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With slim prospects of having an H7N9 influenza vaccine in the near term (see JAMA: Challenges Of Producing An Effective & Timely H7N9 Vaccine) society will have basically two tools to combat H7N9 should it erupt into a pandemic:
- NPIs (Non Pharmaceutical Interventions like social distancing, school closures, hand hygiene & masks)
- neuraminidase (NA) inhibiting antiviral drugs (NAIs) like oseltamivir (Tamiflu ®) and Zanamivir (Relenza ®).
While most of the H7N9 cases reported in China have proven susceptible to these two antiviral drugs, a small but disturbing number of cases have developed resistance while receiving treatment.
Last May, in The Lancet: Antiviral Resistance In Two H7N9 Patients, we looked at a study that found that a mutation R292K (Arginine to Lysine at position 292 in the NA) – also known as Arg292Lys – already known to confer antiviral resistance to seasonal flu (see Resistant influenza A viruses in children treated with oseltamivir: descriptive study), appeared in two patients after several days of oseltamivir therapy.
In the middle of July, in mBio: Antiviral Resistance In H7N9, we saw a study that warned that the standard laboratory tests for antiviral susceptibility can miss `mixed population’ infections (comprised of both resistant and susceptible strains), and that antiviral treatment could suppress the susceptible strains while allowing resistant strains to flourish.
And this particular mutation (R292K) can provide resistance not only to oseltamivir, but to zanamivir and peramivir as well.
While the number of resistant cases identified has remained relatively small, as we saw with the old (pre-2009) seasonal H1N1 virus – resistance can sometimes develop quickly and spread globally (see 2008 CIDRAP NEWS article With H1N1 resistance, CDC changes advice on flu drugs).
Which beings us to an EID Journal study, published this week, that found that (as did the mBio study above) that conventional NI assays may not always reliably detect the R292K substitution.
They believe that the `fitness’ of resistant strains to replicate and spread is less than that of the NAI susceptible `wild type’ H7N9, and in the absence of antiviral exposure they would be unlikely to compete effectively against them.
But when you inhibit the replication of susceptible viruses with antivirals, you create an environment in which resistant strains can flourish.
Not clear at this time is how much of a `fitness’ hit these resistant viruses take. That is, whether they are likely to be a dead-end infection in a host, or can spread efficiently to others.
Also unknown is just how much this R292K mutation actually reduces the effectiveness of antivirals in a clinical setting. Nor what other mutations might exacerbate, or mitigate, this effect.
Follow the link below to read:
Volume 19, Number 9—September 2013
Dispatch
R292K Substitution and Drug Susceptibility of Influenza A(H7N9) Viruses
Katrina Sleeman1, Zhu Guo1, John Barnes, Michael Shaw, James Stevens, and Larisa V. Gubareva
Author affiliations: Centers for Disease Control and Prevention, Atlanta, Georgia, USA
Abstract
Neuraminidase inhibitors are the only licensed antiviral medications available to treat avian influenza A(H7N9) virus infections in humans. According to a neuraminidase inhibition assay, an R292K substitution reduced antiviral efficacy of inhibitors, especially oseltamivir, and decreased viral fitness in cell culture. Monitoring emergence of R292K-carrying viruses using a pH-modified neuraminidase inhibition assay should be considered.
The recent emergence of an avian influenza A(H7N9) virus causing human infections in China (1,2) is of global concern. Most patients infected during this outbreak have experienced severe disease and required hospitalization; the mortality rate is 21% (3). Although epidemiologic investigations have revealed no evidence of sustained human-to-human transmission (4), suspected limited human-to-human transmission has been reported (3).
As with any emergent influenza virus, it is critical to assess the susceptibility of the influenza A(H7N9) outbreak virus to antiviral drugs, which are the first line of defense before an effective vaccine becomes available. Two classes of antiviral drugs are approved for management of influenza A infections, neuraminidase (NA) inhibitors (NAIs) and matrix 2 protein (M2) blockers (adamantanes). The outbreak viruses carry the established adamantane resistance marker, an S31N substitution in the M2 protein (2), leaving NAIs as the only licensed treatment option.
Among the 4 NAIs, oseltamivir and zanamivir are approved in many countries; peramivir has been approved in Japan, South Korea, and China; and laninamivir is approved only in Japan. In contrast to those for adamantanes, genetic markers of resistance to NAIs are often subtype specific and drug specific (5). Therefore, monitoring drug susceptibility of the influenza A(H7N9) viruses requires testing in phenotypic assays using all available NAIs.
<SNIP>
Conclusions
R292 is a highly conserved amino acid across all NA subtypes, and together with 2 other highly conserved residues (R118 and R371), it forms an arginine triad in the enzyme active site (5). R292K is a rare substitution and to date has only been reported in viruses collected from patients treated with oseltamivir (2,5). In addition to A/Shanghai/1/2013 isolate, there is evidence of additional influenza A(H7N9) isolates with the R292K substitution (11).
In this study, propagation of A/Shanghai/1/2013 (E1) isolate in eggs and in MDCK-SIAT1 cells resulted in reversion to wild-type (23% Arg in E1 to 100% in E1/S3), confirming results of previous studies with N2 subtype viruses (12). Therefore, fitness of the A/Shanghai/1/2013 R292K virus is probably compromised when replication occurs in the absence of an NAI. However, propagation of the E1 isolate in the presence of oseltamivir (100 nmol/L) resulted in enrichment of the R292K population (from 77% to 100%), demonstrating a growth advantage over the wild-type.
Replication of the E1 isolate in the presence of any NAI in cell culture might lead to enrichment with R292K, because even a small growth advantage would reduce the proportion of the wild type. The efficacy of NAIs in clinical management of influenza (H7N9) infection remains unknown and may be compromised to a certain extent when R292K is present.
Animal model studies are needed to aid in the understanding of clinical relevance of R292K. Reduction of NA activity caused by R292K may detrimentally affect transmission of the virus, as indicated by an R292K influenza A(H3N2) virus that showed reduced infectivity in mice (13,14) and ferrets (12,13,15) and was not transmitted among ferrets (12,15). The data reported here demonstrate the continued importance of monitoring drug susceptibility in emergent influenza viruses and highlight the challenges involved in laboratory assessment of NAI drug susceptibility testing.
