efficacy (Sissoko et al., 2016), possibly due to the low median trough drug concentrations (46 μg/ml or 293 μM at day 2 posttreatment) (Nguyen et al., 2017). Thus, the optimal dosage and potency of favipiravir merit further investigation.
Favipiravir exhibited antiviral activity against SARS-CoV-2 in vitro  and showed a significantly shorter viral clearance time than the control group (4 vs. 11 days) and a higher improvement in chest imaging in a non-randomized clinical study (Cai et al., 2020). Another small-sized open-label phase II/III clinical trial (NCT04434248) also found that favipiravir enabled SARS-CoV-2 viral clearance in 62.5% of patients within 4 days, as compared to 30% of patients on a standard of care; however, the viral clearance rate by day 10 after favipiravir administration was only marginally improved (Ivashchenko et al., 2020). Although favipiravir has been approved in some countries, large-scale, placebo-controlled, double-blinded clinical trials may be still needed to further evaluate the efficacy and safety of favipiravir. The mechanism of action of inhibiting influenza RdRp by favipiravir involves the conversion to the metabolite favipiravir ribofuranosyl-5′-triphosphate (favipiravir-RTP), which further incorporates into influenza RdRp to inhibit the polymerase activity at nanomolar to micromolar concentrations (Furuta et al., 2013). The inhibition of other viral RdRp by favipiravir may involve a similar mechanism.

Sofosbuvir
Sofosbuvir that targets HCV RdRp NS5B from 15 subtypes in six genotypes with an EC50 ranging from 0.032 to 0.13 μM is an approved oral direct-actin antiviral to treat chronic hepatitis C (Han et al., 2019). A cocktail treatment regimen containing sofosbuvir and HCV protease NS3/4A inhibitors has already been approved for pan-genotypic HCV infection. Sofosbuvir shows antiviral effects against other virus members in  Table 3). A phase II clinical trial of sofosbuvir for HBV treatment (phase II NCT03312023) is also under investigation. Besides, the binding residue of sofosbuvir on coronavirus RdRp is conserved among SARS-CoV, SARS-CoV-2, and MERS-CoV (Jacome et al., 2020), although sofosbuvir did not exhibit the inhibitory effect against MERS-CoV RdRp in a cellbased reporter assay (Min et al., 2020). Sofosbuvir binds to SARS-CoV-2 RdRp and inhibits virus infection in lung and brain cells (Elfiky, 2020a), and clinical trials have initiated in multiple countries (phase II NCT04561063, phase II NCT04532931, phase II/III NCT04460443; phase II/III NCT04497649; phase III NCT04530422, phase III NCT04535869, phase IV NCT04498936).

Galidesivir (BCX4430, Immucillin-A)
Galidesivir, an imino-C-nucleoside analog, was originally developed to combat EBOV infection (Warren et al., 2014). Galidesivir strongly inhibits EBOV RdRp activity in vitro and post-exposure intramuscular administration of galidesivir protects mice or macaques against Ebola virus or Marburg disease (Warren et al., 2014). Currently, a phase I clinical trial (NCT03800173) for Marburg disease is being performed.

Viral Protease Inhibitor
Lopinavir/ritonavir (LPV/r) LPV/r is a fixed-dose combination for HIV prevention and treatment. It combines LPV with a low dose of ritonavir (RTV), both of which are HIV protease inhibitors. In HIV-1 NL4-3 infection system in vitro, LPV and RTV have an EC50 of 0.018 and 0.046 μM, respectively, against HIV-1, but LPV has a much higher potency than RTV does (EC50 0.015 μM for LPV vs. 0.349 μM for RTV) for HIV-2 (Masse et al., 2007). RTV is also a very potent inhibitor of intestinal and hepatic cytochrome P450 3A4 (Eagling et al., 1997), which is involved in LPV catabolism. As with LPV/r, darunavir/cobicistat (DRV/c) is also a dose-fixed combination containing HIV protease inhibitor DRV and CYP3A enzymatic antagonist cobicistat (Masse et al., 2007). Low doses of RTV or cobicistat could slow down the breakdown of HIV protease inhibitors, thereby greatly increases its blood concentration.
Some viruses, like SARS-CoV, MERS-CoV, and SARS-CoV-2, encode proteases, which are structurally and functionally similar to HIV protease. LPV shows an EC50 of 17.1, 8.0, and 26.63 μM, respectively against the three coronaviruses (de Wilde et al., 2014;Choy et al., 2020) (Table 3). LPV/r combination shows greater anti-MERS-CoV activity than LPV does (EC50 of 8.5 vs. 11.6 μM) in vitro (Sheahan et al., 2020). LPV/r alongside IFN-β shows improved clinical and pathological features in a nonhuman primate MERS model (Chan et al., 2015), while prophylactic or therapeutic LPV/ r-IFN-β treatment only slightly improves the disease outcomes in patients (Sheahan et al., 2020). Similarly, SARS patients receiving LPV/r were found to have an improvement in respiratory syndrome (Chu et al., 2004). However, the potency of LPV/r for SARS-CoV is only effective if administrated early but not as rescue or salvage therapy . Despite of its potential for COVID-19 treatment, a randomized trial showed the hospitalized COVID-19 patients did not benefit from LPV/r therapy .

