Virus Susceptibility Analyses from a Phase IV Clinical Trial of Inhaled Zanamivir Treatment in Children Infected with Influenza

A zanamivir postapproval efficacy study was conducted in children (n = 279) in Japan during three influenza seasons. Pharyngeal swab specimens (n = 714) were obtained for detailed resistance analysis. From 371 cultured viruses, 3 viruses (A/H1N1) from two subjects showed reduced susceptibility to zanamivir at day 1 (before treatment), 1 had an N74S amino acid substitution (fold shift, 46), and 2 (day 1 and day 2) had a Q136K amino acid substitution (fold shifts, 292 and 301). Q136K was detected only in cultured virus and not in the swab. From the remaining 118 cultured viruses obtained during or after treatment with zanamivir, no shifts in virus susceptibility were detected. Neuraminidase (NA) population sequencing showed that viruses from 12 subjects had emergent amino acid substitutions, but 3 with susceptibility data were not zanamivir resistant. The remainder may be natural variants. Further analysis is planned. Hemagglutinin (HA) sequencing showed that viruses from 20 subjects had 9 HA amino acid substitutions that were previously implicated in resistance to neuraminidase inhibitors in in vitro assays or that were close to the receptor binding site. Their role in in vivo resistance appears to be less important but is not well understood. NA clonal sequence analysis was undertaken to determine if minority species of resistant viruses were present. A total of 1,682 clones from 90 subjects were analyzed. Single clones from 12 subjects contained amino acid substitutions close to the NA active site. It is unclear whether these single amino acid substitutions could have been amplified after drug pressure or are just chance mutations introduced during PCR.