Characterization of human influenza virus variants selected in vitro in the presence of the neuraminidase inhibitor GS 4071

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.

Fetal calf serum inhibits virus genome expression in Madin-Darby canine kidney cells persistently infected with influenza A virus

A cell line of Madin-Darby canine kidney (MDCK) cells persistently infected with human influenza A virus has been established and designated as MDCK-IVpi cells. Production of progeny virus in MDCK-IVpi cells was suppressed when the cells were incubated in the presence of 10% fetal calf serum (FCS). FCS impaired virus mRNA synthesis in MDCK-IVpi cells, which resulted in a scarcity of virus proteins for virion formation. However, MDCK-IVpi cells well supported the growth of superinfecting heterologous influenza viruses, even in the presence of FCS. A certain fetuin-like substance in FCS might be responsible for the observed inhibition of virus replication.

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.

A cell-based luminescence assay is effective for high-throughput screening of potential influenza antivirals

The spread of highly pathogenic avian influenza across geographical and species barriers underscores the increasing need for novel antivirals to compliment vaccination and existing antiviral therapies. Identification of new antiviral lead compounds depends on robust primary assays for high-throughput screening (HTS) of large compound libraries. We have developed a cell-based screen for potential influenza antivirals that measures the cytopathic effect (CPE) induced by influenza virus (A/Udorn/72, H3N2) infection in Madin Darby canine kidney (MDCK) cells using the luminescent-based CellTiter Glo system. This 72 h assay is validated for HTS in 384-well plates and performs more consistently and reliably than methods using neutral red, with Z values>0.8, signal-to-background>30 and signal-to-noise>10. In a blinded pilot screen (n=10,781) at 10 microM concentration, four compounds (with previously demonstrated efficacy against influenza) inhibited viral-induced CPE by >50%, with EC50/CC50 values comparable to those determined by other cell-based assays, thereby validating this assay accuracy and ability to simultaneously evaluate compound cellular availability and/or toxicity. This assay is translatable for screening against other influenza strains, such as avian flu, and may facilitate identification of antivirals for other viruses that induce CPE, such as West Nile or Dengue.

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.

Synthesis and evaluation of a new series of substituted acyl(thio)urea and thiadiazolo [2,3-a] pyrimidine derivatives as potent inhibitors of influenza virus neuraminidase

A series of substituted acyl(thio)urea and 2H-1,2,4-thiadiazolo [2,3-a] pyrimidine derivatives were prepared and both of their cell culture and enzymatic activity toward influenza virus were tested. Their in vitro neuraminidase inhibitory activities were in good agreement with the corresponding activities in cultured cells and they were evaluated as potent neuraminidase inhibitors. Of the analogues that demonstrated IC(50)s<0.1microM, 16 and 60 were further investigated as candidates with the most potential for future development. The molecular docking work of the representative compound was described to provide more insight into their mechanism of action and further rationalize the observations of this new series herein, which represents a novel class of highly potent and selective inhibitors of influenza virus.

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.

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.

Sialobiology of influenza: molecular mechanism of host range variation of influenza viruses

The gene pool of influenza A viruses in aquatic birds provides all of the genetic diversity required for human and lower animals. Host range selection of the receptor binding specificity of the influenza virus hemagglutinin occurs during maintenance of the virus in different host cells that express different receptor sialo-sugar chains. In this paper, functional roles of the hemagglutinin and neuraminidase spikes of influenza viruses are described in the relation to 1) host range of influenza viruses, 2) receptor binding specificity of human and other animal influenza viruses, 3) recognition of sialyl sugar chains by Spanish influenza virus hemagglutinin, 4) highly pathogenic and potentially pandemic H5N1, H9N2, and H7N7 avian influenza viruses and molecular mechanism of host range variation of influenza viruses, 5) role of the neuraminidase spike for the host range of influenza viruses, and 6) Development of anti-influenza drugs.

Characterization of recombinant influenza B viruses with key neuraminidase inhibitor resistance mutations

OBJECTIVES AND METHODS

An influenza B virus plasmid-based rescue system was used to introduce site-specific mutations, previously observed in neuraminidase (NA) inhibitor-resistant viruses, into the NA protein of six recombinant viruses. Three mutations observed only among in vitro selected zanamivir-resistant influenza A mutants were introduced into the B/Beijing/1/87 virus NA protein, to change residue E116 to glycine, alanine or aspartic acid. Residue E116 was also mutated to valine, a mutation found in the clinic among oseltamivir-resistant viruses. An arginine to lysine change at position 291 (292 N2 numbering) mimicked that seen frequently in influenza A N2 clinical isolates resistant to oseltamivir. Similarly, an arginine to lysine change at position 149 (152 in N2 numbering) was made to reproduce the change found in the only reported zanamivir-resistant clinical isolate of influenza B virus. In vitro selection and prolonged treatment in the clinic leads to resistance pathways that require compensatory mutations in the haemagglutinin gene, but these appear not to be important for mutants isolated from immunocompetent patients. The reverse genetics system was therefore used to generate mutants containing only the NA mutation.

RESULTS AND CONCLUSIONS

With the exception of a virus containing the E116G mutation, mutant viruses were attenuated to different levels in comparison with wild-type virus. This attenuation was a result of altered NA activity or stability depending on the introduced mutation. Mutant viruses displayed increased resistance to zanamivir, oseltamivir and peramivir, with certain viruses displaying cross-resistance to all three drugs.

In vitro and in vivo activities of T-705 and oseltamivir against influenza virus

T-705 (6-fluoro-3-hydroxy-2-pyrazinecarboxamide) has a potent and selective inhibitory activity against influenza virus. We studied the effects of an infectious dose on the anti-influenza virus activities of T-705 and oseltamivir, a commercially available neuraminidase inhibitor, both in vitro and in vivo. Plaque formation of influenza A/PR/8/34 virus was completely inhibited by 10 microg/ml of T-705 after 72 h incubation, whereas visible plaque formation was detected in the plate treated with GS 4071, the active form of oseltamivir (10 microg/ml). The antiviral activity of T-705 was not influenced by an increase in multiplicity of infection (MOI) from 0.0001 to 1, but that of GS 4071 was influenced in a yield reduction assay. No increase in viral yield was seen in either culture supernatant or cells after removal of T-705 (10 microg/ml) but, in contrast, productive infection recurred in culture supernatant and in cells after removal of GS 4071. In mice infected with a high challenge dose of influenza A/PR/8/34 virus, orally administered T-705 (200 and 400 mg/kg/day) completely prevented the death of mice and the survival rates of mice were significantly higher than those in mice treated with oseltamivir (P<0.01). When the treatment was delayed at 1, 13 and 25 h post infection, oral administration of 200 mg/kg of T-705 significantly prevented the death of mice (P<0.01), and the survival rates of mice treated with T-705 were comparable to those of mice treated with oseltamivir. These results suggest that T-705 has the potential to be a potent inhibitor of human influenza virus infections.