Rupintrivir (AG-7088)
Rupintrivir is a peptidomimetic compound inhibiting viral protease activity, it is designed to combat human rhinovirus (HRV, belonging to family Piconaviridae) infection. Rupintrivir shows potent in vitro activity against all 48 HRV serotypes tested, with a range of EC50s of 0.007 to 0.104 μM (Binford et al., 2005). A phase II placebocontrolled randomized, double-blind trial experimentally shows the biosafety and potential efficacy in volunteers (Hayden et al., 2003). Because of a lack of efficacy in natural HRV infection in a subsequent clinical trial, further development of rupintrivir was suspended.
Rupintrivir has poor aqueous solubility and low oral bioavailability in animals, the hydrolyzed metabolites are reportedly 400-fold less active than rupintrivir but predominates the biotransformation pathway. The above features may limit its potential clinical application.

Host-Targeting Antiviral (HTA) Repurposing
HTA repurposing identifies antivirals targeting to host proteins, functions, or pathways, which are required for virus life cycle including viral entry, genome replication, protein translation, and virus assembly and release. As the entire viral life cycle cannot be completed without cells, HTA may exhibit broad antiviral activity against different viruses. Based on the essential steps of a viral life cycle, four major categories of host-targeting repurposed antivirals (HTRA) are classified as below.

HTRA Aiming Virus Entry Step
The first step of the viral life cycle is to enter permissive cells. Some enveloped viruses like HIV, and Nipah virus enter cells via direct membrane fusion with the plasma membrane, resulting in the release of nucleocapsid directly to the cytosol (Bossart et al., 2002;Wilen et al., 2012). Bacteriophages can inject their genomes alone into bacterial cells. Except for the aforementioned two mechanisms, most viruses depend on an endocytic pathway to be internalized into cells. The involved pathways include clathrinmediated endocytosis, caveolar/lipid raft-mediated endocytosis, or micropinocytosis, through which viruses are internalized into the early endosome, intermediate endosome, and then late endosome or lysosome in a stepwise manner. Finally, the exposure of virions either naked or enveloped to low pH and proteolytic enzymes will trigger changes in the naked virions, or membrane fusion between the organelle and enveloped viruses, to help deliver the viral genome or the intact nucleocapsid into cytosol. Aftermath, most RNA viruses replicate in different locations within the cytosol, whereas DNA viruses continue the journey to the nucleus.

Chlorpromazine (CPZ) and Other Dopamine Antagonists
CPZ is a phenothiazine used to treat psychotic disorders including schizophrenia or manic-depression in adults. CPZ can treat in children severe behavioral problems like attention deficit hyperactivity disorder. CPZ is also indicated to treat anxiety before surgery, nausea and vomiting, and chronic hiccups that do not improve following other treatments (Lopez-Munoz et al., 2005). CPZ is on the list of WHO's essential medicines, among the most effective and safest medicines. CPZ antagonizes dopamine receptors, which are divided into two classes based on which G-protein they are coupled: the D1-like class (including D1 and D5) and the D2like class, which comprises D2, D3, and D4 receptors. CPZ can bind to and block two types of dopamine receptors, in particular D2 dopamine receptors, exerting antipsychotic activity.
HIV can either enter cells through direct viral membrane fusion with the plasma membrane, or cell-to-cell transmission and viral synapses between T cells. The latter type of HIV entry is sensitive to CPZ treatment, suggesting the involvement of clathrin-mediated endocytosis (Bosch et al., 2008). Viruses within Flaviviridae family, such as HCV, DENV, ZIKA, and CSFV also enter cells dependent on clathrin-mediated endocytosis and are susceptible to CPZ treatment (Zhu et al., 2012;Shi et al., 2016). Cell surface FcγR was reported to be required for antibody-dependent enhancement of DENV or ZIKV infection (Khandia et al., 2018). Interestingly, the viral entry mediated by FcγRII needs the formation of clathrin-coated vesicles whilst FcγRI-dependent viral entry is independent of clathrin (Carro et al., 2018). On the contrary, naked and enveloped viruses may comparably be sensitive to CPZ. HAV and HEV had been recognized as naked viruses until recently the membrane-trapped viral particles were identified (Feng et al., 2013;Yin et al., 2016a). The naked and enveloped HAV or HEV are both sensitive to CPZ treatment (Yin et al., 2016a;Rivera-Serrano et al., 2019), suggesting the clathrin-mediated endocytosis is equally needed. Coronaviruses such as MERS-CoV and SARS-CoV share the same clathrin-mediated endocytosis for virus entry. In light of this, clinical studies have initiated (phase II/III NCT04354805; phase III NCT04366739) to evaluate the safety and effectiveness of CPZ for COVID-19 treatment, although observative clinical studies have suggested that CPZ at the prescribed dose may not be clinically effective for COVID-19 (Hoertel et al., 2021).

