Reduced sensitivity of influenza A (H5N1) to oseltamivir

Antiviral Susceptibility of Influenza A(H5N1) Clade 2.3.2.1c and 2.3.4.4b Viruses from Humans, 2023-2024

During 2023-2024, highly pathogenic avian influenza A(H5N1) viruses from clade 2.3.2.1c caused human infections in Cambodia and from clade 2.3.4.4b caused human infections in the Americas. We assessed the susceptibility of those viruses to approved and investigational antiviral drugs. Except for 2 viruses isolated from Cambodia, all viruses were susceptible to M2 ion channel-blockers in cell culture-based assays. In the neuraminidase inhibition assay, all viruses displayed susceptibility to neuraminidase inhibitor antiviral drugs oseltamivir, zanamivir, peramivir, laninamivir, and AV5080. Oseltamivir was ≈4-fold less potent at inhibiting the neuraminidase activity of clade 2.3.4.4b than clade 2.3.2.1c viruses. All viruses were susceptible to polymerase inhibitors baloxavir and tivoxavir and to polymerase basic 2 inhibitor pimodivir with 50% effective concentrations in low nanomolar ranges. Because drug-resistant viruses can emerge spontaneously or by reassortment, close monitoring of antiviral susceptibility of H5N1 viruses collected from animals and humans by using sequence-based analysis supplemented with phenotypic testing is essential.

Genotypic and phenotypic susceptibility of emerging avian influenza A viruses to neuraminidase and cap-dependent endonuclease inhibitors

Avian influenza outbreaks, including ones caused by highly pathogenic A(H5N1) clade 2.3.4.4b viruses, have devastated animal populations and remain a threat to humans. Risk elements assessed for emerging influenza viruses include their susceptibility to approved antivirals. Here, we screened >20,000 neuraminidase (NA) or polymerase acidic (PA) protein sequences of potentially pandemic A(H5Nx), A(H7Nx), and A(H9N2) viruses that circulated globally in 2010-2023. The frequencies of NA or PA substitutions associated with reduced inhibition (RI) or highly reduced inhibition (HRI) by NA inhibitors (NAIs) (oseltamivir, zanamivir) or a cap-dependent endonuclease inhibitor (baloxavir) were low: 0.60% (137/22,713) and 0.62% (126/20,347), respectively. All tested subtypes were susceptible to NAIs and baloxavir at sub-nanomolar concentrations. A(H9N2) viruses were the most susceptible to oseltamivir, with IC50s 3- to 4-fold lower than for other subtypes (median IC50: 0.18 nM; n = 22). NA-I222M conferred RI of A(H5N1) viruses by oseltamivir (with a 26-fold IC50 increase), but NA-S246N did not reduce inhibition. PA-E23G, PA-K34R, PA-I38M/T, and the previously unreported PA-A36T caused RI by baloxavir in all subtypes tested. Avian A(H9N2) viruses endemic in Egyptian poultry predominantly acquired PA-I38V, which causes only a <3-fold decrease in the baloxavir EC50 and fails to meet the RI criteria. PA-E199A/D in A(H7Nx) and A(H9N2) viruses caused a 2- to 4-fold decrease in EC50 (close to the borderline for RI) and should be closely monitored. Our data indicate antiviral susceptibility is high among avian influenza A viruses with pandemic potential and present novel markers of resistance to existing antiviral interventions.

Antiviral Susceptibility of Highly Pathogenic Avian Influenza A(H5N1) Viruses Circulating Globally in 2022-2023

The antiviral susceptibility of currently circulating (2022-2023) highly pathogenic avian influenza (HPAI) A(H5N1) viruses was assessed by genotypic and phenotypic approaches. The frequency was low for neuraminidase (NA) and polymerase acidic (PA) substitutions associated with reduced inhibition by NA inhibitors (21/2698, 0.78%) or the PA inhibitor baloxavir (14/2600, 0.54%). Phenotypic testing of 22 clade 2.3.2.1a and 2.3.4.4b viruses revealed broad susceptibility to NA inhibitors and baloxavir for a conclusion that most contemporary HPAI A(H5N1) viruses retain susceptibility to antiviral drugs. Novel NA-K432E and NA-T438I substitutions (N2 numbering) were identified at elevated frequencies (104/2698, 3.85%) and caused reduced zanamivir and peramivir inhibition.

Antiviral Chemotherapy in Avian Medicine-A Review

This review article describes the current knowledge about the use of antiviral chemotherapeutics in avian species, such as farm poultry and companion birds. Specific therapeutics are described in alphabetical order including classic antiviral drugs, such as acyclovir, abacavir, adefovir, amantadine, didanosine, entecavir, ganciclovir, interferon, lamivudine, penciclovir, famciclovir, oseltamivir, ribavirin, and zidovudine, repurposed drugs, such as ivermectin and nitazoxanide, which were originally used as antiparasitic drugs, and some others substances showing antiviral activity, such as ampligen, azo derivates, docosanol, fluoroarabinosylpyrimidine nucleosides, and novel peptides. Most of them have only been used for research purposes and are not widely used in clinical practice because of a lack of essential pharmacokinetic and safety data. Suggested future research directions are also highlighted.

Recombinant A(H6N1)-H274Y avian influenza virus with dual drug resistance does not require permissive mutations to retain the replicative fitness in vitro and in ovo

The possible emergence of drug-resistant avian flu raises concerns over the limited effectiveness of currently approved antivirals (neuraminidase inhibitors - NAIs) in the hypothetical event of a zoonotic spillover. Our study demonstrated that the recombinant avian A(H6N1) viruses showed reduced inhibition (RI) by multiple NAI drugs following the introduction of point mutations found predominantly in the neuraminidase gene (NA) of NAI-resistant human influenza strains (E119V, R292K and H274Y; N2 numbering). Moreover, A(H6N1)-H274Y showed increased replication efficiency in vitro, and a fitness advantage over wild-type (WT) when co-inoculated into embryonated hen's eggs. The results presented in our study together with the zoonotic potential of the A(H6N1) virus as evidenced by the human infection from 2013, highlight the need for enhanced monitoring of NAI resistance-associated signatures in circulating LPAI (low pathogenic avian influenza) globally.

Vaccination and Antiviral Treatment against Avian Influenza H5Nx Viruses: A Harbinger of Virus Control or Evolution

Despite the panzootic nature of emergent highly pathogenic avian influenza H5Nx viruses in wild migratory birds and domestic poultry, only a limited number of human infections with H5Nx viruses have been identified since its emergence in 1996. Few countries with endemic avian influenza viruses (AIVs) have implemented vaccination as a control strategy, while most of the countries have adopted a culling strategy for the infected flocks. To date, China and Egypt are the two major sites where vaccination has been adopted to control avian influenza H5Nx infections, especially with the widespread circulation of clade 2.3.4.4b H5N1 viruses. This virus is currently circulating among birds and poultry, with occasional spillovers to mammals, including humans. Herein, we will discuss the history of AIVs in Egypt as one of the hotspots for infections and the improper implementation of prophylactic and therapeutic control strategies, leading to continuous flock outbreaks with remarkable virus evolution scenarios. Along with current pre-pandemic preparedness efforts, comprehensive surveillance of H5Nx viruses in wild birds, domestic poultry, and mammals, including humans, in endemic areas is critical to explore the public health risk of the newly emerging immune-evasive or drug-resistant H5Nx variants.

