In vitro and in vivo activities of anti-influenza virus compound T-705

In silico structural elucidation of RNA-dependent RNA polymerase towards the identification of potential Crimean-Congo Hemorrhagic Fever Virus inhibitors

The Crimean-Congo Hemorrhagic Fever virus (CCHFV) is a segmented negative single-stranded RNA virus (-ssRNA) which causes severe hemorrhagic fever in humans with a mortality rate of ~50%. To date, no vaccine has been approved. Treatment is limited to supportive care with few investigational drugs in practice. Previous studies have identified viral RNA dependent RNA Polymerase (RdRp) as a potential drug target due to its significant role in viral replication and transcription. Since no crystal structure is available yet, we report the structural elucidation of CCHFV-RdRp by in-depth homology modeling. Even with low sequence identity, the generated model suggests a similar overall structure as previously reported RdRps. More specifically, the model suggests the presence of structural/functional conserved RdRp motifs for polymerase function, the configuration of uniform spatial arrangement of core RdRp sub-domains, and predicted positively charged entry/exit tunnels, as seen in sNSV polymerases. Extensive pharmacophore modeling based on per-residue energy contribution with investigational drugs allowed the concise mapping of pharmacophoric features and identified potential hits. The combination of pharmacophoric features with interaction energy analysis revealed functionally important residues in the conserved motifs together with in silico predicted common inhibitory binding modes with highly potent reference compounds.

Mechanism of Inhibition of Ebola Virus RNA-Dependent RNA Polymerase by Remdesivir

Remdesivir (GS-5734) is a 1'-cyano-substituted adenosine nucleotide analogue prodrug that shows broad-spectrum antiviral activity against several RNA viruses. This compound is currently under clinical development for the treatment of Ebola virus disease (EVD). While antiviral effects have been demonstrated in cell culture and in non-human primates, the mechanism of action of Ebola virus (EBOV) inhibition for remdesivir remains to be fully elucidated. The EBOV RNA-dependent RNA polymerase (RdRp) complex was recently expressed and purified, enabling biochemical studies with the relevant triphosphate (TP) form of remdesivir and its presumptive target. In this study, we confirmed that remdesivir-TP is able to compete for incorporation with adenosine triphosphate (ATP). Enzyme kinetics revealed that EBOV RdRp and respiratory syncytial virus (RSV) RdRp incorporate ATP and remdesivir-TP with similar efficiencies. The selectivity of ATP against remdesivir-TP is ~4 for EBOV RdRp and ~3 for RSV RdRp. In contrast, purified human mitochondrial RNA polymerase (h-mtRNAP) effectively discriminates against remdesivir-TP with a selectivity value of ~500-fold. For EBOV RdRp, the incorporated inhibitor at position i does not affect the ensuing nucleotide incorporation event at position i+1. For RSV RdRp, we measured a ~6-fold inhibition at position i+1 although RNA synthesis was not terminated. Chain termination was in both cases delayed and was seen predominantly at position i+5. This pattern is specific to remdesivir-TP and its 1'-cyano modification. Compounds with modifications at the 2'-position show different patterns of inhibition. While 2'-C-methyl-ATP is not incorporated, ara-ATP acts as a non-obligate chain terminator and prevents nucleotide incorporation at position i+1. Taken together, our biochemical data indicate that the major contribution to EBOV RNA synthesis inhibition by remdesivir can be ascribed to delayed chain termination. The long distance of five residues between the incorporated nucleotide analogue and its inhibitory effect warrant further investigation.

Drug Repurposing Approaches for the Treatment of Influenza Viral Infection: Reviving Old Drugs to Fight Against a Long-Lived Enemy

Influenza viruses still constitute a real public health problem today. To cope with the emergence of new circulating strains, but also the emergence of resistant strains to classic antivirals, it is necessary to develop new antiviral approaches. This review summarizes the state-of-the-art of current antiviral options against influenza infection, with a particular focus on the recent advances of anti-influenza drug repurposing strategies and their potential therapeutic, regulatory and economic benefits. The review will illustrate the multiple ways to reposition molecules for the treatment of influenza, from adventitious discovery to in silico-based screening. These novel antiviral molecules, many of which targeting the host cell, in combination with conventional antiviral agents targeting the virus, will ideally enter the clinics and reinforce the therapeutic arsenal to combat influenza virus infections.

RNAi-based small molecule repositioning reveals clinically approved urea-based kinase inhibitors as broadly active antivirals

Influenza viruses (IVs) tend to rapidly develop resistance to virus-directed vaccines and common antivirals targeting pathogen determinants, but novel host-directed approaches might preclude resistance development. To identify the most promising cellular targets for a host-directed approach against influenza, we performed a comparative small interfering RNA (siRNA) loss-of-function screen of IV replication in A549 cells. Analysis of four different IV strains including a highly pathogenic avian H5N1 strain, an influenza B virus (IBV) and two human influenza A viruses (IAVs) revealed 133 genes required by all four IV strains. According to gene enrichment analyses, these strain-independent host genes were particularly enriched for nucleocytoplasmic trafficking. In addition, 360 strain-specific genes were identified with distinct patterns of usage for IAVs versus IBV and human versus avian IVs. The strain-independent host genes served to define 43 experimental and otherwise clinically approved drugs, targeting reportedly fourteen of the encoded host factors. Amongst the approved drugs, the urea-based kinase inhibitors (UBKIs) regorafenib and sorafenib exhibited a superior therapeutic window of high IV antiviral activity and low cytotoxicity. Both UBKIs appeared to block a cell signaling pathway involved in IV replication after internalization, yet prior to vRNP uncoating. Interestingly, both compounds were active also against unrelated viruses including cowpox virus (CPXV), hantavirus (HTV), herpes simplex virus 1 (HSV1) and vesicular stomatitis virus (VSV) and showed antiviral efficacy in human primary respiratory cells. An in vitro resistance development analysis for regorafenib failed to detect IV resistance development against this drug. Taken together, the otherwise clinically approved UBKIs regorafenib and sorafenib possess high and broad-spectrum antiviral activity along with substantial robustness against resistance development and thus constitute attractive host-directed drug candidates against a range of viral infections including influenza.

Conventional medications against influenza infections, including vaccination and antiviral drug therapy, are targeted against viral determinants–an approach collectively referred to as pathogen-directed. However, influenza viruses mutate fast and quickly develop resistance to these pathogen-directed treatments. An alternative, yet not well established, is to block host cellular molecules required by the virus to successfully multiply. Such a host-directed approach is anticipated to be more robust against the development of drug resistance. This notion is founded on the different modes of action of the two principal approaches: Virus-directed therapeutics target the virus itself. Thus, just a single mutation could abrogate sensitivity to a virus-directed therapeutic. In contrast, it is unlikely that viruses can easily circumvent a pharmacological blockage of a cellular factor by means of just a few mutations. Instead, the virus needs to either exploit an immediate parallel cellular pathway or adjust its replication cycle to a different cellular factor–the latter being a process likely to require multiple mutations, if possible at all. To identify the most promising targets for a host-directed therapy, we performed a small interfering RNA (siRNA) screen with four different influenza virus strains using a lung epithelial cell line. Subsequently, we tested a series of drugs, specific for the products of the genes that are required for replication of all four influenza virus strains tested. Regorafenib and sorafenib, two chemically related urea-based kinase inhibitors already clinically approved for cancer treatment, turned out to be effective inhibitors of all influenza viruses and displayed low cytotoxicity. These drugs blocked viral replication at an early stage of the life cycle not only in cell lines but also in human primary respiratory cells. Moreover, these drugs exhibited high efficacy even against unrelated viruses. In addition, no development of resistance was observed against regorafenib, which was used in an in vitro assay representatively of urea-based kinase inhibitors. Our results suggest that regorafenib and sorafenib are promising drug candidates for a host-directed therapy of influenza and other viral infections.