Sunitinib, Erlotinib (Receptor Tyrosine Kinase Inhibitors)
Sunitinib and erlotinib are inhibitors to receptor tyrosine kinases (RTK) that play important roles in both tumor angiogenesis and tumor cell proliferation. Sunitinib has been approved for the treatment of cancers, such as gastrointestinal stromal cell tumor, renal cell carcinoma, and imatinib-resistant gastrointestinal stromal tumor; while erlotinib is licensed to treat non-small cell lung cancer, and pancreatic cancer (Hartmann and Kanz, 2008;Neveu et al., 2015). Erlotinib is on the list of WHO's essential medicines.
EGFR is involved in multiple virus entry processes such as DNA viruses HBV, HPV, and RNA viruses HCV, RSV, and porcine reproductive and respiratory syndrome virus in cell cultures (Lupberger et al., 2011;Wang et al., 2016a;Iwamoto et al., 2019;Lingemann et al., 2019;Mikuličić et al., 2019). Specifically, EGFR mediates HCV entry by regulating CD81-claudin-1 associations and viral glycoprotein-dependent membrane fusion (Lupberger et al., 2011). EGFR reportedly associates with sodium taurocholate cotransporting polypeptide (NTCP), the HBV receptor on the hepatocyte cell surface, and inhibition of EGFR dramatically impairs HBV virion internalization (Iwamoto et al., 2019;Gan et al., 2020). However, a recent clinical study suggests that HBV reactivation may occur in lung cancer patients receiving erlotinib treatment (Yao et al., 2019). Therefore, the safety and efficacy of sunitinib/erlotinib need to be cautiously investigated.

Chloroquine (CQ) (Lysosomotropic Agents)
CQ is a medication primarily used to treat or prevent a nonresistant malaria infection, it is also occasionally used for amebiasis treatment. Additionally, CQ has shown antiinflammatory properties for the clinical management of some autoimmune diseases such as rheumatoid arthritis and lupus erythematosus (Rainsford et al., 2015). CQ is on the list of WHO's essential medicines. The anti-malarial mechanism of action involves the lysosomotropic feature, which allows CQ to accumulate in an acidic digestive vacuole inside red blood cells, where CQ binds to hemes to form a toxic product resulting in cell lysis and ultimately parasite cell autodigestion. Also, because of the involvement of lysosomes in the autophagy process, the inhibition by CQ of lysosomal enzymes leads to the accumulation of the autophagy cargos that are supposed to break down in lysosomes, leading to the impairment of autophagy machinery.
CQ exerts antiviral activity against DENV infection either in vitro (Farias et al., 2014) and in vivo in an infected monkey model (Farias et al., 2015). However, a double-blind and placebocontrolled trial in 307 dengue patients found that CQ does not reduce the duration of viremia and the viral NS1 antigenemia (Tricou et al., 2010). Although CQ shows antiviral activity against EBOV in vitro; however, CQ fails to protect against EBOV infection and disease pathogenesis in the in vivo guinea pig model (Dowall et al., 2015). CQ shows antiviral activity against some strains of HIV in vitro (Tsai et al., 1990) with an EC50 of 0.4-0.9 μM when treated in combination with hydroxyurea plus didanosine in a lymphocytic cell line (Boelaert et al., 1999). And HCQ exhibits anti-HIV activity in vivo (Sperber et al., 1995). The mechanism may involve the inhibition of cell-to-cell transmission of HIV, which requires clathrin-mediated endocytosis (Bosch et al., 2008), or the inhibition of p120 production at a post-transcriptional level, possibly by impairing hydrolases or other enzymes in acidic vesicles (Savarino et al., 2001).
CQ shows broad and good antiviral activity against coronavirus infection in vitro. However, no efficacy is observed to reduce SARS-CoV virus titers in a nonlethal mouse model (Barnard et al., 2006a). Although CQ and HCQ had drawn a lot of attention for the treatment of COVID-19, the safety or severe side effects of CQ or HCQ at a high dose is  . . .

