Influenza A virus (IAV) is the sole cause of the unpredictable influenza pandemics and deadly zoonotic outbreaks and constitutes at least half of the cause of regular annual influenza epidemics in humans. Two classes of anti-IAV drugs, adamantanes and neuraminidase (NA) inhibitors (NAIs) targeting the viral components M2 ion channel and NA, respectively, have been approved to treat IAV infections. However, IAV rapidly acquired resistance against both classes of drugs by mutating these viral components. The adamantane-resistant IAV has established itself in nature, and a majority of the IAV subtypes, especially the most common H1N1 and H3N2, circulating globally are resistant to adamantanes. Consequently, adamantanes have become practically obsolete as anti-IAV drugs. Similarly, up to 100% of the globally circulating IAV H1N1 subtypes were resistant to oseltamivir, the most commonly used NAI, until 2009. However, the 2009 pandemic IAV H1N1 subtype, which was sensitive to NAIs and has now become one of the dominant seasonal influenza virus strains, has replaced the pre-2009 oseltamivir-resistant H1N1 variants. This review traces the epidemiology of both adamantane- and NAI-resistant IAV subtypes since the approval of these drugs and highlights the susceptibility status of currently circulating IAV subtypes to NAIs. Further, it provides an overview of currently and soon to be available control measures to manage current and emerging drug-resistant IAV. Finally, this review outlines the research directions that should be undertaken to manage the circulation of IAV in intermediate hosts and develop effective and alternative anti-IAV therapies.

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Drug resistance in influenza A virus: the epidemiology and management.

Infection with influenza viruses, including seasonal, avian and pandemic viruses, remains a worldwide public health problem. Although influenza virus infection is both vaccine preventable and drug treatable, high rates of mutation and reassortment of viruses can result in reduced effectiveness of vaccines or drugs. Currently, two classes of drugs, adamantanes (M2 blockers) and neuraminidase (NA) inhibitors (NAIs), are available for treatment and chemoprophylaxis of influenza infections. Given these limited antiviral therapy options, resistance to anti-influenza drugs is a constant concern. The emergence and global spread of adamantane-resistant H3N2 viruses in 2003-2004 and oseltamivir-resistant seasonal H1N1 viruses in 2007-2009 demonstrated the ability of drug-resistant variants to rapidly become predominant worldwide. Since the 2009 H1N1 pandemic, all influenza viruses circulating in humans are M2-blocker-resistant and, in general, NAI-susceptible. However, pandemic H1N1 viruses with resistance to the NAI oseltamivir have been reported. 'Permissive' drift mutations and reassortment of viral gene segments have been proposed as mechanisms underlying the retained replicative fitness of resistant viruses. Nevertheless, the precise role of these genetic changes in the efficient transmission and maintenance of resistant viruses in the absence of drug pressure remains poorly understood. In this review, we summarize NAI resistance in influenza viruses and discuss recent challenges in laboratory testing methods. Close monitoring of antiviral resistance among all influenza viruses, both locally and globally, are essential to inform public health strategies for the control of influenza infections.

Neuraminidase inhibitor resistance in influenza viruses and laboratory testing methods.

VX-787 is a novel inhibitor of influenza virus replication that blocks the PB2 cap-snatching activity of the influenza viral polymerase complex. Viral genetics and X-ray crystallography studies provide support for the idea that VX-787 occupies the 7-methyl GTP (m7GTP) cap-binding site of PB2. VX-787 binds the cap-binding domain of the PB2 subunit with a KD (dissociation constant) of 24 nM as determined by isothermal titration calorimetry (ITC). The cell-based EC50 (the concentration of compound that ensures 50% cell viability of an uninfected control) for VX-787 is 1.6 nM in a cytopathic effect (CPE) assay, with a similar EC50 in a viral RNA replication assay. VX-787 is active against a diverse panel of influenza A virus strains, including H1N1pdm09 and H5N1 strains, as well as strains with reduced susceptibility to neuraminidase inhibitors (NAIs). VX-787 was highly efficacious in both prophylaxis and treatment models of mouse influenza and was superior to the neuraminidase inhibitor, oseltamivir, including in delayed-start-to-treat experiments, with 100% survival at up to 96 h postinfection and partial survival in groups where the initiation of therapy was delayed up to 120 h postinfection. At different doses, VX-787 showed a 1-log to >5-log reduction in viral load (relative to vehicle controls) in mouse lungs. Overall, these favorable findings validate the PB2 subunit of the viral polymerase as a drug target for influenza therapy and support the continued development of VX-787 as a novel antiviral agent for the treatment of influenza infection.

