Efficacy of orally administered T-705 pyrazine analog on lethal West Nile virus infection in rodents

Oral Favipiravir Exposure and Pharmacodynamic Effects in Adult Outpatients With Acute Influenza

BACKGROUND

The pharmacokinetics of oral favipiravir and the relationships of plasma concentrations to antiviral effects are incompletely studied in influenza.

METHODS

Serial plasma samples were collected from adults with uncomplicated influenza who were randomized to favipiravir (1800 mg twice a day on day 1, 800 mg twice a day on days 2 to 5; n = 827) or placebo (n = 419) in 2 phase 3 trials. Post hoc analyses assessed the frequency of reaching an average minimum concentration (Cmin) ≥20 µg/mL, its association with antiviral efficacy, and factors associated with reduced favipiravir exposure.

RESULTS

Wide interindividual variability existed in favipiravir concentrations, and this regimen failed to reach an average Cmin>20 µg/mL in 41%-43% of participants. Those attaining this threshold showed greater reductions in nasopharyngeal infectious virus titers on treatment days 2 and 3 and lower viral titer area under the curve compared to those who did not. Those with average Cmin <20 µg/mL had over 2-fold higher mean ratios of the metabolite T-705M1 to favipiravir, consistent with greater metabolism, and were more likely to weigh >80 kg (61.5%-64%).

CONCLUSIONS

Higher favipiravir levels with average Cmin>20 µg/mL were associated with larger antiviral effects and more rapid illness alleviation compared to placebo and to favipiravir recipients with lower average Cmin values in uncomplicated influenza. Clinical Trials Registration . NCT1068912 and NCT01728753.

N-Heterocycles as Promising Antiviral Agents: A Comprehensive Overview

Viruses are a real threat to every organism at any stage of life leading to extensive infections and casualties. N-heterocycles can affect the viral life cycle at many points, including viral entrance into host cells, viral genome replication, and the production of novel viral species. Certain N-heterocycles can also stimulate the host's immune system, producing antiviral cytokines and chemokines that can stop the reproduction of viruses. This review focused on recent five- or six-membered synthetic N-heterocyclic molecules showing antiviral activity through SAR analyses. The review will assist in identifying robust scaffolds that might be utilized to create effective antiviral drugs with either no or few side effects.

In vitro antiviral activity of favipiravir and its 6- and 3-O-substituted derivatives against coronavirus: Acetylation leads to improvement of antiviral activity

Favipiravir is currently approved for the treatment of the influenza virus and has shown encouraging results in terms of antiviral capacity in clinical studies against severe acute respiratory syndrome coronavirus 2. Favipiravir is a prodrug, where its favipiravir-ribofuranosyl-5B-triphosphate metabolite is capable of blocking RNA replication of the virus. However, the antiviral efficiency of favipiravir is limited by two factors: (i) low accumulation in plasma and rapid excretion/elimination post-administration and (ii) low conversion rate into the active metabolite. To tackle these problems, herein, we have designed new favipiravir analogues focusing on the replacement of the fluorine atom at the 6-position by halogen or hydrogen atoms and 3-O-functionalization with labile groups. The first type of functionalization seeks to increase the antiviral activity because of the better ability of the keto-tautomer as a function of the halogen, and it is hypothesized that the keto-tautomer tends to promote the formation of the ribofuranosyl-5B-triphosphate (RTP) metabolite. Meanwhile, the second type of functionalization seeks to promote lipophilicity and increase accumulation in cells. From the in vitro antiviral activity against two coronavirus models (bovine and human 229E), it was identified that the replacement did not improve the antiviral activity against both the models, which seems to be attributable to the low water solubility of these new 6-functionalized analogues. Meanwhile, with 3-O-functionalization, acetylation provided the most active compounds with higher half-maximal inhibitory concentration and selectivity than favipiravir, whereas benzylation/methanosulfonation yielded the least active compounds. In summary, acetylation is found to be a convenient functionalization to enhance the antiviral profile of favipiravir.

Tautomerism and Rotamerism of Favipiravir and Halogenated Analogues in Solution and in the Solid State

Favipiravir is an important selective antiviral against RNA-based viruses, and currently, it is being repurposed as a potential drug for the treatment of COVID-19. This type of chemical system presents different carboxamide-rotameric and hydroxyl-tautomeric states, which could be essential for interpreting its selective antiviral activity. Herein, the tautomeric 3-hydroxypyrazine/3-pyrazinone pair of favipiravir and its 6-substituted analogues, 6-Cl, 6-Br, 6-I, and 6-H, were fully investigated in solution and in the solid state through ultraviolet-visible, 1H nuclear magnetic resonance, infrared spectroscopy, and X-ray diffraction techniques. Also, a study of the gas phase was performed using density functional theory calculations. In general, the keto-enol balance in these 3-hydroxy-2-pyrazinecarboxamides is finely modulated by external and internal electrical variations via changes in solvent polarity or by replacement of substituents at position 6. The enol tautomer was prevalent in an apolar environment, whereas an increase in the level of the keto tautomer was favored by an increase in solvent polarity and, even moreso, with a strong hydrogen-donor solvent. Keto tautomerization was favored either in solution or in the solid state with a decrease in 6-substituent electronegativity as follows: H ≫ I ≈ Br > Cl ≥ F. Specific rotameric states based on carboxamide, "cisoide" and "transoide", were identified for the enol and keto tautomer, respectively; their rotamerism is dependent on the tautomerism and not the aggregation state.

California Serogroup Viruses in a Changing Canadian Arctic: A Review

The Arctic is warming at four times the global rate, changing the diversity, activity and distribution of vectors and associated pathogens. While the Arctic is not often considered a hotbed of vector-borne diseases, Jamestown Canyon virus (JCV) and Snowshoe Hare virus (SSHV) are mosquito-borne zoonotic viruses of the California serogroup endemic to the Canadian North. The viruses are maintained by transovarial transmission in vectors and circulate among vertebrate hosts, both of which are not well characterized in Arctic regions. While most human infections are subclinical or mild, serious cases occur, and both JCV and SSHV have recently been identified as leading causes of arbovirus-associated neurological diseases in North America. Consequently, both viruses are currently recognised as neglected and emerging viruses of public health concern. This review aims to summarise previous findings in the region regarding the enzootic transmission cycle of both viruses. We identify key gaps and approaches needed to critically evaluate, detect, and model the effects of climate change on these uniquely northern viruses. Based on limited data, we predict that (1) these northern adapted viruses will increase their range northwards, but not lose range at their southern limits, (2) undergo more rapid amplification and amplified transmission in endemic regions for longer vector-biting seasons, (3) take advantage of northward shifts of hosts and vectors, and (4) increase bite rates following an increase in the availability of breeding sites, along with phenological synchrony between the reproduction cycle of theorized reservoirs (such as caribou calving) and mosquito emergence.