Antiviral susceptibility of clade 2.3.4.4b highly pathogenic avian influenza A(H5N1) viruses isolated from birds and mammals in the United States, 2022

Clade 2.3.4.4b highly pathogenic avian influenza (HPAI) A(H5N1) viruses that are responsible for devastating outbreaks in birds and mammals pose a potential threat to public health. Here, we evaluated their susceptibility to influenza antivirals. Of 1,015 sequences of HPAI A(H5N1) viruses collected in the United States during 2022, eight viruses (∼0.8%) had a molecular marker of drug resistance to an FDA-approved antiviral: three adamantane-resistant (M2-V27A), four oseltamivir-resistant (NA-H275Y), and one baloxavir-resistant (PA-I38T). Additionally, 31 viruses contained mutations that may reduce susceptibility to inhibitors of neuraminidase (NA) (n = 20) or cap-dependent endonuclease (CEN) (n = 11). A panel of 22 representative viruses was tested phenotypically. Overall, clade 2.3.4.4b A(H5N1) viruses lacking recognized resistance mutations were susceptible to FDA-approved antivirals. Oseltamivir was least potent at inhibiting NA activity, while the investigational NA inhibitor AV5080 was most potent, including against NA mutants. A novel NA substitution T438N conferred 12-fold reduced inhibition by zanamivir, and in combination with the known marker N295S, synergistically affected susceptibility to all five NA inhibitors. In cell culture-based assays HINT and IRINA, the PA-I38T virus displayed 75- to 108-fold and 37- to 78-fold reduced susceptibility to CEN inhibitors, baloxavir and the investigational AV5116, respectively. Viruses with PA-I38M or PA-A37T showed 5- to 10-fold reduced susceptibilities. As HPAI A(H5N1) viruses continue to circulate and evolve, close monitoring of drug susceptibility is needed for risk assessment and to inform decisions regarding antiviral stockpiling.

Antiviral Approaches against Influenza Virus

Preventing and controlling influenza virus infection remains a global public health challenge, as it causes seasonal epidemics to unexpected pandemics. These infections are responsible for high morbidity, mortality, and substantial economic impact. Vaccines are the prophylaxis mainstay in the fight against influenza. However, vaccination fails to confer complete protection due to inadequate vaccination coverages, vaccine shortages, and mismatches with circulating strains. Antivirals represent an important prophylactic and therapeutic measure to reduce influenza-associated morbidity and mortality, particularly in high-risk populations. Here, we review current FDA-approved influenza antivirals with their mechanisms of action, and different viral- and host-directed influenza antiviral approaches, including immunomodulatory interventions in clinical development. Furthermore, we also illustrate the potential utility of machine learning in developing next-generation antivirals against influenza.

Pharmacokinetics, Pharmacodynamics and Antiviral Efficacy of the MEK Inhibitor Zapnometinib in Animal Models and in Humans

The mitogen-activated protein kinase (MEK) inhibitor zapnometinib is in development to treat acute viral infections like COVID-19 and influenza. While the antiviral efficacy of zapnometinib is well documented, further data on target engagement/pharmacodynamics (PD) and pharmacokinetics (PK) are needed. Here, we report zapnometinib PK and PD parameters in mice, hamsters, dogs, and healthy human volunteers. Mice received 25 mg/kg/day zapnometinib (12.5 mg/kg p. o. twice daily, 8 h interval). Syrian hamsters received 30 mg/kg (15 mg/kg twice daily) or 60 mg/kg/day once daily. Beagle dogs were administered 300 mg/kg/day, and healthy human volunteers were administered 100, 300, 600 and 900 mg zapnometinib (once daily p. o.). Regardless of species or formulation, zapnometinib maximum plasma concentration (C_max) was reached between 2–4 h after administration with an elimination half-life of 4–5 h in dogs, 8 h in mice or hamsters and 19 h in human subjects. Doses were sufficient to cause up to 80% MEK inhibition. Across all species approximately 10 μg/ml zapnometinib was appropriate to inhibit 50% of peripheral blood mononuclear cells (PBMC) MEK activity. In mice, a 50%–80% reduction of MEK activity was sufficient to reduce influenza virus titer in the lungs by more than 90%. In general, while >50% MEK inhibition was reached in vivo at most doses, 80% inhibition in PBMCs required significantly higher doses and appeared to be the practical maximal level obtained in vivo. However, the period of reduced phosphorylated extracellular-signal regulated kinase (pERK), a measure of MEK inhibition, was maintained even after elimination of zapnometinib from plasma, suggesting a sustained effect on MEK consistent with regulatory effects or a slow off-rate. These data suggest a target plasma C_max of at least 10 μg/ml zapnometinib in further clinical studies.

Optimisation of Neuraminidase Expression for Use in Drug Discovery by Using HEK293-6E Cells

Influenza virus is a highly contagious virus that causes significant human mortality and morbidity annually. The most effective drugs for treating influenza are the neuraminidase inhibitors, but resistance to these inhibitors has emerged, and additional drug discovery research on neuraminidase and other targets is needed. Traditional methods of neuraminidase production from embryonated eggs are cumbersome, while insect cell derived protein is less reflective of neuraminidase produced during human infection. Herein we describe a method for producing neuraminidase from a human cell line, HEK293-6E, and demonstrate the method by producing the neuraminidase from the 1918 H1N1 pandemic influenza strain. This method produced high levels of soluble neuraminidase expression (>3000 EU/mL), was enhanced by including a secretion signal from a viral chemokine binding protein, and does not require co-expression of additional proteins. The neuraminidase produced was of sufficient quantity and purity to support high resolution crystal structure determination. The structure solved using this protein conformed to the previously reported structure. Notably the glycosylation at three asparagine residues was superior in quality to that from insect cell derived neuraminidase. This method of production of neuraminidase should prove useful in further studies, such as the characterisation of inhibitor binding.

Viral Factors Associated With the High Mortality Related to Human Infections With Clade 2.1 Influenza A/H5N1 Virus in Indonesia

Background

Since their emergence in Indonesia in 2005, 200 human infections with clade 2.1 highly pathogenic avian influenza A/H5N1 virus have been reported, associated with exceptionally high mortality (84%) compared to regions affected by other genetic clades of this virus. To provide potential clues towards understanding this high mortality, detailed clinical virological analyses were performed in specimens from 180 H5N1 patients, representing 90% of all Indonesian patients and 20% of reported H5N1-infected patients globally.

Methods

H5N1 RNA was quantified in available upper- and lower-respiratory tract specimens as well as fecal and blood samples from 180 patients with confirmed infection between 2005 and 2017. Mutations in the neuraminidase and M2 genes that confer resistance to oseltamivir and adamantanes were assessed. Fatal and nonfatal cases were compared.

Results

High viral RNA loads in nasal and pharyngeal specimens were associated with fatal outcome. Mortality increased over time during the study period, which correlated with increasing viral RNA loads on admission. Furthermore, the prevalence of amantadine resistance–conferring M2 mutations increased over time, and viral loads were higher in patients infected with viruses that harbored these mutations. Compared to observations from other regions, viral RNA was detected more frequently in feces (80%) and particularly in blood (85%), and antiviral responses to oseltamivir appeared less pronounced.

Conclusions

These observations confirm the association of viral load with outcome of human H5N1 infections and suggest potential differences in virulence and antiviral responses to oseltamivir that may explain the exceptionally high mortality related to clade 2.1 H5N1 infections in Indonesia.

Phase 2a, open-label, dose-escalating, multi-center pharmacokinetic study of favipiravir (T-705) in combination with oseltamivir in patients with severe influenza

BACKGROUND

The pharmacokinetics and appropriate dose regimens of favipiravir are unknown in hospitalized influenza patients; such data are also needed to determine dosage selection for favipiravir trials in COVID-19.

