Catalytic and framework mutations in the neuraminidase active site of influenza viruses that are resistant to 4-guanidino-Neu5Ac2en

Pandemic 2009 H1N1 influenza A virus carrying a Q136K mutation in the neuraminidase gene is resistant to zanamivir but exhibits reduced fitness in the guinea pig transmission model

Resistance of influenza A viruses to neuraminidase inhibitors can arise through mutations in the neuraminidase (NA) gene. We show here that a Q136K mutation in the NA of the 2009 pandemic H1N1 virus confers a high degree of resistance to zanamivir. Resistance is accompanied by reduced numbers of NA molecules in viral particles and reduced intrinsic enzymatic activity of mutant NA. Interestingly, the Q136K mutation strongly impairs viral fitness in the guinea pig transmission model.

Rescue of a H3N2 influenza virus containing a deficient neuraminidase protein by a hemagglutinin with a low receptor-binding affinity

Influenza viruses possess at their surface two glycoproteins, the hemagglutinin and the neuraminidase, of which the antagonistic functions have to be well balanced for the virus to grow efficiently. Ferraris et al. isolated in 2003-2004 viruses lacking both a NA gene and protein (H3NA- viruses) (Ferraris O., 2006, Vaccine, 24(44-46):6656-9). In this study we showed that the hemagglutinins of two of the H3NA- viruses have reduced affinity for SAα2.6Gal receptors, between 49 and 128 times lower than that of the A/Moscow/10/99 (H3N2) virus and no detectable affinity for SAα2.3Gal receptors. We also showed that the low hemagglutinin affinity of the H3NA- viruses compensates for the lack of NA activity and allows the restoration of the growth of an A/Moscow/10/99 virus deficient in neuraminidase. These observations increase our understanding of H3NA- viruses in relation to the balance between the functional activities of the neuraminidase and hemagglutinin.

Combinatorial effect of two framework mutations (E119V and I222L) in the neuraminidase active site of H3N2 influenza virus on resistance to oseltamivir

Neuraminidase (NA) inhibitors (NIs) are the first line of defense against influenza virus. Reverse genetics experiments allow the study of resistance mechanisms by anticipating the impacts of mutations to the virus. To look at the possibility of an increased effect on the resistance phenotype of a combination of framework mutations, known to confer resistance to oseltamivir or zanamivir, with limited effect on virus fitness, we constructed 4 viruses by reverse genetics in the A/Moscow/10/99 H3N2 background containing double mutations in their neuraminidase genes: E119D+I222L, E119V+I222L, D198N+I222L, and H274Y+I222L (N2 numbering). Among the viruses produced, the E119D+I222L mutant virus was not able to grow without bacterial NA complementation and the D198N+I222L mutant and H274Y+I222L mutant were not stable after passages in MDCK cells. The E119V+I222L mutant was stable after five passages in MDCK cells. This E119V-and-I222L combination had a combinatorial effect on oseltamivir resistance. The total NA activity of the E119V+I222L mutant was low (5% compared to that of the wild-type virus). This drop in NA activity resulted from a decreased NA quantity in the virion in comparison to that of the wild-type virus (1.4% of that of the wild type). In MDCK-SIAT1 cells, the E119V+I222L mutant virus did not present a replicative advantage over the wild-type virus, even in the presence of oseltamivir. Double mutations combining two framework mutations in the NA gene still have to be monitored, as they could induce a high level of resistance to NIs, without impairing the NA affinity. Our study allows a better understanding of the diversity of the mechanisms of resistance to NIs.

Human influenza vaccines and assessment of immunogenicity

Influenza is responsible for the infection of approximately 20% of the population every season and for an annual death toll of approximately half a million people. The most effective means for controlling infection and thereby reducing morbidity and mortality is vaccination by injection with an inactivated vaccine, or by intranasal administration of a live-attenuated vaccine. Protection is not always optimal and there is a need for the development of new vaccines with improved efficacy and for the expansion of enrollment into vaccination programs. An overview of old and new vaccines is presented. Methods of monitoring immune responses such as hemagglutination-inhibition, ELISA and neutralization tests are evaluated for their accuracy in the assessment of current and new-generation vaccines.

Frequency of drug-resistant viruses and virus shedding in pediatric influenza patients treated with neuraminidase inhibitors

BACKGROUND

Although influenza virus resistance to the neuraminidase inhibitor zanamivir is reported less frequently than is resistance to the neuraminidase inhibitor oseltamivir in clinical settings, it is unknown whether this difference is due to the limited use of zanamivir or to an inherent property of the drug. We therefore compared the prevalence of drug-resistant viruses and virus shedding in seasonal influenza virus-infected children treated with either oseltamivir or zanamivir.

METHODS

Clinical specimens (throat or nasal swab) were collected from a total of 144 pediatric influenza patients during the 2005-2006, 2006-2007, 2007-2008, and 2008-2009 influenza seasons. Neuraminidase inhibitor-resistant mutants were detected among the isolated viruses by sequencing the viral hemagglutinin and neuraminidase genes. Sensitivity of the viruses to neuraminidase inhibitors was tested by neuraminidase inhibition assay.

RESULTS

In oseltamivir- or zanamivir-treated influenza patients who were statistically comparable in their age distribution, vaccination history, and type or subtype of virus isolates, the virus-shedding period in zanamivir-treated patients was significantly shorter than that in oseltamivir-treated patients. Furthermore, the frequency of zanamivir-resistant viruses was significantly lower than that of oseltamivir-resistant viruses.

CONCLUSION

In comparison with treatment with oseltamivir, treatment of pediatric patients with zanamivir resulted in the emergence of fewer drug-resistant influenza viruses and a shorter virus-shedding period. We conclude that zanamivir shows promise as a better therapy for pediatric influenza patients.

Generation and characterization of recombinant pandemic influenza A(H1N1) viruses resistant to neuraminidase inhibitors

BACKGROUND

Neuraminidase inhibitors (NAIs) play a key role in the management of influenza epidemics and pandemics. Given the novel pandemic influenza A(H1N1) (pH1N1) virus and the restricted number of approved anti-influenza drugs, evaluation of potential drug-resistant variants is of high priority.

METHODS

Recombinant pH1N1 viruses were generated by reverse genetics, expressing either the wild-type or any of 9 mutant neuraminidase (NA) proteins (N2 numbering: E119G, E119V, D198G, I222V, H274Y, N294S, S334N, I222V-H274Y, and H274Y-S334N). We evaluated these recombinant viruses for their resistance phenotype to 4 NAIs (oseltamivir, zanamivir, peramivir, and A-315675), NA enzymatic activity, and replicative capacity.

RESULTS

The E119G and E119V mutations conferred a multidrug resistance phenotype to many NAIs but severely compromised viral fitness. The oseltamivir- and peramivir-resistance phenotype was confirmed for the H274Y and N294S mutants, although both viruses remained susceptible to zanamivir. Remarkably, the I222V mutation had a synergistic effect on the oseltamivir- and peramivir-resistance phenotype of H274Y and compensated for reduced viral fitness, raising concerns about the potential emergence and dissemination of this double-mutant virus.

CONCLUSIONS

This study highlights the importance of continuous monitoring of antiviral drug resistance in clinical samples as well as the need to develop new agents and combination strategies.