Camostat or Nafamostat Mesylate (TMPRSS2 Inhibitor)
Camostat and nafamostat are serine protease inhibitors that inhibit the transmembrane protease, serine 2 (TMPRSS2). Camostat is approved in Japan for the treatment of postoperative reflux esophagitis and chronic pancreatitis, while nafamostat is approved for pancreatitis, an anticoagulant in patients with disseminative blood vessel coagulation, hemorrhagic lesions, and hemorrhagic tendencies (Jia et al., 2005;Maruyama et al., 2011). Camostat and nafamostat exhibit effectiveness against a broad spectrum of viruses from different families including IAV (Hosoya et al., 1992;Hosoya et al., 1993;Lee et al., 1996), myxoviruses (Hosoya et al., 1992), DENV (Rathore et al., 2019), MERS-CoV (Shirato et al., 2013), SARS-CoV (Kawase et al., 2012), and SARS-CoV-2 (Hoffmann et al., 2020b) ( Table 4). Particularly, camostat and nafamostat are effective to inhibit type A or B IAV replication in vitro at a micromolar level, while showing no efficacy against other respiratory viruses tested including RSV and parainfluenza virus (Hosoya et al., 1992). Additionally, camostat mesylate at an ED50 (the survival rate of influenza virus-infected chick embryos by 50%) of 0.8 μg/g can increase the survival rate of influenza virus-infected chick embryos (Lee et al., 1996). The mechanism of camostat antiviral activity involves the inhibition of hemagglutinin (HA) cleavage, which is essential for the process of IAV infection and is achieved intracellularly or extracellularly by host proteases like TMPRSS2 (Yamaya et al., 2015).
SARS-CoV and SARS-CoV-2 both use angiotensin-converting enzyme 2 (ACE2) as a receptor, whereas MERS-CoV utilizes DDP4 as the receptor for entry (Li et al., 2003;Raj et al., 2013;Lan et al., 2020). These three coronaviruses all use two mechanisms to enter cells. One involves the direct membrane fusion at the cell surface after the virions binding to receptors, the membrane fusion is triggered by TMPRSS2 serine protease to generate a cleaved form of spike (S) protein for fusion step (Kawase et al., 2012;Shirato et al., 2013;Yamamoto et al., 2016;Hoffmann et al., 2020a). Alternatively, these viruses enter cells through the endocytic pathway in the absence of TMPRSS2, involving the cleavage and priming of S protein by cysteine protease cathepsin B/L in acidic endosomes (Simmons et al., 2005;Qian et al., 2013;Hoffmann et al., 2020a). Inhibition of both TMPRSS2 serine protease and cathepsin B/L cysteine protease activities are required for full blockade of the above coronavirus infection (Kawase et al., 2012;Hoffmann et al., 2020a), while TMPRSS2 inhibition by camostat or nafamostat only results in partial impairment in virus entry (Kawase et al., 2012;Shirato et al., 2013). Strikingly, camostat at a dose of 30 mg/g shows antiviral efficacy by improving the survival rate in a lethal SARS-CoVinfected mouse model; however, a cysteine protease inhibitor SMDC256160 at a higher dose of 50 mg/g is no effective (Zhou et al., 2015). The result suggests that TMPRSS2 rather than cathepsin B/L facilitates the spread of SARS-CoV in the infected mice. It would be of interest to evaluate the treatment efficacy of camostat or nafamostat for SARS-CoV-2 infection, and multiple clinical studies have been available for camostat (phase I/II NCT04321096; phase I/II NCT04435015; phase II  NCT04353284; phase II NCT04374019; phase II NCT04470544;  phase II/III NCT04455815; phase III NCT04355052; phase IV  NCT04338906) and nafamostat (phase II/III NCT04352400; phase II/III NCT04418128; phase II/III NCT04473053).