METHODS

In this dose-escalating study, favipiravir pharmacokinetics and tolerability were assessed in critically ill influenza patients. Participants received one of two dosing regimens; Japan licensed dose (1600 mg BID on day 1 and 600 mg BID on the following days) and the higher dose (1800 mg/800 mg BID) trialed in uncomplicated influenza. The primary pharmacokinetic endpoint was the proportion of patients with a minimum observed plasma trough concentration (Ctrough) ≥20 mg/L at all measured time points after the second dose.

RESULTS

Sixteen patients were enrolled into the low dose group and 19 patients into the high dose group of the study. Favipiravir Ctrough decreased significantly over time in both groups (p <0.01). Relative to day 2 (48 hrs), concentrations were 91.7% and 90.3% lower in the 1600/600 mg group and 79.3% and 89.5% lower in the 1800/800 mg group at day 7 and 10, respectively. In contrast, oseltamivir concentrations did not change significantly over time. A 2-compartment disposition model with first-order absorption and elimination described the observed favipiravir concentration-time data well. Modeling demonstrated that less than 50% of patients achieved Ctrough ≥20 mg/L for >80% of the duration of treatment of the two dose regimens evaluated (18.8% and 42.1% of patients for low and high dose regimen, respectively). Increasing the favipravir dosage predicted a higher proportion of patients reaching this threshold of 20 mg/L, suggesting that dosing regimens of ≥3600/2600 mg might be required for adequate concentrations. The two dosing regimens were well-tolerated in critical ill patients with influenza.

CONCLUSION

The two dosing regimens proposed for uncomplicated influenza did not achieve our pre-defined treatment threshold.

Synthesis and anti-H5N1 virus activity of triazole- and oxadiazole-pyrimidine hybrids and their nucleoside analogs

New 1,2,3-triazole glycosides and 1,2,4-thioglycosides incorporating a substituted pyrimidinedione ring system were synthesized via click dipolar cycloaddition and heterocyclization of hydrazine-1-carbodithioate derivatives, respectively. The sugar hydrazine derivatives linked aminodimethyluracil were also prepared. In addition, the oxadiazoline substituted with acyclic sugar moieties linked to the pyrimidinedione as acyclic nucleoside analogs were synthesized. The antiviral activity of the synthesized compounds against avian influenza H5N1 virus was investigated and compounds 18, 13 and 19 showed good activities against the virus strains.

Neuraminidase inhibitors susceptibility profiles of highly pathogenic influenza A (H5N1) viruses isolated from avian species in India (2006-2015)

We tested 65 highly pathogenic avian influenza (HPAI) A(H5N1) viruses, isolated from avian species in India between 2006 and 2015, for susceptibility to the FDA approved neuraminidase (NA) inhibitors (NAIs), oseltamivir and zanamivir using a phenotypic fluorescence-based assay. The overall incidence of resistant variants among HPAI A(H5N1) viruses was 7.69% (5/65). The NA inhibition assay identified 3 viruses resistant to oseltamivir (N294S substitution, N2 numbering) and 2 cross-resistant to oseltamivir and zanamivir (E119A or I117V+E119A substitutions), all of which belonged to hemagglutinin (HA) clade 2.2 (5/17) and predominantly circulated in Indian poultry during 2006-2010. In comparison to E119A substitution alone, viruses with I117V+E119A double substitutions showed greater reduction in susceptibility to both oseltamivir and zanamivir. The NAI resistance-associated NA markers, identified in this study, were as a result of naturally occurring mutations. Of note, 48 viruses of HA clade 2.3.2.1 that circulated in Indian poultry during 2011-2015 were susceptible to both oseltamivir and zanamivir. It is essential to monitor NAI susceptibility among human and avian HPAI A(H5N1) viruses that would provide baseline data to develop strategies for pandemic preparedness and therapeutic interventions.

Zoonotic Influenza and Human Health-Part 2: Clinical Features, Diagnosis, Treatment, and Prevention Strategies

Purpose of Review

Zoonotic influenza viruses are those influenza viruses that cross the animal-human barrier and can cause disease in humans, manifesting from minor respiratory illnesses to multiorgan dysfunction. The increasing incidence of infections caused by these viruses worldwide has necessitated focused attention to improve both diagnostic as well as treatment modalities. In this second part of a two-part review, we discuss the clinical features, diagnostic modalities, and treatment of zoonotic influenza, and provide an overview of prevention strategies.

Recent Findings

Illnesses caused by novel reassortant avian influenza viruses continue to be detected and described; most recently, a human case of avian influenza A(H7N4) has been described from China. We continue to witness increasing rates of A(H7N9) infections, with the latest (fifth) wave, from late 2016 to 2017, being the largest to date. The case fatality rate for A(H7N9) and A(H5N1) infections among humans is much higher than that of seasonal influenza infections. Since the emergence of the A(H1N1) 2009 pandemic, and subsequently A(H7N9), testing and surveillance for novel influenzas have become more effective. Various newer treatment options, including peramivir, favipiravir (T-705), and DAS181, and human or murine monoclonal antibodies have been evaluated in vitro and in animal models.

Summary

Armed with robust diagnostic modalities, antiviral medications, vaccines, and advanced surveillance systems, we are today better prepared to face a new influenza pandemic and to limit the burden of zoonotic influenza than ever before. Sustained efforts and robust research are necessary to efficiently deal with the highly mutagenic zoonotic influenza viruses.

Identification of Indonesian clade 2.1 highly pathogenic influenza A(H5N1) viruses with N294S and S246N neuraminidase substitutions which further reduce oseltamivir susceptibility

We have tested the in vitro susceptibility to the neuraminidase (NA) inhibitors of 96 highly pathogenic clade 2.1 A(H5N1) viruses from Indonesia, isolated between 2008 and 2011. HPAI virus samples obtained through the Influenza Virus Monitoring (IVM) surveillance program in Indonesia were tested for susceptibility to oseltamivir and zanamivir. The NAs of four viruses were identified as extreme outliers to oseltamivir, based on statistical analysis by box plots, with IC₅₀ values ranging from 46 to 62 nM. The NAs of two of these viruses from Sumatra and Aceh, had an N294S substitution, while one virus from Sulawesi had an S246N NA substitution. The NAs of all four viruses showed a specific loss of slow binding to oseltamivir in an IC₅₀ kinetics assay. As observed in our previous surveillance, there was only a minimal effect on the sensitivity to zanamivir or peramivir for these mutants or any of the other isolates tested. The continued circulation of subtype H5N1 viruses in avian species poses an on-going zoonotic threat. The fact that we continue to identify avian isolates with naturally occurring mutations conferring reduced oseltamivir susceptibility remains a concern, given oseltamivir will be a key antiviral in the event of a new pandemic emerging.

Structural and Functional Analysis of Anti-Influenza Activity of 4-, 7-, 8- and 9-Deoxygenated 2,3-Difluoro- N-acetylneuraminic Acid Derivatives

Competitive inhibitors of the influenza neuraminidase (NA) were discovered almost 20 years ago, with zanamivir and oseltamivir licensed globally. These compounds are based on a transition state analogue of the sialic acid substrate. We recently showed that 5- N-(acetylamino)-2,3,5-trideoxy-2,3-difluoro-d-erythro-β-l-manno-2-nonulopyranosonic acid (DFSA) and its derivatives are also potent inhibitors of the influenza NA. They are mechanism based inhibitors, forming a covalent bond between the C2 of the sugar ring and Y406 in the NA active site, thus inactivating the enzyme. We have now synthesized a series of deoxygenated DFSA derivatives in order to understand the contribution of each hydroxyl in DFSA to binding and inhibition of the influenza NA. We have investigated their relative efficacy in enzyme assays in vitro, in cell culture, and by X-ray crystallography. We found loss of the 8- and 9-OH had the biggest impact on the affinity of binding and antiviral potency.