Resistant influenza A viruses in children treated with oseltamivir: descriptive study

BACKGROUND

Oseltamivir is an effective inhibitor of influenza virus neuraminidase. Although viruses resistant to oseltamivir emerge less frequently than those resistant to amantadine or rimantadine, information on oseltamivir-resistant viruses arising during clinical use of the drug in children is limited. Our aim was to investigate oseltamivir resistance in a group of children treated for influenza.

METHODS

We analysed influenza A viruses (H3N2) collected from 50 children before and during treatment with oseltamivir. We sequenced the genes for neuraminidase and haemagglutinin and studied the mutant neuraminidases for their sensitivity to oseltamivir carboxylate.

FINDINGS

We found neuraminidase mutations in viruses from nine patients (18%), six of whom had mutations at position 292 (Arg292Lys) and two at position 119 (Glu119Val), which are known to confer resistance to neuraminidase inhibitors. We also identified another mutation (Asn294Ser) in one patient. Sensitivity testing to oseltamivir carboxylate revealed that the neuraminidases of viruses that have an Arg292Lys, Glu119Val, or Asn294Ser mutation were about 10(4)-10(5)-fold, 500-fold, or 300-fold more resistant than their pretreatment neuraminidases, respectively. Oseltamivir-resistant viruses were first detected at day 4 of treatment and on each successive day of the study. More than 10(3) infectious units per mL of virus were detected in some of the patients who did not shed drug-resistant viruses, even after 5 days of treatment.

INTERPRETATION

Oseltamivir-resistant mutants in children being treated for influenza with oseltamivir arise more frequently than previously reported. Furthermore, children can be a source of viral transmission, even after 5 days of treatment with oseltamivir.

Susceptibility of human influenza viruses from Australasia and South East Asia to the neuraminidase inhibitors zanamivir and oseltamivir

Human influenza viruses isolated from Australasia (Australia and New Zealand) and South East Asia were analysed to determine their sensitivity to the NA inhibitor drugs, zanamivir and oseltamivir. A total of 532 strains isolated between 1998 and 2002 were tested using a fluorescence-based assay to measure the relative inhibition of NA activity over a range of drug concentrations. Based on median IC50 values, influenza A viruses (with neuraminidase subtypes N1 and N2) were more sensitive to both the NA inhibitors than were influenza B strains. Influenza A viruses with a N1 subtype and influenza B strains both demonstrated a greater sensitivity to zanamivir than to oseltamivir carboxylate, whereas influenza A strains with a N2 subtype were more susceptible to oseltamivir carboxylate. For each of the neuraminidase types, IC50 values for viruses from Australasia and South East Asia were found to be comparable. Based on the data prior to and following the licensing of the drugs into the respective regions, the use of the NA inhibitors did not appear to have a significant impact on the susceptibility of the viruses tested to zanamivir or oseltamivir carboxylate.

A novel means of identifying the neuraminidase type of currently circulating human A(H1) influenza viruses

With the recent emergence and spread of influenza A(H1N2) viruses which appear to have arisen by reassortment of circulating A(H1N1) and A(H3N2) strains, there is a need in epidemiological studies to determine the neuraminidase type in order to differentiate between influenza A(H1N2) and A(H1N1) strains. A fluorescence-based neuraminidase enzyme inhibition assay that has been developed to screen influenza viruses for potential resistance to the neuraminidase inhibitor drugs appears to be suitable for this purpose. When used with the neuraminidase inhibitor zanamivir the assay was able to provide a positive predictive value of 93.5% for the identification of neuraminidase type N1 or N2. This assay enables a large number of influenza A viruses to be screened at low cost to determine relative levels of A(H1N2) or A(H1N1) viruses circulating in the population.

Molecular mechanisms of influenza virus resistance to neuraminidase inhibitors

A wide use of inhibitors of influenza virus neuraminidase (NAIs) to control influenza in humans demands a better understanding of the mechanisms involved in the resistance emergence. In vitro studies demonstrate that both neuraminidase (NA) and hemagglutinin (HA) influence virus susceptibility to NAIs. Drug resistance conferred due to changes in the NA could be monitored in the NA inhibition assays. Zanamivir-selected viruses acquired the NA substitutions at residues 119 and 292; oseltamivir-selected--at 274 and 292; peramivir-selected--at 292; and A-315675-selected--at 119. The HA binding efficiency and therefore susceptibility to NAIs are affected by the amino acids forming the HA receptor-binding site, the location and number of oligosaccharide chains, and structure of the neuraminic acid-containing cellular receptors. The lack of suitable cell culture-based assays hampers the assessment of virus susceptibility in humans. Emergence of the viruses with the NAI-induced substitutions in the NA is uncommon in drug-treated humans, however a compromised state of the immune system promotes emergence of drug resistance. In vivo, the zanamivir-selected mutant contained a substitution at 152 (B/NA); the oseltamivir-selected mutants-at residues 119 (A/N2), 198 (B/NA), 274 (A/N1), and 292 (A/N2). Substitutions in the NA were often accompanied by impairment of virus infectivity and virulence in animal models. Because of complexity of mechanisms of virus resistance, further analysis of the viruses recovered from the drug-treated humans is warranted.

Antiviral chemotherapeutic agents against respiratory viruses: where are we now and what's in the pipeline?