HTRA Targeting Virus Replication Step
After the viral genome is uncoated from nucleocapsid, viral genome replication and protein translation occur. Positivesense RNA viruses, for instance, coronaviruses and flaviviruses, directly use the viral genome as a template for viral protein translation using host machinery. Negative-sense RNA viruses like filoviruses and myxoviruses, need to generate positive-sense RNA by the virally encoded polymerase, and then protein translation is initiated. Retrovirus and HBV replication involve one additional step, copying RNA to DNA by using virally encoded reverse transcriptase. DNA viruses need to use a host RNA polymerase to generate RNA from the viral DNA genome for protein translation. A number of viruses replicate in specific compartments, so-called replication organelles, in the cytoplasm, involving the aberrant lipid-rich membrane rearrangement (de Wilde et al., 2018). Specifically, some flaviviruses or alphaviruses replicate on an architecture composed of single-membrane spherules (den Boon and Ahlquist, 2010); while coronaviruses or picornaviruses form double-membrane vesicles as replicase sites (de Wilde et al., 2013;van der Schaar et al., 2016). To facilitate viral genome amplification, a variety of host proteins or related pathways are required to generate a favorable environment for virus production. These host proteins or pathways that interact with viral proteins are ideal host-targeting antivirals with a potential comprehensive antiviral efficacy.
Statins efficiently inhibit flaviviral replication either in cell cultures or in animal models. Lovastatin impairs HCV RNA replication by blocking geranylgeranylation of a host protein required for HCV replication. Statins inhibit infectious ZIKV production as well as virus spread, possibly through the inhibition at either the early stage and the post-entry steps (Españo et al., 2019). Lovastatin impairs DENV replicase complex formation or virus assembly (Martinez-Gutierrez et al., 2011;Soto-Acosta et al., 2017). In addition, lovastatin, either prophylactically or even 48 hr post-infection significantly prolongs the survival rate of DENV-infected mice (Martinez-Gutierrez et al., 2014). Lovastatin is able to inhibit HIV reverse transcriptase activity (Mazière et al., 1994), HIV attachment (Giguère and Tremblay, 2004), or HIV virion release (Amet et al., 2008). However, drugdrug interactions are reported to exist for statins and viral protease inhibitors like HCV NS3/4A antagonists or HIV protease inhibitors (Busti et al., 2004;Chauvin et al., 2013). Thus, the efficacy of statins against Flavivirus or HIV infection in the real world needs to be further assessed.
Negative strand RNA viruses are susceptible to statins. Lovastatin shows antiviral potency in RSV-infected mice prophylactically and prevents the illness-associated weight loss (Gower and Graham, 2001), which is consistent with the observation that RSV induces HMG-CoA reductase activity and lovastatin is able to inhibit RSV replication in vitro (Ravi et al., 2013b). Lovastatin impairs coxackie virus B3 infection through downregulating coxsackie and adenovirus receptor expression (Werner et al., 2014). Lovastatin inhibits hPIV assembly and release but no other steps (Bajimaya et al., 2017). Statins are capable of impairing EBOV glycoprotein processing and infectious EBOV production and the glycoprotein-induced attachment (Hacke et al., 2015;Shrivastava-Ranjan et al., 2018). Retrospective clinical studies found that statins may help improve the outcome in hospitalized flu patients (Rothberg et al., 2012), although no effect was observed in IAV-infected rodent models (Kumaki et al., 2012;Belser et al., 2013;Glück et al., 2013).
Statins may benefit COVID-19 patients according to a retrospective study in 13,981 COVID-19 patients . Currently, at least eight clinical trials are being on the way to continue the investigation into the efficacy of statins for COVID-19 (NCT04333407; phase II NCT04348695; phase II NCT04380402; phase II/III NCT04466241; phase III NCT04486508; phase III NCT04472611; phase III NCT04343001; phase IV NCT02735707).
Cardiac glycoside efficiently inhibits DNA virus replication. Digoxin or ouabain exhibits herpes virus, such as HSV and HCMV, at the immediate-early or early gene expression stage, the antiviral activity replies on the inhibition in Na + /K + -ATPase activity (Dodson et al., 2007;Su et al., 2008;Kapoor et al., 2012), Besides, digoxin also reportedly inhibits HSV release step (Su et al., 2008). Digoxin also inhibits HAdV DNA synthesis (Grosso et al., 2017).
Other cardiac glycosides but not digoxin are able to inhibit HBV infection in vitro, possibly through blocking HBV preS1 protein binding to its receptor NTCP (Okuyama-Dobashi et al., 2015). Digoxin inhibits HIV protein expression in peripheral blood mononuclear cells, by a mechanism involving the impaired activity of the CLK family of SR protein kinases (Wong et al., 2013), or the modulation of MEK1/2-ERK1/2 signaling (Wong et al., 2018). In addition, digoxin may also negatively affect HIV integration in T cells (Zhyvoloup et al., 2017). As digoxin exhibits anti-HIV activity with an excellent EC50 (1.1-1.3 nM) at which it is far below the concentration in clinical use, cardiac glycosides merit further investigation to validate the efficacy for HIV treatment.