Commentary: A Historical Review of Centers for Disease Control and Prevention Antiviral Treatment and Postexposure Chemoprophylaxis Guidance for Human Infections With Novel Influenza A Viruses Associated With Severe Human Disease

Human infections with novel influenza A viruses are of global public health concern, and antiviral medications have a potentially important role in treatment and prevention of human illness. Initial guidance was developed by the U.S. Centers for Disease Control and Prevention after the emergence of human infections with avian influenza A(H5N1) and has evolved over time, with identification of influenza A(H7N9) virus infections in humans, as well as detection of avian influenza viruses in birds in the United States. This commentary describes the historical context and current guidance for the use of influenza antiviral medications for treatment and post-exposure chemoprophylaxis of human infections with novel influenza A viruses associated with severe human illness, or with the potential to cause severe human disease, and provides the scientific rationale behind current recommendations.

DNAzymes Dz13 target the c-jun possess antiviral activity against influenza A viruses

The emergence of anti-influenza A virus drugs resistant strain highlights the need for more effective therapy. Our earlier study demonstrated that c-jun, a downstream molecule of JNK, might be important in viral infections and inflammatory responses. In the present study, we explored the function of DNAzymes Dz13 that target c-jun in influenza A virus infected mice. Dz13 displayed non-toxic side effects on A549 cells and BALB/c mice. Moreover, Dz13-treated mice had enhanced survival after influenza compared with untreated mice. Simultaneously, the pulmonary inflammatory responses and viral burden were decreased in Dz13 treated mice. Furthermore, proliferation levels of infection-induced CD4⁺ and CD8⁺ T cells were impaired. These data demonstrated that Dz13 could reduce viral replication and inflammatory response in vivo, suggesting that Dz13 may potentially be used to treat influenza A viral infection.

Identification of Residues That Affect Oligomerization and/or Enzymatic Activity of Influenza Virus H5N1 Neuraminidase Proteins

Influenza A virus (IAV) attachment to and release from sialoside receptors is determined by the balance between hemagglutinin (HA) and neuraminidase (NA). The molecular determinants that mediate the specificity and activity of NA are still poorly understood. In this study, we aimed to design the optimal recombinant soluble NA protein to identify residues that affect NA enzymatic activity. To this end, recombinant soluble versions of four different NA proteins from H5N1 viruses were compared with their full-length counterparts. The soluble NA ectodomains were fused to three commonly used tetramerization domains. Our results indicate that the particular oligomerization domain used does not affect the Km value but may affect the specific enzymatic activity. This particularly holds true when the stalk domain is included and for NA ectodomains that display a low intrinsic ability to oligomerize. NA ectodomains extended with a Tetrabrachion domain, which forms a nearly parallel four-helix bundle, better mimicked the enzymatic properties of full-length proteins than when other coiled-coil tetramerization domains were used, which probably distort the stalk domain. Comparison of different NA proteins and mutagenic analysis of recombinant soluble versions thereof resulted in the identification of several residues that affected oligomerization of the NA head domain (position 95) and therefore the specific activity or sialic acid binding affinity (Km value; positions 252 and 347). This study demonstrates the potential of using recombinant soluble NA proteins to reveal determinants of NA assembly and enzymatic activity.

IMPORTANCE

The IAV HA and NA glycoproteins are important determinants of host tropism and pathogenicity. However, NA is relatively understudied compared to HA. Analysis of soluble versions of these glycoproteins is an attractive way to study their activities, as they are easily purified from cell culture media and applied in downstream assays. In the present study, we analyzed the enzymatic activity of different NA ectodomains with three commonly used tetramerization domains and compared them with full-length NA proteins. By performing a mutagenic analysis, we identified several residues that affected NA assembly, activity, and/or substrate binding. In addition, our results indicate that the design of the recombinant soluble NA protein, including the particular tetramerization domain, is an important determinant for maintaining the enzymatic properties within the head domain. NA ectodomains extended with a Tetrabrachion domain better mimicked the full-length proteins than when the other tetramerization domains were used.

Avian influenza virus in pregnancy

The unprecedented epizootic of avian influenza viruses, such as H5N1, H5N6, H7N1 and H10N8, has continued to cause disease in humans in recent years. In 2013, another novel influenza A (H7N9) virus emerged in China, and 30% of those patients died. Pregnant women are particularly susceptible to avian influenza and are more likely to develop severe complications and to die, especially when infection occurs in the middle and late trimesters. Viremia is believed to occur infrequently, and thus vertical transmission induced by avian influenza appears to be rare. However, avian influenza increases the risk of adverse pregnancy outcomes, including spontaneous abortion, preterm birth and fatal distress. This review summarises 39 cases of pregnant women and their fetuses from different countries dating back to 1997, including 11, 15 and 13 infections with H7N9, H5N1 and the 2009 pandemic influenza (H1N1), respectively. We analysed the epidemic features, following the geographical, population and pregnancy trimester distributions; underlying diseases; exposure history; medical timelines; human-to-human transmission; pathogenicity and vertical transmission; antivirus treatments; maternal severity and mortality and pregnancy outcome. The common experiences reported in different countries and areas suggest that early identification and treatment are imperative. In the future, vigilant virologic and epidemiologic surveillance systems should be developed to monitor avian influenza viruses during pregnancy. Furthermore, extensive study on the immune mechanisms should be conducted, as this will guide safe, rational immunomodulatory treatment among this high-risk population. Most importantly, we should develop a universal avian influenza virus vaccine to prevent outbreaks of the different subtypes. Copyright © 2016 John Wiley & Sons, Ltd.

Safety, potential efficacy, and pharmacokinetics of specific polyclonal immunoglobulin F(ab')₂ fragments against avian influenza A (H5N1) in healthy volunteers: a single-centre, randomised, double-blind, placebo-controlled, phase 1 study

BACKGROUND

Human infection with the avian influenza A H5N1 virus results in disease with a high fatality rate, against which antiviral treatments have limited efficacy. We aimed to investigate the safety, pharmacokinetics, and therapeutic potential of specific polyclonal immunoglobulin equine F(ab')₂ fragments raised against influenza A/Vietnam/1194/2004 virus (H5N1 subtype) in healthy volunteers.

METHODS

We did a randomised, double-blind, placebo-controlled, single-centre phase 1 study. In stage 1 (one infusion) and stage 2 (five infusions) of the trial, we randomly assigned healthy male volunteers to receive once-daily intravenous infusions of 0·85 U/kg body weight of F(ab')₂ or once-daily saline placebo. Randomisation was done centrally, with one block of four patients and one block for substitutes (three actives, one placebo) in stage 1, and two blocks of six patients (five actives and one placebo) and the same block for substitutes in stage 2. The primary objective was assessment of the clinical and laboratory safety of F(ab')₂, which was monitored for 22 days in the group that received one dose (assessments on days 0-2, 4, 8, 15, and 22) and 33 days in the group that received five doses (days 0-6, 8, 10, 12, 19, 26, and 33). A final post-study safety assessment was done at 120 days. We also assessed pharmacokinetic outcomes, and assayed haemagglutination and seroneutralisation activity. Analysis was done according to intention-to-treat. This trial is registered with ClinicalTrials.gov, number NCT02295813.