Potent and broad-spectrum cycloheptathiophene-3-carboxamide compounds that target the PA-PB1 interaction of influenza virus RNA polymerase and possess a high barrier to drug resistance

Influenza viruses are major respiratory pathogens responsible for both seasonal epidemics and occasional pandemics worldwide. The current available treatment options have limited efficacy and thus the development of new antivirals is highly needed. We previously reported the identification of a series of cycloheptathiophene-3-carboxamide compounds as influenza A virus inhibitors that act by targeting the protein-protein interactions between the PA-PB1 subunits of the viral polymerase. In this study, we characterized the antiviral properties of the most promising compounds as well as investigated their propensity to induce drug resistance. Our results show that some of the selected compounds possess potent, broad-spectrum anti-influenza activity as they efficiently inhibited the replication of several strains of influenza A and B viruses, including an oseltamivir-resistant clinical isolate, with nanomolar or low-micromolar potency. The most promising compounds specifically inhibited the PA-PB1 binding in vitro and interfered with the influenza A virus polymerase activity in a cellular context, without showing cytotoxicity. The most active PA-PB1 inhibitors showed to possess a drug resistance barrier higher than that of oseltamivir. Indeed, no viral variants with reduced susceptibility to the selected compounds emerged after serial passages of influenza A virus under drug selective pressure. Overall, our studies identified potent PA-PB1 inhibitors as promising candidates for the development of new anti-influenza drugs.

New World alphavirus protein interactomes from a therapeutic perspective

The New World alphaviruses, Venezuelan, eastern and western equine encephalitis viruses (VEEV, EEEV, and WEEV), are important human pathogens due to their ability to cause varying levels of morbidity and mortality in humans. There is also concern about VEEV and EEEV being used as bioweapons. Currently, a FDA-approved antiviral is lacking for New World alphaviruses. In this review, the function of each viral protein is discussed with an emphasis on how these functions can be targeted by therapeutics. Both direct acting antivirals as well as inhibitors that impact host protein interactions with viral proteins are described. Non-structural protein 3 (nsP3), capsid, and E2 proteins have garnered attention in recent years, whereas little is known regarding host protein interactions of the other viral proteins and is an important avenue for future study.

Determining the Mutation Bias of Favipiravir in Influenza Virus Using Next-Generation Sequencing

New antiviral drugs are needed as a first line of defense in the event of a novel influenza pandemic. Favipiravir is a broad-spectrum antiviral which is effective against influenza. The exact mechanism of how favipiravir works to inhibit influenza is still unclear. We used next-generation sequencing (NGS) to demonstrate that favipiravir causes mutations in influenza RNA. The greater depth of NGS sequence information over traditional sequencing methods allowed us to precisely determine the bias of particular mutations caused by favipiravir. NGS can also be used in a standard diagnostic pipeline to show that favipiravir is acting on the virus by revealing the mutation bias pattern typical to the drug. Our work will aid in testing whether viruses are resistant to favipiravir and may help demonstrate the effect of favipiravir on viruses in a clinical setting. This will be important if favipiravir is used during a future influenza pandemic.

Favipiravir is a broad-spectrum antiviral drug that may be used to treat influenza. Previous research has identified that favipiravir likely acts as a mutagen, but the precise mutation bias that favipiravir induces in influenza virus RNAs has not been described. Here, we use next-generation sequencing (NGS) with barcoding of individual RNA molecules to accurately and quantitatively detect favipiravir-induced mutations and to sample orders of magnitude more mutations than would be possible through Sanger sequencing. We demonstrate that favipiravir causes mutations and show that favipiravir primarily acts as a guanine analogue and secondarily as an adenine analogue resulting in the accumulation of transition mutations. We also use a standard NGS pipeline to show that the mutagenic effect of favipiravir can be measured by whole-genome sequencing of virus.

IMPORTANCE New antiviral drugs are needed as a first line of defense in the event of a novel influenza pandemic. Favipiravir is a broad-spectrum antiviral which is effective against influenza. The exact mechanism of how favipiravir works to inhibit influenza is still unclear. We used next-generation sequencing (NGS) to demonstrate that favipiravir causes mutations in influenza RNA. The greater depth of NGS sequence information over traditional sequencing methods allowed us to precisely determine the bias of particular mutations caused by favipiravir. NGS can also be used in a standard diagnostic pipeline to show that favipiravir is acting on the virus by revealing the mutation bias pattern typical to the drug. Our work will aid in testing whether viruses are resistant to favipiravir and may help demonstrate the effect of favipiravir on viruses in a clinical setting. This will be important if favipiravir is used during a future influenza pandemic.

Rhinovirus RNA Polymerase

Human rhinovirus is responsible for causing 50% of common cold infections in infants and adults. It belongs to the picornavirus family of nonenveloped positive-strand RNA viruses. The RNA synthesis of rhinovirus is carried out by RNA-dependent RNA polymerase, also known as 3D^Pol. It catalyzes the synthesis of negative-strand RNA using a positive-strand template. The structure of the enzyme consists of three domains: palm, fingers, and thumb domains and Mg²⁺ in the active site. These conserved structural features of the enzyme help in catalyzing phosphodiester bond formation between the two consecutive nucleotide units complimentary to the template RNA using a VPg primer. Owing to the presence of over 100 serotypes of the enzyme, designing specific inhibitors targeting the polymerase is a challenging task and until now no clinically approved antirhino viral drug is reported. In this review, we have given detailed information about the structure and function of the enzyme and also discussed some of the inhibitors and their in vivo activity against 3D^Pol.

Novel antiviral drug discovery strategies to tackle drug-resistant mutants of influenza virus strains

INTRODUCTION

The emergence of drug-resistant influenza virus strains highlights the need for new antiviral therapeutics to combat future pandemic outbreaks as well as continuing seasonal cycles of influenza. Areas covered: This review summarizes the mechanisms of current FDA-approved anti-influenza drugs and patterns of resistance to those drugs. It also discusses potential novel targets for broad-spectrum antiviral drugs and recent progress in novel drug design to overcome drug resistance in influenza. Expert opinion: Using the available structural information about drug-binding pockets, research is currently underway to identify molecular interactions that can be exploited to generate new antiviral drugs. Despite continued efforts, antivirals targeting viral surface proteins like HA, NA, and M2, are all susceptible to developing resistance. Structural information on the internal viral polymerase complex (PB1, PB2, and PA) provides a new avenue for influenza drug discovery. Host factors, either at the initial step of viral infection or at the later step of nuclear trafficking of viral RNP complex, are being actively pursued to generate novel drugs with new modes of action, without resulting in drug resistance.

