Antiviral activities and selectivity indices of anti-<i>orthopoxvirus</i> compounds against cowpox (CPXV) strains.

aViral strains tested. bFor full chemical names, see Materials and Methods. cEC50 values or 50% effective concentrations are given as the mean ±SD of at least three independent experiments, and were defined as the concentration of compound required to reduce viral cytopathic effect by 50%. Briefly, confluent HEL cells were infected at a multiplicity of infection of 0.01 PFU/cell for 2 h after which residual virus was removed and replaced with fresh medium containing serial dilutions of the test compounds (in duplicate). After 2 to 4 days, viral cytopathic effect was recorded and EC50 values were determined. dSI or selectivity index is the ratio CC50/EC50. CC50 or 50% cytostatic concentration was defined as the concentration required to reduce cell growth by 50%. CC50 values were of 301±171 µM for cidofovir, 108±72 µM for (S)-HPMP-5-azaC, ≥2.1±1.6 µM for CMX001 and ≥287±176 µM for ST-246.