PURPOSE OF REVIEW

The emergence of severe acute respiratory syndrome in late 2002 and the recent outbreaks of avian influenza in Asia are timely reminders of the ever present risks from respiratory viral diseases. Apart from influenza, there are no vaccines and very few antiviral chemotherapeutic agents available for the prevention and treatment of respiratory viral infections-the most common cause of human illness. If the current H5N1 avian influenza outbreak ever assumes the role of a pandemic, formidable technical difficulties relating to the properties of the agent, itself, will ensure that vaccines will only become available after a significant lead time and then only to a relatively small percentage of the population. The use of existing antivirals could be critical in limiting the initial spread of a pandemic, although their use in the control of epidemics caused by nonpandemic viruses has not been evaluated. It is against this background that a review of recent developments in respiratory antivirals has been undertaken.

RECENT FINDINGS

The late 1990s were a period of unprecedented activity in the development of new and much superior antivirals for the treatment of influenza infections. However, during the past 2 to 3 years and largely for commercial reasons, there has been a decline in interest in their further development by major drug companies. This situation may soon change with the possible advent of new pandemic viruses, and moves are afoot in several countries to consider the stockpiling of antivirals. The neuraminidase inhibitors zanamivir and oseltamivir, and the M2 inhibitors amantadine and rimantadine, remain the only options for controlling respiratory disease caused by influenza viruses, although the latter two could not be used against very recent H5N1 strains. There are several other neuraminidase inhibitors in development. Compounds with activity against other respiratory viruses, notably rhinoviruses, are also in development, many based on a newer knowledge of viral protein structure and function (rational drug design).

SUMMARY

The following is an overview of recent papers on the further development of neuraminidase inhibitors against influenza viruses and on recent development of newer antivirals against RSV and rhinoviruses. Where possible, comparisons are made with existing antivirals. For considerations of space, this review has been structured around stages in the replication cycle of significant respiratory viruses that have been traditionally used as targets for inhibition.

A Reverse Genetics Study of Resistance to Neuraminidase Inhibitors in An Influenza A/H1N1 Virus

A system of reverse genetics was used to generate influenza A/H1N1 viruses harbouring neuraminidase (NA) mutations previously associated with resistance to NA inhibitors in various viral subtypes. The His274Tyr and Glu119Gln mutants were rescued whereas the Arg292Lys and Glu119→Gly, Val, Ala or Asp mutants could not be generated. In NA inhibition assays, the His274Tyr mutant was resistant to oseltamivir (430-fold over wild-type) and BCX-1812 (50-fold) but was sensitive to zanamivir. A similar trend was seen when the mutant was evaluated by plaque reduction assay (PRA). The Glu119Gln mutant expressed a low level of resistance to oseltamivir (ninefold) and zanamivir (fourfold) in NA inhibition assay but was only marginally resistant to oseltamivir (fourfold) in PRA. The replication capacity of both mutants, in particular that of the His274Tyr virus, was impaired when compared with the wild-type virus in vitro.

Evaluation of neuraminidase enzyme assays using different substrates to measure susceptibility of influenza virus clinical isolates to neuraminidase inhibitors: report of the neuraminidase inhibitor susceptibility network

The increasing use of influenza virus neuraminidase (NA) inhibitors (NIs) necessitates the development of reliable methods for assessing the NI susceptibility of clinical isolates. We evaluated three NA inhibition assays against a panel of five clinical isolates each of influenza virus A/H1N1, A/H3N2, and B strains and four viruses with a defined resistance genotype (R292K, H274Y, R152K, and E119V). For fluorometric enzyme assay (FA) 1 (FA-1), 2'-(4-methylumbelliferyl)-alpha-D-N-acetylneuraminic acid (MUNANA) at 100 microM was used as the substrate, with pretitration of the virus input. For FA-2, MUNANA at 200 microM was used as the substrate, with a fixed 1:10 dilution of input virus. For the chemiluminescence (CL) assay, the 1,2-dioxetane derivative of sialic acid at 100 microM was used as the substrate, with pretitration of the virus. Four different operators repeated the assays several times in a blinded fashion with both zanamivir and oseltamivir carboxylate (GS4071) to determine intra- and interassay variations. Mean 50% inhibitory concentration (IC(50)) values were lower and generally less variable with the CL assay. FA-1 displayed greater variation than the CL assay or FA-2 and the highest IC(50) values with zanamivir; FA-2 showed the highest values with oseltamivir, particularly for influenza virus B, and was more variable with zanamivir than was the CL assay. All three assays detected 40-fold or greater changes in IC(50) values for the resistant viruses with at least one drug. Mixing experiments, whereby increasing fractions (0, 20, 40, 60, 80, and 100%) of NA from a known NI-resistant virus were mixed with the corresponding NI-sensitive parental NA, indicated that the resolution of IC(50) values was clearer with the CL assay than with FA-2 for two of the resistant variants (R152K and E119V). The FA and CL methods were reliable for the detection of NI resistance, but all assays have certain limitations. Based on reproducibility, ease of automation, time required for the assay, and greater sensitivity, the CL assay was selected for future susceptibility testing of influenza virus isolates circulating globally.

Mechanism by which mutations at his274 alter sensitivity of influenza a virus n1 neuraminidase to oseltamivir carboxylate and zanamivir

Oseltamivir carboxylate is a potent and specific inhibitor of influenza neuraminidase (NA). An influenza A/H1N1 variant selected in vitro with reduced susceptibility to oseltamivir carboxylate contains a His274Tyr mutation. To understand the mechanism by which a His274Tyr mutation gives rise to drug resistance, we studied a series of NA variant proteins containing various substitutions at position 274. Replacement of His274 with larger side chain residues (Tyr or Phe) reduced the NA sensitivity to oseltamivir carboxylate. In contrast, replacement of His274 with smaller side chain residues (Gly, Asn, Ser, and Gln) resulted in enhanced or unchanged sensitivity to oseltamivir carboxylate. Previous studies have suggested that the slow-binding inhibition of NA by oseltamivir carboxylate is a result of the reorientation of Glu276. Loss of this slow-binding inhibition in the His274Tyr and His274Phe mutant NA but not in His274Asn, His274Gly, His274Ser, or His274Gln supports the conclusion that the conformational change of Glu276 is restricted in the His274Tyr and His274Phe mutant NA upon oseltamivir carboxylate binding. Interestingly, His274Asn, as well as His274Gly, His274Ser, and His274Gln, also displayed reduced sensitivity to zanamivir and its analogue, 4-amino-Neu5Ac2en. Substitution of His274 with Tyr in influenza A/Tokyo/3/67 (H3N2) recombinant NA did not affect the susceptibility to oseltamivir carboxylate. These data indicate that the volume occupied by the amino acid side chain at position 274 can influence the sensitivities of influenza N1 NA but not of N2 NA to both oseltamivir carboxylate and zanamivir.