FINDINGS

We enrolled 16 healthy Asian men between Sept 28 and Dec 28, 2012, and randomly assigned 13 to one or five doses of F(ab')₂ and three to placebo. F(ab')₂ was well tolerated, and no deaths or serious adverse events occurred. Three patients had mild adverse events (one each of blepharospasm, sinusitis, and pyrexia). The pyrexia (38°C) was regarded as probably related to the infusion, and resolved after 37 min. Our laboratory assessments of blood and urine samples and physical examinations of heart rate, electrocardiogram readings, and weight showed no clinically significant safety issues. Mean peak plasma concentrations were 19·3 μg/mL (SD 3·5) with the one dose schedule and 23·0 μg/mL (4·5) with the five-dose schedule. F(ab')₂ were still detectable in plasma on average up to 5 days after five doses. Haemagglutination inhibition was only increased after the third dose, but in-vitro seroneutralisation activity was transiently increased after each of the five doses to concentrations regarded as clinically beneficial in infected patients.

INTERPRETATION

F(ab')₂ showed good safety, tolerability, and therapeutic potential for managing of H5N1 exposed patients.

FUNDING

Fab'entech.

Single H5N1 influenza A neuraminidase mutation develops resistance to oseltamivir due to distorted conformational and drug binding landscape: multiple molecular dynamics analyses

Clinical studies showed that a single mutation, I117V, develops severe resistance to oseltamivir, the first orally active influenza A neuraminidase inhibitor, in highly pathogenic H5N1 influenza A viruses.

Effects of flavonoid-induced oxidative stress on anti-H5N1 influenza a virus activity exerted by baicalein and biochanin A

Background

Different flavonoids are known to interfere with influenza A virus replication. Recently, we showed that the structurally similar flavonoids baicalein and biochanin A inhibit highly pathogenic avian H5N1 influenza A virus replication by different mechanisms in A549 lung cells. Here, we investigated the effects of both compounds on H5N1-induced reactive oxygen species (ROS) formation and the role of ROS formation during H5N1 replication.

Findings

Baicalein and biochanin A enhanced H5N1-induced ROS formation in A549 cells and primary human monocyte-derived macrophages. Suppression of ROS formation induced by baicalein and biochanin A using the antioxidant N-acetyl-L-cysteine strongly increased the anti-H5N1 activity of both compounds in A549 cells but not in macrophages.

Conclusions

These findings emphasise that flavonoids induce complex pharmacological actions some of which may interfere with H5N1 replication while others may support H5N1 replication. A more detailed understanding of these actions and the underlying structure-activity relationships is needed to design agents with optimised anti-H5N1 activity.

Solid phase assay for comparing reactivation rates of neuraminidases of influenza wild type and resistant mutants after inhibitor removal

The influenza virus neuraminidase inhibitors are normally slow binding inhibitors, but many mutations leading to resistance, also result in the loss of the slow binding phenotype. Mutations can also affect the rate of dissociation of the inhibitors from the neuraminidase, but the assays to measure this require large amounts of virus and are time consuming. To more fully understand the impacts of mutations on the binding and dissociation of the neuraminidase inhibitors we have developed a solid phase reactivation assay, which can use small amounts of crude virus sample bound to an ELISA plate. Multiple viruses can be assayed simultaneously against multiple inhibitors. Using this assay we have demonstrated differences in the relative rates of dissociation of the inhibitors and reactivation of enzyme activity among different influenza A and B viruses for zanamivir, oseltamivir and peramivir. In general oseltamivir dissociated the fastest, and dissociation of peramivir was much slower than both the other inhibitors. Viruses with H274Y, E119V and E119G mutations demonstrated faster dissociation of the inhibitor to which they were resistant. Dissociation of zanamivir and oseltamivir were faster from the D197E mutant, but not of peramivir.

Specific polyclonal F(ab')2 neutralize a large panel of highly pathogenic avian influenza A viruses (H5N1) and control infection in mice

AIM

There is still no specific therapy for infection with the highly pathogenic avian influenza A virus (HPAI) H5N1, which caused 39 human cases with a 64% fatality rate in 2013.

MATERIALS & METHODS

We prepared highly purified specific equine polyclonal immunoglobulin fragments (F(ab')2) against H5N1 and tested them for efficacy in vitro and with different administration schedules in H5N1-challenged BALB/c mice.

RESULTS

in vitro, F(ab')2 neutralized 21 different H5N1 strains from different areas, representative of 11 different clades and sub-clades and 9 years of evolution of the virus. In vivo mouse experiments identified that the most efficient administration protocol consists of five consecutive daily injections after infection; 10 mg/kg giving a 60% increase in survival.

CONCLUSION

These data demonstrate the ability of anti-H5N1 F(ab')2 to markedly reduce the mortality and morbidity associated with infection of mice with HPAI H5N1 virus, and their potential for human therapy.

Antiviral susceptibility of highly pathogenic avian influenza A(H5N1) viruses isolated from poultry, Vietnam, 2009-2011

We assessed drug susceptibilities of 125 avian influenza A(H5N1) viruses isolated from poultry in Vietnam during 2009-2011. Of 25 clade 1.1 viruses, all possessed a marker of resistance to M2 blockers amantadine and rimantadine; 24 were inhibited by neuraminidase inhibitors. One clade 1.1 virus contained the R430W neuraminidase gene and reduced inhibition by oseltamivir, zanamivir, and laninamivir 12-, 73-, and 29-fold, respectively. Three of 30 clade 2.3.4 viruses contained a I223T mutation and showed 7-fold reduced inhibition by oseltamivir. One of 70 clade 2.3.2.1 viruses had the H275Y marker of oseltamivir resistance and exhibited highly reduced inhibition by oseltamivir and peramivir; antiviral agents DAS181 and favipiravir inhibited H275Y mutant virus replication in MDCK-SIAT1 cells. Replicative fitness of the H275Y mutant virus was comparable to that of wildtype virus. These findings highlight the role of drug susceptibility monitoring of H5N1 subtype viruses circulating among birds to inform antiviral stockpiling decisions for pandemic preparedness.

Immunomodulatory Activity and Protective Effects of Polysaccharide from Eupatorium adenophorum Leaf Extract on Highly Pathogenic H5N1 Influenza Infection

The development of novel broad-spectrum, antiviral agents against H5N1 infection is urgently needed. In this study, we evaluated the immunomodulatory activities and protective effect of Eupatorium adenophorum polysaccharide (EAP) against the highly pathogenic H5N1 subtype influenza virus. EAP treatment significantly increased the production of IL-6, TNF-α, and IFN-γ both in vivo and in vitro as measured by qPCR and ELISA. In a mouse infection model, intranasal administration of EAP at a dose of 25 mg/kg body weight prior to H5N1 viral challenge efficiently inhibited viral replication, decreased lung lesions, and increased survival rate. We further evaluated the innate immune recognition of EAP, as this process is regulated primarily Dectin-1 and mannose receptor (MR). These results indicate that EAP may have immunomodulatory properties and a potential prophylactic effect against H5N1 influenza infection. Our investigation suggests an alternative strategy for the development of novel antiinfluenza agents and benefits of E. adenophorum products.

Influenza neuraminidase inhibitors: antiviral action and mechanisms of resistance

There are two major classes of antivirals available for the treatment and prevention of influenza, the M2 inhibitors and the neuraminidase inhibitors (NAIs). The M2 inhibitors are cheap, but they are only effective against influenza A viruses, and resistance arises rapidly. The current influenza A H3N2 and pandemic A(H1N1)pdm09 viruses are already resistant to the M2 inhibitors as are many H5N1 viruses. There are four NAIs licensed in some parts of the world, zanamivir, oseltamivir, peramivir, and a long-acting NAI, laninamivir. This review focuses on resistance to the NAIs. Because of differences in their chemistry and subtle differences in NA structures, resistance can be both NAI- and subtype specific. This results in different drug resistance profiles, for example, the H274Y mutation confers resistance to oseltamivir and peramivir, but not to zanamivir, and only in N1 NAs. Mutations at E119, D198, I222, R292, and N294 can also reduce NAI sensitivity. In the winter of 2007-2008, an oseltamivir-resistant seasonal influenza A(H1N1) strain with an H274Y mutation emerged in the northern hemisphere and spread rapidly around the world. In contrast to earlier evidence of such resistant viruses being unfit, this mutant virus remained fully transmissible and pathogenic and became the major seasonal A(H1N1) virus globally within a year. This resistant A(H1N1) virus was displaced by the sensitive A(H1N1)pdm09 virus. Approximately 0.5-1.0% of community A(H1N1)pdm09 isolates are currently resistant to oseltamivir. It is now apparent that variation in non-active site amino acids can affect the fitness of the enzyme and compensate for mutations that confer high-level oseltamivir resistance resulting in minimal impact on enzyme function.

