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Concept: Oseltamivir

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Without baseline human immunity to the emergent avian influenza A(H7N9) virus, neuraminidase inhibitors are vital for controlling viral replication in severe infections. An amino acid change in the viral neuraminidase associated with drug resistance, NA-R292K (N2 numbering), has been found in some H7N9 clinical isolates. Here we assess the impact of the NA-R292K substitution on antiviral sensitivity and viral replication, pathogenicity and transmissibility of H7N9 viruses. Our data indicate that an H7N9 isolate encoding the NA-R292K substitution is highly resistant to oseltamivir and peramivir and partially resistant to zanamivir. Furthermore, H7N9 reassortants with and without the resistance mutation demonstrate comparable viral replication in primary human respiratory cells, virulence in mice and transmissibility in guinea pigs. Thus, in stark contrast to oseltamivir-resistant seasonal influenza A(H3N2) viruses, H7N9 virus replication and pathogenicity in these models are not substantially altered by the acquisition of high-level oseltamivir resistance due to the NA-R292K mutation.

Concepts: Virus, Antiviral drug, Influenza, Oseltamivir, Neuraminidase, Viral neuraminidase, Zanamivir, Neuraminidase inhibitor

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Influenza antiviral agents play important roles in modulating disease severity and in controlling pandemics while vaccines are prepared, but the development of resistance to agents like the commonly used neuraminidase inhibitor oseltamivir may limit their future utility. We report here a new class of specific, mechanism-based anti-influenza drugs that function via the formation of a stabilized covalent intermediate in the influenza neuraminidase enzyme, and confirm this mode of action via structural and mechanistic studies. These compounds function in cell-based assays and in animal models, with efficacies comparable to that of the neuraminidase inhibitor zanamivir and with broad spectrum activity against drug-resistant strains in vitro. The similarity of their structure to that of the natural substrate and their mechanism-based design make these attractive antiviral candidates.

Concepts: Enzyme, Antiviral drug, Influenza, Oseltamivir, Neuraminidase, Viral neuraminidase, Zanamivir, Neuraminidase inhibitor

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The wizard of OS (resistance): The binding difference of neuraminidase inhibitors (zanamivir versus oseltamivir (OS)) was used to establish an assay to identify the influenza subtypes that are resistant to OS but still sensitive to zanamivir. This assay used a zanamivir-biotin conjugate to determine the OS susceptibility of a wide range of influenza viruses and over 200 clinical isolates.

Concepts: Antibiotic resistance, Antiviral drug, Influenza, Oseltamivir, Neuraminidase, Viral neuraminidase, Zanamivir, Neuraminidase inhibitor

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Oseltamivir, an anti-influenza virus drug, has strong antipyretic effects in mice (Ono et al., 2008) and influenza patients. In addition, hypothermia has been reported as an adverse event. The prodrug oseltamivir is converted to oseltamivir carboxylate (OC), an active metabolite of influenza virus neuraminidase. In this study, core body temperature was measured in mice, and oseltamivir and OC were administered intracerebroventricularly (i.c.v.) or intraperitoneally (i.p). Low i.c.v. doses of oseltamivir and OC dose-dependently produced hypothermia. Zanamivir (i.c.v.), another neuraminidase inhibitor, did not produce hypothermia. These results suggested that the hypothermic effects of oseltamivir (i.p. and i.c.v.) and OC (i.c.v.) are not due to neuraminidase inhibition. OC (i.p.) did not lower body temperature. Although mecamylamine (i.c.v.) blocked the hypothermic effect of nicotine administered i.c.v., the hypothermic effects of oseltamivir and OC (i.c.v.) were not blocked by mecamylamine (i.c.v.). The effect of oseltamivir (i.p.) was markedly increased by s.c.-preadministered mecamylamine and also hexamethonium, a peripherally acting ganglionic blocker, suggesting their potentiating interaction at peripheral sites. The hypothermic effect of nicotine (i.c.v.) was decreased by lower doses of oseltamivir (i.c.v.), suggesting the anti-nicotinic action of oseltamivir. These results suggest that oseltamivir (i.p.) causes hypothermia through depression of sympathetic temperature regulatory mechanisms via inhibition of nicotinic receptor function and through unknown central mechanisms.