A dispersion-corrected DFT calculation on encapsulation of favipiravir drug used as antiviral against COVID-19 into carbon-, boron-, and aluminum-nitride nanotubes for optimal drug delivery systems combined with molecular docking simulations

Favipiravir (FAV) (6-fluoro-3-oxo-3,4-dihydropyrazine-2-carboxamide) is one of the most effective antiviral drugs which is cited for action against RNA-viral infections of COVID-19. In this study, density functional theory (DFT) calculations were used to investigate three nanotubes (NTs) with FAV drug as delivery systems. The encapsulated systems (ESs) consist of FAV drug inside carbon-carbon, aluminum nitride, and boron nitride. At B3LYP-D/6-31G(d,p) and CPCM/B3LYP-D/6-31G(d,p), the optimization of NTs, FAV, and its tautomeric forms and six ESs was investigated in gas and water environments. Five tautomeric forms of FAV were investigated, two keto forms (K1 and K2) and three enol forms (E1, E2, and E3). The results revealed that E3 and K2 isomeric forms represented the most stable structures in both media; thus, these two forms were encapsulated into the NTs. The stability and the synthesis feasibility of NTs have been proven by calculating their interaction energies. Non-covalent interactions (NCIs) were investigated in the ESs to show the type of NCI with the molecular voids. The binding energies, thermochemical parameters, and recovery times were investigated to understand the mechanism of FAV encapsulation and release. The encapsulated AlNNT systems are more favorable than those of BNNTs and CNTs in gas and aqueous environments with much higher binding energies. The quantum theory of atoms in molecules (QTAIM) and recovery time analysis revealed the easier releasing of E3 from AlNNT over K2 form. Based on molecular docking simulations, we found that E3 and K2 FAV forms showed a high level of resistance to SARS-CoV-6M3M/6LU7/6W9C proteases.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11224-023-02182-4.

Identification of West Nile virus RNA-dependent RNA polymerase non-nucleoside inhibitors by real-time high throughput fluorescence screening

West Nile virus (WNV) is a re-emergent mosquito-borne RNA virus that causes major outbreaks of encephalitis around the world. However, there is no therapeutic treatment to struggle against WNV, and the current treatment relies on alleviating symptoms. Therefore, due to the threat virus poses to animal and human health, there is an urgent need to come up with fast strategies to identify and assess effective antiviral compounds. A relevant target when developing drugs against RNA viruses is the viral RNA-dependent RNA polymerase (RdRp), responsible for the replication of the viral genome within a host cell. RdRps are key therapeutic targets based on their specificity for RNA and their essential role in the propagation of the infection. We have developed a fluorescence-based method to measure WNV RdRp activity in a fast and reliable real-time way. Interestingly, rilpivirine has shown in our assay inhibition of the WNV RdRp activity with an IC₅₀ value of 3.3 μM and its antiviral activity was confirmed in cell cultures. Furthermore, this method has been extended to build up a high-throughput screening platform to identify WNV polymerase inhibitors. By screening a small chemical library, novel RdRp inhibitors 1-4 have been identified. When their antiviral activity was tested against WNV in cell culture, 4 exhibited an EC₅₀ value of 2.5 μM and a selective index of 12.3. Thus, rilpivirine shows up as an interesting candidate for repurposing against flavivirus. Moreover, the here reported method allows the rapid identification of new WNV RdRp inhibitors.

Analysis of the responsible site for favipiravir resistance in RNA-dependent RNA polymerase of influenza virus A/PR/8/34 (H1N1) using site-directed mutagenesis

Favipiravir (T-705, 6-fluoro-3-hydroxy-2-pyrazinecarboxamide) selectively and strongly inhibits the replication of influenza virus in vitro and in vivo. Favipiravir is converted to favipiravir-4-ribofuranosyl-5-triphosphate (favipiravir RTP) by intracellular enzymes and functions as a nucleotide analog to selectively inhibit RNA-dependent RNA polymerase (RdRP) of influenza virus. Our previous experiments failed in an attempt to obtain a favipiravir-resistant influenza virus in vitro using influenza virus A/PR/8/34(H1N1). Conversely, Goldhill et al. reported a favipiravir-resistant influenza virus generated by in vitro passage of influenza virus A/England/195/2009 (H1N1), an early isolate from the 2009 H1N1 pandemic (pdm09), in the presence of favipiravir with K229R mutation in PB1. This study focused on K229R mutation near the NTP cross-linked region in PB1 based on the above conflicting findings to confirm whether K229R mutation brings favipiravir resistance to influenza virus A/PR/8/34. Thirty PB1 mutants generated by site-directed mutagenesis of the NTP cross-linked region were evaluated using an influenza virus A/PR/8/34 replicon system. Among the 30 mutants, 10 possessed but 20 lost replicon activity. When susceptibility to favipiravir in 10 mutants was further assessed, the PB1 E491D mutant was five times more sensitive than the wild-type (WT), while only the PB1 K229R mutant was resistant to favipiravir. Results suggested that the evaluated region was essential for polymerase activity, and K229 mutation was responsible for polymerase inhibition of favipiravir in the influenza virus A/PR/8/34. Interestingly, the tested K229X series mutants entirely lost replicon activity, except for K229R. This suggested that the amino acid at position 229 in PB1 of influenza virus may play a pivotal role in polymerase activity. Moreover, this lysine residue is highly conserved among positive- and negative-sense single-stranded RNA viruses, in which favipiravir showed potent activity, suggesting that this mutation may determine the characterization of the in vitro broad-spectrum activity of favipiravir. Additionally, this mutation acquisition greatly influences the viral replication and the susceptibility to favipiravir.