Oseltamivir resistance among influenza viruses: surveillance in northern Viet Nam, 2009-2012

INTRODUCTION

Antiviral resistance has been reported in seasonal influenza A viruses and avian influenza A(H5N1) viruses in Viet Nam, raising concerns about the efficacy of treatment.

METHODS

We analysed specimens from two sources during the period 2009-2012: influenza-positive samples from influenza-like illness patients at sentinel clinics in northern Viet Nam and isolates from patients with confirmed A(H5N1) infections. Pyrosequencing was used to detect mutations: H275Y [for A(H1N1) and A(H5N1)], E119V [for A(H3N2)] and I117V [for A(H5N1)]. A neuraminidase inhibition assay was used to determine the Inhibitory Concentration 50 (IC₅₀) values for all influenza A and B isolates.

RESULTS

There were 341 influenza A positive samples identified; influenza A(H1N1)pdm09 was identified most frequently (n = 215). In 2009, oseltamivir resistance was observed in 100% (19 of 19) of seasonal A(H1N1) isolates and 1.4% (3/215) of A(H1N1)pdm09 isolates. This H275Y mutation was not found in influenza subtypes A(H5N1) or A(H3N2) isolates.

DISCUSSION

In Viet Nam, seasonal and A(H5N1) influenza vaccines are not currently available; thus, effective treatment is required. The presence of oseltamivir-resistant viruses is therefore a concern. Active surveillance for oseltamivir resistance among influenza viruses circulating in Viet Nam should be continued.

I222 Neuraminidase mutations further reduce oseltamivir susceptibility of Indonesian Clade 2.1 highly pathogenic Avian Influenza A(H5N1) viruses

We have tested the susceptibility to neuraminidase inhibitors of 155 clade 2.1 H5N1 viruses from Indonesia, isolated between 2006-2008 as well as 12 clade 1 isolates from Thailand and Cambodia from 2004-2007 using a fluorometric MUNANA-based enzyme inhibition assay. The Thailand and Cambodian clade 1 isolates tested here were all susceptible to oseltamivir and zanamivir, and sequence comparison indicated that reduced oseltamivir susceptibility we observed previously with clade 1 Cambodian isolates correlated with an S246G neuraminidase mutation. Eight Indonesian viruses (5%), all bearing I222 neuraminidase mutations, were identified as mild to extreme outliers for oseltamivir based on statistical analysis by box plots. IC50s were from 50 to 500-fold higher than the reference clade 1 virus from Viet Nam, ranging from 43-75 nM for I222T/V mutants and from 268-349 nM for I222M mutants. All eight viruses were from different geographic locales; all I222M variants were from central Sumatra. None of the H5N1 isolates tested demonstrated reduced susceptibility to zanamivir (IC50s all <5 nM). All I222 mutants showed loss of slow binding specifically for oseltamivir in an IC50 kinetics assay. We identified four other Indonesian isolates with higher IC50s which also demonstrated loss of slow binding, including one virus with an I117V mutation. There was a minimal effect on the binding of zanamivir and peramivir for all isolates tested. As H5N1 remains a potential pandemic threat, the incidence of mutations conferring reduced oseltamivir susceptibility is concerning and emphasizes the need for greater surveillance of drug susceptibility.

Pseudovirus-based neuraminidase inhibition assays reveal potential H5N1 drug-resistant mutations

The use of antiviral drugs such as influenza neuraminidase (NA) inhibitors is a critical strategy to prevent and control flu pandemic, but this strategy faces the challenge of emerging drug-resistant strains. For a highly pathogenic avian influenza (HPAI) H5N1 virus, biosafety restrictions have significantly limited the efforts to monitor its drug responses and mechanisms involved. In this study, a rapid and biosafe assay based on NA pseudovirus was developed to study the resistance of HPAI H5N1 virus to NA inhibitor drugs. The H5N1 NA pseudovirus was comprehensively tested using oseltamivir-sensitive strains and their resistant mutants. Results were consistent with those in previous studies, in which live H5N1 viruses were used. Several oseltamivir-resistant mutations reported in human H1N1 were also identified to cause decreased oseltamivir sensitivity in H5N1 NA by using the H5N1 NA pseudovirus. Thus, H5N1 NA pseudoviruses could be used to monitor HPAI H5N1 drug resistance rapidly and safely.

Effect of oseltamivir carboxylate consumption on emergence of drug-resistant H5N2 avian influenza virus in Mallard ducks

Oseltamivir carboxylate (OC) has been detected in environmental waters at various levels during recent influenza seasons in humans, reflecting levels of usage and stability of this drug. In consideration of the role of waterfowl as hosts for influenza viruses that may contribute to human infections, we evaluated the effect of consumption of low doses of OC on development of oseltamivir-resistant influenza virus mutants in mallard ducks (Anas platyrhynchos) infected with two different low-pathogenic (LP) H5N2 avian influenza viruses (AIV). We detected development of virus variants carrying a known molecular marker of oseltamivir resistance (neuraminidase E119V) in 4 out of 6 mallards infected with A/Mallard/Minnesota/182742/1998 (H5N2) and exposed to 1,000 ng/liter OC. The mutation first appeared as a minor population on days 5 to 6 and was the dominant genotype on days 6 to 8. Oseltamivir-resistant mutations were not detected in virus from ducks not exposed to the drug or in ducks infected with a second strain of virus and similarly exposed to OC. Virus isolates carrying the E119V mutation displayed in vitro replication kinetics similar to those of the wild-type virus, but in vivo, the E119V virus rapidly reverted back to wild type in the absence of OC, and only the wild-type parental strain was transmitted to contact ducks. These results indicate that consumption by wild waterfowl of OC in drinking water may promote selection of the E119V resistance mutation in some strains of H5N2 AIV that could contribute to viruses infecting human populations.

Oseltamivir and inhaled zanamivir as influenza prophylaxis in Thai health workers: a randomized, double-blind, placebo-controlled safety trial over 16 weeks

OBJECTIVES

Long-term chemoprophylaxis using neuraminidase inhibitors may be needed during influenza epidemics but safety data are limited to several weeks. We sought to assess the tolerability of oseltamivir and zanamivir as primary prophylaxis over 16 weeks.

METHODS

We conducted a parallel group, double blind, 2 (active drug) :1 (placebo) randomized trial of oral oseltamivir/placebo or inhaled zanamivir/placebo over 16 weeks in healthy, Thai hospital professionals at two Bangkok hospitals. The primary endpoint was study withdrawal due to drug-related (possibly, probably, definitely) serious or adverse events (AEs) graded ≥ 2.

RESULTS

Recruited subjects numbered 129 oseltamivir/65 placebo and 131 zanamivir/65 placebo. A total of 102 grade ≥ 2 AEs were reported or detected in 69 subjects: 23/129 (17.8%) versus 15/65 (23.1%) (P=0.26), and 23/131 (17.6%) versus 8/65 (12.3%) (P=0.28). Intercurrent infections/fevers [26/102 (25.5%)], abnormal biochemistry [25/102 (24.5%)] and gastrointestinal symptoms [18/102 (17.6%)] were the most frequently reported AEs. There were no drug-related study withdrawals. Eight serious AEs were all due to intercurrent illnesses. Laboratory, lung function and ECG parameters were similar between drugs and placebos.