Detection of resistance mutations to antivirals oseltamivir and zanamivir in avian influenza A viruses isolated from wild birds

The neuraminidase (NA) inhibitors oseltamivir and zanamivir are the first-line of defense against potentially fatal variants of influenza A pandemic strains. However, if resistant virus strains start to arise easily or at a high frequency, a new anti-influenza strategy will be necessary. This study aimed to investigate if and to what extent NA inhibitor–resistant mutants exist in the wild population of influenza A viruses that inhabit wild birds. NA sequences of all NA subtypes available from 5490 avian, 379 swine and 122 environmental isolates were extracted from NCBI databases. In addition, a dataset containing 230 virus isolates from mallard collected at Ottenby Bird Observatory (Öland, Sweden) was analyzed. Isolated NA RNA fragments from Ottenby were transformed to cDNA by RT-PCR, which was followed by sequencing. The analysis of genotypic profiles for NAs from both data sets in regard to antiviral resistance mutations was performed using bioinformatics tools. All 6221 sequences were scanned for oseltamivir- (I117V, E119V, D198N, I222V, H274Y, R292K, N294S and I314V) and zanamivir-related mutations (V116A, R118K, E119G/A/D, Q136K, D151E, R152K, R224K, E276D, R292K and R371K). Of the sequences from the avian NCBI dataset, 132 (2.4%) carried at least one, or in two cases even two and three, NA inhibitor resistance mutations. Swine and environmental isolates from the same data set had 18 (4.75%) and one (0.82%) mutant, respectively, with at least one mutation. The Ottenby sequences carried at least one mutation in 15 cases (6.52%). Therefore, resistant strains were more frequently found in Ottenby samples than in NCBI data sets. However, it is still uncertain if these mutations are the result of natural variations in the viruses or if they are induced by the selective pressure of xenobiotics (e.g., oseltamivir, zanamivir).

Phenotypic and genotypic characterization of influenza virus mutants selected with the sialidase fusion protein DAS181

BACKGROUND

influenza viruses (IFVs) frequently achieve resistance to antiviral drugs, necessitating the development of compounds with novel mechanisms of action. DAS181 (Fludase), a sialidase fusion protein, may have a reduced potential for generating drug resistance due to its novel host-targeting mechanism of action.

METHODS

IFV strains B/Maryland/1/59 and A/Victoria/3/75 (H3N2) were subjected to >30 passages under increasing selective pressure with DAS181. The DAS181-selected IFV isolates were characterized in vitro and in mice.

RESULTS

despite extensive passaging, DAS181-selected viruses exhibited a very low level of resistance to DAS181, which ranged between 3- and 18-fold increase in EC(50). DAS181-selected viruses displayed an attenuated phenotype in vitro, as exhibited by slower growth, smaller plaque size and increased particle to pfu ratios relative to wild-type virus. Further, the DAS181 resistance phenotype was unstable and was substantially reversed over time upon DAS181 withdrawal. In mice, the DAS181-selected viruses exhibited no greater virulence than their wild-type counterparts. Genotypic and phenotypic analysis of DAS181-selected viruses revealed mutations in the haemagglutinin (HA) and neuraminidase (NA) molecules and also changes in HA and NA function.

CONCLUSIONS

results indicate that resistance to DAS181 is minimal and unstable. The DAS181-selected IFV isolates exhibit reduced fitness in vitro, likely due to altered HA and NA functions.

Rapid evolution of low-pathogenic H9N2 avian influenza viruses following poultry vaccination programmes

To investigate whether currently circulating H9N2 avian influenza viruses (AIVs) in domestic poultry have evolved in Korean poultry since 2007, genetic and serological comparisons were conducted of H9N2 isolates from poultry slaughterhouses from January 2008 to December 2009. The isolation rate was relatively low in 2008 but increased gradually from January 2009 onwards. Genetic and phylogenetic analyses revealed that reassortant viruses had emerged, generating at least five novel genotypes, mostly containing segments of a previously prevalent domestic H9N2 virus lineage (Ck/Korea/04116/04-like). It was noteworthy that the N2 genes of some H9N2 isolates (genotypes D, E and F) were derived from those of H3N2-like viruses commonly isolated among domestic ducks in live-poultry markets. Animal challenge studies demonstrated that the pathogenicity of Ck/Korea/SH0906/09 (genotype B) and Ck/Korea/SH0912/09 (genotype F) in domestic avian species was altered due to reassortment. Furthermore, serological analysis revealed that the isolates were antigenically distinct from previous Korean H9N2 viruses including Ck/Korea/01310/01. Such antigenic diversity was illustrated further in experiments using H9N2-immunized chickens, which could not inhibit the replication and transmission of challenge viruses from each genotype. These results suggest that H9N2 viruses from domestic poultry have undergone substantial evolution since 2007 by immune selection as a result of vaccinal and natural immunity, coupled with reassortment. Taken together, this study demonstrates that periodical updating of vaccine strains, based on continuous surveillance data, is an important issue in order to provide sufficient protectivity against AIV infections.

Attaching zanamivir to a polymer markedly enhances its activity against drug-resistant strains of influenza a virus

Effects of the commercial drug zanamivir (Relenza) covalently attached to poly-l-glutamine on the infectivity of influenza A viruses are examined using the plaque reduction assay and binding affinity to viral neuraminidase (NA). These multivalent drug conjugates exhibit (i) up to a 20,000-fold improvement in anti-influenza potency compared with the zanamivir parent against human and avian viral strains, including both wild-type and drug-resistant mutants, and (ii) superior neuraminidase (NA) inhibition constants, especially for the mutants. These findings provide a basis for exploring polymer-attached inhibitors as more efficacious therapeutics, particularly against drug-resistant influenza strains.

Antiviral strategies for pandemic and seasonal influenza

While vaccines are the primary public health response to seasonal and pandemic flu, short of a universal vaccine there are inherent limitations to this approach. Antiviral drugs provide valuable alternative options for treatment and prophylaxis of influenza. Here, we will review drugs and drug candidates against influenza with an emphasis on the recent progress of a host-targeting entry-blocker drug candidate, DAS181, a sialidase fusion protein.

Antiviral susceptibility of avian and swine influenza virus of the N1 neuraminidase subtype

Influenza viruses of the N1 neuraminidase (NA) subtype affecting both animals and humans caused the 2009 pandemic. Anti-influenza virus NA inhibitors are crucial early in a pandemic, when specific influenza vaccines are unavailable. Thus, it is urgent to confirm the antiviral susceptibility of the avian viruses, a potential source of a pandemic virus. We evaluated the NA inhibitor susceptibilities of viruses of the N1 subtype isolated from wild waterbirds, swine, and humans. Most avian viruses were highly or moderately susceptible to oseltamivir (50% inhibitory concentration [IC(50)], <5.1 to 50 nM). Of 91 avian isolates, 7 (7.7%) had reduced susceptibility (IC(50), >50 nM) but were sensitive to the NA inhibitors zanamivir and peramivir. Oseltamivir susceptibility ranged more widely among the waterbird viruses (IC(50), 0.5 to 154.43 nM) than among swine and human viruses (IC(50), 0.33 to 2.56 nM). Swine viruses were sensitive to oseltamivir, compared to human seasonal H1N1 isolated before 2007 (mean IC(50), 1.4 nM). Avian viruses from 2007 to 2008 were sensitive to oseltamivir, in contrast to the emergence of resistant H1N1 in humans. Susceptibility remained high to moderate over time among influenza viruses. Sequence analysis of the outliers did not detect molecular markers of drug-resistance (e.g., H275Y NA mutation [N1 numbering]) but revealed mutations outside the NA active site. In particular, V267I, N307D, and V321I residue changes were found, and structural analyses suggest that these mutations distort hydrophobic pockets and affect residues in the NA active site. We determined that natural oseltamivir resistance among swine and wild waterbirds is rare. Minor naturally occurring variants in NA can affect antiviral susceptibility.