An overview on small molecules acting as broad spectrum-agents for yellow fever infection

INTRODUCTION

Yellow Fever virus (YFV) is a mosquito-borne flavivirus, endemic in 47 countries in Africa and South America, which causes febrile symptoms that can evolve in 15% of the patients to serious haemorrhagic conditions, liver injury, and multiorgan failure. Although a highly effective vaccine (YF-17D vaccine) is available, to date, no antiviral drugs have been approved for the prevention and treatment of YFV infections.

AREAS COVERED

This review article focuses on the description of viral targets that have been considered within YFV and flavivirus drug discovery studies and on the most relevant candidates reported so far that elicit broad-spectrum inhibition against relevant strains and mutants of YFV.

EXPERT OPINION

Considering the growing interest on (re)emerging vector-borne viral infections, it is expected that flavivirus drug discovery will quickly deliver potential candidates for clinical evaluation. Due to similarity among flaviviral targets, several candidates identified against different flaviviruses have shown broad-spectrum activity, thus exhibiting anti-YFV activity, as well. In this regard, it would be desirable to routinely include the assessment of antiviral activity against different YFV strains. On the other hand, the development of host targeting agents are still at an initial stage and deserve further focused efforts.

Antivirals against (Re)emerging Flaviviruses: Should We Target the Virus or the Host?

The COVID pandemic has evidenced how vulnerable we are to emerging infectious diseases and how short our current armamentarium is. Flavivirus, single stranded RNA viruses transmitted by arthropods, are considered a global health challenge. No drugs to treat these infections have been approved. In this Viewpoint, we analyze the advantages and disadvantages of two different, but probably also complementary, therapeutic approaches: virus-targeting antivirals and host-targeting drugs.

Broad-spectrum prodrugs with anti-SARS-CoV-2 activities: Strategies, benefits, and challenges

In this era, broad-spectrum prodrugs with anti-severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) activities are gaining considerable attention owing to their potential clinical benefits and role in combating the fast-spreading coronavirus disease 2019 (COVID-19) pandemic. The last 2 years have seen a surge of reports on various broad-spectrum prodrugs against SARS-CoV-2, and in in vitro studies, animal models, and clinical practice. Currently, only remdesivir (with many controversies and limitations) has been approved by the U.S. FDA for the treatment of SARS-CoV-2 infection, and additional potent anti-SARS-CoV-2 drugs are urgently required to enrich the defense arsenals. The world has ubiquitously grappled with the COVID-19 pandemic, and the availability of broad-spectrum prodrugs provides great hope for us to subdue this global threat. This article reviews promising treatment strategies, antiviral mechanisms, potential benefits, and daunting clinical challenges of anti-SARS-CoV-2 agents to provide some important guidance for future clinical treatment.

MMV-db: vaccinomics and RNA-based therapeutics database for infectious hemorrhagic fever-causing mammarenaviruses

The recent viral outbreaks and the current pandemic situation urges us to timely address any emerging viral infections by designing therapeutic strategies. Multi-omics and therapeutic data are of great interest to develop early remedial interventions. This work provides a therapeutic data platform (Mammarenavirus (MMV)-db) for pathogenic mammarenaviruses with potential catastrophic effects on human health around the world. The database integrates vaccinomics and RNA-based therapeutics data for seven human pathogenic MMVs associated with severe viral hemorrhagic fever and lethality in humans. Protein-specific cytotoxic T lymphocytes, B lymphocytes, helper T-cell and interferon-inducing epitopes were mapped using a cluster of immune-omics-based algorithms and tools for the seven human pathogenic viral species. Furthermore, the physiochemical and antigenic properties were also explored to guide protein-specific multi-epitope subunit vaccine for each species. Moreover, highly efficacious RNAs (small Interfering RNA (siRNA), microRNA and single guide RNA (sgRNA)) after extensive genome-based analysis with therapeutic relevance were explored. All the therapeutic RNAs were further classified and listed on the basis of predicted higher efficacy. The online platform (http://www.mmvdb.dqweilab-sjtu.com/index.php) contains easily accessible data sets and vaccine designs with potential utility in further computational and experimental work. Conclusively, the current study provides a baseline data platform to secure better future therapeutic interventions against the hemorrhagic fever causing mammarenaviruses. Database URL: http://www.mmvdb.dqweilab-sjtu.com/index.php.

Favipiravir treatment prolongs the survival in a lethal mouse model intracerebrally inoculated with Jamestown Canyon virus

Background

Jamestown Canyon virus (JCV) is a mosquito-borne orthobunyavirus that causes acute febrile illness, meningitis, and meningoencephalitis, primarily in North American adults. Currently, there are no available vaccines or specific treatments against JCV infections.

Methodology/Principal findings

The antiviral efficacy of favipiravir (FPV) against JCV infection was evaluated in vitro and in vivo in comparison with that of ribavirin (RBV) and 2’-fluoro-2’-deoxycytidine (2’-FdC). The in vitro inhibitory effect of these drugs on JCV replication was evaluated in Vero and Neuro-2a (N2A) cells. The efficacy of FPV in the treatment of JCV infection in vivo was evaluated in C57BL/6J mice inoculated intracerebrally with JCV, as per the survival, viral titers in the brain, and viral RNA load in the blood. The 90% inhibitory concentrations (IC₉₀) of FPV, RBV, and 2’-FdC were 41.0, 61.8, and 13.6 μM in Vero cells and 20.7, 25.8, and 8.8 μM in N2A cells, respectively. All mice infected with 1.0×10⁴ TCID₅₀ died or were sacrificed within 10 days post-infection (dpi) without treatment. However, mice treated with FPV for 5 days [initiated either 2 days prior to infection (−2 dpi–2 dpi) or on the day of infection (0 dpi–4 dpi)] survived significantly longer than control mice, administered with PBS (p = 0.025 and 0.011, respectively). Moreover, at 1 and 3 dpi, the virus titers in the brain were significantly lower in FPV-treated mice (0 dpi–4 dpi) versus PBS-treated mice (p = 0.002 for both 1 and 3 dpi).