The Antihistamine Drugs Carbinoxamine Maleate and Chlorpheniramine Maleate Exhibit Potent Antiviral Activity Against a Broad Spectrum of Influenza Viruses

Influenza A viruses (IAV) comprise some of the most common infectious pathogens in humans, and they cause significant mortality and morbidity in immunocompromised people as well as children and the elderly. After screening an FDA-approved drug library containing 1280 compounds by cytopathic effect (CPE) reduction assay using the Cell Counting Kit-8, we found two antihistamines, carbinoxamine maleate (CAM) and S-(+)-chlorpheniramine maleate (SCM) with potent antiviral activity against A/Shanghai/4664T/2013(H7N9) infection with IC₅₀ (half-maximal inhibitory concentration) of 3.56 and 11.84 μM, respectively. Further studies showed that CAM and SCM could also inhibit infection by other influenza A viruses, including A/Shanghai/37T/2009(H1N1), A/Puerto Rico/8/1934(H1N1), A/Guizhou/54/1989(H3N2), and one influenza B virus, B/Shanghai/2017(BY). Mice were challenged intranasally with A/H7N9/4664T/2013 (H7N9) virus and intraperitoneally injected with CAM (10 mg/kg per day) or SCM (1 mg/kg per day) for 5 days. CAM or SCM (10 mg/kg per day) were fully protected against challenge with A/Shanghai/4664T/2013(H7N9). The results from mechanistic studies indicate that both could inhibit influenza virus infection by blocking viral entry into the target cell, the early stage of virus life cycle. However, CAM and SCM neither blocked virus attachment, characteristic of HA activity, nor virus release, characteristic of NA activity. Such data suggest that these two compounds may interfere with the endocytosis process. Thus, we have identified two FDA-approved antihistamine drugs, CAM and SCM, which can be repurposed for inhibiting infection by divergent influenza A strains and one influenza B strain with potential to be used for treatment and prevention of influenza virus infection.

Combination Therapy with Oseltamivir and Favipiravir Delays Mortality but Does Not Prevent Oseltamivir Resistance in Immunodeficient Mice Infected with Pandemic A(H1N1) Influenza Virus

Immunosuppressed individuals can shed influenza virus for prolonged periods of time, leading to the frequent emergence of antiviral resistance. We evaluated the benefits of oseltamivir and favipiravir combination therapy compared to single antiviral agents and monitored the emergence of drug-resistant variants in a pharmacologically immunosuppressed mouse model infected with the A(H1N1) pandemic influenza virus. C57BL/6 mice were immunosuppressed with cyclophosphamide and infected with a lethal dose of pandemic influenza A(H1N1) virus. Forty-eight hours post-infection, mice were treated with oseltamivir (20 mg/kg), favipiravir (20 or 50 mg/kg) or both agents BID for 5 or 10 days. Body weight losses, survival rates, lung viral titers, cytokine levels and emergence of resistant viruses were evaluated. Treatment of immunosuppressed mice with high (50 mg/kg) but not low (20 mg/kg) doses of favipiravir in combination with oseltamivir (20 mg/kg) significantly delayed mortality and reduced lung viral titers compared to treatment with a single drug regimen with oseltamivir but did not prevent the emergence of oseltamivir-resistant H275Y neuraminidase variants. Combination therapy with oseltamivir and favipiravir should be considered for evaluation in clinical trials.

Favipiravir inhibits in vitro Usutu virus replication and delays disease progression in an infection model in mice

Usutu virus (USUV) is an emerging flavivirus that causes Usutu disease mainly in birds, but infection of mammals such as rodents, bats and horses has also been demonstrated. In addition, human cases (both in immunocompromised and -competent individuals) were also reported. Large outbreaks with other flaviviruses, such as West Nile virus and Zika virus, indicate that one should be vigilant for yet other outbreaks. To allow the identification of inhibitors of USUV replication, we established in vitro antiviral assays, which were validated using a small selection of known flavivirus inhibitors, including the broad-spectrum viral RNA polymerase inhibitor favipiravir (T-705). Next, an USUV infection model in AG129 (IFN-α/β and IFN-γ receptor knockout) mice was established. AG129 mice proved highly susceptible to USUV; an inoculum as low as 10² PFU (1.3 × 10⁵ TCID₅₀) resulted in the development of symptoms as early as 3 days post infection with viral RNA being detectable in various tissues. Treatment of mice with favipiravir (150 mg/kg/dose, BID, oral gavage) significantly reduced viral load in blood and tissues and significantly delayed virus-induced disease. This USUV mouse model is thus amenable for assessing the potential in vivo efficacy of (novel) USUV/flavivirus inhibitors.

Recent advances in understanding Crimean-Congo hemorrhagic fever virus

Crimean-Congo hemorrhagic fever virus (CCHFV) is a widely distributed hemorrhagic fever virus and the cause of hemorrhagic disease in Africa, Southern and Eastern Europe, the Middle East, India and Asia. Recent emergence of CCHFV into Spain indicates that the geographic range of this virus is expanding and the presence of its tick vector in several countries without reported disease suggest that CCHFV will continue to spread. Research into CCHFV was historically limited by a lack of suitable animal models and tools to study viral pathogenesis. However, in the past few years the toolset for studying CCHFV has expanded with small animal and non-human primate models for CCHFV being developed along with a reverse genetics system that allows for investigation of viral determinants of disease. These tools have been utilized to understand how CCHFV antagonizes host restriction factors and to develop novel vaccine candidates that may help limit the substantial morbidity and mortality in humans caused by CCHFV.

The mechanism of resistance to favipiravir in influenza

Favipiravir is a broad-spectrum antiviral that has shown promise in treatment of influenza virus infections, in particular due to the apparent lack of emergence of resistance mutations against the drug in cell culture or animal studies. We demonstrate here that a mutation in a conserved region of the viral RNA polymerase confers resistance to favipiravir in vitro and in cell culture. The resistance mutation has a cost to viral fitness, but this can be restored by a compensatory mutation in the polymerase. Our findings support the development of favipiravir-resistance diagnostic and surveillance testing strategies and reinforce the importance of considering combinations of therapies to treat influenza infections.

Favipiravir is a broad-spectrum antiviral that has shown promise in treatment of influenza virus infections. While emergence of resistance has been observed for many antiinfluenza drugs, to date, clinical trials and laboratory studies of favipiravir have not yielded resistant viruses. Here we show evolution of resistance to favipiravir in the pandemic H1N1 influenza A virus in a laboratory setting. We found that two mutations were required for robust resistance to favipiravir. We demonstrate that a K229R mutation in motif F of the PB1 subunit of the influenza virus RNA-dependent RNA polymerase (RdRP) confers resistance to favipiravir in vitro and in cell culture. This mutation has a cost to viral fitness, but fitness can be restored by a P653L mutation in the PA subunit of the polymerase. K229R also conferred favipiravir resistance to RNA polymerases of other influenza A virus strains, and its location within a highly conserved structural feature of the RdRP suggests that other RNA viruses might also acquire resistance through mutations in motif F. The mutations identified here could be used to screen influenza virus-infected patients treated with favipiravir for the emergence of resistance.

In vitro characterization of baloxavir acid, a first-in-class cap-dependent endonuclease inhibitor of the influenza virus polymerase PA subunit

Cap-dependent endonuclease (CEN) resides in the PA subunit of the influenza virus and mediates the critical "cap-snatching" step of viral RNA transcription, which is considered to be a promising anti-influenza target. Here, we describe in vitro characterization of a novel CEN inhibitor, baloxavir acid (BXA), the active form of baloxavir marboxil (BXM). BXA inhibits viral RNA transcription via selective inhibition of CEN activity in enzymatic assays, and inhibits viral replication in infected cells without cytotoxicity in cytopathic effect assays. The antiviral activity of BXA is also confirmed in yield reduction assays with seasonal type A and B viruses, including neuraminidase inhibitor-resistant strains. Furthermore, BXA shows broad potency against various subtypes of influenza A viruses (H1N2, H5N1, H5N2, H5N6, H7N9 and H9N2). Additionally, serial passages of the viruses in the presence of BXA result in isolation of PA/I38T variants with reduced BXA susceptibility. Phenotypic and genotypic analyses with reverse genetics demonstrate the mechanism of BXA action via CEN inhibition in infected cells. These results reveal the in vitro characteristics of BXA and support clinical use of BXM to treat influenza.