Concepts: Hypothermia, Influenza, Acetylcholine, Nicotinic acetylcholine receptor, Oseltamivir, Neuraminidase, Viral neuraminidase, Mecamylamine

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Introduction: At present, the key public health concern is to define the way in which the next influenza pandemic should be controlled. While influenza vaccines are available, their effectiveness could be significantly reduced if new strains differ significantly from those of the vaccines. Therefore, antiviral drugs play an important role in the prevention and management of influenza. The influenza neuraminidase (NA), a surface-glycoprotein enzyme involved in releasing the virus from the host cell, has been considered as an essential therapeutic target for treatment and prophylaxis of influenza infection. It is a highly conserved feature of the active site across all influenza A and B viruses and is of particular interest because compounds NA inhibitors (NAIs) can be cross-reactive against multiple types and subtypes of influenza. Currently, there are two NAI drugs which are licensed worldwide: oseltamivir and zanamivir, and two more drugs which have received recent approval in Japan: peramivir and laninamivir. Sudden changes in NAI susceptibility have stressed the urgent need in searching for novel inhibitors. Areas covered: In this review, a potential pitfall in NA-based assays and the progress in the chemical synthesis of all patented NAIs from February 2006 to July 2012 are discussed. Expert opinion: Both NA enzyme inhibition and X-ray crystallography data have suggested that the strategy of designing NAIs binding to the highly conserved region of NA can lead to inhibitors that are effective against all influenza NA subtypes. A number of new synthetic entries having unique structural frameworks that were designed based on computational study of X-ray structures of NA. They strongly exhibited the activity of NA in the low nanomolar range such as phosphonate congeners of zanamivir and oseltarmivir. Screening strategies based on the chemical diversity of natural products have revealed that flavonoids are the most prominent scaffolds possessing NA inhibitory activity. However, these substituted phenyl-benzopyrane compounds have been reported to exert a considerable quenching effect, causing false-positive results in the commonly used method of enzyme-based NA inhibition assays, and thus, reliability of the flavonoid-based NAIs reported in the literature.

Concepts: Virus, Antiviral drug, Influenza, Influenza pandemic, Influenza vaccine, Oseltamivir, Neuraminidase, Neuraminidase inhibitor

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The first structure determination of a neuraminidase inhibitor, oseltamivir phosphate, the active component of the anti-influenza agent Tamiflu, was achieved by single crystal X-ray diffraction with synchrotron radiation. The structure reproduces to a great extent the binding of the inhibitor in the neuraminidase active site and provides more accurate structural parameters as well as detailed insight into the binding preferences of the inhibitor.

Concepts: Electron, X-ray, Influenza, Oseltamivir, Neuraminidase, Viral neuraminidase, Zanamivir, Neuraminidase inhibitor

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BACKGROUND: On March 30, a novel influenza A subtype H7N9 virus (A/H7N9) was detected in patients with severe respiratory disease in eastern China. Virological factors associated with a poor clinical outcome for this virus remain unclear. We quantified the viral load and analysed antiviral resistance mutations in specimens from patients with A/H7N9. METHODS: We studied 14 patients with A/H7N9 disease admitted to the Shanghai Public Health Clinical Centre (SPHCC), China, between April 4, and April 20, 2013, who were given antiviral treatment (oseltamivir or peramivir) for less than 2 days before admission. We investigated the viral load in throat, stool, serum, and urine specimens obtained sequentially from these patients. We also sequenced viral RNA from these specimens to study the mutations associated with resistance to neuraminidase inhibitors and their association with disease outcome. FINDINGS: All patients developed pneumonia, seven of them required mechanical ventilation, and three of them further deteriorated to become dependent on extracorporeal membrane oxygenation (ECMO), two of whom died. Antiviral treatment was associated with a reduction of viral load in throat swab specimens in 11 surviving patients. Three patients with persistently high viral load in the throat in spite of antiviral therapy became ECMO dependent. An Arg292Lys mutation in the virus neuraminidase (NA) gene known to confer resistance to both zanamivir and oseltamivir was identified in two of these patients, both also received corticosteroid treatment. In one of them, wild-type sequence Arg292 was noted 2 days after start of antiviral treatment, and the resistant mutant Lys292 dominated 9 days after start of treatment. INTERPRETATION: Reduction of viral load following antiviral treatment correlated with improved outcome. Emergence of NA Arg292Lys mutation in two patients who also received corticosteroid treatment led to treatment failure and a poor clinical outcome. The emergence of antiviral resistance in A/H7N9 viruses, especially in patients receiving corticosteroid therapy, is concerning, needs to be closely monitored, and considered in pandemic preparedness planning. FUNDING: National Megaprojects of China for Infectious Diseases, Shanghai Municipal Health and Family Planning Commission, the National Key Basic Research Program of China, Ministry of Science and Technology, and National Natural Science Foundation of China.