Assessing the viral fitness of oseltamivir-resistant influenza viruses in ferrets, using a competitive-mixtures model

To determine the relative fitness of oseltamivir-resistant strains compared to susceptible wild-type viruses, we combined mathematical modeling and statistical techniques with a novel in vivo "competitive-mixtures" experimental model. Ferrets were coinfected with either pure populations (100% susceptible wild-type or 100% oseltamivir-resistant mutant virus) or mixed populations of wild-type and oseltamivir-resistant influenza viruses (80%:20%, 50%:50%, and 20%:80%) at equivalent infectivity titers, and the changes in the relative proportions of those two viruses were monitored over the course of the infection during within-host and over host-to-host transmission events in a ferret contact model. Coinfection of ferrets with mixtures of an oseltamivir-resistant R292K mutant A(H3N2) virus and a R292 oseltamivir-susceptible wild-type virus demonstrated that the R292K mutant virus was rapidly outgrown by the R292 wild-type virus in artificially infected donor ferrets and did not transmit to any of the recipient ferrets. The competitive-mixtures model was also used to investigate the fitness of the seasonal A(H1N1) oseltamivir-resistant H274Y mutant and showed that within infected ferrets the H274Y mutant virus was marginally outgrown by the wild-type strain but demonstrated equivalent transmissibility between ferrets. This novel in vivo experimental method and accompanying mathematical analysis provide greater insight into the relative fitness, both within the host and between hosts, of two different influenza virus strains compared to more traditional methods that infect ferrets with only pure populations of viruses. Our statistical inferences are essential for the development of the next generation of mathematical models of the emergence and spread of oseltamivir-resistant influenza in human populations.

Effect of neuraminidase inhibitor-resistant mutations on pathogenicity of clade 2.2 A/Turkey/15/06 (H5N1) influenza virus in ferrets

The acquisition of neuraminidase (NA) inhibitor resistance by H5N1 influenza viruses has serious clinical implications, as this class of drugs can be an essential component of pandemic control measures. The continuous evolution of the highly pathogenic H5N1 influenza viruses results in the emergence of natural NA gene variations whose impact on viral fitness and NA inhibitor susceptibility are poorly defined. We generated seven genetically stable recombinant clade 2.2 A/Turkey/15/06-like (H5N1) influenza viruses carrying NA mutations located either in the framework residues (E119A, H274Y, N294S) or in close proximity to the NA enzyme active site (V116A, I117V, K150N, Y252H). NA enzyme inhibition assays showed that NA mutations at positions 116, 117, 274, and 294 reduced susceptibility to oseltamivir carboxylate (IC(50)s increased 5- to 940-fold). Importantly, the E119A NA mutation (previously reported to confer resistance in the N2 NA subtype) was stable in the clade 2.2 H5N1 virus background and induced cross-resistance to oseltamivir carboxylate and zanamivir. We demonstrated that Y252H NA mutation contributed for decreased susceptibility of clade 2.2 H5N1 viruses to oseltamivir carboxylate as compared to clade 1 viruses. The enzyme kinetic parameters (V(max), K(m) and K(i)) of the avian-like N1 NA glycoproteins were highly consistent with their IC(50) values. None of the recombinant H5N1 viruses had attenuated virulence in ferrets inoculated with 10(6) EID(50) dose. Most infected ferrets showed mild clinical disease signs that differed in duration. However, H5N1 viruses carrying the E119A or the N294S NA mutation were lethal to 1 of 3 inoculated animals and were associated with significantly higher virus titers (P<0.01) and inflammation in the lungs compared to the wild-type virus. Our results suggest that highly pathogenic H5N1 variants carrying mutations within the NA active site that decrease susceptibility to NA inhibitors may possess increased virulence in mammalian hosts compared to drug-sensitive viruses. There is a need for novel anti-influenza drugs that target different virus/host factors and can limit the emergence of resistance.

Surveillance and characterization of low pathogenic H5 avian influenza viruses isolated from wild migratory birds in Korea

Migratory waterfowls are the natural reservoir of influenza A viruses. However, interspecies transmission had occasionally caused outbreaks in various hosts including humans. To characterize the genetic origins of H5 avian influenza viruses isolated from migratory birds in South Korea, phylogenetic analysis were conducted. A total of 53 H5 viruses were isolated between October 2005 and November 2008. Full genetic characterization indicated that most of these viruses belong to the Eurasian-like avian lineage. However, some segments of the AB/Korea/W235/07 and the AB/Korea/W236/07 isolates were clustered with North American lineage viruses rather than those of the Eurasian lineage, suggesting the occurrence of reassortment between these two avian virus lineages. Phylogenetic analysis further demonstrated that the H5N2 and H5N3 virus isolates were of the low pathogenicity H5 phenotype. The H5 viruses appear to be antigenically similar to each other, but could be distinguished from a recent HPAI H5N1 (EM/Korea/W149/06) virus by hemagglutinin inhibition (HI) assays. Experimental inoculation of representative viruses indicated that certain isolates, particularly AB/Korea/W163/07 (H5N2), could be detected in trachea and lungs of chickens but none could be transmitted by direct contact. Furthermore, all of the viruses could be detected in mice lung without prior adaptation which is indicative of their pathogenic potential in a mammalian host. Overall, our results emphasize the important role that migratory birds play in the perpetuation, transport, and reassortment of avian influenza viruses stressing the need for continued surveillance of influenza virus activity in these avian populations.

Detection of E119V and E119I mutations in influenza A (H3N2) viruses isolated from an immunocompromised patient: challenges in diagnosis of oseltamivir resistance

The clinical use of the neuraminidase inhibitor (NAI) oseltamivir is associated with the emergence of drug resistance resulting from subtype-specific neuraminidase (NA) mutations. The influenza A/Texas/12/2007 (H3N2) virus isolated from an oseltamivir-treated immunocompromised patient exhibited reduced susceptibility to oseltamivir in the chemiluminescent neuraminidase inhibition (NI) assay (approximately 60-fold increase in its 50% inhibitory concentration [IC(50)] compared to that for a control virus). When further propagated in cell culture, the isolate maintained reduced susceptibility to oseltamivir in both chemiluminescent and fluorescent NI assays (approximately 50- and 350-fold increases in IC(50), respectively). Sequencing analysis of the isolate revealed a mix of nucleotides coding for amino acids at position 119 of the NA [E119(V/I)]. Plaque purification of the isolate yielded E119V and E119I variants, both exhibiting reduced susceptibility to oseltamivir. The E119I variant also showed decreased susceptibility to zanamivir and the investigational NAIs peramivir and A-315675. The emergence of E119V variants in oseltamivir-treated patients has been previously reported; however, the E119I mutation detected here is a novel one which reduces susceptibility to several NAIs. Both mutations were not detected in unpropagated original clinical specimens using either conventional sequencing or pyrosequencing, suggesting that these variants were present in very low proportions (<10%) in clinical specimens and gained dominance after virus propagation in MDCK cells. All virus isolates recovered from the patient were resistant to adamantanes. Our findings highlight the potential for emergence and persistence of multidrug-resistant influenza viruses in oseltamivir-treated immunocompromised subjects and also highlight challenges for drug resistance diagnosis due to the genetic instability of the virus population upon propagation in cell culture.