Existing antivirals are effective against influenza viruses with genes from the 1918 pandemic virus

The 1918 influenza pandemic caused more than 20 million deaths worldwide. Thus, the potential impact of a re-emergent 1918 or 1918-like influenza virus, whether through natural means or as a result of bioterrorism, is of significant concern. The genetic determinants of the virulence of the 1918 virus have not been defined yet, nor have specific clinical prophylaxis and/or treatment interventions that would be effective against a re-emergent 1918 or 1918-like virus been identified. Based on the reported nucleotide sequences, we have reconstructed the hemagglutinin (HA), neuraminidase (NA), and matrix (M) genes of the 1918 virus. Under biosafety level 3 (agricultural) conditions, we have generated recombinant influenza viruses bearing the 1918 HA, NA, or M segments. Strikingly, recombinant viruses possessing both the 1918 HA and 1918 NA were virulent in mice. In contrast, a control virus with the HA and NA from a more recent human isolate was unable to kill mice at any dose tested. The recombinant viruses were also tested for their sensitivity to U.S. Food and Drug Administration-approved antiinfluenza virus drugs in vitro and in vivo. Recombinant viruses possessing the 1918 NA or both the 1918 HA and 1918 NA were inhibited effectively in both tissue culture and mice by the NA inhibitors, zanamivir and oseltamivir. A recombinant virus possessing the 1918 M segment was inhibited effectively both in tissue culture and in vivo by the M2 ion-channel inhibitors amantadine and rimantadine. These data suggest that current antiviral strategies would be effective in curbing the dangers of a re-emergent 1918 or 1918-like virus.

The H274Y mutation in the influenza A/H1N1 neuraminidase active site following oseltamivir phosphate treatment leave virus severely compromised both in vitro and in vivo

Oseltamivir carboxylate is a potent and specific inhibitor of influenza A and B neuraminidase (NA). Oseltamivir phosphate, the ethyl ester prodrug of oseltamivir carboxylate, is the first orally active NA inhibitor available for the prophylaxis and treatment of influenza A and B. It offers an improvement over amantadine and rimantadine which are active only against influenza A and rapidly generate resistant virus. The emergence of virus resistant to oseltamivir carboxylate in the treatment of naturally acquired influenza infection is low (about 1%). The types of NA mutation to arise are sub-type specific and largely predicted from in vitro drug selection studies. A substitution of the conserved histidine at position 274 for tyrosine in the NA active site has been selected via site directed mutagenesis, serial passage in culture under drug pressure in H1N1 and during the treatment of experimental H1N1 infection in man. Virus carrying H274Y NA enzyme selected in vivo has reduced sensitivity to oseltamivir carboxylate. The replicative ability in cell culture was reduced up to 3 logs, as was infectivity in animal models of influenza virus infection. Additionally, pathogenicity of the mutant virus is significantly compromised in ferret, compared to the corresponding wild type virus. Virus carrying a H274Y mutation is unlikely to be of clinical consequence in man.

In vitro selection and characterization of influenza A (A/N9) virus variants resistant to a novel neuraminidase inhibitor, A-315675

With the recent introduction of neuraminidase (NA) inhibitors into clinical practice for the treatment of influenza virus infections, considerable attention has been focused on the potential for resistance development and cross-resistance between different agents from this class. A-315675 is a novel influenza virus NA inhibitor that has potent enzyme activity and is highly active in cell culture against a variety of strains of influenza A and B viruses. To further assess the therapeutic potential of this compound, in vitro resistance studies have been conducted and a comparative assessment has been made relative to oseltamivir carboxylate. The development of viral resistance to A-315675 was studied by in vitro serial passage of influenza A/N9 virus strains grown in MDCK cells in the presence of increasing concentrations of A-315675. Parallel passaging experiments were conducted with oseltamivir carboxylate, the active form of a currently marketed oral agent for the treatment of influenza virus infections. Passage experiments with A-315675 identified a variant at passage 8 that was 60-fold less susceptible to the compound. Sequencing of the viral population identified an E119D mutation in the NA gene, but no mutations were observed in the hemagglutinin (HA) gene. However, by passage 10 (2.56 microM A-315675), two mutations (R233K, S339P) in the HA gene appeared in addition to the E119D mutation in the NA gene, resulting in a 310-fold-lower susceptibility to A-315675. Further passaging at higher drug concentrations had no effect on the generation of further NA or HA mutations (20.5 microM A-315675). This P15 virus displayed 355-fold-lower susceptibility to A-315675 and >175-fold-lower susceptibility to zanamivir than did wild-type virus, but it retained a high degree of susceptibility to oseltamivir carboxylate. By comparison, virus variants recovered from passaging against oseltamivir carboxylate (passage 14) harbored an E119V mutation and displayed a 6,000-fold-lower susceptibility to oseltamivir carboxylate and a 175-fold-lower susceptibility to zanamivir than did wild-type virus. Interestingly, this mutant still retained susceptibility to A-315675 (42-fold loss). This suggests that cross-resistance between A-315675- and oseltamivir carboxylate-selected variants in vitro is minimal.

Virulence and reduced fitness of simian immunodeficiency virus with the M184V mutation in reverse transcriptase

Drug-resistant mutants with a methionine-to-valine substitution at position 184 of reverse transcriptase (M184V) emerged within 5 weeks of initiation of therapy in four newborn macaques infected with simian immunodeficiency virus (SIVmac251) and treated with lamivudine (3TC) or emtricitabine [(-)-FTC] (two animals per drug). Thus, this animal model mimics the rapid emergence of M184V mutants of HIV-1 during 3TC therapy of human patients. One animal of each treatment group developed fatal immunodeficiency at 12 weeks of age, which is similar to the rapid disease course seen in most untreated SIVmac251-infected infant macaques. To further evaluate the effect of the M184V mutation on viral fitness and virulence, groups of juvenile macaques were inoculated with the molecular clone SIVmac239 with either the wild-type sequence (group A [n = 5]) or the M184V sequence (SIVmac239-184V; group B [n = 5] and group C [n = 2]). The two SIVmac239-184V-infected animals of group C did not receive any drug treatment, and in both animals the virus population reverted to predominantly wild type (184M) by 8 weeks after inoculation. The other five SIVmac239-184V-infected animals (group B) were treated with (-)-FTC to prevent reversion. Although virus levels 1 week after inoculation were lower in the SIVmac239-184V-infected macaques than in the SIVmac239-infected animals, no significant differences were observed from week 2 onwards. Two animals in each group developed AIDS and were euthanized, while all other animals were clinically stable at 46 weeks of infection. These data demonstrate that the M184V mutation in SIV conferred a slightly reduced fitness but did not affect disease outcome.