CONCLUSIONS

Oseltamivir and zanamivir were well tolerated in healthy hospital professionals. Both drugs can be recommended for primary influenza prophylaxis for up to 16 weeks.

Antiviral resistance among highly pathogenic influenza A (H5N1) viruses isolated worldwide in 2002-2012 shows need for continued monitoring

Highly pathogenic (HP) H5N1 influenza viruses are evolving pathogens with the potential to cause sustained human-to-human transmission and pandemic virus spread. Specific antiviral drugs can play an important role in the early stages of a pandemic, but the emergence of drug-resistant variants can limit control options. The available data on the susceptibility of HP H5N1 influenza viruses to neuraminidase (NA) inhibitors and adamantanes is scarce, and there is no extensive analysis. Here, we systematically examined the prevalence of NA inhibitor and adamantane resistance among HP H5N1 influenza viruses that circulated worldwide during 2002-2012. The phenotypic fluorescence-based assay showed that both human and avian HP H5N1 viruses are susceptible to NA inhibitors oseltamivir and zanamivir with little variability over time and ∼5.5-fold less susceptibility to oseltamivir of viruses of hemagglutinin (HA) clade 2 than of clade 1. Analysis of available sequence data revealed a low incidence of NA inhibitor-resistant variants. The established markers of NA inhibitor resistance (E119A, H274Y, and N294S, N2 numbering) were found in 2.4% of human and 0.8% of avian isolates, and the markers of reduced susceptibility (I117V, K150N, I222V/T/K, and S246N) were found in 0.8% of human and 2.9% of avian isolates. The frequency of amantadine-resistant variants was higher among human (62.2%) than avian (31.6%) viruses with disproportionate distribution among different HA clades. As in human isolates, avian H5N1 viruses carry double L26I and S31N M2 mutations more often than a single S31N mutation. Overall, both human and avian HP H5N1 influenza viruses are susceptible to NA inhibitors; some proportion is still susceptible to amantadine in contrast to ∼100% amantadine resistance among currently circulating seasonal human H1N1 and H3N2 viruses. Continued antiviral susceptibility monitoring of H5N1 viruses is needed to maintain therapeutic approaches for control of disease.

Increasing oral absorption of polar neuraminidase inhibitors: a prodrug transporter approach applied to oseltamivir analogue

Poor oral absorption is one of the limiting factors in utilizing the full potential of polar antiviral agents. The neuraminidase target site requires a polar chemical structure for high affinity binding, thus limiting oral efficacy of many high affinity ligands. The aim of this study was to overcome this poor oral absorption barrier, utilizing prodrug to target the apical brush border peptide transporter 1 (PEPT1). Guanidine oseltamivir carboxylate (GOCarb) is a highly active polar antiviral agent with insufficient oral bioavailability (4%) to be an effective therapeutic agent. In this report we utilize a carrier-mediated targeted prodrug approach to improve the oral absorption of GOCarb. Acyloxy(alkyl) ester based amino acid linked prodrugs were synthesized and evaluated as potential substrates of mucosal transporters, e.g., PEPT1. Prodrugs were also evaluated for their chemical and enzymatic stability. PEPT1 transport studies included [(3)H]Gly-Sar uptake inhibition in Caco-2 cells and cellular uptake experiments using HeLa cells overexpressing PEPT1. The intestinal membrane permeabilities of the selected prodrugs and the parent drug were then evaluated for epithelial cell transport across Caco-2 monolayers, and in the in situ rat intestinal jejunal perfusion model. Prodrugs exhibited a pH dependent stability with higher stability at acidic pHs. Significant inhibition of uptake (IC(50) <1 mM) was observed for l-valyl and l-isoleucyl amino acid prodrugs in competition experiments with [(3)H]Gly-Sar, indicating a 3-6 times higher affinity for PEPT1 compared to valacyclovir, a well-known PEPT1 substrate and >30-fold increase in affinity compared to GOCarb. The l-valyl prodrug exhibited significant enhancement of uptake in PEPT1/HeLa cells and compared favorably with the well-absorbed valacyclovir. Transepithelial permeability across Caco-2 monolayers showed that these amino acid prodrugs have a 2-5-fold increase in permeability as compared to the parent drug and showed that the l-valyl prodrug (P(app) = 1.7 × 10(-6) cm/s) has the potential to be rapidly transported across the epithelial cell apical membrane. Significantly, only the parent drug (GOCarb) appeared in the basolateral compartment, indicating complete activation (hydrolysis) during transport. Intestinal rat jejunal permeability studies showed that l-valyl and l-isoleucyl prodrugs are highly permeable compared to the orally well absorbed metoprolol, while the parent drug had essentially zero permeability in the jejunum, consistent with its known poor low absorption. Prodrugs were rapidly converted to parent in cell homogenates, suggesting their ability to be activated endogenously in the epithelial cell, consistent with the transport studies. Additionally, l-valyl prodrug was found to be a substrate for valacyclovirase (K(m) = 2.37 mM), suggesting a potential cell activation mechanism. Finally we determined the oral bioavailability of our most promising candidate, GOC-l-Val, in mice to be 23% under fed conditions and 48% under fasted conditions. In conclusion, GOC-l-Val prodrug was found to be a very promising antiviral agent for oral delivery. These findings indicate that the carrier-mediated prodrug approach is an excellent strategy for improving oral absorption of polar neuraminidase inhibitors. These promising results demonstrate that the oral peptide transporter-mediated prodrug strategy has enormous promise for improving the oral mucosal cell membrane permeability of polar, poorly absorbed antiviral agents and treating influenza via the oral route of administration.

Insight into alternative approaches for control of avian influenza in poultry, with emphasis on highly pathogenic H5N1

Highly pathogenic avian influenza virus (HPAIV) of subtype H5N1 causes a devastating disease in poultry but when it accidentally infects humans it can cause death. Therefore, decrease the incidence of H5N1 in humans needs to focus on prevention and control of poultry infections. Conventional control strategies in poultry based on surveillance, stamping out, movement restriction and enforcement of biosecurity measures did not prevent the virus spreading, particularly in developing countries. Several challenges limit efficiency of the vaccines to prevent outbreaks of HPAIV H5N1 in endemic countries. Alternative and complementary approaches to reduce the current burden of H5N1 epidemics in poultry should be encouraged. The use of antiviral chemotherapy and natural compounds, avian-cytokines, RNA interference, genetic breeding and/or development of transgenic poultry warrant further evaluation as integrated intervention strategies for control of HPAIV H5N1 in poultry.