Conclusions/Significance

Although the intracerebral inoculation route is thought to be a challenging way to evaluate drug efficacy, FPV inhibits the in vitro replication of JCV and prolongs the survival of mice intracerebrally inoculated with JCV. These results will enable the development of a specific antiviral treatment against JCV infections and establishment of an effective animal model.

Jamestown Canyon virus (JCV) is a mosquito-borne virus (arbovirus) classified into the California serogroup. JCV is distributed widely throughout North America and is considered one of the potentially re-emerging viruses due to the recent spurt in JCV cases in the region. JCV infection often leads to an acute febrile illness, meningitis, and meningoencephalitis mainly among adults. Currently, no antiviral therapy against JCV is approved. In this study, we evaluated the antiviral efficacy of favipiravir (FPV), ribavirin (RBV), and 2’-fluoro-2’-deoxycytidine (2’-FdC) against JCV infection in cultured cells and mice. As a result, FPV, RBV, and 2’-FdC effectively inhibited JCV replication in Vero and Neuro-2a cells. Furthermore, FPV delayed the onset of neurological symptoms in mice intracerebrally inoculated with JCV. Notably, although most patients infected with JCV do not present severe disease, neuroinvasive cases are not rare and may result in residual neurological sequelae such as persisting cognitive deficits. Therefore, this study contributes to the development of a specific antiviral treatment for patients with JCV infection.

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.

Clinical effect and antiviral mechanism of T-705 in treating severe fever with thrombocytopenia syndrome

Severe fever with thrombocytopenia syndrome (SFTS) virus (SFTSV) is an emerging tick-borne virus with high fatality and an expanding endemic. Currently, effective anti-SFTSV intervention remains unavailable. Favipiravir (T-705) was recently reported to show in vitro and in animal model antiviral efficacy against SFTSV. Here, we conducted a single-blind, randomized controlled trial to assess the efficacy and safety of T-705 in treating SFTS (Chinese Clinical Trial Registry website, number ChiCTR1900023350). From May to August 2018, laboratory-confirmed SFTS patients were recruited from a designated hospital and randomly assigned to receive oral T-705 in combination with supportive care or supportive care only. Fatal outcome occurred in 9.5% (7/74) of T-705 treated patients and 18.3% (13/71) of controls (odds ratio, 0.466, 95% CI, 0.174-1.247). Cox regression showed a significant reduction in case fatality rate (CFR) with an adjusted hazard ratio of 0.366 (95% CI, 0.142-0.944). Among the low-viral load subgroup (RT-PCR cycle threshold ≥26), T-705 treatment significantly reduced CFR from 11.5 to 1.6% (P = 0.029), while no between-arm difference was observed in the high-viral load subgroup (RT-PCR cycle threshold <26). The T-705-treated group showed shorter viral clearance, lower incidence of hemorrhagic signs, and faster recovery of laboratory abnormities compared with the controls. The in vitro and animal experiments demonstrated that the antiviral efficacies of T-705 were proportionally induced by SFTSV mutation rates, particularly from two transition mutation types. The mutation analyses on T-705-treated serum samples disclosed a partially consistent mutagenesis pattern as those of the in vitro or animal experiments in reducing the SFTSV viral loads, further supporting the anti-SFTSV effect of T-705, especially for the low-viral loads.

Pathogenicity and virulence of West Nile virus revisited eight decades after its first isolation

West Nile virus (WNV) is a flavivirus which transmission cycle is maintained between mosquitoes and birds, although it occasionally causes sporadic outbreaks in horses and humans that can result in serious diseases and even death. Since its first isolation in Africa in 1937, WNV had been considered a neglected pathogen until its recent spread throughout Europe and the colonization of America, regions where it continues to cause outbreaks with severe neurological consequences in humans and horses. Although our knowledge about the characteristics and consequences of the virus has increased enormously lately, many questions remain to be resolved. Here, we thoroughly update our knowledge of different aspects of the WNV life cycle: virology and molecular classification, host cell interactions, transmission dynamics, host range, epidemiology and surveillance, immune response, clinical presentations, pathogenesis, diagnosis, prophylaxis (antivirals and vaccines), and prevention, and we highlight those aspects that are still unknown and that undoubtedly require further investigation.

Lassa Virus Treatment Options

Lassa fever causes an approximate 5000 to 10,000 deaths annually in West Africa and cases have been imported into Europe and the Americas, challenging public health. Although Lassa virus was first described over 5 decades ago in 1969, no treatments or vaccines have been approved to treat or prevent infection. In this review, we discuss current therapeutics in the development pipeline for the treatment of Lassa fever, focusing on those that have been evaluated in humans or animal models. Several treatments, including the antiviral favipiravir and a human monoclonal antibody cocktail, have shown efficacy in preclinical rodent and non-human primate animal models and have potential for use in clinical settings. Movement of the promising preclinical treatment options for Lassa fever into clinical trials is critical to continue addressing this neglected tropical disease.

Understanding the clinical utility of favipiravir (T-705) in coronavirus disease of 2019: a review

The coronavirus disease of 2019 (COVID-19) has caused significant morbidity and mortality among infected individuals across the world. High transmissibility rate of the causative virus - Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) - has led to immense strain and bottlenecking of the health care system. While noteworthy advances in vaccine development have been made amid the current global pandemic, most therapeutic agents are repurposed from use in other viral infections and are being evaluated for efficacy in COVID-19. Favipiravir, an orally administered drug originally developed in Japan against emerging influenza viral strains, has been shown to have widespread application and safety across multiple ribonucleic acid (RNA) viral infections. With a strong affinity toward the viral RNA-dependent RNA polymerase (RdRp), favipiravir could be a promising therapy against SARS-CoV-2, by targeting downstream viral RNA replication. Initial trials for usage in COVID-19 have suggested that favipiravir administration during initial infection stages, in individuals with mild to moderate infection, has a strong potential to improve clinical outcomes. However, additional well-designed clinical trials are required to closely examine ideal timing of drug administration, dosage, and duration, to assess the role of favipiravir in COVID-19 therapy. This review provides evidence-based insights and throws light on the current clinical trials examining the efficacy of favipiravir in tackling COVID-19, including its mechanism, pharmacodynamics, and pharmacokinetics.

