Stimulation of murine interferon by a substituted pyrimidine.

2-Amino-5-bromo-6-methyl-4-pyrimidinol (U-25,166) induced high levels of circulating interferon in mice when administered either parenterally or orally. Peak titers of interferon were found in the serum between 6 and 12 h after inoculation of the drug. Lower but significant levels of interferon were found in rat serum after administration of U-25,166 by either the intraperitoneal or oral route, and good levels of circulating interferon were observed in cats after oral treatment. Repeated intraperitoneal doses (50 mg/kg) of U-25,166 protected mice against intranasal encephalomyocarditis virus challenge. The minimal effective acute oral dose for antiviral activity was approximately 250 mg/kg. This was also the minimal dose that produced detectable levels of interferon. Maximum tolerated doses in mice were four to six times the minimal effective doses. A single oral treatment was protective in mice against challenge virus inoculated 24 h later. The compound protected mice from challenge with high levels of encephalomyocarditis virus, up to 20,000 mean lethal doses. Antiviral activity in mice was retained when certain minor substitutions were made in the U-25,166 structure.