The anti-influenza virus ahent 4-GU-DANA (Zanamivir) inhibits cell fusion mediated by human parainfluenza virus and influenza virus HA

4-GU-DANA (zanamivir) (as well as DANA and 4-AM-DANA) was found to inhibit the neuraminidase activity of human parainfluenza virus type 3 (HPF3). The viral neuraminidase activity is attributable to hemagglutinin-neuraminidase (HN), an envelope protein essential for viral attachment and for fusion me diated by the other envelope protein, F. While there is no evidence that HN 's neuraminidase activity is essential for receptor binding and syncytium f ormation, we found that 4-GU-DANA prevented hemadsorption and fusion of per sistently infected cells with uninfected cells. In plaque assays, 4-GU-DANA reduced the number (but not the area) of plaques if present only during th e adsorption period and reduced plaque area (but not number) if added only after the 90-min adsorption period. 4-GU-DANA also reduced the area of plaq ues formed by a neuraminidase-deficient variant, confirming that its interf erence with cell-cell fusion is unrelated to inhibition of neuraminidase ac tivity. The order-of-magnitude lower 50% inhibitory concentrations of 4-GU- DANA (and also DANA and 4-AM-DANA) for plaque area reduction and for inhibi tion in the fusion assay than for reducing plaque number or blocking hemads orption indicate the particular efficacy of these sialic acid analogs in in terfering with cell-cell fusion. In cell lines expressing influenza virus h emagglutinin (RA) as the only viral protein, we found that 4-GU-DANA had no effect on hemadsorption but did inhibit HA2b-red blood cell fusion, as jud ged by both lipid mixing and content mixing. Thus, 4-GU-DANA can interfere with both influenza virus- and HPF3-mediated fusion. The results indicate t hat (i) in HPF3, 4-GU-DANA and its analogs have an affinity not only for th e neuraminidase active site of HN but also for sites important for receptor binding and cell fusion and (ii) sialic acid-based inhibitors of influenza virus neuraminidase can also exert a direct, negative effect on the fusoge nic function of the other envelope protein, HA.