Viral Sepsis in Children

Sepsis in children is typically presumed to be bacterial in origin until proven otherwise, but frequently bacterial cultures ultimately return negative. Although viruses may be important causative agents of culture-negative sepsis worldwide, the incidence, disease burden and mortality of viral-induced sepsis is poorly elucidated. Consideration of viral sepsis is critical as its recognition carries implications on appropriate use of antibacterial agents, infection control measures, and, in some cases, specific, time-sensitive antiviral therapies. This review outlines our current understanding of viral sepsis in children and addresses its epidemiology and pathophysiology, including pathogen-host interaction during active infection. Clinical manifestation, diagnostic testing, and management options unique to viral infections will be outlined.

Treating Influenza Infection, From Now and Into the Future

Influenza viruses (IVs) are a continual threat to global health. The high mutation rate of the IV genome makes this virus incredibly successful, genetic drift allows for annual epidemics which result in thousands of deaths and millions of hospitalizations. Moreover, the emergence of new strains through genetic shift (e.g., swine-origin influenza A) can cause devastating global outbreaks of infection. Neuraminidase inhibitors (NAIs) are currently used to treat IV infection and act directly on viral proteins to halt IV spread. However, effectivity is limited late in infection and drug resistance can develop. New therapies which target highly conserved features of IV such as antibodies to the stem region of hemagglutinin or the IV RNA polymerase inhibitor: Favipiravir are currently in clinical trials. Compared to NAIs, these treatments have a higher tolerance for resistance and a longer therapeutic window and therefore, may prove more effective. However, clinical and experimental evidence has demonstrated that it is not just viral spread, but also the host inflammatory response and damage to the lung epithelium which dictate the outcome of IV infection. Therapeutic regimens for IV infection should therefore also regulate the host inflammatory response and protect epithelial cells from unnecessary cell death. Anti-inflammatory drugs such as etanercept, statins or cyclooxygenase enzyme 2 inhibitors may temper IV induced inflammation, demonstrating the possibility of repurposing these drugs as single or adjunct therapies for IV infection. IV binds to sialic acid receptors on the host cell surface to initiate infection and productive IV replication is primarily restricted to airway epithelial cells. Accordingly, targeting therapies to the epithelium will directly inhibit IV spread while minimizing off target consequences, such as over activation of immune cells. The neuraminidase mimic Fludase cleaves sialic acid receptors from the epithelium to inhibit IV entry to cells. While type III interferons activate an antiviral gene program in epithelial cells with minimal perturbation to the IV specific immune response. This review discusses the above-mentioned candidate anti-IV therapeutics and others at the preclinical and clinical trial stage.

Favipiravir (T-705) but not ribavirin is effective against two distinct strains of Crimean-Congo hemorrhagic fever virus in mice

Crimean-Congo hemorrhagic fever virus (CCHFV) is a cause of serious hemorrhagic disease in humans. Humans infected with CCHFV develop a non-specific febrile illness and then progress to the hemorrhagic phase where case fatality rates can be as high as 30%. Currently there is lack of vaccines and the recommended antiviral treatment, ribavirin, has inconsistent efficacy in both human and animal studies. In this study we developed a model of CCHFV infection in type I interferon deficient mice using the clinical CCHFV isolate strain Hoti. Mice infected with strain Hoti develop a progressively worsening and ultimately fatal disease. We utilized this model along with our established model using the prototypical CCHFV strain 10200 to evaluate treatment with ribavirin or the antiviral favipiravir. While ribavirin treatment was able to suppress viral loads at early time points it was ultimately unable to prevent development of terminal disease in mice infected with either strain of CCHFV. In contrast, favipiravir showed clinical benefit even when administered late in the clinical progression of CCHF. Interestingly, in a small subset of mice, late-onset of CCHF was observed after favipiravir treatment was stopped and persistence of viral RNA in favipiravir treated survivors was also seen. Nevertheless, favipiravir showed significant clinical benefit against two distinct strains of CCHFV suggesting it may be a potent antiviral for treatment of human CCHFV infections.

Clinical Regimens of Favipiravir Inhibit Zika Virus Replication in the Hollow-Fiber Infection Model

Zika virus (ZIKV) infection is associated with serious, long-term neurological manifestations. There are currently no approved therapies for the treatment or prevention of ZIKV infection. Favipiravir (FAV) is a viral polymerase inhibitor with broad-spectrum activity. Our prior studies used static FAV concentrations and demonstrated promising activity. However, the anti-ZIKV activity of dynamic FAV concentrations has never been evaluated in a human cell line. Here we employed the hollow-fiber infection model (HFIM) to simulate the human pharmacokinetic (PK) profiles associated with the clinically utilized FAV dosage regimens against influenza and Ebola viruses and assessed the viral burden profiles. Clinically achievable FAV concentrations inhibited ZIKV replication in HUH-7 cells in a dose-dependent fashion (50% effective concentration = 236.5 μM). The viral burden profiles under dynamic FAV concentrations were predicted by use of a mechanism-based mathematical model (MBM) and subsequently successfully validated in the HFIM. This validated, translational MBM can now be used to predict the anti-ZIKV activity of other FAV dosage regimens in the presence of between-patient variability in pharmacokinetics. This approach can be extended to rationally optimize FAV combination dosage regimens which hold promise to treat ZIKV infections in nonpregnant patients.

Advances in Designing and Developing Vaccines, Drugs, and Therapies to Counter Ebola Virus

Ebola virus (EBOV), a member of the family Filoviridae, is responsible for causing Ebola virus disease (EVD) (formerly named Ebola hemorrhagic fever). This is a severe, often fatal illness with mortality rates varying from 50 to 90% in humans. Although the virus and associated disease has been recognized since 1976, it was only when the recent outbreak of EBOV in 2014-2016 highlighted the danger and global impact of this virus, necessitating the need for coming up with the effective vaccines and drugs to counter its pandemic threat. Albeit no commercial vaccine is available so far against EBOV, a few vaccine candidates are under evaluation and clinical trials to assess their prophylactic efficacy. These include recombinant viral vector (recombinant vesicular stomatitis virus vector, chimpanzee adenovirus type 3-vector, and modified vaccinia Ankara virus), Ebola virus-like particles, virus-like replicon particles, DNA, and plant-based vaccines. Due to improvement in the field of genomics and proteomics, epitope-targeted vaccines have gained top priority. Correspondingly, several therapies have also been developed, including immunoglobulins against specific viral structures small cell-penetrating antibody fragments that target intracellular EBOV proteins. Small interfering RNAs and oligomer-mediated inhibition have also been verified for EVD treatment. Other treatment options include viral entry inhibitors, transfusion of convalescent blood/serum, neutralizing antibodies, and gene expression inhibitors. Repurposed drugs, which have proven safety profiles, can be adapted after high-throughput screening for efficacy and potency for EVD treatment. Herbal and other natural products are also being explored for EVD treatment. Further studies to better understand the pathogenesis and antigenic structures of the virus can help in developing an effective vaccine and identifying appropriate antiviral targets. This review presents the recent advances in designing and developing vaccines, drugs, and therapies to counter the EBOV threat.

