Description: IC50 Value: Pirodavir inhibits 80 percentage of viruses at 0.064 micrograms/ml(EC80)[2]. pirodavir was one of the most promising capsid-binding compounds to show efficacy in human clinical trials for chemoprophylaxis of the common cold. Susceptibility to hydrolysis precluded its use as an oral agent [1]. in vitro: R-77975's predecessor, R 61837, a substituted phenyl-pyridazinamine, was effective in inhibiting 80% of 100 serotypes tested (EC80) at concentrations above 32 micrograms/ml, pirodavir inhibits the same percentage of viruses at 0.064 micrograms/ml. Pirodavir is also effective in inhibiting 16 enteroviruses, with an EC80 of 1.3 micrograms/ml. Pirodavir acts at an early stage of the viral replication cycle (up to 40 min after infection) and reduces the yield of selected rhinoviruses 1,000- to 100,000-fold in a single round of replication [2]. in vivo: Adults with symptoms of < or = 2 days' duration were randomly assigned to intranasal spr