Host cell selection of influenza neuraminidase variants: implications for drug resistance monitoring in A(H1N1) viruses

The neuraminidase inhibitors (NAIs), oseltamivir and zanamivir, are essential for treatment and prevention of influenza A and B infections. Oseltamivir resistance among influenza A (H1N1) viruses rapidly emerged and spread globally during the 2007-2008 and 2008-2009 influenza seasons. Approximately 20% and 90% of viruses tested for NAI susceptibility at CDC during these seasons, respectively, were resistant to oseltamivir (IC(50) approximately 100-3000 time>those of sensitive viruses), based on the chemiluminescent NA inhibition assay. Pyrosequencing analysis confirmed H274Y mutation (H275Y in N1 numbering) in the neuraminidase (NA) gene of oseltamivir-resistant viruses. Full NA sequence analysis of a subset of oseltamivir-resistant and sensitive virus isolates from both seasons (n=725) showed that 53 (7.3%) had mutations at residue D151 (D-->E/G/N), while 9 (1.2%) had mutations at Q136 (Q-->K) and 2 (0.3%) had mutations at both residues. Viruses with very high IC(50) for oseltamivir and peramivir, and elevated IC(50) for zanamivir, had H274Y in addition to mutations at D151 and/or Q136, residues which can potentially confer NAI resistance based on recent N1 NA crystal structure data. Mutations at D151 without H274Y, did not elevate IC(50) for any tested NAI, however, Q136K alone significantly reduced susceptibility to zanamivir (36-fold), peramivir (80-fold) and A-315675 (114-fold) but not oseltamivir. Mutations at D151 and Q136 were present only in MDCK grown viruses but not in matching original clinical specimens (n=33) which were available for testing, suggesting that these variants were the result of cell culture selection or they were present in very low proportions. Our findings provide evidence that propagation of influenza virus outside its natural host may lead to selection of virus variants with mutations in the NA that affect sensitivity to NAIs and thus poses implications for drug resistance monitoring and diagnostics.

The polymerase acidic protein gene of influenza a virus contributes to pathogenicity in a mouse model

Adaptation of influenza A viruses to a new host species usually involves the mutation of one or more of the eight viral gene segments, and the molecular basis for host range restriction is still poorly understood. To investigate the molecular changes that occur during adaptation of a low-pathogenic avian influenza virus subtype commonly isolated from migratory birds to a mammalian host, we serially passaged the avirulent wild-bird H5N2 strain A/Aquatic bird/Korea/W81/05 (W81) in the lungs of mice. The resulting mouse-adapted strain (ma81) was highly virulent (50% mouse lethal dose = 2.6 log(10) 50% tissue culture infective dose) and highly lethal. Nonconserved mutations were observed in six viral genes (those for PB2, PB1, PA, HA, NA, and M). Reverse genetic experiments substituting viral genes and mutations demonstrated that the PA gene was a determinant of the enhanced virulence in mice and that a Thr-to-Iso substitution at position 97 of PA played a key role. In growth kinetics studies, ma81 showed enhanced replication in mammalian but not avian cell lines; the PA(97I) mutation in strain W81 increased its replicative fitness in mice but not in chickens. The high virulence associated with the PA(97I) mutation in mice corresponded to considerably enhanced polymerase activity in mammalian cells. Furthermore, this characteristic mutation is not conserved among avian influenza viruses but is prevalent among mouse-adapted strains, indicating a host-dependent mutation. To our knowledge, this is the first study that the isoleucine residue at position 97 in PA plays a key role in enhanced virulence in mice and is implicated in the adaptation of avian influenza viruses to mammalian hosts.

In vitro generation of neuraminidase inhibitor resistance in A(H5N1) influenza viruses

To identify mutations that can arise in highly pathogenic A(H5N1) viruses under neuraminidase inhibitor selective pressure, two antigenically different strains were serially passaged with increasing levels of either oseltamivir or zanamivir. Under oseltamivir pressure, both A(H5N1) viruses developed a H274Y neuraminidase mutation, although in one strain the mutation occurred in combination with an I222M neuraminidase mutation. The H274Y neuraminidase mutation reduced oseltamivir susceptibility significantly (900- to 2,500-fold compared to the wild type). However the dual H274Y/I222M neuraminidase mutation had an even greater impact on resistance, with oseltamivir susceptibility reduced significantly further (8,000-fold compared to the wild type). A similar affect on oseltamivir susceptibility was observed when the dual H274Y/I222M mutations were introduced, by reverse genetics, into a recombinant seasonal human A(H1N1) virus and also when an alternative I222 substitution (I222V) was generated in combination with H274Y in A(H5N1) and A(H1N1) viruses. These viruses remained fully susceptible to zanamivir but demonstrated reduced susceptibility to peramivir. Following passage of the A(H5N1) viruses in the presence of zanamivir, the strains developed a D198G neuraminidase mutation, which reduced susceptibility to both zanamivir and oseltamivir, and also an E119G neuraminidase mutation, which demonstrated significantly reduced zanamivir susceptibility (1,400-fold compared to the wild type). Mutations in hemagglutinin residues implicated in receptor binding were also detected in many of the resistant strains. This study identified the mutations that can arise in A(H5N1) under either oseltamivir or zanamivir selective pressure and the potential for dual neuraminidase mutations to result in dramatically reduced drug susceptibility.

Zanamivir-resistant influenza viruses with a novel neuraminidase mutation

The neuraminidase inhibitors zanamivir and oseltamivir are marketed for the treatment and prophylaxis of influenza and have been stockpiled by many countries for use in a pandemic. Although recent surveillance has identified a striking increase in the frequency of oseltamivir-resistant seasonal influenza A (H1N1) viruses in Europe, the United States, Oceania, and South Africa, to date there have been no reports of significant zanamivir resistance among influenza A (H1N1) viruses or any other human influenza viruses. We investigated the frequency of oseltamivir and zanamivir resistance in circulating seasonal influenza A (H1N1) viruses in Australasia and Southeast Asia. Analysis of 391 influenza A (H1N1) viruses isolated between 2006 and early 2008 from Australasia and Southeast Asia revealed nine viruses (2.3%) that demonstrated markedly reduced zanamivir susceptibility and contained a previously undescribed Gln136Lys (Q136K) neuraminidase mutation. The mutation had no effect on oseltamivir susceptibility but caused approximately a 300-fold and a 70-fold reduction in zanamivir and peramivir susceptibility, respectively. The role of the Q136K mutation in conferring zanamivir resistance was confirmed using reverse genetics. Interestingly, the mutation was not detected in the primary clinical specimens from which these mutant isolates were grown, suggesting that the resistant viruses either occurred in very low proportions in the primary clinical specimens or arose during MDCK cell culture passage. Compared to susceptible influenza A (H1N1) viruses, the Q136K mutant strains displayed greater viral fitness than the wild-type virus in MDCK cells but equivalent infectivity and transmissibility in a ferret model.