Differential antiviral and anti-inflammatory mechanisms of the flavonoids biochanin A and baicalein in H5N1 influenza A virus-infected cells

From a panel of 22 flavonoids, we identified six compounds (apigenin, baicalein, biochanin A, kaempferol, luteolin, naringenin) that inhibited influenza A nucleoprotein production in human lung epithelial (A549) cells infected with the highly pathogenic avian influenza H5N1 virus strain A/Thailand/Kan-1/04 in non-toxic concentrations. Baicalein (IC(50): 18.79±1.17μM, selectivity index 5.82) and biochanin A (IC(50) 8.92±1.87μM, selectivity index 5.60) were selected for further experiments. Both compounds reduced H5N1 infectious titres (baicalein 40μM: 29-fold reduction, biochanin A 40μM: 55-fold reduction after infection at MOI 0.01), virus-induced caspase 3 cleavage, nuclear export of viral RNP complexes, and enhanced the effects of the neuraminidase inhibitor zanamivir. Biochanin A and baicalein also inhibited the replication of the H5N1 strain A/Vietnam/1203/04. Time of addition experiments indicated that both compounds interfere with H5N1 replication after the adsorption period. Further mechanistic investigations revealed clear differences between these two flavonoids. Only baicalein interfered with the viral neuraminidase activity (39±7% inhibition at 100μM, the maximum concentration tested). In contrast to baicalein, biochanin A affected cellular signalling pathways resulting in reduced virus-induced activation of AKT, ERK 1/2, and NF-kB. Moreover, biochanin A inhibited the virus-induced production of IL-6, IL-8, and IP-10 while baicalein inhibited IL-6 and IL-8 production without affecting IP-10 levels. In primary human monocyte-derived macrophages, only baicalein but not biochanin A impaired H5N1 virus replication. Both flavonoids interfered with the H5N1-induced production of IL-6, IP-10, and TNF-α but not of IL-8 in macrophages. These findings indicate that closely related flavonoids can exert anti-H5N1 effects by different molecular mechanisms.

Molecular mechanisms underlying oseltamivir resistance mediated by an I117V substitution in the neuraminidase of subtype H5N1 avian influenza A viruses

BACKGROUND

The neuraminidase (NA) inhibitor oseltamivir is widely used to treat patients infected with influenza viruses. An Ile-to-Val change at position 117 in influenza A virus subtype H5N1 NA (NA-I117V) confers a reduction in susceptibility to oseltamivir carboxylate. However, the in vivo relevance and molecular basis of the decreased sensitivity mediated by this mutation are poorly understood.

METHODS

We created single-point-mutant viruses with 3 genetically different backgrounds (ie, 1 belonging to clade 1 and 2 belonging to clade 2.3.4) and evaluated the effects of the I117V mutation on oseltamivir susceptibility in vitro, in vivo, and in silico.

RESULTS

The NA-I117V mutation conferred a slight reduction in susceptibility to oseltamivir in vitro (1.3- to 6.3-fold changes), although it did not substantially compromise NA enzymatic activity. Mice infected with I117V virus exhibited reduced susceptibility to oseltamivir and decreased survival in 2 of 3 virus pairs tested. Molecular dynamics simulations revealed that I117V caused the loss of hydrogen bonds between an arginine at position 118 and the carboxyl group of oseltamivir, leading to a lower binding affinity for oseltamivir.

CONCLUSIONS

Our findings provide new insight into the mechanism of NA-I117V-mediated oseltamivir resistance in highly pathogenic H5N1 avian influenza viruses.

Avian influenza A H5N1 virus: a continuous threat to humans

We report the first case of severe pneumonia due to co-infection with the emerging avian influenza A (H5N1) virus subclade 2.3.2.1 and Mycoplasma pneumoniae. The patient was a returning traveller who had visited a poultry market in South China. We then review the epidemiology, virology, interspecies barrier limiting poultry-to-human transmission, clinical manifestation, laboratory diagnosis, treatment and control measures of H5N1 clades that can be transmitted to humans. The recent controversy regarding the experiments involving aerosol transmission of recombinant H5N1 virus between ferrets is discussed. We also review the relative contribution of the poor response to antiviral treatment and the virus-induced hyperinflammatory damage to the pathogenesis and the high mortality of this infection. The factors related to the host, virus or medical intervention leading to the difference in disease mortality of different countries remain unknown. Because most developing countries have difficulty in instituting effective biosecurity measures, poultry vaccination becomes an important control measure. The rapid evolution of the virus would adversely affect the efficacy of poultry vaccination unless a correctly matched vaccine was chosen, manufactured and administered in a timely manner. Vigilant surveillance must continue to allow better preparedness for another poultry or human pandemic due to new viral mutants.

New treatments for influenza

Influenza has a long history of causing morbidity and mortality in the human population through routine seasonal spread and global pandemics. The high mutation rate of the RNA genome of the influenza virus, combined with assortment of its multiple genomic segments, promote antigenic diversity and new subtypes, allowing the virus to evade vaccines and become resistant to antiviral drugs. There is thus a continuing need for new anti-influenza therapy using novel targets and creative strategies. In this review, we summarize prospective future therapeutic regimens based on recent molecular and genomic discoveries.

Influenza outbreaks in Singapore: epidemiology, diagnosis, treatment and prevention

With the recent influenza A/H1N1 2009 pandemic still spreading through global populations, there has been an increased focus on optimizing the prevention, diagnosis and treatment of influenza infections, as well as the epidemiology of the virus. Clinical and epidemiological data on influenza infections in tropical countries have been relatively sparse until fairly recently, and it is the aim of this review to close some of these gaps by examining the behavior of influenza viruses in the tropical Singaporean population.

Mast cell-induced lung injury in mice infected with H5N1 influenza virus

Although an important role for mast cells in several viral infections has been demonstrated, its role in the invasion of highly pathogenic H5N1 influenza virus is unknown. In the present study, we demonstrate that mast cells were activated significantly by H5N1 virus (A/chicken/Henan/1/2004) infection both in vivo and in vitro. Mast cells could possibly intensify the lung injury that results from H5N1 infection by releasing proinflammatory mediators, including histamine, tryptase, and gamma interferon (IFN-γ). Lung lesions and apoptosis induced by H5N1 infection were reduced dramatically by treatment with ketotifen, which is a mast cell degranulation inhibitor. A combination of ketotifen and the neuraminidase inhibitor oseltamivir protected 100% of the mice from death postinfection. In conclusion, our data suggest that mast cells play a crucial role in the early stages of H5N1 influenza virus infection and provide a new approach to combat highly pathogenic influenza virus infection.

Decreased neuraminidase activity is important for the adaptation of H5N1 influenza virus to human airway epithelium

Highly pathogenic avian H5N1 influenza viruses remain a pandemic threat. Antiviral drugs such as neuraminidase (NA) inhibitors will be crucial for disease control in the event of a pandemic. Should drug-resistant H5N1 viruses develop, all defense strategies will be compromised. To determine the likelihood and mechanisms of emergence of NA inhibitor-resistant H5N1 variants in humans, we serially passaged two H5N1 viruses, A/Hong Kong/213/03 and A/Turkey/65-1242/06, in normal human bronchial epithelial (NHBE) cells in the presence of oseltamivir, zanamivir, or peramivir. To monitor the emergence of changes associated with the adaptation of H5N1 viruses to humans, we passaged the strains in the absence of drugs. Under pressure of each NA inhibitor, A/Turkey/65-1242/06 developed mutations in the hemagglutinin (HA) (H28R and P194L/T215I) and NA (E119A) proteins that reduced virus binding to α2,3-sialyl receptor and NA activity. Oseltamivir pressure selected a variant of A/Hong Kong/213/03 virus with HA P194S mutation that decreased viral binding to α2,6 receptor. Under peramivir pressure, A/Hong Kong/213/03 virus developed a novel NA mutation, R156K, that reduced binding to all three drugs, caused about 90% loss of NA activity, and compromised replication in NHBE cells. Both strains were eliminated in NHBE cells when they were cultivated in the absence of drugs. Here, we show for the first time that decreased NA activity mediated through NA inhibitors is essential for the adaptation of pandemic H5N1 influenza virus to humans. This ability of decreased NA activity to promote H5N1 infection underlines the necessity to optimize management strategies for a plausible H5N1 pandemic.

XIth International Symposium on Respiratory Viral Infections

Please cite this paper as: Belser et al. (2011) XIth International Symposium on Respiratory Viral Infections. Influenza and Other Respiratory Viruses 5(6), 443–e457.