Digoxin and ouabain at nanomolar inhibit JEV infection in multiple cell culture systems, and ouabain protects against the JEV infection-induced lethality in mice (Guo et al., 2020). The inhibitory mechanism of digoxin or ouabain against (+) ssRNA virus infection may involve impaired RNA replication or virus entry (Ashbrook et al., 2016;Guo et al., 2020). Na + /K + -ATPase is implicated in the entry process of coronaviruses including MHV and FIPV, cardiac glycosides like ouabain or bufalin inhibit MHV, FIPV, and MERS-CoV infections, by inhibiting virus entry and membrane fusion steps (Burkard et al., 2015). By contrast, cardiac glycosides inhibit SARS-CoV-2 at the post-entry step rather than S proteinmediated virus fusion or syncytia formation (Cho et al., 2020;Musarrat et al., 2020). Despite the excellent antiviral activity of cardiac glycosides against SARS-CoV-2 in vitro, no clinical trials have been initiated. One of the possible concerns is the association between digoxin use and increased all-cause mortality.
MPA displays anti-HIV activity both in vitro and in HIV patients (Margolis et al., 1999;Chapuis et al., 2000), and a phase II clinical study (NCT03262441) is currently under investigation. Additionally, MPA potentiates the antiviral effect of reverse transcriptase inhibitors such as abacavir (Margolis et al., 1999). The antiviral mechanisms involve the depleted guanosine pool, as well as the induction of T cell apoptosis (Chapuis et al., 2000).
MPA is also effective for HBV, at 31.2 μΜ (10 μg/ml, the therapeutic concentration in serum for immunosuppressive effect) in primary hepatocytes drastically reduces the secretion of HBV DNA and HBsAg, as well as the intracellular cccDNA level (Gong et al., 1999). Moreover, MPA and RBV, another IMPDH inhibitor, enhance the anti-HBV activity of nucleoside analogs including entecavir (Ying et al., 2000;Ying et al., 2007).
CsA inhibits HCV RNA replication and the antiviral capacity seems independent of its immunosuppressive effect (Watashi et al., 2003;Henry et al., 2006), as ALV, a nonimmunosuppressive CsA analog, maintains the anti-HCV capacity (Crouchet et al., 2018). ALV in follow-up clinical trials (phase I to III) shows a higher sustained virological response rate than IFN and ribavirin do (Stanciu et al., 2019). However, serious side effects were more frequent in ALV-treated patients, the clinical studies were then halted. CyPA interacts with flavivirus proteins and is required for viral replication (Chatterji et al., 2009;Chatterji et al., 2010), and CsA is capable of inhibiting the infections of DENV, WNV, and YFV (Qing et al., 2009). Interestingly, CsA shows strong adulticidal activity against mosquitos, although no direct anti-ZIKV activity was found in a mosquito cell culture system (Dong et al., 2019).
CsA displays anti-HBV activity in vitro, and the inhibition is independent of CyPA or calcineurin (Watashi et al., 2014;Phillips et al., 2015). The inhibitory mechanism involves the block of the interactions of NTCP, the HBV entry receptor, with HBV large envelope protein (Watashi et al., 2014). CsA derivatives that maintain the anti-HBV activities but lose the impact on NTCP transporter activity have been successfully developed (Shimura et al., 2017). CyPA interacts with IAV matrix (M) protein ), but CsA inhibits IAV or IBV infection at the steps of viral replication, IAV protein translation, or virus assembly/ release, in a CyPA-independent manner Hamamoto et al., 2013;Ma et al., 2016). Further preclinical and clinical studies for either virus infection are warranted later.
CyPA specifically binds to the nucleocapsid and Nsp1 proteins of SARS-CoV and is detectable in the SARS-CoV particles (Chen et al., 2005;Pfefferle et al., 2011). CsA inhibits diverse coronaviruses including SARS-CoV, MERS-CoV, HCoV-229E, HCoV-NL63, and MHV (de Wilde et al., 2011;Pfefferle et al., 2011;de Wilde et al., 2013). 16 μM CsA drastically inhibits infectious SARS-CoV production by >3log, although CsA less than 4 μM seems to have pro-viral activity (de Wilde et al., 2011). CsA inhibits SARS-CoV RNA replication or post-entry steps (Pfefferle et al., 2011;Softic et al., 2020), and the early step is also possibly affected (de Wilde et al., 2011). Despite the encouraging results in cell culture systems, treatment with RBV and ALV does not protect from SARS-CoV infection in a mouse model (de Wilde et al., 2017). 9 μM CsA treatment completely blocks the MERS-CoV-induced cytopathology in Vero cells, and CsA in combination with IFN-α display more effective anti-MERS-CoV activity (de Wilde et al., 2013;Li et al., 2018). ALV displays antiviral activity against SARS-CoV-2 with an EC50 of 0.46 μM in vitro (Softic et al., 2020), and CsA in a cohort study showed a 4fold decrease in observed mortality in hospitalized COVID-19 patients (Guisado-Vasco et al., 2020). Currently, at least four clinical trials have been in the process to evaluate the efficacy of CsA or ALV to treat COVID-19 (NCT04451239; phase I NCT04412785; phase II NCT04492891; phase IV NCT04392531). More results will be available soon.