Small interfering RNA targeting m2 gene induces effective and long term inhibition of influenza A virus replication

RNA interference (RNAi) provides a powerful new means to inhibit viral infection specifically. However, the selection of siRNA-resistant viruses is a major concern in the use of RNAi as antiviral therapeutics. In this study, we conducted a lentiviral vector with a H1-short hairpin RNA (shRNA) expression cassette to deliver small interfering RNAs (siRNAs) into mammalian cells. Using this vector that also expresses enhanced green fluorescence protein (EGFP) as surrogate marker, stable shRNA-expressing cell lines were successfully established and the inhibition efficiencies of rationally designed siRNAs targeting to conserved regions of influenza A virus genome were assessed. The results showed that a siRNA targeting influenza M2 gene (siM2) potently inhibited viral replication. The siM2 was not only effective for H1N1 virus but also for highly pathogenic avian influenza virus H5N1. In addition to its M2 inhibition, the siM2 also inhibited NP mRNA accumulation and protein expression. A long term inhibition effect of the siM2 was demonstrated and the emergence of siRNA-resistant mutants in influenza quasispecies was not observed. Taken together, our study suggested that M2 gene might be an optimal RNAi target for antiviral therapy. These findings provide useful information for the development of RNAi-based prophylaxis and therapy for human influenza virus infection.

Isolation and genetic characterization of H5N2 influenza viruses from pigs in Korea

Due to dual susceptibility to both human and avian influenza A viruses, pigs are believed to be effective intermediate hosts for the spread and production of new viruses with pandemic potential. In early 2008, two swine H5N2 viruses were isolated from our routine swine surveillance in Korea. The sequencing and phylogenetic analysis of surface proteins revealed that the Sw/Korea/C12/08 and Sw/Korea/C13/08 viruses were derived from avian influenza viruses of the Eurasian lineage. However, although the Sw/Korea/C12/08 isolate is an entirely avian-like virus, the Sw/Korea/C13/08 isolate is an avian-swine-like reassortant with the PB2, PA, NP, and M genes coming from a 2006 Korean swine H3N1-like virus. The molecular characterization of the two viruses indicated an absence of significant mutations that could be associated with virulence or binding affinity. However, animal experiments showed that the reassortant Sw/Korea/C13/08 virus was more adapted and was more readily transmitted than the purely avian-like virus in a swine experimental model but not in ferrets. Furthermore, seroprevalence in swine sera from 2006 to 2008 suggested that avian H5 viruses have been infecting swine since 2006. Although there are no known potential clinical implications of the avian-swine reassortant virus for pathogenicity in pigs or other species, including humans, at present, the efficient transmissibility of the swine-adapted H5N2 virus could facilitate virus spread and could be a potential model for pandemic, highly pathogenic avian influenza (e.g., H5N1 and H7N7) virus outbreaks or a pandemic strain itself.

Isolation and genetic characterization of H5N2 influenza viruses from pigs in Korea

Due to dual susceptibility to both human and avian influenza A viruses, pigs are believed to be effective intermediate hosts for the spread and production of new viruses with pandemic potential. In early 2008, two swine H5N2 viruses were isolated from our routine swine surveillance in Korea. The sequencing and phylogenetic analysis of surface proteins revealed that the Sw/Korea/C12/08 and Sw/Korea/C13/08 viruses were derived from avian influenza viruses of the Eurasian lineage. However, although the Sw/Korea/C12/08 isolate is an entirely avian-like virus, the Sw/Korea/C13/08 isolate is an avian-swine-like reassortant with the PB2, PA, NP, and M genes coming from a 2006 Korean swine H3N1-like virus. The molecular characterization of the two viruses indicated an absence of significant mutations that could be associated with virulence or binding affinity. However, animal experiments showed that the reassortant Sw/Korea/C13/08 virus was more adapted and was more readily transmitted than the purely avian-like virus in a swine experimental model but not in ferrets. Furthermore, seroprevalence in swine sera from 2006 to 2008 suggested that avian H5 viruses have been infecting swine since 2006. Although there are no known potential clinical implications of the avian-swine reassortant virus for pathogenicity in pigs or other species, including humans, at present, the efficient transmissibility of the swine-adapted H5N2 virus could facilitate virus spread and could be a potential model for pandemic, highly pathogenic avian influenza (e.g., H5N1 and H7N7) virus outbreaks or a pandemic strain itself.

Combining crystallographic information and an aspherical-atom data bank in the evaluation of the electrostatic interaction energy in an enzyme-substrate complex: influenza neuraminidase inhibition

Although electrostatic interactions contribute only a part of the interaction energies between macromolecules, unlike dispersion forces they are highly directional and therefore dominate the nature of molecular packing in crystals and in biological complexes and contribute significantly to differences in inhibition strength among related enzyme inhibitors. In the reported study, a wide range of complexes of influenza neuraminidases with inhibitor molecules (sialic acid derivatives and others) have been analyzed using charge densities from a transferable aspherical-atom data bank. The strongest interactions of the residues are with the acidic group at the C2 position of the inhibitor ( approximately -300 kJ mol(-1) for -COO(-) in non-aromatic inhibitors, approximately -120-210 kJ mol(-1) for -COO(-) in aromatic inhibitors and approximately -450 kJ mol(-1) for -PO(3)(2-)) and with the amino and guanidine groups at C4 ( approximately -250 kJ mol(-1)). Other groups contribute less than approximately 100 kJ mol(-1). Residues Glu119, Asp151, Glu227, Glu276 and Arg371 show the largest variation in electrostatic energies of interaction with different groups of inhibitors, which points to their important role in the inhibitor recognition. The Arg292-->Lys mutation reduces the electrostatic interactions of the enzyme with the acidic group at C2 for all inhibitors that have been studied (SIA, DAN, 4AM, ZMR, G20, G28, G39 and BCZ), but enhances the interactions with the glycerol group at C6 for inhibitors that contain it. This is in agreement with the lower level of resistance of the mutated virus to glycerol-containing inhibitors compared with the more hydrophobic derivatives.

Animal models for the study of influenza pathogenesis and therapy

Influenza A viruses causes a variety of illnesses in humans. The most common infection, seasonal influenza, is usually a mild, self-limited febrile syndrome, but it can be more severe in infants, the elderly, and immunodeficient persons, in whom it can progress to severe viral pneumonitis or be complicated by bacterial superinfection, leading to pneumonia and sepsis. Seasonal influenza also occasionally results in neurologic complications. Rarely, viruses that have spread from wild birds to domestic poultry can infect humans; such “avian influenza” can range in severity from mild conjunctivitis through the rapidly lethal disease seen in persons infected with the H5N1 virus that first emerged in Hong Kong in 1997. To develop effective therapies for this wide range of diseases, it is essential to have laboratory animal models that replicate the major features of illness in humans. This review describes models currently in use for elucidating influenza pathogenesis and evaluating new therapeutic agents.

Emergence of oseltamivir-resistant influenza A/H3N2 virus with altered hemagglutination pattern in a hematopoietic stem cell transplant recipient

BACKGROUND

Persistent influenza virus replication during antiviral therapy in patients undergoing hematopoietic stem cell transplantation (HSCT) could promote the emergence of antiviral drug resistance.

OBJECTIVES

To follow the viral genotypic and drug susceptibility changes in a patient who developed progressive influenza A/H3N2 pneumonia despite oseltamivir therapy after haploidentical HSCT.

STUDY DESIGN

Direct genotypic analysis of the neuraminidase (NA) and hemagglutinin (HA) genes in successive bronchoalveolar lavage specimens was employed in combination with hemagglutination and NA enzymatic activity assays of the corresponding viral isolates.

RESULTS

The emergence of NA oseltamivir-resistance mutation R292K was detected by 12 days of oseltamivir treatment with 44,286-fold increase in oseltamivir IC50. Resurgence of wild type viral population was identified by 7 days after cessation of oseltamivir. Sequential HA mutations R228S and A138S were identified and associated with a shift in the HA receptor binding pattern reflected by loss of the ability to agglutinate chicken erythrocytes.