Improving properties of the nucleobase analogs T-705/T-1105 as potential antiviral

In this minireview we describe our work on the improvement of the nucleobase analogs Favipiravir (T-705) und its non-fluorinated derivative T-1105 as influenza and SARS-CoV-2 active compounds. Both nucleobases were converted into nucleotides and then included in our nucleotide prodrugs technologies cycloSal-monophosphates, DiPPro-nucleoside diphosphates and TriPPPro-nucleoside triphosphates. Particularly the DiPPro-derivatives of T-1105-RDP proved to be very active against influenza viruses. T-1105-derivatives in general were found to be more antivirally active as compared to their T-705 counterpart. This may be due to the low chemical stability of all ribosylated derivatives of T-705. The ribosyltriphosphate derivative of T-1105 was studied for the potential to act as a inhibitor of the SARS-CoV-2 RdRp and was found to be an extremely potent compound causing lethal mutagenesis. The pronucleotide technologies, the chemical synthesis, the biophysical properties and the biological effects of the compounds will be addressed as well.

A review on favipiravir: the properties, function, and usefulness to treat COVID-19

INTRODUCTION

At this time, there is no specific therapeutic or vaccine for treatment of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Hence, available drugs for treatment of other viral infections may be useful to treat COVID-19.

AREAS COVERED

The focus of the current review was studying the main characteristics of favipiravir and its usefulness to treat COVID-19. An electronic search was done by using Pubmed and Google scholar.

EXPERT OPINION

Based on the mechanism of action and safety of favipiravir, the drug may be a promising candidate for compassionate use against the SARS-CoV-2 infection. Favipiravir has a wide range of activity against many single-stranded RNA viruses, is well tolerated in humans and has a high barrier to resistance. However, high doses of the agent are necessary to obtain an efficient antiviral activity. Favipiravir is teratogen in pregnant women and associated with the hyperuricemia. Therefore, the administration of the drug should be well controlled. Investigating the antiviral prophylactic potency of favipiravir and search for its pro-drugs and/or analogs showing improved activity and/or safety are critical.

Unexpected complete recovery of a patient with severe tick-borne encephalitis treated with favipiravir

We report a case of tick-borne encephalitis (TBE) in a 22-year-old man, who was admitted to the Medical University of Vienna hospital with severe meningoencephalitis, unresponsive and dependent on a respirator. He had given a history of a recent tick bite, but because he had previously received a full course of vaccination against TBE, West Nile virus infection was suspected. Because the antiviral drug favipiravir has been reported to be active against WNV, therapy was initiated, and continued even after a diagnosis of TBE was confirmed, due to significant improvement of symptoms. Within days, the patient's symptoms resolved, and he was discharged after complete recovery at 15 days after onset. Although this single case does not permit any conclusion as to the role of favipiravir in the favorable outcome, it suggests that the drug should be further evaluated in laboratory animal models and in appropriate clinical settings.

In vitro inhibition of mumps virus replication by favipiravir (T-705)

During the last decade multiple mumps outbreaks have occurred in the U.S. despite high two dose MMR coverage with most cases detected among two dose MMR vaccine recipients. Waning immunity, the evolution of wild-type virus strains, and settings with intense exposure have contributed to the resurgence of mumps. Typically, mumps virus infections resolve without serious clinical sequelae; however, serious complications may occur among unvaccinated or severely immunocompromised individuals. Favipiravir (T-705) has been shown to have in vitro anti-viral activity against a broad range of positive and negative strand RNA viruses. Here, we demonstrate that T-705 inhibits the growth of wildtype and vaccine strains of mumps virus in vitro at low micro-molar concentrations (EC₅₀ 8-10μM). We did not observe the development of resistance after five subsequent passages at low concentrations of drug. Both viral RNA and protein synthesis were selectively reduced compared to host mRNA and protein synthesis. Antiviral treatment options for mumps virus infection may be valuable, especially for areas with a high disease burden or for cases with severe complications. These results presented here suggest that further studies are warranted.

Interim Guidelines on Antiviral Therapy for COVID-19

Since the first case was reported in Wuhan, Hubei Province, China on December 12, 2019, Coronavirus disease 2019 (COVID-19) has spread widely to other countries since January 2020. As of April 16, 2020, 10635 confirmed cases have been reported, with 230 deaths in Korea. COVID-19 patients may be asymptomatic or show various clinical manifestations, including acute symptoms such as fever, fatigue, sore throat; pneumonia presenting as acute respiratory distress syndrome; and multiple organ failure. As COVID-19 has such varied clinical manifestations and case fatality rates, no standard antiviral therapy regimen has been established other than supportive therapy. In the present guideline, we aim to introduce potentially helpful antiviral and other drug therapies based on in vivo and in vitro research and clinical experiences from many countries.

Next-generation direct-acting influenza therapeutics

Influenza viruses are a major threat to human health globally. In addition to further improving vaccine prophylaxis, disease management through antiviral therapeutics constitutes an important component of the current intervention strategy to prevent advance to complicated disease and reduce case-fatality rates. Standard-of-care is treatment with neuraminidase inhibitors that prevent viral dissemination. In 2018, the first mechanistically new influenza drug class for the treatment of uncomplicated seasonal influenza in 2 decades was approved for human use. Targeting the PA endonuclease subunit of the viral polymerase complex, this class suppresses viral replication. However, the genetic barrier against viral resistance to both drug classes is low, pre-existing resistance is observed in circulating strains, and resistant viruses are pathogenic and transmit efficiently. Addressing the resistance problem has emerged as an important objective for the development of next-generation influenza virus therapeutics. This review will discuss the status of influenza therapeutics including the endonuclease inhibitor baloxavir marboxil after its first year of clinical use and evaluate a subset of direct-acting antiviral candidates in different stages of preclinical and clinical development.

Lack of Efficacy of High-Titered Immunoglobulin in Patients with West Nile Virus Central Nervous System Disease

Immunoglobulin administered to adults with neuroinvasive disease appeared to be safe but was not demonstrated to improve clinical outcomes.

