Comprehensive assessment of 2009 pandemic influenza A (H1N1) virus drug susceptibility in vitro

Background: Antiviral drugs are an important option for managing infections caused by influenza viruses. This study assessed the drug susceptibility of 2009 pandemic influenza A (H1N1) viruses collected globally between April 2009 and January 2010. Methods: Virus isolates were tested for adamantane susceptibility, using pyrosequencing to detect the S31N marker of adamantane resistance in the M2 protein and biological assays to assess viral replication in cell culture. To assess neuraminidase (NA) inhibitor (NAI) susceptibility, virus isolates were tested in chemiluminescent NA inhibition assays and by pyrosequencing to detect the H275Y (H274Y in N2 numbering) marker of oseltamivir resistance in the NA. Results: With the exception of three, all viruses that were tested for adamantane susceptibility (n=3,362) were resistant to this class of drugs. All viruses tested for NAI susceptibility (n=3,359) were sensitive to two US Food and Drug Administration-approved NAIs, oseltamivir (mean +/- SD 50% inhibitory concentration (IC50] 0.25 +/- 0.12 nM) and zanamivir (mean IC50 0.29 +/- 0.09 nM), except 23 (0.7%), which were resistant to oseltamivir, but sensitive to zanamivir. Oseltamivir-resistant viruses had the H275Y mutation in their NA and were detected in patients exposed to the drug through prophylaxis or treatment. NA activity of all viruses was inhibited by the NAls peramivir, laninamivir (R-125489) and A-315675, except for H275Y variants, which exhibited approximately 100-fold reduction in peramivir susceptibility. Conclusions: This report provides data regarding antiviral susceptibility of 2009 pandemic influenza A (H1N1) surveillance viruses, the majority of which were resistant to adamantanes and sensitive to NAls. These findings provide information essential for antiviral resistance monitoring and development of novel diagnostic tests for detecting influenza antiviral resistance.