Influenza A(H7N9) virus gains neuraminidase inhibitor resistance without loss of in vivo virulence or transmissibility

ECDC comment

​Without baseline human immunity to the emergent avian influenza A(H7N9) virus, neuraminidase inhibitors are vital for controlling viral replication in severe infections. An amino acid change in the viral neuraminidase associated with drug resistance, NA-R292K (N2 numbering), has been found in some H7N9 clinical isolates.

By Rong Hai, Mirco Schmolke, Victor H. Leyva-Grado, Rajagowthamee R. Thangavel, Irina Margine, Eric L. Jaffe, Florian Krammer,  Alicia Solórzano,  Adolfo García-Sastre, Peter Palese & Nicole M. BouvierNature Communications Volume:4,Article number:2854 DOI: doi:10.1038/ncomms3854 Published10 December 2013

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

ECDC comment

Since the emergence of the novel A(H7N9) influenza virus in China, the public health experts have been interested in the neuraminidase (NA) susceptibility of these viruses. Of the first three A(H7N9) isolates collected from Chinese patients, one (A/Shanghai/1/2013), was found to encode the NA mutation R292K. Earlier, this mutation had been shown to confer decreased susceptibility to NA inhibitors. Therefore, Hai et al. (2013) studied the characteristics of the A/Shanghai/1/2013 (H7N9) in detail. The researchers assessed the impact of the R292K mutation on the susceptibility of this novel virus to neuraminidase (NA) inhibitors, enzymatic activity, virus replication, virulence and transmissibility. Hai et al. compared the Shanghai/1 strain to Anhui/1, another early A(H7N9) strain. They also generated recombinant viruses based on A/Puerto Rico/8/1934 (H1N1) with both Shanghai and Anhui strain NA molecules.
 
The arginine in position 292 is critical for interaction with sialic acid and with the sialic acid analogues that are used as antiviral drugs. Earlier studies with other influenza viruses have shown that mutations at position 292 impair NA activity and therefore also the replicative activity of the viruses. Compared to the Anhui/1 strain the Shanghai/1 strain demonstrated high resistance to oseltamivir, moderate resistance to peramivir and mild resistance to zanamivir in inhibitory concentration (IC50) testing. Shanghai/1 showed also a reduction in enzymatic activity of NA. Despite of the enzymatic changes and the resistance, the oseltamivir resistance of Shanghai/1did not lead to decreased replication of the virus in human respiratory tract cells or in mouse lungs. In the study by Hai et al. the resistant strain of A(H7N9) showed to be as virulent in mice and as transmissible in guinea pigs as a sensitive strain. There was also no effect on tissue tropism.

The findings of this study are of clinical relevance as a high resistance has been shown to develop without loss of replicative ability. Therefore, the prudent use of NA inhibitors will be critical to ensure correct dose and length of treatment. Intravenous zanamivir maintains the best efficacy against R292K mutation viruses and should be used where available and indicated as treatment.
 
Further transmissibility studies in ferrets are needed although the current study demonstrates that the transmissibility of the Shanghai/1 virus did not show any increase in the guinea pig model. Currently, the study does not change ECDC risk assessment of the H7N9 viruses for Europe as no clusters of oseltamivir resistant viruses have been detected.  The fitness of influenza viruses is dependent on multiple factors, NA activity only being one of those. Influenza viruses also compensate effectively for the reduced sialidase activity and oseltamivir resistance is therefore not always affecting viral replication, virulence and transmissibility, as seen in this study.
 
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