An easy, rapid, and sensitive method for detection of drug-resistant influenza virus by using a sialidase fluorescent imaging probe, BTP3-Neu5Ac

Immunochromatographic kits and RT-PCR are widely used as diagnostic tools for influenza detection in clinical and hygiene fields. Immunochromatographic kits are useful for differential typing of influenza A and influenza B but cannot show if the detected virus strains have acquired drug resistance against neuraminidase inhibitors that target sialidase activity of viral neuraminidase. Although RT-PCR enables determination of drug-resistant mutants, its efficacy is limited to viruses carrying a known substitution in their neuraminidase genome sequence. In the present study, an easy, rapid and sensitive method for detection of drug-resistant influenza viruses regardless of major antigenic changes or genomic mutations was developed. By using the method in combination with virus-concentrated membranes in centrifugal filter units and a sialidase imaging probe, 2-(benzothiazol-2-yl)-4-bromophenyl-N-acetylneuraminic acid (BTP3-Neu5Ac), sialidase activity of influenza neuraminidase was visualized on membranes by the green fluorescence of produced hydrophobic BTP3 under UV irradiation with a handheld UV flashlight. Fluorescence images in the presence or absence of neuraminidase inhibitors clearly discriminated drug-resistant influenza viruses from drug-sensitive ones. The assay can be done within 15 min. The detection sensitivity was shown to be equal to or higher than the sensitivities of commercial immunochromatographic kits. The assay will be a powerful tool for screening and monitoring of emerging drug-resistant influenza viruses and would help clinicians decide effective antiviral treatment strategies when such mutants have become prevalent.

Prodrugs of the Phosphoribosylated Forms of Hydroxypyrazinecarboxamide Pseudobase T-705 and Its De-Fluoro Analogue T-1105 as Potent Influenza Virus Inhibitors

We here disclose chemical synthesis of ribonucleoside 5'-monophosphate (RMP), -diphosphate (RDP), and -triphosphate (RTP) and cycloSal-, Di PPro-, and Tri PPPro nucleotide prodrugs of the antiviral pseudobase T-1105. Moreover, we include one nucleoside diphosphate prodrug of the chemically less stable T-705. We demonstrate efficient T-1105-RDP and -RTP release from the Di PPro and Tri PPPro compounds by esterase activation. Using crude enzyme extracts, we saw rapid phosphorylation of T-1105-RDP into T-1105-RTP. In sharp contrast, phosphorylation of T-1105-RMP was not seen, indicating a yet unrecognized bottleneck in T-1105's metabolic activation. Accordingly, Di PPro and Tri PPPro compounds displayed improved cell culture activity against influenza A and B virus, which they retained in a mutant cell line incapable of activating the nucleobase parent. T-1105-RTP had a strong inhibitory effect against isolated influenza polymerase, and Di PPro-T-1105-RDP showed 4-fold higher potency in suppressing one-cycle viral RNA synthesis versus T-1105. Hence, our T-1105-RDP and -RTP prodrugs improve antiviral potency and achieve efficient metabolic bypass.

Characterization of influenza virus variants induced by treatment with the endonuclease inhibitor baloxavir marboxil

Baloxavir acid (BXA), derived from the prodrug baloxavir marboxil (BXM), potently and selectively inhibits the cap-dependent endonuclease within the polymerase PA subunit of influenza A and B viruses. In clinical trials, single doses of BXM profoundly decrease viral titers as well as alleviating influenza symptoms. Here, we characterize the impact on BXA susceptibility and replicative capacity of variant viruses detected in the post-treatment monitoring of the clinical studies. We find that the PA I38T substitution is a major pathway for reduced susceptibility to BXA, with 30- to 50-fold and 7-fold EC₅₀ changes in A and B viruses, respectively. The viruses harboring the I38T substitution show severely impaired replicative fitness in cells, and correspondingly reduced endonuclease activity in vitro. Co-crystal structures of wild-type and I38T influenza A and B endonucleases bound to BXA show that the mutation reduces van der Waals contacts with the inhibitor. A reduced affinity to the I38T mutant is supported by the lower stability of the BXA-bound endonuclease. These mechanistic insights provide markers for future surveillance of treated populations.

Favipiravir (T-705) protects against Nipah virus infection in the hamster model

Nipah and Hendra viruses are recently emerged bat-borne paramyxoviruses (genus Henipavirus) causing severe encephalitis and respiratory disease in humans with fatality rates ranging from 40–75%. Despite the severe pathogenicity of these viruses and their pandemic potential, no therapeutics or vaccines are currently approved for use in humans. Favipiravir (T-705) is a purine analogue antiviral approved for use in Japan against emerging influenza strains; and several phase 2 and 3 clinical trials are ongoing in the United States and Europe. Favipiravir has demonstrated efficacy against a broad spectrum of RNA viruses, including members of the Paramyxoviridae, Filoviridae, Arenaviridae families, and the Bunyavirales order. We now demonstrate that favipiravir has potent antiviral activity against henipaviruses. In vitro, favipiravir inhibited Nipah and Hendra virus replication and transcription at micromolar concentrations. In the Syrian hamster model, either twice daily oral or once daily subcutaneous administration of favipiravir for 14 days fully protected animals challenged with a lethal dose of Nipah virus. This first successful treatment of henipavirus infection in vivo with a small molecule drug suggests that favipiravir should be further evaluated as an antiviral treatment option for henipavirus infections.

Gemcitabine and Nucleos(t)ide Synthesis Inhibitors Are Broad-Spectrum Antiviral Drugs that Activate Innate Immunity

Nucleoside analogs have been frequently identified as antiviral agents. In recent years, gemcitabine, a cytidine analog in clinical use for the treatment of many solid tumors, was also shown to have antiviral activity against a broad range of viruses. Nucleoside analogs generally interfere with cellular nucleos(t)ide synthesis pathways, resulting in the depletion or imbalance of (d)NTP pools. Intriguingly, a few recent reports have shown that some nucleoside analogs, including gemcitabine, activated innate immunity, inducing the expression of interferon-stimulated genes, through nucleos(t)ide synthesis inhibition. The precise crosstalk between these two independent processes remains to be determined. Nonetheless, we summarize the current knowledge of nucleos(t)ide synthesis inhibition-related innate immunity and propose it as a newly emerging antiviral mechanism of nucleoside analogs.

Nucleobases and corresponding nucleosides display potent antiviral activities against dengue virus possibly through viral lethal mutagenesis

Dengue virus affects millions of people worldwide each year. To date, there is no drug for the treatment of dengue-associated disease. Nucleosides are effective antivirals and work by inhibiting the accurate replication of the viral genome. Nucleobases offer a cheaper alternative to nucleosides for broad antiviral applications. Metabolic activation of nucleobases involves condensation with 5-phosphoribosyl-1-pyrophosphate to give the corresponding nucleoside-5’-monophosphate. This could provide an alternative to phosphorylation of a nucleoside, a step that is often rate limiting and inefficient in activation of nucleosides. We evaluated more than 30 nucleobases and corresponding nucleosides for their antiviral activity against dengue virus. Five nucleobases and two nucleosides were found to induce potent antiviral effects not previously described. Our studies further revealed that nucleobases were usually more active with a better tissue culture therapeutic index than their corresponding nucleosides. The development of viral lethal mutagenesis, an antiviral approach that takes into account the quasispecies behavior of RNA viruses, represents an exciting prospect not yet studied in the context of dengue replication. Passage of the virus in the presence of the nucleobase 3a (T-1105) and corresponding nucleoside 3b (T-1106), favipiravir derivatives, induced an increase in apparent mutations, indicating lethal mutagenesis as a possible antiviral mechanism. A more concerted and widespread screening of nucleobase libraries is a very promising approach to identify dengue virus inhibitors including those that may act as viral mutagens.

