MODE OF INHIBITION OF RESPIRATOR SYNCYTIAL VIRUS BY A PLANT FLAVONOID, SP-303

A natural-product polyphenolic polymer of molecular weight 2,100 dalto ns designated SP-303, was found to have antiviral activity against res piratory syncytial virus (RSV) in the 2-10 muM range in plaque reducti on assays and cytopathic-effect-inhibition assays. The material was al so virucidal. The 50% effective concentration (EC50) for virucidal act ivity was 28 muM. Experiments were done to determine the mode(s) of RS V inhibition by SP-303. Interferon was not induced. SP-303 did not inh ibit attachment at antiviral concentrations. However, the EC50 for inh ibition of virus penetration by SP-303 was 0.48 +/- 0.19 muM. These da ta suggest that abolition of RSV penetration into host cells is one me chanism whereby SP-303 inhibits RSV replication.