West Nile Virus (WNV) can result in clinically severe neurologic disease. There is no treatment for WNV infection, but administration of anti-WNV polyclonal human antibody has demonstrated efficacy in animal models. We compared Omr-IgG-am, an immunoglobulin product with high titers of anti-WNV antibody, with intravenous immunoglobulin (IVIG) and normal saline to assess safety and efficacy in patients with WNV neuroinvasive disease as part of a phase I/II, randomized, double-blind, multicenter study in North America. During 2003–2006, a total of 62 hospitalized patients were randomized to receive Omr-IgG-am, standard IVIG, or normal saline (3:1:1). The primary endpoint was medication safety. Secondary endpoints were morbidity and mortality, measured using 4 standardized assessments of cognitive and functional status. The death rate in the study population was 12.9%. No significant differences were found between groups receiving Omr-IgG-am compared with IVIG or saline for either the safety or efficacy endpoints.

Favipiravir, an anti-influenza drug against life-threatening RNA virus infections

Favipiravir has been developed as an anti-influenza drug and licensed as an anti-influenza drug in Japan. Additionally, favipiravir is being stockpiled for 2 million people as a countermeasure for novel influenza strains. This drug functions as a chain terminator at the site of incorporation of the viral RNA and reduces the viral load. Favipiravir cures all mice in a lethal influenza infection model, while oseltamivir fails to cure the animals. Thus, favipiravir contributes to curing animals with lethal infection. In addition to influenza, favipiravir has a broad spectrum of anti-RNA virus activities in vitro and efficacies in animal models with lethal RNA viruses and has been used for treatment of human infection with life-threatening Ebola virus, Lassa virus, rabies, and severe fever with thrombocytopenia syndrome. The best feature of favipiravir as an antiviral agent is the apparent lack of generation of favipiravir-resistant viruses. Favipiravir alone maintains its therapeutic efficacy from the first to the last patient in an influenza pandemic or an epidemic lethal RNA virus infection. Favipiravir is expected to be an important therapeutic agent for severe influenza, the next pandemic influenza strain, and other severe RNA virus infections for which standard treatments are not available.

VAMP8 Contributes to the TRIM6-Mediated Type I Interferon Antiviral Response during West Nile Virus Infection

Several members of the tripartite motif (TRIM) family of E3 ubiquitin ligases regulate immune pathways, including the antiviral type I interferon (IFN-I) system. Previously, we demonstrated that TRIM6 is involved in IFN-I induction and signaling. In the absence of TRIM6, optimal IFN-I signaling is reduced, allowing increased replication of interferon-sensitive viruses. Despite having evolved numerous mechanisms to restrict the vertebrate host's IFN-I response, West Nile virus (WNV) replication is sensitive to pretreatment with IFN-I. However, the regulators and products of the IFN-I pathway that are important in regulating WNV replication are incompletely defined. Consistent with WNV's sensitivity to IFN-I, we found that in TRIM6 knockout (TRIM6-KO) A549 cells, WNV replication is significantly increased and IFN-I induction and signaling are impaired compared to wild-type (wt) cells. IFN-β pretreatment was more effective in protecting against subsequent WNV infection in wt cells than TRIM6-KO, indicating that TRIM6 contributes to the establishment of an IFN-induced antiviral response against WNV. Using next-generation sequencing, we identified VAMP8 as a potential factor involved in this TRIM6-mediated antiviral response. VAMP8 knockdown resulted in reduced JAK1 and STAT1 phosphorylation and impaired induction of several interferon-stimulated genes (ISGs) following WNV infection or IFN-β treatment. Furthermore, VAMP8-mediated STAT1 phosphorylation required the presence of TRIM6. Therefore, the VAMP8 protein is a novel regulator of IFN-I signaling, and its expression and function are dependent on TRIM6 activity. Overall, these results provide evidence that TRIM6 contributes to the antiviral response against WNV and identify VAMP8 as a novel regulator of the IFN-I system.IMPORTANCE WNV is a mosquito-borne flavivirus that poses a threat to human health across large discontinuous areas throughout the world. Infection with WNV results in febrile illness, which can progress to severe neurological disease. Currently, there are no approved treatment options to control WNV infection. Understanding the cellular immune responses that regulate viral replication is important in diversifying the resources available to control WNV. Here, we show that the elimination of TRIM6 in human cells results in an increase in WNV replication and alters the expression and function of other components of the IFN-I pathway through VAMP8. Dissecting the interactions between WNV and host defenses both informs basic molecular virology and promotes the development of host- and virus-targeted antiviral strategies.

New Nucleoside Analogues for the Treatment of Hemorrhagic Fever Virus Infections

Eight different compounds, all nucleoside analogues, could presently be considered as potential drug candidates for the treatment of Ebola virus (EBOV) and/or other hemorrhagic fever virus (HFV) infections. They can be considered as either (i) adenine analogues (3-deazaneplanocin A, galidesivir, GS-6620 and remdesivir) or (ii) guanine analogues containing the carboxamide entity (ribavirin, EICAR, pyrazofurin and favipiravir). All eight owe their mechanism of action to hydrogen bonded base pairing with either (i) uracil or (ii) cytosine. Four out of the eight compounds (galidesivir, GS-6620, remdesivir and pyrazofurin) are C-nucleosides, and two of them (GS-6620, remdesivir) also contain a phosphoramidate part. The C-nucleoside and phosphoramidate (and for the adenine analogues the 1'-cyano group as well) may be considered as essential attributes for their antiviral activity.

Synergistic lethal mutagenesis of hepatitis C virus

Lethal mutagenesis is an antiviral approach that consists in extinguishing a virus by an excess of mutations acquired during replication in the presence of a mutagenic agent, often a nucleotide analogue. One of its advantages is its broad spectrum nature that renders the strategy potentially effective against emergent RNA viral infections. Here we describe synergistic lethal mutagenesis of hepatitis C virus (HCV) by a combination of favipiravir (T-705) and ribavirin. Synergy has been documented over a broad range of analogue concentrations using the Chou-Talalay method as implemented in the CompuSyn graphics, with average dose reduction index (DRI) above 1 (68.02±101.6 for favipiravir, and 5.83±6.07 for ribavirin), and average combination indices (CI) below 1 (0.52±0.28). Furthermore, analogue concentrations that individually did not extinguish high fitness HCV in ten serial infections, when used in combination they extinguished high fitness HCV in one to two passages. Although both analogues display a preference for G→A and C→U transitions, deep sequencing analysis of mutant spectra indicated a different preference of the two analogues for the mutation sites, thus unveiling a new possible synergy mechanism in lethal mutagenesis. Prospects of synergy among mutagenic nucleotides as a strategy to confront emerging viral infections are discussed.