Dengue virus is a world-wide public health menace estimated to infect hundreds of millions of people per year. Vaccines to prevent dengue virus infection have had limited success due in part to the requirement to elicit effective immune responses against the four dengue serotypes. There is an urgent unmet need for anti-dengue virus therapies. Nucleosides are effective antiviral small molecules which usually work by inhibiting the accurate replication of the viral genome. Typically, nucleosides must be converted within the cell to their triphosphate form to inhibit virus replication, thus inefficient phosphorylation often leads to suboptimal activity. We screened a small library of nucleobases that require an activation pathway different from nucleosides to achieve the same active form. We identified some known and previously undescribed dengue virus nucleobase inhibitors and their corresponding nucleosides. Our investigation of the mechanism of action of one nucleobase and its corresponding nucleoside found evidence for enhanced mutagenesis of the dengue virus genome in the presence of the compounds in cell culture. A wide screening of nucleobases libraries is a promising strategy to discover dengue virus inhibitors including potential viral mutagens.

Stimulation of alpha2-adrenergic receptors impairs influenza virus infection

Influenza A viruses cause seasonal epidemics and occasional pandemics. The emergence of viruses resistant to neuraminidase (NA) inhibitors and M2 ion channel inhibitors underlines the need for alternate anti-influenza drugs with novel mechanisms of action. Here, we report the discovery of a host factor as a potential target of anti-influenza drugs. By using cell-based virus replication screening of a chemical library and several additional assays, we identified clonidine as a new anti-influenza agent in vitro. We found that clonidine, which is an agonist of the alpha2-adrenergic receptor (α2-AR), has an inhibitory effect on the replication of various influenza virus strains. α2-AR is a Gi-type G protein-coupled receptor that reduces intracellular cyclic AMP (cAMP) levels. In-depth analysis showed that stimulation of α2-ARs leads to impairment of influenza virus replication and that α2-AR agonists inhibit the virus assembly step, likely via a cAMP-mediated pathway. Although clonidine administration did not reduce lung virus titers or prevent body weight loss, it did suppress lung edema and improve survival in a murine lethal infection model. Clonidine may thus protect against lung damage caused by influenza virus infection. Our results identify α2-AR-mediated signaling as a key pathway to exploit in the development of anti-influenza agents.

Favipiravir as a potential countermeasure against neglected and emerging RNA viruses

Favipiravir, also known as T-705, is an antiviral drug that has been approved in 2014 in Japan to treat pandemic influenza virus infections. The drug is converted intracellularly into its active, phosphoribosylated form, which is recognized as a substrate by the viral RNA-dependent RNA polymerase. Interestingly, besides its anti-influenza virus activity, this molecule is also able to inhibit the replication of flavi-, alpha-, filo-, bunya-, arena-, noro-, and of other RNA viruses, which include neglected and (re)emerging viruses for which no antiviral therapy is currently available. We will discuss the potential of favipiravir as a broad-spectrum countermeasure against infections caused by such neglected RNA viruses. Favipiravir has already been used off-label to treat patients infected with the Ebola virus and the Lassa virus. Because of the particular set-up of the clinical trials during these outbreaks, clear conclusions on the efficacy of favipiravir could not be made. For several viruses, it was demonstrated that the barrier of resistance development against favipiravir is high. Favipiravir has been shown to be well tolerated in healthy volunteers and in influenza virus-infected patients; however, caution is needed because of the teratogenic risks of this molecule. Because of its antiviral activity against different RNA viruses and its high barrier for resistance, the potential of favipiravir as a broad-spectrum antiviral seems promising, but safety and potency issues should be overcome before this drug or similar molecules could be used to treat large patient groups.

Inhibition of influenza virus replication by adlay tea

BACKGROUND

The present study was conducted aiming to examine the antiviral activity of adlay tea and its components against influenza viruses. We further aimed to clarify the mechanism by which these components regulate virus replication.

RESULTS

Adlay tea at a concentration suitable for drinking inhibited the multiplication of influenza viruses. Moreover, our results suggest that individual components of the tea had antiviral activities against the influenza A/PR/8/34 virus. Adlay tea inhibited multiplication of the H1N1, H3N2 and B types of influenza virus, including oseltamivir-resistant viruses. In addition, adlay tea inhibited influenza infection during the periods of virus adsorption to the cell and virus replication. Adlay tea did not suppress hemagglutination inhibition or cell fusion, although it slightly inhibited virus binding to Malin Darby canine kidney cells. Furthermore, our findings suggest that the antiviral compounds included in adlay tea were ingredients other than polyphenols and that there were several types of effective compounds in adlay tea inhibiting several steps of viral replication.

CONCLUSION

The results of the present study demonstrate that adlay tea had antiviral effects against influenza viruses. Our findings with respect to adlay tea suggest that the polyphenols might have a small influence on its antiviral activity and that other ingredients might have more influence. © 2017 Society of Chemical Industry.

Anti-influenza virus activity of a salcomine derivative mediated by inhibition of viral RNA synthesis

Influenza virus infection is a major threat to global health. Although vaccines and anti-influenza virus drugs are available, annual influenza virus epidemics result in severe illness, and an influenza pandemic occurs every 20-30 years. To identify candidate anti-influenza virus compounds, we screened approximately 5,000 compounds in an in-house library. We identified MZ7465, a salcomine derivative, as a potent inhibitor of influenza virus propagation. We analyzed the antiviral propagation mechanism of the hit compound by determining the amounts of viral proteins and RNA in infected cells treated with or without the hit compound. Treatment of infected cells with MZ7465 decreased both viral protein and RNA synthesis. In addition, an in vitro assay showed that viral RNA synthesis was directly inhibited by MZ7465. These results suggest that salcomine and its derivatives are potential candidates for the treatment of influenza virus infections.

Favipiravir and Ribavirin Inhibit Replication of Asian and African Strains of Zika Virus in Different Cell Models

Zika virus (ZIKV) has recently emerged as a new public health threat. ZIKV infections have caused a wide spectrum of neurological diseases, such as Guillain-Barré syndrome, myelitis, meningoencephalitis, and congenital microcephaly. No effective therapies currently exist for treating patients infected with ZIKV. Herein, we evaluated the anti-viral activity of favipiravir (T-705) and ribavirin against Asian and African strains of ZIKV using different cell models, including human neuronal progenitor cells (hNPCs), human dermal fibroblasts (HDFs), human lung adenocarcinoma cells (A549) and Vero cells. Cells were treated with favipiravir or ribavirin and effects on ZIKV replication were determined using quantitative real-time PCR and plaque assay. Our results demonstrate that favipiravir or ribavirin treatment significantly inhibited ZIKV replication in a dose-dependent manner. Moreover, favipiravir treatment of ZIKV-infected hNPCs led to reduced cell death, enhanced AKT pathway phosphorylation, and increased expression of anti-apoptotic factor B cell lymphoma 2. In conclusion, our results demonstrate conclusively that favipiravir inhibits ZIKV replication and prevents cell death, and can be a promising intervention for ZIKV-associated disease.

Influenza A virus polymerase: an attractive target for next-generation anti-influenza therapeutics

The influenza RNA-dependent RNA polymerase (RdRP) is conserved among different types of influenza virus, playing an important part in transcription and replication. In this regard, influenza RdRP is an attractive target for novel anti-influenza drug discovery. Herein, we will introduce the structural and functional information of influenza polymerase; and an overview of inhibitors targeting the PA endonuclease and PB2 cap-binding site is provided, along with the approaches utilized for identification of these inhibitors. The protein-protein interactions (PPIs) of the three polymerase subunits: PA, PB1 and PB2, are described based on the published crystal structures, and inhibitors targeting the PA-PB1 interaction are introduced briefly.