CONCLUSIONS

These rapid evolutionary changes warrant close virologic monitoring of immunocompromised patients treated for influenza infection, and raise concern about the efficacy of mono-drug therapy for influenza-associated disease in HSCT recipients.

Anti-influenza drugs: the development of sialidase inhibitors

Viruses, particularly those that are harmful to humans, are the 'silent terrorists' of the twenty-first century. Well over four million humans die per annum as a result of viral infections alone. The scourge of influenza virus has plagued mankind throughout the ages. The fact that new viral strains emerge on a regular basis, particularly out of Asia, establishes a continual socio-economic threat to mankind. The arrival of the highly pathogenic avian influenza H5N1 heightened the threat of a potential human pandemic to the point where many countries have put in place 'preparedness plans' to defend against such an outcome. The discovery of the first designer influenza virus sialidase inhibitor and anti-influenza drug Relenza, and subsequently Tamiflu, has now inspired a number of continuing efforts towards the discovery of next generation anti-influenza drugs. Such drugs may act as 'first-line-of-defence' against the spread of influenza infection and buy time for necessary vaccine development particularly in a human pandemic setting. Furthermore, the fact that influenza virus can develop resistance to therapeutics makes these continuing efforts extremely important. An overview of the role of the virus-associated glycoprotein sialidase (neuraminidase) and some of the most recent developments towards the discovery of anti-influenza drugs based on the inhibition of influenza virus sialidase is provided in this chapter.

Pyrosequencing as a tool to detect molecular markers of resistance to neuraminidase inhibitors in seasonal influenza A viruses

Pyrosequencing has been successfully used to monitor resistance in influenza A viruses to the first class of anti-influenza drugs, M2 blockers (adamantanes). In contrast to M2 blockers, resistance to neuraminidase (NA) inhibitors (NAIs) is subtype- and drug-specific. Here, we designed a pyrosequencing assay for detection of the most commonly reported mutations associated with resistance to NAIs, a newer class of anti-influenza drugs. These common mutations occur at residues: H274 (N1), E119 (N2), R292 (N2), and N294 (N2) in seasonal influenza A viruses. Additionally, we designed primers to detect substitutions at D151 in NAs of N1 and N2 subtypes. This assay allows detection of mutations associated with resistance not only in grown viruses but also in clinical specimens, thus reducing the time needed for testing and providing an advantage for disease outbreak investigation and management. The pyrosequencing approach also allows the detection of mixed populations of virus variants at positions of interest. Analysis of viruses in the original clinical specimens reduces the potential for introducing genetic variance in the virus population due to selection by cell culture. Our results showed that, in at least one instance, a D151E change seen in N1NA after virus propagation in cell culture was not detected in the original clinical specimen. Although the pyrosequencing assay allows high throughput screening for established genetic markers of antiviral resistance, it is not a replacement for the NA inhibition assays due to insufficient knowledge of the molecular mechanisms of the NAI-resistance.

Rapid molecular detection of the H275Y oseltamivir resistance gene mutation in circulating influenza A (H1N1) viruses

In early 2008, drug susceptibility surveillance of influenza viruses in Europe revealed that some influenza A viruses (subtype H1N1) circulating during the winter season of 2007 and 2008 were resistant to the neuraminidase inhibitor, oseltamivir. This resistance arises due to a histidine to tyrosine substitution in the neuraminidase active site (H275Y in N1 nomenclature). Current methods to detect this mutation involve an end-point reverse transcription polymerase chain reaction followed by nucleotide sequencing. While accurate, this approach has the limitation of being time-consuming, labour-intensive and expensive. Herein we describe a one-step allelic discrimination assay which rapidly (2 h) detects this resistance mutation. The sensitivity of the assay was as low as 10 copies per reaction and is capable of detecting the antiviral resistance mutation in a mixture of wild type H275 and mutant H275Y targets.

Oseltamivir-resistant influenza A viruses are transmitted efficiently among guinea pigs by direct contact but not by aerosol

Influenza viruses resistant to the neuraminidase (NA) inhibitor oseltamivir arise under drug selection pressure both in vitro and in vivo. Several mutations in the active site of the viral NA are known to confer relative resistance to oseltamivir, and influenza viruses with certain oseltamivir resistance mutations have been shown to transmit efficiently among cocaged ferrets. However, it is not known whether NA mutations alter aerosol transmission of drug-resistant influenza virus. Here, we demonstrate that recombinant human influenza A/H3N2 viruses without and with oseltamivir resistance mutations (in which NA carries the mutation E119V or the double mutations E119V I222V) have similar in ovo growth kinetics and infectivity in guinea pigs. These viruses also transmit efficiently by the contact route among cocaged guinea pigs, as in the ferret model. However, in an aerosol transmission model, in which guinea pigs are caged separately, the oseltamivir-resistant viruses transmit poorly or not at all; in contrast, the oseltamivir-sensitive virus transmits efficiently even in the absence of direct contact. The present results suggest that oseltamivir resistance mutations reduce aerosol transmission of influenza virus, which could have implications for public health measures taken in the event of an influenza pandemic.

Mutations of neuraminidase implicated in neuraminidase inhibitors resistance

Influenza constitutes one of the most important upper respiratory tract infections regarding morbidity, and mortality. Prevention and treatment of influenza rely on inactivated vaccines and antiviral drugs. Zanamivir and Oseltamivir, the currently available influenza neuraminidase inhibitors (NAI) can be used in clinical practice for the treatment of influenza infection. These drugs have also shown their efficacy against highly pathogenic avian influenza. Recent transmission of avian H7N7 and H5N1 influenza virus to human emphasized the need for active antiviral against emerging influenza viruses. Since their introduction in clinical practice, numerous studies have been implemented to determine the rate of emergence of NAI resistant isolates. These studies describe mechanisms of resistance associated to mutations in the neuraminidase protein, and their consequence in virus fitness and transmission. This review is summarizing the mutations described in human and avian influenza neuraminidases that are associated to resistance or reduction in sensitivity.

Neuraminidase inhibitor-resistant recombinant A/Vietnam/1203/04 (H5N1) influenza viruses retain their replication efficiency and pathogenicity in vitro and in vivo

Effective antiviral drugs are essential for early control of an influenza pandemic. It is therefore crucial to evaluate the possible threat posed by neuraminidase (NA) inhibitor-resistant influenza viruses with pandemic potential. Four NA mutations (E119G, H274Y, R292K, and N294S) that have been reported to confer resistance to NA inhibitors were each introduced into recombinant A/Vietnam/1203/04 (VN1203) H5N1 influenza virus. For comparison, the same mutations were introduced into recombinant A/Puerto Rico/8/34 (PR8) H1N1 influenza virus. The E119G and R292K mutations significantly compromised viral growth in vitro, but the H274Y and N294S mutations were stably maintained in VN1203 and PR8 viruses. In both backgrounds, the H274Y and N294S mutations conferred resistance to oseltamivir carboxylate (50% inhibitory concentration [IC(50)] increases, >250-fold and >20-fold, respectively), and the N294S mutation reduced susceptibility to zanamivir (IC(50) increase, >3.0-fold). Although the H274Y and N294S mutations did not compromise the replication efficiency of VN1203 or PR8 viruses in vitro, these mutations slightly reduced the lethality of PR8 virus in mice. However, the VN1203 virus carrying either the H274Y or N294S mutation exhibited lethality similar to that of the wild-type VN1203 virus. The different enzyme kinetic parameters (V(max) and K(m)) of avian-like VN1203 NA and human-like PR8 NA suggest that resistance-associated NA mutations can cause different levels of functional loss in NA glycoproteins of the same subtype. Our results suggest that NA inhibitor-resistant H5N1 variants may retain the high pathogenicity of the wild-type virus in mammalian species. Patients receiving NA inhibitors for H5N1 influenza virus infection should be closely monitored for the emergence of resistant variants.

