Zanamivir is renally excreted as unchanged drug. No metabolites have been detected in humans. The serum half- life of zanamivir following administration by oral inhalation ranges from 2.5 to 5.1 hours. It is excreted unchanged in the urine with excretion of a single dose completed within 24 hours.
Unabsorbed drug is excreted in the feces.
Safety and efficacy have not been documented in the presence of severe renal insufficiency. There were no significant correlations between measures of systemic exposure and safety parameters. The proposed mechanism of action of zanamivir is via inhibition of influenza virus neuraminidase with the possibility of alteration of virus particle aggregation and release. The concentrations of zanamivir required for inhibition of influenza virus were highly variable depending on the assay method used and virus isolate tested. The relationship between the in vitro inhibition of influenza virus by zanamivir and the inhibition of influenza virus replication in humans has not been established.