In vitro neuraminidase inhibitory activities of four neuraminidase inhibitors against influenza viruses isolated in the 2010–2011 season in Japan

The half maximal inhibitory concentration (IC50) of four neuraminidase inhibitors (NAIs), oseltamivir, zanamivir, laninamivir, and peramivir; was measured using influenza viruses isolated in the 2010–2011 influenza season in Japan. Clinical samples for viral isolation were obtained from nasal aspiration, nasopharyngeal swab, or self-blown nasal discharge and cultured with Madin–Darby canine kidney cells. The type and subtype of H3N2 or B were determined by reverse transcriptase polymerase chain reaction (RT-PCR). For the A(H1N1)pdm09 virus, the subtype was determined by real-time RT-PCR. IC50s to oseltamivir carboxylate, zanamivir, laninamivir, and peramivir were determined by a fluorescence-based neuraminidase inhibition assay. Influenza viruses were isolated from 269 patients. A(H1N1)pdm09, H3N2, and B were isolated from 185, 54, and 30 patients, respectively. The geometric means of IC50 for oseltamivir were 0.86 and 0.73 nM to A (H1N1) pdm09, except for the two outlier viruses described below and H3N2, respectively, and 33.12 nM for B. The geometric means of IC50 for the other three NAIs were lowest to A(H1N1)pdm09 and highest to B. Two A(H1N1)pdm09 isolates showed very high IC50 values for oseltamivir (840 and 600 nM) and peramivir (19 and 24 nM). No isolate showed significantly high IC50 values for zanamivir or laninamivir. Continuous surveillance against the emergence or spread of influenza virus with high IC50 values for anti-influenza drugs is important.