Functional balance between haemagglutinin and neuraminidase in influenza virus infections

Influenza A and B viruses carry two surface glycoproteins, the haemagglutinin (HA) and the neuraminidase (NA). Both proteins have been found to recognise the same host cell molecule, sialic acid. HA binds to sialic acid-containing receptors on target cells to initiate virus infection, whereas NA cleaves sialic acids from cellular receptors and extracellular inhibitors to facilitate progeny virus release and to promote the spread of the infection to neighbouring cells. Numerous studies performed recently have revealed that an optimal interplay between these receptor-binding and receptor-destroying activities of the surface glycoproteins is required for efficient virus replication. An existing balance between the antagonistic HA and NA functions of individual viruses can be disturbed by various events, such as reassortment, virus transmission to a new host, or therapeutic inhibition of neuraminidase. The resulting decrease in the viral replicative fitness is usually overcome by restoration of the functional balance due to compensatory mutations in HA, NA or both proteins.

Influenza virus carrying an R292K mutation in the neuraminidase gene is not transmitted in ferrets

A model of influenza transmission has been established in ferrets in which wild-type influenza infection in a donor ferret can be transmitted sequentially to other ferrets. We have studied the transmission in ferrets of a clinical isolate of A/Sydney/5/97 (H3N2) carrying the neuraminidase 292K mutation compared with the corresponding wild-type virus from the same subject. Donor ferrets (n=four per group) were inoculated intranasally with mutant or wild-type virus and each housed with three naïve contact ferrets. All donor ferrets inoculated with wildtype virus were productively infected and transmitted virus to all 12 contacts, who in turn had high viral titres in their nasal washes. In contrast, only two of the donor ferrets inoculated with mutant virus were productively infected. There was little or no evidence that the two infected donor animals transmitted mutant virus to their contact animals. This ferret model has demonstrated that the mutant influenza virus with lysine at position 292 of the neuraminidase is of reduced infectivity and does not transmit under conditions in which the wild-type virus with arginine at position 292 readily transmits.

Influenza virus carrying neuraminidase with reduced sensitivity to oseltamivir carboxylate has altered properties in vitro and is compromised for infectivity and replicative ability in vivo

Oseltamivir phosphate (Tamiflu, Ro 64-0796) is the first orally administered neuraminidase (NA) inhibitor approved for use in treatment and prevention of influenza virus infection in man. Oseltamivir phosphate is the pro-drug of the active metabolite oseltamivir carboxylate (Ro 64-0802). Extensive monitoring throughout the oseltamivir development programme has identified a very low incidence of patients who have carried drug-resistant virus. The predominant mutation seen is the substitution of arginine for lysine at position 292 of the viral NA. The fitness of clinically isolated influenza virus A/Sydney/5/97 (H3N2) carrying this mutation was markedly reduced in animal models of influenza virus infection. The infectivity and replicative abilities of R292K mutant virus were reduced by at least 2 logs in a mouse model of influenza infection and by 2 and 4 logs, respectively, in the ferret model. Pathogenicity of R292K influenza virus A/Sydney/5/97 was reduced in ferrets as measured by inflammatory and febrile responses at least in parallel to the decrease in replicative ability. The data indicate that the R292K NA mutation compromises viral fitness such that virus carrying this mutation is unlikely to be of significant clinical consequence in man.

In vitro and in vivo activities of anti-influenza virus compound T-705

T-705 (6-fluoro-3-hydroxy-2-pyrazinecarboxamide) has been found to have potent and selective inhibitory activity against influenza virus. In an in vitro plaque reduction assay, T-705 showed potent inhibitory activity against influenza A, B, and C viruses, with 50% inhibitory concentrations (IC(50)s) of 0.013 to 0.48 microg/ml, while it showed no cytotoxicity at concentrations up to 1,000 microg/ml in Madin-Darby canine kidney cells. The selectivity index for influenza virus was more than 2,000. It was also active against a neuraminidase inhibitor-resistant virus and some amantadine-resistant viruses. T-705 showed weak activity against non-influenza virus RNA viruses, with the IC(50)s being higher for non-influenza virus RNA viruses than for influenza virus, and it had no activity against DNA viruses. Orally administered T-705 at 100 mg/kg of body weight/day (four times a day) for 5 days significantly reduced the mean pulmonary virus yields and the rate of mortality in mice infected with influenza virus A/PR/8/34 (3 x 10(2) PFU). These results suggest that T-705 may be a compound that is useful and highly selective against influenza virus infections and that has a mode of action different from those of commercially available drugs, such as amantadine, rimantadine, and neuraminidase inhibitors.

In vitro characterization of A-315675, a highly potent inhibitor of A and B strain influenza virus neuraminidases and influenza virus replication

A-315675 is a novel, pyrrolidine-based compound that was evaluated in this study for its ability to inhibit A and B strain influenza virus neuraminidases in enzyme assays and influenza virus replication in cell culture. A-315675 effectively inhibited influenza A N1, N2, and N9 and B strain neuraminidases with inhibitor constant (K(i)) values between 0.024 and 0.31 nM. These values were comparable to or lower than the K(i) values measured for oseltamivir carboxylate (GS4071), zanamivir, and BCX-1812, except for the N1 enzymes that were found to be the most sensitive to BCX-1812. The time-dependent inhibition of neuraminidase catalytic activity observed with A-315675 is likely due to its very low rate of dissociation from the active site of neuraminidase. The half times for dissociation of A-315675 from B/Memphis/3/89 and A/Tokyo/3/67 (H3N2) influenza virus neuraminidases of 10 to 12 h are significantly slower than the half times measured for oseltamivir carboxylate (33 to 60 min). A-315675 inhibited the replication of several laboratory strains of influenza virus in cell culture with potencies that were comparable or superior to those for oseltamivir carboxylate and BCX-1812, except for the A/H1N1 viruses that were found to be two- to fourfold more susceptible to BCX-1812. A-315675 and oseltamivir carboxylate exhibited comparable potencies against a panel of A/H1N1 and A/H3N2 influenza virus clinical isolates, but A-315675 was found to be significantly more potent than oseltamivir carboxylate against the B strain isolates. The favorable in vitro results relative to other clinically effective agents provide strong support for the further investigation of A-315675 as a potential therapy for influenza virus infections.

