Background: RFI-641, a small dendrimer-like compound, is a potent and selective inhibitor of respiratory syncytial virus (RSV), which is currently a clinical candidate for the treatment of upper and lower respiratory tract infections caused by RSV. RFI-641 inhibits RSV growth with an IC₅₀ value of 50 nM and prevents syncytia formation in tissue culture. RSV contains of three surface glycoproteins, a small hydrophobic (SH) protein of unknown function, and attachment (G) and fusion (F) proteins that enable binding and fusion of virus, respectively, with target cells. Because of their role in attachment and fusion, the G and F surface proteins are prominent targets for therapeutic intervention. RFI-641 was previously shown to bind purified preparations of RSV fusion protein. Based on this observation, in conjunction with the biological results, it was speculated that the fusion event might be the target of these inhibitors.

Results: A fusion assay based upon the relief of self-quenching of octadecyl rhodamine R18 was used to determine effects of the inhibitors on binding and fusion of RSV. The results show that RFI-641 inhibits both RSV–cell binding and fusion events. The inhibition of RSV is mediated via binding to the fusion protein on the viral surface. A closely related analog, WAY-158830, which is much less active in the virus-infectivity assay does not inhibit binding and fusion of RSV with Vero cells.

Conclusions: RFI-641, an in vivo active RSV inhibitor, is shown to inhibit both binding and fusion of RSV with cells, events that are early committed steps in RSV entry and pathogenicity. The results described here demonstrate that a non-peptidic, small molecule can inhibit binding and fusion of enveloped virus specifically via interaction with the viral fusion protein.