Antiviral Drugs Against Alphaherpesvirus

The discovery of acyclovir and penciclovir has led to the development of a successful systemic therapy for treating herpes simplex virus infection and varicella-zoster virus infection, and the orally available prodrugs, valacyclovir and famciclovir, have improved antiviral treatment compliance. Acyclovir and penciclovir are phosphorylated by viral thymidine kinase and are incorporated into the DNA chain by viral DNA polymerase, resulting in chain termination. Helicase-primase plays an initial step in DNA synthesis to separate the double strand into two single strands (replication fork) and is a new target of antiviral therapy. The helicase-primase inhibitors (HPIs) pritelivir and amenamevir have novel mechanisms of action, drug resistance properties, pharmacokinetic characteristics, and clinical efficacy for treating genital herpes. The clinical study of amenamevir in herpes zoster has been completed, and amenamevir has been submitted for approval for treating herpes zoster in Japan. The clinical use of HPIs will be the beginning of a new era of anti-herpes therapy.

Nucleosides for the treatment of respiratory RNA virus infections

Influenza virus, respiratory syncytial virus, human metapneumovirus, parainfluenza virus, coronaviruses, and rhinoviruses are among the most common viruses causing mild seasonal colds. These RNA viruses can also cause lower respiratory tract infections leading to bronchiolitis and pneumonia. Young children, the elderly, and patients with compromised cardiac, pulmonary, or immune systems are at greatest risk for serious disease associated with these RNA virus respiratory infections. In addition, swine and avian influenza viruses, together with severe acute respiratory syndrome-associated and Middle Eastern respiratory syndrome coronaviruses, represent significant pandemic threats to the general population. In this review, we describe the current medical need resulting from respiratory infections caused by RNA viruses, which justifies drug discovery efforts to identify new therapeutic agents. The RNA polymerase of respiratory viruses represents an attractive target for nucleoside and nucleotide analogs acting as inhibitors of RNA chain synthesis. Here, we present the molecular, biochemical, and structural fundamentals of the polymerase of the four major families of RNA respiratory viruses: Orthomyxoviridae, Pneumoviridae/Paramyxoviridae, Coronaviridae, and Picornaviridae. We summarize past and current efforts to develop nucleoside and nucleotide analogs as antiviral agents against respiratory virus infections. This includes molecules with very broad antiviral spectrum such as ribavirin and T-705 (favipiravir), and others targeting more specifically one or a few virus families. Recent advances in our understanding of the structure(s) and function(s) of respiratory virus polymerases will likely support the discovery and development of novel nucleoside analogs.

Zika Virus Replication Is Substantially Inhibited by Novel Favipiravir and Interferon Alpha Combination Regimens

Zika virus (ZIKV) is a major public health concern due to its overwhelming spread into the Americas. Currently, there are neither licensed vaccines nor antiviral therapies available for the treatment of ZIKV. We aimed to identify and rationally optimize effective therapeutic regimens for ZIKV by evaluating the antiviral potentials of the approved broad-spectrum antiviral agents favipiravir (FAV), interferon alpha (IFN), and ribavirin (RBV) as single agents and in combinations. For these studies, Vero cells were infected with ZIKV in the presence of increasing concentrations of FAV, IFN, or/and RBV for 4 days. Supernatants were harvested daily, and the viral burden was quantified by a plaque assay on Vero cells. The time course of the viral burden during treatment in vitro was characterized by a novel translational, mechanism-based model, which was subsequently used to rationally optimize combination dosage regimens. The combination regimen of FAV plus IFN provided the greatest extent of viral inhibition without cytotoxicity, reducing the viral burden by 4.4 log₁₀ PFU/ml at concentrations of 250 μM FAV and 100 IU/ml IFN. Importantly, these concentrations are achievable in humans. The translational, mechanism-based model yielded unbiased and reasonably precise curve fits. Simulations with the model predicted that clinically relevant regimens of FAV plus IFN would markedly reduce viral burdens in humans, resulting in at least a 10,000-fold reduction in the amount of the virus during the first 4 days of treatment. These findings highlight the substantial promise of rationally optimized combination dosage regimens of FAV plus IFN, which should be further investigated to combat ZIKV.

Extinction of West Nile Virus by Favipiravir through Lethal Mutagenesis

Favipiravir is an antiviral agent effective against several RNA viruses. The drug has been shown to protect mice against experimental infection with a lethal dose of West Nile virus (WNV), a mosquito-borne flavivirus responsible for outbreaks of meningitis and encephalitis for which no antiviral therapy has been licensed; however, the mechanism of action of the drug is still not well understood. Here, we describe the potent in vitro antiviral activity of favipiravir against WNV, showing that it decreases virus-specific infectivity and drives the virus to extinction. Two passages of WNV in the presence of 1 mM favipiravir-a concentration that is more than 10-fold lower than its 50% cytotoxic concentration (CC₅₀)-resulted in a significant increase in mutation frequency in the mutant spectrum and in a bias toward A→G and G→A transitions relative to the population passaged in the absence of the drug. These data, together with the fact that the drug is already licensed in Japan against influenza virus and in a clinical trial against Ebola virus, point to favipiravir as a promising antiviral agent to fight medically relevant flaviviral infections, such as that caused by WNV.

Recent advances in the development of antiviral therapeutics for Rift Valley fever virus infection

Rift Valley fever virus (RVFV) is a mosquito-borne bunyavirus endemic to sub-Saharan Africa and the Arabian Peninsula and the etiological agent of Rift Valley fever. Rift Valley fever is a disease of major public health and economic concern, affecting livestock and humans. In ruminants, RVFV infection is characterized by high mortality rates in newborns and near 100% abortion rates in pregnant animals. Infection in humans is typically manifested as a self-limiting febrile illness, but can lead to severe and fatal hepatitis, encephalitis, hemorrhagic fever or retinitis with partial or complete blindness. Currently, there are no specific treatment options available for RVFV infection. This review presents a summary of the therapeutic approaches that have been explored on the treatment of RVFV infection.

Understanding the Pathogenesis of Zika Virus Infection Using Animal Models

Zika virus (ZIKV) is a member of Flaviviridae family that has emerged as a pathogen of significant public health importance. The rapid expansion of ZIKV in the South and Central America has recently gained medical attention emphasizing the capacity of ZIKV to spread to non-endemic regions. ZIKV infection during pregnancy has been demonstrated to cause microcephaly and other fetal developmental abnormalities. An increased incidence of Guillain-Barre syndrome, an immune mediated neuropathy of the peripheral nervous system, has also been reported in ZIKV-infected patients in French Polynesia and Brazil. No effective therapies currently exist for treating patients infected with ZIKV. Despite the relatively short time interval, an intensive effort by the global scientific community has resulted in development of animal models to study multiple aspects of ZIKV biology. Several animal models have been established to investigate pathogenesis of ZIKV in adults, pregnant mothers, and developing fetuses. Here we review the remarkable progress of newly developed small and large animal models for understanding ZIKV pathogenesis.