Neuraminidase inhibitors for the treatment and prevention of influenza

The impact of influenza virus infection is estimated to run into billions of dollars worldwide. Vaccination plays a key role in prevention; however, vaccines do not provide complete protection against influenza due to the constant mutation of the virus responsible. Unlike amantadine and rimantadine, which are only effective against influenza A, the new neuraminidase inhibitors zanamivir (Relenza), GlaxoSmithKline) and oseltamivir (Tamiflu), Gilead/Roche) are potent and specific inhibitors of influenza types A and B and have minimal side effects. The greatest benefit is derived if treatment commences as soon as possible after symptoms develop. In order for these inhibitors to have a significant impact on the disease, clinicians and the general public need to be made more aware of the symptoms of influenza and the availability of these new drugs.

Detection of influenza virus resistance to neuraminidase inhibitors by an enzyme inhibition assay

We previously characterized influenza viruses whose selection in the presence of neuraminidase (NA) inhibitors resulted in a substituted residue (position 119,152,274, or 292) in the NA active center. To identify the most favorable conditions for detecting NA inhibitor-resistant viruses we compared the results of four modifications of the NA inhibition assay utilizing a fluorogenic substrate. The IC50 values were highly dependent upon assay conditions, and most mutant enzymes were more sensitive to changes in assay conditions (e.g. addition of PO4(3-), Ca2+, DMSO, or EDTA) than wild-type enzymes or a mutant NA with an Arg292-->Lys substitution. Although the levels of resistance to zanamivir, oseltamivir carboxylate, and BCX-1812 (RWJ-270201) for each mutant varied among assays, mutants with substitutions at framework residues 119 or 274 exhibited sensitivity to at least one inhibitor. Viruses with substitutions at catalytic residues 152 or 292 were resistant to each inhibitor in all assays. Monitoring resistance in a clinical setting will require a panel of resistant viruses to ensure that assay conditions are favorable for detecting variants with substituted residues in the NA active center. Because variants selected in the presence of one NA inhibitor could be variably resistant to other inhibitors, all three inhibitors should be used in drug susceptibility testing.

RWJ-270201 (BCX-1812): a novel neuraminidase inhibitor for influenza

The influenza virus neuraminidase (NA) is important in the pathogenesis of infection and, thus, is an attractive target for agents used in the treatment and prophylaxis of influenza. This article describes preclinical and early clinical data related to RWJ-270201 (BCX-1812), a novel, orally active NA inhibitor that was rationally designed for having potent and selective activity against influenza A and B viruses. RWJ-270201 is a unique NA inhibitor with a cyclopentane ring structure and high selectivity for the influenza NA. RWJ-270201 has efficacy comparable to or better than earlier NA inhibitors against a wide range of influenza A and B isolates, including recently emerged and avian strains, both in vitro and in a lethal murine model of influenza. Based on the high selectivity and efficacy of RWJ-270201 against both type A and B influenza strains in preclinical studies as well as murine pharmacodynamic studies supporting the potential for once-daily administration, clinical trials were initiated in order to determine the tolerability and antiviral activity of RWJ-270201 in humans. To date, clinical studies have indicated that RWJ-270201 is well tolerated and has antiviral activity in human experimental influenza models when administered orally once daily.

Treatment of influenza with neuraminidase inhibitors: virological implications

Evaluation of the emergence of influenza virus resistance to neuraminidase inhibitors (NAIs) is now demanded following experience with amantadinamines. Preliminary data have indicated that NAI-resistant virus is unlikely to emerge readily in the clinic and this is consistent with the difficulty experienced in selecting resistant virus in vitro. Resistance mutations can occur in both neuraminidase and haemagglutinin genes. The neuraminidase mutations are viral subtype specific and, therefore, clinically relevant subtypes must be employed for in vitro studies if pre-clinical data are to have predictive value. Haemagglutinin mutations generated in vitro are probably both subtype and cell culture system specific and, therefore, may not be predictive of clinical findings. Analysis of influenza-positive samples from NAI-treated patients in the clinical setting must include samples from late treatment time-points (day 4 and later) in order for resistant virus to be detected as in vitro studies and current clinical experience have indicated that resistant virus is slow to emerge and is transient.

Comparison of the activities of zanamivir, oseltamivir, and RWJ-270201 against clinical isolates of influenza virus and neuraminidase inhibitor-resistant variants

RWJ-270201 is a novel cyclopentane inhibitor of influenza A and B virus neuraminidases (NAs). We compared the ability of RWJ-270201 to inhibit NA activity of clinical influenza isolates and viruses with defined resistance mutations with that of zanamivir and oseltamivir carboxylate. In NA inhibition assays with influenza A viruses, the median 50% inhibitory concentration (IC(50)) of RWJ-270201 (approximately 0.34 nM) was comparable to that of oseltamivir carboxylate (0.45 nM) but lower than that of zanamivir (0.95 nM). For influenza B virus isolates, the IC(50) of RWJ-270201 (1.36 nM) was comparable to that of zanamivir (2.7 nM) and less than that of oseltamivir carboxylate (8.5 nM). A zanamivir-resistant variant bearing a Glu119-to-Gly (Glu119-->Gly) or Glu119-->Ala substitution in an NA (N2) remained susceptible to RWJ-270201 and oseltamivir carboxylate. However, a zanamivir-selected variant with an Arg292-->Lys substitution in an NA (N2) showed a moderate level of resistance to RWJ-270201 (IC(50) = 30 nM) and zanamivir (IC(50) = 20 nM) and a high level of resistance to oseltamivir carboxylate (IC(50) > 3,000 nM). The zanamivir-resistant influenza B virus variant bearing an Arg152-->Lys substitution was resistant to each NA inhibitor (IC(50) = 100 to 750 nM). The oseltamivir-selected variant (N1) with the His274-->Tyr substitution exhibited resistance to oseltamivir carboxylate (IC(50) = 400 nM) and to RWJ-270201 (IC(50) = 40 nM) but retained full susceptibility to zanamivir (IC(50) = 1.5 nM). Thus, drug-resistant variants with substitutions in framework residues 119 or 274 can retain susceptibility to other NA inhibitors, whereas replacement of functional residue 152 or 292 leads to variable levels of cross-resistance. We conclude that RWJ-270201 is a potent inhibitor of NAs of wild-type and some zanamivir-resistant or oseltamivir-resistant influenza A and B virus variants.