HTRA Targeting Virus Assembly/Release
Step After a sufficient viral structure protein pool is available, viral assembly, a dynamic process driven by programmed sequential reactions is initiated, which involves interactions between the viral genomes and viral capsid proteins, and virus-host protein associations. The newly assembled nonenveloped virions disrupt the cytoskeleton to facilitate dispersal of viral progenies, whilst enveloped viruses gain their envelope from an intracellular organelle or plasma membrane to exit the cells by a budding or exocytosis process, albeit the dividing line between nonenveloped and enveloped viruses has become blurred given that non-lytic spread mechanisms have been identified for HAV, HEV, and some enteroviruses (Feng et al., 2013;Bird et al., 2014;Chen et al., 2015;Yin et al., 2016a). The host endosomal sorting complexes required for transport (ESCRT) and autophagy machinery have emerged roles to mediate the virus release despite the envelopment.

Imatinib (STI-571) (c-Abl Inhibitors)
Imatinib is a 2-phenyl amino pyrimidine derivative that functions as a specific inhibitor of many tyrosine kinases, including c-Abl, c-Kit, and platelet-derived growth factor receptor. It replaces ATP in the enzymatically active site, leading to the decreased activity of these tyrosine kinases. Imatinib is a medication used to treat cancer including chronic myelogenous leukemia, acute lymphocytic leukemia, and gastrointestinal stromal tumors. Imatinib is on the list of WHO's essential medicines. c-Abl is also implicated in the lifecycle of different viruses, and imatinib has been reported to inhibit infection of EBOV, DENV, MERS-CoV, SARS-CoV, coxsackievirus, and VacV (Table 4).
c-Abl1 inhibitor imatinib or nilotinib drastically decreases the budding or release of EBOV, as the inhibition of phosphorylation of the viral matrix protein VP40 (Garcia et al., 2012). Similarly, imatinib significantly dampens the extracellular enveloped VacV virion release without affecting cell-associated enveloped virions, and imatinib shows prophylactical or therapeutic antiviral effect in VacV-infected mice (Reeves et al., 2011). In addition, imatinib significantly inhibits DENV replication at the post-entry steps, reducing the production of infectious DENV (Clark et al., 2016).
Interestingly, imatinib appears to inhibit the entry step of group B coxsackieviruses (CVBs), blocking the aggregation of virions to the tight junction, where the virions subsequently initiate the internalization step to finally surmount the epithelial barrier (Coyne and Bergelson, 2006).
Imatinib or other c-Abl inhibitors nilotinib and dasatinib are able to inhibit MERS-CoV or SARS-CoV infection (Dyall et al., 2014). Specifically, imatinib and dasatinib show effectiveness against both viruses, while nilotinib is only effective for SARS-CoV (Dyall et al., 2014). Recently, imatinib was reported to inhibit SARS-CoV-2 in stem cell-differentiated lung organoids (EC50 4.86 μM) (Han et al., 2021). The detailed mechanism for this inhibition warrants further investigation. Currently, at least five clinical trials including three phase III studies (NCT04394416; NCT04422678; NCT04356495) have been carried out to investigate the treatment efficacy of imatinib for COVID-19.

Repurposed Agents With Multiple Targets
Some repurposed agents have more than three potential targets, either viral or host proteins. The most documented example is IFN-α/β, which is a crucial member of innate immunity, the first line to defend pathogens including viruses. Another instance is nitazoxanide, which has shown a very broad antiviral efficiency, representing divergent antiviral mechanisms for different viruses.