Low pathogenicity H5N2 avian influenza outbreak in Japan during the 2005-2006

At the end of May 2005, a low-pathogenicity avian influenza (LPAI) virus of subtype H5N2 was isolated for the first time from chickens in Japan. Through active and epidemiological surveillance, 5.78 million chickens on 41 farms were found to be affected and 16 H5N2 viruses were isolated. Antigenic analysis revealed antigenic similarity of these isolates. Phylogenetic analysis showed that they originated from a common ancestor and clustered with the H5N2 strains prevalent in Central America that have been circulating since 1994. Experimental infection of chickens with the index isolate (A/chicken/Ibaraki/1/05) demonstrated that this virus replicated efficiently in the respiratory tract without clinical signs, and dust-borne and/or droplet-borne transmission was considered as a possible mode of transmission. These results suggested that the H5N2 LPAI viruses isolated in Japan were highly adapted to chickens.

Resistance to anti-influenza drugs: adamantanes and neuraminidase inhibitors

Development of effective drugs for the treatment or prevention of epidemic and pandemic influenza is important in order to reduce its impact. Adamantanes and neuraminidase inhibitors are two classes of anti-influenza drugs available for influenza therapy currently. However, emergence of resistance to these drugs has been detected, which raises concerns regarding their widespread use. In this review, resistance to the adamantanes and neuraminidase inhibitors will be discussed in relation to both epidemic and pandemic influenza viruses.

Influenza Virus Susceptibility and Resistance to Oseltamivir

Oseltamivir phosphate is a prodrug of oseltamivir carboxylate, a highly specific inhibitor of influenza virus neuraminidases. Given that oseltamivir carboxylate binds to highly conserved, essential amino acids in the catalytic site of the enzyme, and that the activity of neuraminidase is critical for virus release from infected cells and subsequent virus spread, the drug was expected to have a low propensity to select for viable resistant mutants. Indeed, viruses with neuraminidase (and haemagglutinin) substitutions conferring reduced susceptibility to oseltamivir have been generated with difficulty in vitro, and these mutants generally have reduced infectivity and transmissibility compared with wild-type virus in animal models. Studies of seasonal influenza isolates collected before the introduction of oseltamivir show an absence of naturally occurring resistance. Few resistant mutants have arisen during clinical trials of oseltamivir in seasonal influenza, with cumulative data from all Roche-sponsored studies indicating an incidence of resistance of 0.32% in adults (0.4%, including low-level mutants detected by genotyping alone in mixed virus populations) and 4.1% (5.4%) in children. Higher incidences of resistance were observed in two small Japanese studies, in which children received a different dosing schedule from their Western counterparts. In summary, the overall incidence of influenza virus resistance associated with the seasonal use of oseltamivir is currently low and resistant viruses might be of little clinical significance, except perhaps in immunocompromised individuals. However, continued vigilance, especially of emerging avian H5N1 strains, combined with careful, systematic laboratory-based monitoring, is essential.

Impact of Neuraminidase Mutations Conferring Influenza Resistance to Neuraminidase Inhibitors in the N1 and N2 Genetic Backgrounds

Subtype-specific neuraminidase (NA) mutations conferring resistance to NA inhibitors (NAIs) have been reported during in vitro passages and in clinic. In this study, we evaluated the impact of various NA mutations (E119A/G/V, H274Y, R292K and N294S) on the susceptibility profiles to different NAIs (oseltamivir, zanamivir and peramivir) using recombinant NA proteins of influenza A/WSN/33 (H1N1) and A/Sydney/5/97-like (H3N2) viruses. In the N1 subtype, the E119V mutation conferred cross-resistance to oseltamivir, zanamivir and peramivir [1,727–2,144 and 5,050-fold increase in IC50 values compared with wild-type (WT)] whereas only oseltamivir-resistance (1,028-fold increase in IC50) was conferred by the same mutation in the N2 subtype. The N294S mutation conferred resistance to oseltamivir in both the N1 and N2 subtypes (197- and 1,879-fold increase in IC50 values, respectively) whereas the H274Y mutation conferred resistance to oseltamivir (754-fold increase) and peramivir (260-fold increase) in the N1 subtype only. The virulence of reverse genetics-rescued A/WSN/33 viruses harbouring H274Y and N294S NA mutations was investigated in Balb/c mice. The WT and H274Y recombinants had identical LD50 values (103 PFUs) and generated similar viral lung titres, whereas a higher LD50 (104 PFUs) and a 1-log decrease in viral lung titres were obtained with the N294S mutant. This study shows that some NA mutations at framework residues may confer resistance to one or three NAIs depending on the viral subtype. It suggests that certain drug-resistant NA mutants may still be virulent although additional studies using clinical isolates are needed to confirm our results.

Zanamivir: an influenza virus neuraminidase inhibitor

Zanamivir is the first of two registered neuraminidase inhibitors for the treatment and prophylaxis of influenza. Relenza, an orally inhaled powder form of zanamivir, is currently approved in 19 countries for treatment, and in two for prophylaxis. Relenza reduces the time to alleviation of symptoms by 1 to 2 days in the influenza-positive population, if taken within 48 h of symptom onset, and in prophylaxis in family settings, it confers an 80% reduction in the odds of contracting influenza. The resistance profile of zanamivir is encouraging in the sense that there are still no reports of patients on acute therapy shedding drug-resistant virus. However, patient uptake of the inhaled drug has been insufficient to conclude that drug resistance will not be an issue in the future. All zanamivir-resistant variants selected in the laboratory so far have diminished viability.

Neuraminidase inhibitor-resistant influenza viruses may differ substantially in fitness and transmissibility

Mutations of the conserved residues of influenza virus neuraminidase (NA) that are associated with NA inhibitor (NAI) resistance decrease the sialidase activity and/or stability of the NA, thus compromising viral fitness. In fact, clinically derived NAI-resistant variants with different NA mutations have shown different transmissibilities in ferrets (M. L. Herlocher, R. Truscon, S. Elias, H. Yen, N. A. Roberts, S. E. Ohmit, and A. S. Monto, J. Infect. Dis. 190:1627-1630, 2004). Molecular characterization of mutant viruses that have a homogeneous genetic background is required to determine the effect of single mutations at conserved NA residues. We generated recombinant viruses containing either the wild-type NA (RG WT virus) or a single amino acid change at NA residue 119 (RG E119V-NA virus) or 292 (RG R292K-NA virus) in the A/Wuhan/359/95 (H3N2) influenza virus background by reverse genetics. Both mutants showed decreased sensitivity to oseltamivir carboxylate, and the RG R292K-NA virus showed cross-resistance to zanamivir. We also observed differences between the two mutants in NA enzymatic activity and thermostability. The R292K mutation caused greater reduction of sialidase activity and thermostability than the E119V mutation. The NA defect caused by the R292K mutation was associated with compromised growth and transmissibility, whereas the growth and transmissibility of the RG E119V-NA virus were comparable to those of RG WT virus. Our results suggest that NAI-resistant influenza virus variants may differ substantially in fitness and transmissibility, depending on different levels of NA functional loss.

