In vitro evaluation of the antiviral activity of SP‐303, an euphorbiaceae shrub extract, against a panel of respiratory viruses

The antiviral dn cytotoxic activity of SP‐303, a naturally occurring polyphenolic polymer (average M.W. = 2,100 daltons) isolated from a Euphorbiaceae shrub, was evaluated in tissue culture. The compound exhibited selective antiviral activity (ratio IC 50 /EC 50 ≥10) against parainfluenza virus type 1, respiratory syncytial virus, influenza A viruses, and influenza B viruses; marginal selective antiviral activity (IC 50 /EC 50 ratio >2 to <10) against parinfluenza virus type 3; and no selective antiviral activity (IC 50 /EC 50 ratio ≤1) against measles virus or adenovirus type 5. Hemagglutination and other studies suggested that SP‐303 may at least partially inactivate virus by direct interaction with virus or host cell lipid membranes.