Interferon
Almost all viruses can induce interferon response that is mediated by different sensors including cGAS for DNA viruses; RIG-I, MDA5 for RNA viruses (Tan et al., 2018). These pattern recognition receptors recognize the invaders containing pathogen-associated molecular patterns to induce IFN, which in turn secretes out of cells and binds to receptors to induce the activation of JAK-STAT pathway. As a result, a broad spectrum of interferon-stimulated genes (ISG) is induced and exert antiviral effects through different mechanisms (Liu et al., 2013;Bailey et al., 2014;Li et al., 2017a). Clearly, the ISG network is diverse and complicated, each ISG functions in concert with others, in a combinatorial or even redundant way to combat virus infection. There are three classes of IFNs, type I, type II, and type III, distinguished by their different receptors. The type I IFNs include IFN-α, IFN-β, IFN-ε, IFN-κ, and IFN-ω; type II IFN comprises IFN-γ; and type III IFNs refer to IFN-λ1 (IL-29), IFN-λ2 (IL-28A), IFN-λ3 (IL-28B), and IFN-λ4 (Stanifer et al., 2019). IFN-α has been used for clinical purposes against HCV and HBV for a long time, and IFN-β is approved to treat multiple sclerosis (Rice et al., 2001;Heim, 2013;Ezzikouri et al., 2020).
IFNs enable to inhibit SARS-CoV or MERS-CoV in cell cultures (Cinatl et al., 2003;Falzarano et al., 2013a), and show potency in macaques infected with SARS-CoV or MERS-CoV (Haagmans et al., 2004;Falzarano et al., 2013b). A retrospective cohort study in MERS-CoV patients shows that IFN-α-2a plus RBV results in a significant survival rate at 14 days than control supportive care (Omrani et al., 2014). However, the same treatment regimen did not benefit MERS patients in another clinical study (Al-Tawfiq et al., 2014). A preliminary, uncontrolled study shows that IFN plus corticosteroids are associated with better disease outcomes in SARS patients compared to corticosteroid treatment alone (Loutfy et al., 2003). Interestingly, a dual role of IFN for SARS pathology in mice was recently reported, while delayed IFN response correlates with severe lung immunopathology and reduced survival rate, early IFN administration ameliorates immunopathology (Channappanavar et al., 2016). IFN-α or IFN-β efficiently inhibits SARS-CoV-2 infection in vitro (EC50 1.35 and 0.76 IU/ml, respectively) (Mantlo et al., 2020). However, the Solidarity clinical trials launched by WHO concluded that IFN does not affect overall mortality in hospitalized COVID-19 patients. In spite of that, multiple clinical studies are still in progress to evaluate the efficacy of IFNA for COVID-19 including four phase III (NCT04492475; NCT04320238; NCT04324463; NCT04315948) and five phase IV (NCT04350671; NCT04254874; NCT04350684; NCT04291729; NCT02735707) trials.
HBV or HCV is susceptible to nitazoxanide treatment. An open-label small-scale clinical trial shows the preliminary efficacy of nitazoxanide in treating chronic hepatitis B . A further phase II clinical study (NCT03905655) is currently instigated. Clinical trials in hepatitis C patients show the improved SVR rate when treated alone or in combination with IFN and/or RBV Elazar et al., 2009;Rossignol et al., 2010).

CHALLENGES AND PERSPECTIVE
Currently, most of the approved antivirals are used to treat infections of HIV, HCV, HBV, and IAV, very few novel antivirals for recently emerging viruses including SARS-CoV-2, MERS-CoV, EBOV, ZIKV, and DENV. Drug repurposing has played a crucial role in pushing the approved or investigational therapeutics through clinical trials, because of higher success rate, less investment, and faster approval.
Drug repurposing is not risk-free, the success rate is reported around 30%. There are still a lot of hurdles before the repurposed drug is approved. Even though repurposed drugs could be exempted from phase I clinical trial, which mainly focuses on the drug safety evaluation, drug safety still represents one of the biggest concerns for repurposing. For instance, the safety of the drug that has been evaluated in a group of participants for the original indication does not necessarily guarantee safety in another group of people. In this scenario, drug safety may need to re-evaluate. Moreover, the dosing regimen of the repurposed drug validated previously may be different in new indications. A major obstacle to successful repurposing attributes to the higher effective concentrations in the new indication than those in the original indications. It suggests that greater harm and less benefit may be instigated. To overcome the obstacle, cocktailbased combinatorial regimens that contains at least two repurposed drugs targeting different steps of the viral lifecycle would be beneficial. In addition, drug-drug interactions may be another challenge when a repurposed drug has to use in combination with other drugs and no drug-drug interaction data is available. Thus, drug safety issue needs to be carefully appraised and addressed if necessary.
Apart from the safety issue, the intellectual property barrier is another important issue that needs to address. Commonly, drug repurposing focuses on drugs for which the patents on the original indication have been expired. For the off-patent generic drugs, a new method-of-use patent can be obtained for the new indication. However, enforceability or market exclusivity can be a major issue, as other company-manufactured generic drugs may be prescribed as off-label use for the new indication, which would be hardly prohibited. That may reduce the profit and financial incentive for drug repurposing. The off-label use can be limited if a new formulation or dosing regimen is required for the novel indication so that it cannot be easily fulfilled with the available generic versions of the drug. On the other hand, with the appropriate licensing, repurposing of drugs that are still covered by existing patent property is also achievable, even though many repurposed uses of patent drugs have been reported in the literature, which may limit the ability to gain the new patent protection. The reliable and novel evidence to support the new indication of the repurposed drug is a necessity to obtain the granted patents. Other challenges include self-medication, misuse, drug shortage, and price hike of the drugs with potential repurposed indications (Mallhi et al., 2020a;Mallhi et al., 2020b). The misuse of repurposed drugs would be devastating and should be discouraged particularly during a pandemic. The availability and affordability of these repurposed agents should also be vigilantly monitored.
With more approaches to address the safety, efficacy, and patent issues by deploying recombination therapies and reinforcing collaboration and negotiation, drug repurposing for a novel, efficient, and broad-spectrum antiviral development would strengthen the efforts to combat the emerging and re-emerging viruses.

AUTHOR CONTRIBUTIONS
XL drafted the manuscript. TP edited and revised the manuscript. All authors reviewed and approved the final version of the manuscript for publication.