Antiviral efficacy of favipiravir against canine distemper virus infection in vitro

Background

Canine distemper (CD) is an acute infectious disease with high morbidity rates caused by a highly contagious pathogen (Canine Morbillivirus, also known as canine distemper virus, CDV). CDV can infect a broad range of carnivores resulting in complex clinical signs. Currently, there is no effective method to treat for CDV infections. Favipiravir (T-705), a pyrazine derivative, was shown to be an effective antiviral drug against RNA viruses, acting on RNA-dependent RNA polymerase (RdRp). However, whether the T-705 has antiviral effects following CDV infection is unclear. Here, we investigated the antiviral effect of T-705 against CDV-3 and CDV-11 strains in Vero and DH82 cell lines.

Results

Our data demonstrated that T-705 significantly inhibited the replication of CDV-3 and CDV-11 in both Vero and DH82 cells at different concentrations, ranging from 2.441 μg/ml to 1250 μg/ml. Additionally, T-705 exhibited efficacious antiviral effects when administered at different time points after virus infection. Cytotoxicity tests showed a slight decline in viability in Vero cells after T-705 treatment, and no apparent cytotoxicity was detected in T-705 treated DH82 cells. Comparison of anti-CDV polyclonal serum only inhibition of CDV in supernatant, T-705 directly inhibited viral replication in cells, and indirectly reduced the amount of virions in supernatant. The combination application of T-705 and anti-CDV polyclonal serum exhibited a rapid and robust inhibition against virions in supernatant and virus replication in cells.

Conclusions

Our data strongly indicated that T-705 effectively inhibited viral replication following CDV infection in vitro, and could be a potential candidate for treatment for CD.

Current and Novel Approaches in Influenza Management

Influenza is a disease that poses a significant health burden worldwide. Vaccination is the best way to prevent influenza virus infections. However, conventional vaccines are only effective for a short period of time due to the propensity of influenza viruses to undergo antigenic drift and antigenic shift. The efficacy of these vaccines is uncertain from year-to-year due to potential mismatch between the circulating viruses and vaccine strains, and mutations arising due to egg adaptation. Subsequently, the inability to store these vaccines long-term and vaccine shortages are challenges that need to be overcome. Conventional vaccines also have variable efficacies for certain populations, including the young, old, and immunocompromised. This warrants for diverse efficacious vaccine developmental approaches, involving both active and passive immunization. As opposed to active immunization platforms (requiring the use of whole or portions of pathogens as vaccines), the rapidly developing passive immunization involves administration of either pathogen-specific or broadly acting antibodies against a kind or class of pathogens as a treatment to corresponding acute infection. Several antibodies with broadly acting capacities have been discovered that may serve as means to suppress influenza viral infection and allow the process of natural immunity to engage opsonized pathogens whilst boosting immune system by antibody-dependent mechanisms that bridge the innate and adaptive arms. By that; passive immunotherapeutics approach assumes a robust tool that could aid control of influenza viruses. In this review, we comment on some improvements in influenza management and promising vaccine development platforms with an emphasis on the protective capacity of passive immunotherapeutics especially when coupled with the use of antivirals in the management of influenza infection.

Calicivirus RNA-Dependent RNA Polymerases: Evolution, Structure, Protein Dynamics, and Function

The Caliciviridae are viruses with a positive-sense, single-stranded RNA genome that is packaged into an icosahedral, environmentally stable protein capsid. The family contains five genera (Norovirus, Nebovirus, Sapovirus, Lagovirus, and Vesivirus) that infect vertebrates including amphibians, reptiles, birds, and mammals. The RNA-dependent RNA polymerase (RdRp) replicates the genome of RNA viruses and can speed up evolution due to its error-prone nature. Studying calicivirus RdRps in the context of genuine virus replication is often hampered by a lack of suitable model systems. Enteric caliciviruses and RHDV in particular are notoriously difficult to propagate in cell culture; therefore, molecular studies of replication mechanisms are challenging. Nevertheless, research on recombinant proteins has revealed several unexpected characteristics of calicivirus RdRps. For example, the RdRps of RHDV and related lagoviruses possess the ability to expose a hydrophobic motif, to rearrange Golgi membranes, and to copy RNA at unusually high temperatures. This review is focused on the structural dynamics, biochemical properties, kinetics, and putative interaction partners of these RdRps. In addition, we discuss the possible existence of a conserved but as yet undescribed structural element that is shared amongst the RdRps of all caliciviruses.

Synthesis of [18F]Favipiravir and Biodistribution in C3H/HeN Mice as Assessed by Positron Emission Tomography

Favipiravir (T705; 6-fluoro-3-hydroxypyrazine-2-carboxamide) is a pyrazine analog that has demonstrated potent antiviral activity against a broad spectrum of viruses in multiple in vivo disease models. To better understand the compounds anti-viral activity, assessment of the drug’s biodistribution and kinetics in vivo may lend insight into how best to evaluate the compound efficacy preclinically and to contribute to the design of clinical studies to take into account the compound’s pharmacokinetic distribution and kinetics. In the current study, a method for synthesis of [¹⁸F]favipiravir was developed and the biodistribution in mice naïve to and pre-dosed with favipiravir was assessed by PET and gamma counting of tissue samples. Fluorine-18 labeling of favipiravir was achieved in a one-pot, two-step synthesis using a commercially available precursor, methyl-5-chloroisoxazolo[4,5-b]pyrazine-3-carboxylate, with an overall radiochemical yield of 15–24%, a molar activity of 37–74 GBq/µmol in a 70 minute synthesis time. [¹⁸F]favipiravir tissue uptake and distribution was similar in naïve and pre-dosed mice; however, in the pre-dosed animals plasma clearance was more rapid and tissue clearance appeared to be prolonged. In conclusion, application of PET to the evaluation of favipiravir has demonstrated the importance of dosing regimen on the distribution and tissue uptake and clearance of the molecule. Favipiravir is cleared through the kidney as previously reported but the liver and intestinal excretion may also play an important role in compound elimination. Measurement of the tissue uptake of favipiravir as determined by PET may be a more important indicator of a compound’s potential efficacy than purely monitoring plasma parameters such as viremia and drug levels.