Anti-influenza drugs and neuraminidase inhibitors

Each year, influenza viruses are responsible for considerable illness, complications and mortality. An effective treatment will have a major impact on the severe personal and economic burden that this disease incurs. There are several points in the influenza life cycle that may be potentially inhibited. One critical point is the release of newly synthesized virions from the host cell surface. Viral neuraminidase (NA) cleaves the virus from host cell sialic acid residues allowing infection of other host cells. Rationally designed NA inhibitors that block the viral life cycle are now in the clinic and these molecules are effective and safe for the treatment of influenza. Compared with other anti-influenza agents the NA inhibitors are well tolerated, effective against all influenza types and there has been little evidence of the emergence of viral resistance. NA inhibitors provide an important new therapeutic weapon for the management of influenza infection.

Novel alpha- and beta-amino acid inhibitors of influenza virus neuraminidase

In an effort to discover novel, noncarbohydrate inhibitors of influenza virus neuraminidase we hypothesized that compounds which contain positively charged amino groups in an appropriate position to interact with the Asp 152 or Tyr 406 side chains might be bound tightly by the enzyme. Testing of 300 alpha- and beta-amino acids led to the discovery of two novel neuraminidase inhibitors, a phenylglycine and a pyrrolidine, which exhibited K(i) values in the 50 microM range versus influenza virus A/N2/Tokyo/3/67 neuraminidase but which exhibited weaker activity against influenza virus B/Memphis/3/89 neuraminidase. Limited optimization of the pyrrolidine series resulted in a compound which was about 24-fold more potent than 2-deoxy-2,3-dehydro-N-acetylneuraminic acid in an anti-influenza cell culture assay using A/N2/Victoria/3/75 virus. X-ray structural studies of A/N9 neuraminidase-inhibitor complexes revealed that both classes of inhibitors induced the Glu 278 side chain to undergo a small conformational change, but these compounds did not show time-dependent inhibition. Crystallography also established that the alpha-amino group of the phenylglycine formed hydrogen bonds to the Asp 152 carboxylate as expected. Likewise, the beta-amino group of the pyrrolidine forms an interaction with the Tyr 406 hydroxyl group and represents the first compound known to make an interaction with this absolutely conserved residue. Phenylglycine and pyrrolidine analogs in which the alpha- or beta-amino groups were replaced with hydroxyl groups were 365- and 2,600-fold weaker inhibitors, respectively. These results underscore the importance of the amino group interactions with the Asp 152 and Tyr 406 side chains and have implications for anti-influenza drug design.

The importance of specific virus diagnosis and monitoring for antiviral treatment

Antiviral treatment, rapid viral diagnosis and point-of-care diagnostics are all relatively new, and their appropriate use not fully appreciated or evaluated. In this article, the need for laboratory diagnosis in relation to antiviral treatment, and practical approaches are discussed, with influenza and herpes simplex virus (HSV) as examples.

Anti-influenza drugs and neuraminidase inhibitors

Each year, influenza viruses are responsible for considerable illness, complications and mortality. An effective treatment will have a major impact on the severe personal and economic burden that this disease incurs. There are several points in the influenza life cycle that may be potentially inhibited. One critical point is the release of newly synthesized virions from the host cell surface. Viral neuraminidase (NA) cleaves the virus from host cell sialic acid residues allowing infection of other host cells. Rationally designed NA inhibitors that block the viral life cycle are now in the clinic and these molecules are effective and safe for the treatment of influenza. Compared with other anti-influenza agents the NA inhibitors are well tolerated, effective against all influenza types and there has been little evidence of the emergence of viral resistance. NA inhibitors provide an important new therapeutic weapon for the management of influenza infection.

A single amino acid alteration in the human parainfluenza virus type 3 hemagglutinin-neuraminidase glycoprotein confers resistance to the inhibitory effects of zanamivir on receptor binding and neuraminidase activity

Entry and fusion of human parainfluenza virus type 3 (HPF3) requires interaction of the viral hemagglutinin-neuraminidase (HN) glycoprotein with its sialic acid receptor. 4-Guanidino-2,4-dideoxy-2,3-dehydro-N-acetylneuraminic acid (4-GU-DANA; zanamivir), a sialic acid transition-state analog designed to fit the influenza virus neuraminidase catalytic site, possesses antiviral activity at nanomolar concentrations in vitro. We have shown previously that 4-GU-DANA also inhibits both HN-mediated binding of HPF3 to host cell receptors and HN's neuraminidase activity. In the present study, a 4-GU-DANA-resistant HPF3 virus variant (ZM1) was generated by serial passage in the presence of 4-GU-DANA. ZM1 exhibited a markedly fusogenic plaque morphology and harbored two HN gene mutations resulting in two amino acid alterations, T193I and I567V. Another HPF3 variant studied in parallel, C-0, shared an alteration at T193 and exhibited similar plaque morphology but was not resistant to 4-GU-DANA. Neuraminidase assays revealed a 15-fold reduction in 4-GU-DANA sensitivity for ZM1 relative to the wild type (WT) and C-0. The ability of ZM1 to bind sialic acid receptors was inhibited 10-fold less than for both WT and C-0 in the presence of 1 mM 4-GU-DANA. ZM1 also retained infectivity at 15-fold-higher concentrations of 4-GU-DANA than WT and C-0. A single amino acid alteration at HN residue 567 confers these 4-GU-DANA-resistant properties. An understanding of ZM1 and other escape variants provides insight into the effects of this small molecule on HN function as well as the role of the HN glycoprotein in HPF3 pathogenesis.

