Zanamivir is a dihydropyran-based, sialic acid analog inhibitor of the viral enzyme neuraminidase. Zanamivir's mode of action is blockage of sialic acid cleavage, thereby preventing further virus distribution. For type A viruses zanamivir has IC50s of 0.35 nM for the H1N1 subtype, and 1.1 nM for the H3N2 subtype. [1]

In plaque reduction in vitro assays, zanamivir inhibits a wide range of Type A and Type B influenza viruses with IC50s of plaque formation ranging from 5 to 14 nM in laboratory-passaged strains, and 20 to 1600 nM for clinical isolates. In human respiratory epithelial cells, EC90s of virus yields were determined to be less than 0.01 ug/mL at 24h for both H1N1 and H3N2 subtypes. [2]