Comparative inhibitory effects of various nucleoside and nonnucleoside analogues on replication of influenza virus types A and B in vitro and in ovo.

Six nucleoside analogues, two sulfated polysaccharides, and four protease inhibitors were evaluated in vitro as inhibitors of influenza virus replication. Four guanosine analogues (mizoribine, ribavirin, pyrazofurin, and 5-ethynyl-1-beta-D-ribofuranosylimidazole-4-carboxamide), the sulfated polysaccharide dextran sulfate (molecular weight 500,000), and two protease inhibitors (camostat mesilate and nafamostat mesilate) were inhibitory to the replication of strains of influenza virus types A and B at concentrations down to 0.3 micrograms/mL. Of these seven compounds, ribavirin, camostat mesilate, and nafamostat mesilate were efficacious in both reducing the virus titer and increasing the survival rate of influenza virus-infected chick embryos. For camostat mesilate, the ED50 (required to improve the survival rate of influenza virus-infected chick embryos by 50%) was 0.80 micrograms/g, and its selectivity index, based on the ratio of the 50% toxic dose (required to reduce the viability of chick embryos by 50%) to ED50, was 280. Camostat mesilate deserves further exploration for its potential in the treatment of influenza virus infection.