Oseltamivir: a review of its use in influenza

Oseltamivir is a prodrug of oseltamivir carboxylate (Ro 64-0802, GS4071), a potent and selective inhibitor of the neuraminidase glycoprotein essential for replication of influenza A and B viruses. Studies in volunteers with experimental human influenza A or B showed that administration of oral oseltamivir 20 to 200 mg twice daily for 5 days reduced both the quantity and duration of viral shedding compared with placebo. Subsequent assessment of the drug at a dosage of 75 mg twice daily for 5 days in otherwise healthy adults with naturally acquired febrile influenza showed that oseltamivir reduced the duration of the disease by up to 1.5 days and the severity of illness by up to 38% compared with placebo when initiated within 36 hours of symptom onset (earlier initiation of therapy was associated with faster resolution). The incidence of secondary complications and the use of antibacterials were also reduced significantly in oseltamivir recipients. A liquid formulation of oseltamivir (2 mg/kg twice daily for 5 days) has been shown to be effective in the treatment of children with influenza, and data presented in abstracts suggest that the drug may also be of use in high-risk populations such as the elderly or those with chronic cardiac or respiratory disease. In addition to treatment efficacy, the drug has demonstrated efficacy when used for seasonal or household prophylaxis. Oral oseltamivir (75 mg once or twice daily for 6 weeks) during a period of local influenza activity significantly prevented the development of naturally acquired influenza by >70% compared with placebo in unvaccinated otherwise healthy adults. The drug also demonstrated efficacy when used adjunctively in previously vaccinated high-risk elderly patients (92% protective efficacy). Short term administration of oseltamivir (75 mg once daily for 7 days) may significantly reduce the risk of illness in household contacts of infected persons when administered within 48 hours of symptom onset in the infected person. Oseltamivir 75 mg twice daily for 5 days was well tolerated in clinical trials in healthy adults and high-risk patients, with nausea and vomiting being the most commonly reported events. Gastrointestinal events were mild and transient and both nausea and vomiting were less likely when oseltamivir was taken with food.

CONCLUSIONS

Oseltamivir is a well tolerated orally active neuraminidase inhibitor which significantly reduces the duration of symptomatic illness and hastens the return to normal levels of activity when initiated promptly in patients with naturally acquired influenza. It therefore represents a useful therapeutic alternative to zanamivir (especially in patients who prefer oral administration or who have an underlying respiratory disorder) and the M2 inhibitors amantadine and rimantadine (because of its broader spectrum of anti-influenza activity and lower likelihood of resistance) in patients with influenza. In addition, although annual vaccination remains the best means of influenza prevention, there may be a place for oseltamivir in providing household prophylaxis or adjunctive prophylaxis in high-risk vaccinated patients during an outbreak of the disease or for use in patients in whom vaccination is unsuitable or ineffective.

Zanamivir and oseltamivir: two new options for the treatment and prevention of influenza

BACKGROUND

Influenza infection is responsible for thousands of hospitalizations and deaths in the United States each year. Until recently, management options were limited to vaccination or use of the antiviral agents amantadine and rimantadine. Two antiviral drugs, zanamivir and oseltamivir, have recently been approved by the US Food and Drug Administration for the treatment of influenza A and influenza B.

OBJECTIVE

This article reviews the published data on the pharmacology and clinical utility of zanamivir and oseltamivir in the treatment and prevention of influenza A and influenza B illness.

METHODS

To identify relevant literature, a search of MEDLINE, International Pharmaceutical Abstracts, and the Iowa Drug Information Service was conducted for the period from 1969 to 2000. The search terms used were influenza, neuraminidase, zanamivir, oseltamivir; amantadine, and rimantadine. The reference lists of the articles so obtained were used to identify additional publications.

RESULTS

Zanamivir and oseltamivir inactivate viral neuraminidase, an enzyme responsible for cleaving sialic acid residues on newly formed virions as they bud off from the host cell. This inhibition results in aggregation of virions on the surface of the host cell, which limits the extent of infection and speeds recovery from illness. Clinical studies have shown that neuraminidase inhibitors can decrease the median duration of influenza-related symptoms by approximately 1 day if initiated within 48 hours of the onset of symptoms of influenza.

CONCLUSIONS

Evidence supports the use of zanamivir and oseltamivir in the treatment of influenza; however, additional studies are needed to clarify their utility and tolerability in pediatric and high-risk patients, as well as their utility in the prevention of influenza.

The neuraminidase inhibitor GS4104 (oseltamivir phosphate) is efficacious against A/Hong Kong/156/97 (H5N1) and A/Hong Kong/1074/99 (H9N2) influenza viruses

In 1997, an H5N1 avian influenza A/Hong Kong/156/97 virus transmitted directly to humans and killed six of the 18 people infected. In 1999, another avian A/Hong/1074/99 (H9N2) virus caused influenza in two children. In such cases in which vaccines are unavailable, antiviral drugs are crucial for prophylaxis and therapy. Here we demonstrate the efficacy of the neuraminidase inhibitor GS4104 (oseltamivir phosphate) against these H5N1 and H9N2 viruses. GS4071 (the active metabolite of oseltamivir) inhibited viral replication in MDCK cells (EC(50) values, 7.5-12 microM) and neuraminidase activity (IC(50) values, 7.0-15 nM). When orally administered at doses of 1 and 10 mg/kg per day, GS4104 prevented death of mice infected with A/Hong Kong/156/97 (H5N1), mouse-adapted A/Quail/Hong Kong/G1/97 (H9N2), or human A/Hong Kong/1074/99 (H9N2) viruses and reduced virus titers in the lungs and prevented the spread of virus to the brain of mice infected with A/Hong Kong/156/97 (H5N1) and mouse-adapted A/Quail/Hong Kong/G1/97 (H9N2) viruses. When therapy was delayed until 36 h after exposure to the H5N1 virus, GS4104 was still effective and significantly increased the number of survivors as compared with control. Oral administration of GS4104 (0.1 mg/kg per day) in combination with rimantadine (1 mg/kg per day) reduced the number of deaths of mice infected with 100 MLD(50) of H9N2 virus and prevented the deaths of mice infected with 5 MLD(50) of virus. Thus, GS4104 is efficacious in treating infections caused by H5N1 and H9N2 influenza viruses in mice.