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.

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.

Nucleoside analogs as a rich source of antiviral agents active against arthropod-borne flaviviruses

Nucleoside analogs represent the largest class of small molecule-based antivirals, which currently form the backbone of chemotherapy of chronic infections caused by HIV, hepatitis B or C viruses, and herpes viruses. High antiviral potency and favorable pharmacokinetics parameters make some nucleoside analogs suitable also for the treatment of acute infections caused by other medically important RNA and DNA viruses. This review summarizes available information on antiviral research of nucleoside analogs against arthropod-borne members of the genus Flavivirus within the family Flaviviridae, being primarily focused on description of nucleoside inhibitors of flaviviral RNA-dependent RNA polymerase, methyltransferase, and helicase/NTPase. Inhibitors of intracellular nucleoside synthesis and newly discovered nucleoside derivatives with high antiflavivirus potency, whose modes of action are currently not completely understood, have drawn attention. Moreover, this review highlights important challenges and complications in nucleoside analog development and suggests possible strategies to overcome these limitations.

Efficacy of favipiravir (T-705) in nonhuman primates infected with Ebola virus or Marburg virus

Favipiravir is a broad-spectrum antiviral agent that has demonstrated efficacy against Ebola virus (EBOV) in rodents. However, there are no published reports of favipiravir efficacy for filovirus infection of nonhuman primates (NHPs). Here we evaluated the pharmacokinetic profile of favipiravir in NHPs, as well as in vivo efficacy against two filoviruses, EBOV and Marburg virus (MARV). While no survival benefit was observed in two studies employing once- or twice-daily oral dosing of favipiravir during EBOV infection of NHPs, an antiviral effect was observed in terms of extended time-to-death and reduced levels of viral RNA. However, oral dosing in biosafety level-4 (BSL-4) presents logistical and technical challenges, and repeated anesthesia events may potentially worsen survival outcome in animals. For the third study of treatment of MARV infection, we therefore made use of catheters, jackets, and tethers for intravenous (IV) dosing and blood collection, which minimized the requirement for repeated anesthesia events. When MARV infection was treated with IV favipiravir, five of six animals (83%) survived infection, while all untreated NHPs succumbed. An accompanying report presents the results of favipiravir treatment of EBOV infection in mice.

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.

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.

In vitro susceptibility of geographically and temporally distinct Zika viruses to favipiravir and ribavirin

BACKGROUND

Zika virus, a previously neglected mosquito-borne virus, is prompting worldwide concern because of its connection with congenital defects, Guillain-Barré syndrome, meningoencephalitis and myelitis in infected individuals. However, no specific antiviral therapy is available at present. In this study, we investigated the in vitro susceptibility of geographically and temporally distinct Zika viruses against the RNA polymerase inhibitors, favipiravir (T-705) and ribavirin.

METHODS

The in vitro activity of each drug and a 1:1 mixture combination was assessed against five geographically and temporally distinct Zika strains by plaque reduction assay (PRA), the gold standard phenotypic method.

RESULTS

We showed that both drugs exhibit in vitro inhibitory activity against five different Zika strains isolated in different years and continents, with mean 50% inhibitory concentration (IC₅₀) values of 35 ±14 and 35 ±20 µM, respectively, by PRA. We did not observe a synergistic effect when both drugs were combined at the equimolar concentration (IC₅₀ =33 ±11 µM).

CONCLUSIONS

These results indicate that T-705 has the potential to be used in patients with complicated diseases and/or those individuals presenting with significant comorbidities.

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.

Structure and function of the Zika virus full-length NS5 protein

The recent outbreak of Zika virus (ZIKV) has infected over 1 million people in over 30 countries. ZIKV replicates its RNA genome using virally encoded replication proteins. Nonstructural protein 5 (NS5) contains a methyltransferase for RNA capping and a polymerase for viral RNA synthesis. Here we report the crystal structures of full-length NS5 and its polymerase domain at 3.0 Å resolution. The NS5 structure has striking similarities to the NS5 protein of the related Japanese encephalitis virus. The methyltransferase contains in-line pockets for substrate binding and the active site. Key residues in the polymerase are located in similar positions to those of the initiation complex for the hepatitis C virus polymerase. The polymerase conformation is affected by the methyltransferase, which enables a more efficiently elongation of RNA synthesis in vitro. Overall, our results will contribute to future studies on ZIKV infection and the development of inhibitors of ZIKV replication.

Zika virus infection can cause human birth defects and Guillain-Barré syndrome. Here the authors present the structures of the full-length nonstructural protein 5 and its RNA-dependent RNA polymerase domain of Zika virus, which are targets for inhibitors of virus replication.

Favipiravir (T-705), a broad spectrum inhibitor of viral RNA polymerase

Favipiravir (T-705; 6-fluoro-3-hydroxy-2-pyrazinecarboxamide) is an anti-viral agent that selectively and potently inhibits the RNA-dependent RNA polymerase (RdRp) of RNA viruses. Favipiravir was discovered through screening chemical library for anti-viral activity against the influenza virus by Toyama Chemical Co., Ltd. Favipiravir undergoes an intracellular phosphoribosylation to be an active form, favipiravir-RTP (favipiravir ribofuranosyl-5'-triphosphate), which is recognized as a substrate by RdRp, and inhibits the RNA polymerase activity. Since the catalytic domain of RdRp is conserved among various types of RNA viruses, this mechanism of action underpins a broader spectrum of anti-viral activities of favipiravir. Favipiravir is effective against a wide range of types and subtypes of influenza viruses, including strains resistant to existing anti-influenza drugs. Of note is that favipiravir shows anti-viral activities against other RNA viruses such as arenaviruses, bunyaviruses and filoviruses, all of which are known to cause fatal hemorrhagic fever. These unique anti-viral profiles will make favipiravir a potentially promising drug for specifically untreatable RNA viral infections.