Concepts: Epidemiology, Infectious disease, Virus, Antiviral drug, Influenza, Oseltamivir, Neuraminidase, Neuraminidase inhibitor

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Over the last decade, an increasing proportion of circulating human influenza A(H3N2) viruses exhibited haemagglutination activity that was sensitive to neuraminidase inhibitors. This change in haemagglutination as compared to older circulating A(H3N2) viruses prompted an investigation of the underlying molecular basis. Recent human influenza A(H3N2) viruses were found to agglutinate turkey erythrocytes in a manner that could be blocked with either oseltamivir or neuraminidase-specific antisera, indicating that agglutination was driven by neuraminidase, with a low or negligible contribution of haemagglutinin. Using representative virus recombinants it was shown that the haemagglutinin of a recent A(H3N2) virus indeed had decreased activity to agglutinate turkey erythrocytes, while its neuraminidase displayed increased haemagglutinating activity. Viruses with chimeric and mutant neuraminidases were used to identify the amino acid substitution histidine to arginine at position 150 flanking the neuraminidase catalytic site as the determinant of this neuraminidase-mediated haemagglutination. An analysis of publicly available neuraminidase gene sequences showed that viruses with histidine at position 150 were rapidly replaced by viruses with arginine at this position between 2005 and 2008, in agreement with the phenotypic data. As a consequence of neuraminidase-mediated haemagglutination of recent A(H3N2) viruses and poor haemagglutination via haemagglutinin, haemagglutination inhibition assays with A(H3N2) antisera are no longer useful to characterize the antigenic properties of the haemagglutinin of these viruses for vaccine strain selection purposes. Continuous monitoring of the evolution of these viruses and potential consequences for vaccine strain selection remains important.

Concepts: Protein, Gene, Evolution, Amino acid, Virus, Influenza, Oseltamivir, Viral neuraminidase

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ABSTRACT We characterized the A/Shanghai/1/2013 virus isolated from the first confirmed human case of A/H7N9 disease in China. The A/Shanghai/1/2013 isolate contained a mixed population of R (65%; 15/23 clones) and K (35%; 8/23 clones) at neuraminidase (NA) residue 292, as determined by clonal sequencing. A/Shanghai/1/2013 with mixed R/K at residue 292 exhibited a phenotype that is sensitive to zanamivir and oseltamivir carboxylate by the enzyme-based NA inhibition assay. The plaque-purified A/Shanghai/1/2013 with dominant K292 (94%; 15/16 clones) showed sensitivity to zanamivir that had decreased by >30-fold and to oseltamivir carboxylate that had decreased by >100-fold compared to its plaque-purified wild-type counterpart possessing dominant R292 (93%, 14/15 clones). In Madin-Darby canine kidney (MDCK) cells, the plaque-purified A/Shanghai/1/2013-NA(K292) virus exhibited no reduction in viral titer under conditions of increasing concentrations of oseltamivir carboxylate (range, 0 to 1,000 µM) whereas the replication of the plaque-purified A/Shanghai/1/2013-NA(R292) and the A/Shanghai/2/2013 viruses was completely inhibited at 250 µM and 31.25 µM of oseltamivir carboxylate, respectively. Although the plaque-purified A/Shanghai/1/2013-NA(K292) virus exhibited lower NA enzyme activity and a higher Km for 2'-(4-methylumbelliferryl)-α-d-N-acetylneuraminic acid than the wild-type A/Shanghai/1/2013-NA(R292) virus, the A/Shanghai/1/2013-NA(K292) virus formed large plaques and replicated efficiently in vitro. Our results confirmed that the NA R292K mutation confers resistance to oseltamivir, peramivir, and zanamivir in the novel human H7N9 viruses. Importantly, detection of the resistance phenotype may be masked in the clinical samples containing a mixed population of R/K at NA residue 292 in the enzyme-based NA inhibition assay. IMPORTANCE The neuraminidase (NA) inhibitors oseltamivir and zanamivir are currently the front-line therapeutic options against the novel H7N9 influenza viruses, which possess an S31N mutation that confers resistance to the M2 ion channel blockers. It is therefore important to evaluate the sensitivity of the clinical isolates to NA inhibitors and to monitor for the emergence of resistant variants. We characterized the A/Shanghai/1/2013 (H7N9) isolate which contained a mixed population of R/K at NA residue 292. While the clinical isolate exhibited a phenotype of sensitivity to NA inhibitors using the enzyme-based NA inhibition assay, the plaque-purified A/Shanghai/1/2013 virus with dominant K292 was resistant to zanamivir, peramivir, and oseltamivir. Resistance to NA inhibitors conferred by the R292K mutation in a human influenza virus H7N9 isolate can be masked by a mixed R/K viral population, and this should be taken into consideration while monitoring antiviral resistance in patients with H7N9 infection.

Concepts: Evolution, Virus, Antiviral drug, Influenza, Oseltamivir, Neuraminidase, Viral neuraminidase, Neuraminidase inhibitor