Preclinical Activity of VX-787, a First-in-Class, Orally Bioavailable Inhibitor of the Influenza Virus Polymerase PB2 Subunit

Neuraminidase inhibitors for influenza B virus infection: efficacy and resistance.

Please cite this paper as: Goka et al. (2013) Influenza A viruses dual and multiple infections with other respiratory viruses and risk of hospitalisation and mortality. Influenza and Other Respiratory Viruses 7(6), 1079–1087.



Introduction Recent literature suggests that dual or multiple virus infections may affect disease severity. However, few studies have investigated the effect of co-infection with influenza A viruses.



Objectives To identify the association between influenza A and respiratory viruses co-infections with disease outcome.



Methodology Data for samples from North West England tested between January 2007 and June 2011 was analysed for patterns of co-infection between influenza A viruses and eight respiratory viruses. Risk of hospitalisation to ICU or general ward in single versus co-infections was assessed using logistic regression.



Results Of the 25 596 samples analysed for respiratory viruses 40·7% (10 501) were positive for any virus. Co-infections were detected in 4·7% (137/2879) of all patients with influenza A(H1N1)pdm09, and 7·3% (57/779) of those with other influenza A virus infections. Co-infection between seasonal influenza A viruses and influenza B virus was associated with a significant increase in the risk of admission to ICU/death (OR: 22·0, 95% CI: 2·21–219·8, P = 0·008). Respiratory syncytial virus/influenza A (RSV/Flu A) co-infection also increased this risk but was not statistically significant. For influenza A(H1N1)pdm09, RSV and AdV co-infection increased risk of hospitalisation to general ward whereas Flu B increased risk of admission to ICU, but none of these were statistically significant.



Conclusion Co-infection is a significant predictor of disease outcome; combined treatment, introduction of an integrated vaccine for all respiratory viruses and development of multi-target rapid diagnostic tests is recommended. Integration of respiratory viruses’ co-infections into public health reports could also contribute to the accumulation of evidence.

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Influenza A viruses dual and multiple infections with other respiratory viruses and risk of hospitalisation and mortality

Neuraminidase inhibitors (NAIs) are vital in managing seasonal and pandemic influenza infections. NAI susceptibilities of virus isolates (n = 5540) collected during the 2008–2009 influenza season were assessed in the chemiluminescent neuraminidase inhibition (NI) assay. Box-and-whisker plot analyses of log-transformed IC50s were performed for each virus type/subtype and NAI to identify outliers which were characterized based on a statistical cutoff of IC50 >3 interquartile ranges (IQR) from the 75th percentile. Among 1533 seasonal H1N1 viruses tested, 1431 (93.3%) were outliers for oseltamivir; they all harbored the H275Y mutation in the neuraminidase (NA) and were reported as oseltamivir-resistant. Only 15 (0.7%) of pandemic 2009 H1N1 viruses tested (n = 2259) were resistant to oseltamivir. All influenza A(H3N2) (n = 834) and B (n = 914) viruses were sensitive to oseltamivir, except for one A(H3N2) and one B virus, with D151V and D197E (D198E in N2 numbering) mutations in the NA, respectively. All viruses tested were sensitive to zanamivir, except for six seasonal A(H1N1) and several A(H3N2) outliers (n = 22) which exhibited cell culture induced mutations at residue D151 of the NA. A subset of viruses (n = 1058) tested for peramivir were sensitive to the drug, with exception of H275Y variants that exhibited reduced susceptibility to this NAI. This study summarizes baseline susceptibility patterns of seasonal and pandemic influenza viruses, and seeks to contribute towards criteria for defining NAI resistance.

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Neuraminidase Inhibitor Susceptibility Testing in Human Influenza Viruses: A Laboratory Surveillance Perspective

The authors would like to make the following corrections to their published paper: There was an error in calculation of IC50 fold changes for the NAI-resistant viruses reported in Table 1 of the above-mentioned paper. The corrected values are marked in the updated Table 1 below. [...]

Correction: Okomo-Adhiambo, M. et al. Neuraminidase Inhibitor Susceptibility Testing in Human Influenza Viruses: A Laboratory Surveillance Perspective. Viruses 2010, 2, 2269-2289

Kristina Ballenger

Papers

Neuraminidase Inhibitor Susceptibility Testing in Human Influenza Viruses: A Laboratory Surveillance Perspective

Correction: Okomo-Adhiambo, M. et al. Neuraminidase Inhibitor Susceptibility Testing in Human Influenza Viruses: A Laboratory Surveillance Perspective. Viruses 2010, 2, 2269-2289