Susceptibilities of antiviral-resistant influenza viruses to novel neuraminidase inhibitors

The susceptibilities of five zanamivir-resistant and six oseltamivir-resistant influenza viruses were assessed against four neuraminidase (NA) inhibitors, including peramivir and A-315675, by a fluorometric NA activity inhibition assay. The enzyme activity of a majority of the variants was effectively inhibited by either A-315675 or both peramivir and A-315675 (50% inhibitory concentration, <10 nM). A novel oseltamivir-resistant influenza virus B variant carrying substitution at residue 198 (Asp-->Asn) (N2 numbering) retained susceptibility to peramivir and A-315675. In vivo, the Asn198 variant showed no apparent fitness impairment as judged by its recovery on day 5 from the nasal washes of ferrets coinfected with equal doses of the wild-type virus and the Asn198 variant. Based on the sequence analysis of the virus in the nasal washes, oseltamivir treatment (5 mg/kg twice daily for 5 days) did not provide growth advantage to the Asn198 variant. Nevertheless, treatment with A-315675 (prodrug A-322278) reduced the number of the animals (two of seven) shedding the Asn198 variant. These studies indicate that different patterns of susceptibility and cross-resistance between NA inhibitors may prove important if antiviral resistance to zanamivir and oseltamivir were to emerge.

Sensitivity of influenza viruses to zanamivir and oseltamivir: a study performed on viruses circulating in France prior to the introduction of neuraminidase inhibitors in clinical practice

Influenza virus neuraminidase inhibitors (NAIs) were introduced in clinical practice in various parts of the world since 1999 but were only scarcely distributed in France. Prior to the generalization of zanamivir and oseltamivir utilization in our country, we decided to test a large panel of influenza strains to establish the baseline sensitivity of these viruses to anti-neuraminidase drugs, based upon a fluorometric neuraminidase enzymatic test. Our study was performed on clinical samples collected by practitioners of the GROG network (Groupe Régional d'Observation de la Grippe) in the south of France during the 2002-2003 influenza season. Out of 355 isolates tested in the fluorometric neuraminidase activity assay, 267 isolates could be included in inhibition assay against anti-neuraminidase drugs. Differences in IC50 range were found according to the subtype and the anti-neuraminidase drug. Influenza B and A/H1N1 viruses appeared to be more sensitive to zanamivir than to oseltamivir (mean B IC50 values: 4.19 nM versus 13 nM; mean H1N1 IC50 values: 0.92 nM versus 1.34 nM), while A/H1N2 and A/H3N2 viruses were more sensitive to oseltamivir than to zanamivir (mean H3N2 IC50 values: 0.67 nM versus 2.28 nM; mean H1N2 IC50 values: 0.9 nM versus 3.09 nM). Out of 128 N2 carrying isolates, 10 isolates had zanamivir or oseltamivir IC50 values in upper limits compared to their respective data range. Sequencing of the neuraminidase of these outliers N2 highlighted several mutations, but none of them were associated with resistance to neuraminidase inhibitors.

Enhanced expression of an alpha2,6-linked sialic acid on MDCK cells improves isolation of human influenza viruses and evaluation of their sensitivity to a neuraminidase inhibitor

The extensive use of neuraminidase (NA) inhibitors to treat influenza virus infections mandates close monitoring for resistant variants. Cultured cells do not provide a reliable means of evaluating the susceptibility of human influenza virus isolates to NA inhibitors. That is, the growth of such viruses in cell lines (e.g., Madin-Darby canine kidney [MDCK] cells) is not inhibited by these drugs, even though their sialidase activity is drug-sensitive. Matrosovich et al. (J. Virol. 77:8418-8425, 2003) showed that an MDCK cell line overexpressing the human beta-galactoside alpha2,6-sialyltransferase I (ST6Gal I) gene has the potential to assess the sensitivity of human influenza virus isolates to NA inhibitors, based on studies with a limited number of viruses. Here, we asked whether clinical isolates of influenza virus are universally sensitive to an NA inhibitor (oseltamivir) in an MDCK cell line expressing the ST6Gal I gene. The sensitivity of viruses to oseltamivir correlated with the sensitivity of viral sialidase to the compound, demonstrating the potential utility of this modified cell line for detecting NA inhibitor-resistant viruses. Moreover, in ST6Gal I-overexpressing cells, the growth of human influenza viruses was up to 2 logs higher than in MDCK cells. We conclude that the human ST6Gal I-expressing MDCK cell line is useful not only for evaluating their sensitivity to NA inhibitors, but also for isolation of influenza viruses from clinical samples.

Neuraminidase activity assays for monitoring MDCK cell culture derived influenza virus

Three assay methods were investigated for monitoring the time-course of neuraminidase (NA) activity of tissue culture derived equine influenza A virus from large-scale microcarrier cultivation and several steps of downstream processing required for the production of inactivated vaccines. Measurements of neuraminidase activity by a thiobarbituric acid (TBA) and a fluorometric method using Amplex Red as a fluorogen (FL-AR) did not correlate with the increase of hemagglutinin (HA) during virus replication. Samples analysed by the TBA method showed unspecific interference from low molecular weight compounds (< 3 kDa) of cell growth medium and virus maintenance medium. Further investigations showed that this was probably caused by interfering reactions between reducing sugars and amino acids that can be overcome by dialysis of samples. On the other hand, the sensitivity of the FL-AR method was not sufficient for the required measuring range. However, a reliable and sensitive fluorometric assay method (FL-MU-NANA) was obtained using 4-methylumbelliferyl-alpha-d-N-acetylneuraminic acid (4-MU-NANA) as a substrate, which allowed the detection of neuraminidase activities as low as 0.09 mU/mL. In this assay, time-course of neuraminidase activities correlated well with increasing hemagglutinin activities during virus replication in a bioreactor. Analysis of samples from various downstream processing steps comprising of clarification, inactivation, ultrafiltration (UF) and size-exclusion chromatography for the purification of influenza virus showed that neuraminidase activity was preserved at comparatively high levels. Based on the hemagglutinin and neuraminidase activity of the clarified and inactivated virus harvest, the overall recovery after gel filtration was about 34.4% and 119.5%, respectively.

Development of antivirals against influenza

Morbidity and mortality due to influenza virus infections remain a major problem throughout the world. Yearly, medical costs and loss of productivity resulting from influenza infection are estimated to be in the range of 12 dollars bn in the USA. The predicted increases in the elderly and immune-deficient populations will make influenza an even greater threat in the future. Despite the availability of vaccines, they have been least effective in these high-risk populations. Coupled with the requirement for routine revaccination, the need for effective antiviral agents is illustrated. The currently approved drugs, amantadine, rimantadine and ribavirin (in some countries), have limitations. They are only inhibitory against influenza A viruses, are prone to adverse reactions and quickly give rise to resistant virus. This review examines current drug therapies, antivirals in development and possible future opportunities for anti-influenza drugs.

Sialidase as a target for inhibitors of influenza virus replication

Structure-based drug design has led to the identification of potent and selective inhibitors of influenza virus sialidase, which have antiviral activity in vitro and in experimental animal models of influenza infection. Clinical studies with one such sialidase inhibitor, zanamivir, have shown this compound to be a safe and effective therapy for influenza infections in man. Passage of influenza viruses in the presence of zanamivir in vitro has been shown to result in the selection of viruses with reduced sensitivity to this drug. To date, however, there have been no reports of the isolation of zanamivir-resistant viruses during clinical studies of this compound. Further application of structure-based drug design is yielding novel classes of potent inhibitors of influenza virus sialidase.