Current landscape and future prospects of antiviral drugs derived from microbial products

Viral infections are a major global health threat. Over the last 50 years, significant efforts have been devoted to the development of antiviral drugs and great success has been achieved for some viruses. However, other virus infections, such as epidemic influenza, still spread globally and new threats continue to arise from emerging and re-emerging viruses and drug-resistant viruses. In this review, the contributions of microbial products isolated in Institute of Microbial Chemistry for antiviral research are summarized. In addition, the current state of development of antiviral drugs that target influenza virus and hepatitis B virus, and the future prospects for antivirals from natural products are described and discussed.The Journal of Antibiotics advance online publication, 11 October 2017; doi:10.1038/ja.2017.115.

Oral administration of Euglena gracilis Z and its carbohydrate storage substance provides survival protection against influenza virus infection in mice

Euglena gracilis Z is a micro-algae that is used as a food or nutritional supplement. Paramylon, the carbohydrate storage substance of Euglena gracilis Z has β-1, 3-glucan structure. Euglena gracilis Z and paramylon are reported to affect the immune system. In this study, we investigated the protective effects of Euglena gracilis Z and paramylon against influenza virus infection in mice. Euglena gracilis Z and paramylon were administered to mice as a 2% dietary mixture ad libitum. At 2 weeks after initiation of dietary administration, mice were infected intranasally with influenza virus A/PR/8/34 (H1N1). Survival rate was monitored 10 days after infection. In addition, we performed virus titer and cytokine profiles in the lung. High survival rates were observed for Euglena gracilis Z and paramylon-treated groups compared to the control group. Significantly lower virus titer in the lung was observed in the Euglena gracilis Z and paramylon-treated groups compared to the control group from day 1 after infection. Higher amount of IL-1β, IL-6, IL-12 (p70), IFN-γ, and IL-10 was observed in the paramylon groups compared to the control group. Our data therefore reveals a novel immunoregulatory role of the Euglena gracilis Z and paramylon which provides protection against influenza virus infection.

Iminosugars: Promising therapeutics for influenza infection

Influenza virus causes three to five million severe respiratory infections per year in seasonal epidemics, and sporadic pandemics, three of which occurred in the twentieth century and are a continuing global threat. Currently licensed antivirals exclusively target the viral neuraminidase or M2 ion channel, and emerging drug resistance necessitates the development of novel therapeutics. It is believed that a host-targeted strategy may combat the development of antiviral drug resistance. To this end, a class of molecules known as iminosugars, hydroxylated carbohydrate mimics with the endocyclic oxygen atom replaced by a nitrogen atom, are being investigated for their broad-spectrum antiviral potential. The influenza virus glycoproteins, hemagglutinin and neuraminidase, are susceptible to inhibition of endoplasmic reticulum α-glucosidases by certain iminosugars, leading to reduced virion production or infectivity, demonstrated by in vitro and in vivo studies. In some experiments, viral strain-specific effects are observed. Iminosugars may also inhibit other host and virus targets with antiviral consequences. While investigations of anti-influenza iminosugar activities have been conducted since the 1980s, recent successes of nojirimycin derivatives have re-invigorated investigation of the therapeutic potential of iminosugars as orally available, low cytotoxicity, effective anti-influenza drugs.

Clinical Evaluation of Ebola Virus Disease Therapeutics

Ebola virus disease (EVD) was first described over 40 years ago, but no treatment has been approved for humans. The 2013-2016 EVD outbreak in West Africa has expedited the clinical evaluation of several candidate therapeutics that act through different mechanisms, but with mixed results. Nevertheless, these studies are important because the accumulation of clinical data and valuable experience in conducting efficacy trials under emergency circumstances will lead to better implementation of similar studies in the future. Here, we summarize the results of EVD clinical trials, focus on the discussion of factors that may have potentially impeded the effectiveness of existing candidate therapeutics, and highlight considerations that may help meet the challenges ahead in the quest to develop clinically approved drugs.

Discovering Drugs for the Treatment of Ebola Virus

Purpose of review

Ebola virus, a member of the Filoviridae family, is a causative agent of severe viral hemorrhagic fever in humans. Over the past 40 years, the virus has been linked to several high mortality outbreaks in Africa with the recent West African outbreak resulting in over 11,000 deaths. This review provides a summary of the status of the drug discovery and development process for therapeutics for Ebola virus disease, with a focus on the strategies being used and the challenges facing each stage of the process.

Recent findings

Despite the wealth of in vitro efficacy data, preclinical data in animal models, and human clinical data, no therapeutics have been approved for the treatment of Ebola virus disease. However, several promising candidates, such as ZMapp and GS-5734, have advanced into ongoing clinical trials.

Summary

The gravity of the 2014-2016 outbreak spurred a heightened effort to identify and develop new treatments for Ebola virus disease, including small molecules, immunotherapeutics, host factors, and clinical disease management options.

Disclaimer

Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endoresed by the U.S. Army.

H7N9 avian influenza A virus in China: a short report on its circulation, drug resistant mutants and novel antiviral drugs

INTRODUCTION

The first human H7N9 avian influenza virus case was reported in Shanghai in 2013. Shortly thereafter, this virus spread to other regions in China. Molecular analysis indicated that the H7N9 virus is a reassortant virus containing internal genes from the H9N2 virus and previously described mammalian adaption markers, which could allow the virus to adapt efficiently to a mammalian host. Fortunately, there is no evidence of sustained person-to-person spread. Most of the human H7N9 cases have a history of exposure to live poultry markets (LPMs). The circulating H7N9 were low pathogenic viruses, however highly pathogenic H7N9 viruses were recently identified in human cases. Areas covered: In the present article, the circulation of H7N9 in LPMs of China, the five waves of H7N9 infection in humans, recently identified drug resistant mutants and potential antiviral drugs against H7N9 are discussed; this may provide further understanding of the evolution and pandemic potential of the H7N9 influenza viruses. Expert commentary: All the data reveal that the major source of H7N9 viruses are LPMs and the H7N9 virus is still circulating widely in China. It is concerning that the recent emergence of highly pathogenic H7N9 viruses may result in highly transmissible viruses in mammalian species.

Characterization of susceptibility variants of poliovirus grown in the presence of favipiravir

BACKGROUND

T-705 (favipiravir) is a potent inhibitor of RNA-dependent RNA polymerases of influenza viruses and no favipiravir-resistant virus has been isolated. Poliovirus RNA polymerase has been well characterized and isolation of resistant virus was examined in poliovirus.

METHODS

Susceptibility variants of poliovirus I (Sabin strain) were isolated during passages in the presence of favipiravir and characterized for their susceptibility and the sequence of RNA polymerase.

RESULTS

Five variants with 0.47-1.88 times the 50% inhibitory concentration for plaque formation of the parent poliovirus had amino acid variations in the 3D gene of the RNA polymerase. The distribution of amino acid variations was not related to ribavirin resistance, and two amino acid variation sites were found near the finger domain.

CONCLUSION

Favipiravir as a chain terminator would not be incorporated and replicate to cause lethal mutagenesis as a mutagen like ribavirin, and resistant mutants were not isolated. A high replication level would generate mutations leading to favipiravir resistance as ribavirin resistance was generated, but generated mutations would be lethal to the RNA polymerase function.

Viral polymerase inhibitors T-705 and T-1105 are potential inhibitors of Zika virus replication

Since 2015, 69 countries and territories have reported evidence of vector-borne Zika virus (ZIKV) transmission. Currently, there are no effective licensed vaccines or drugs available for the treatment or prevention of ZIKV infection. We tested a series of compounds for their ability to inhibit ZIKV replication in cell culture. The compounds in T-705 (favipiravir) and T-1105 were found to have antiviral activity, suggesting that these compounds are promising candidates for further development as specific antiviral drugs against ZIKV.