Activities of oseltamivir and ribavirin used alone and in combination against infections in mice with recent isolates of influenza A (H1N1) and B viruses.

Mouse models have been widely used for evaluating potential influenza virus inhibitors. However, the viral strains traditionally used in these models are fairly old and do not represent currently circulating viruses in nature. We developed two new lethal infection models in mice using mouse-adapted influenza A/New Caledonia/20/99 (H1N1) and influenza B/Sichuan/379/99 viruses. Both virus infections were used to study oral treatment with oseltamivir and ribavirin, both alone and in combination. Oral treatments were given twice daily for 5 days starting 4 h before infection in initial studies. Against influenza A, oseltamivir was active at 10, 20, and 40 mg/kg/day, protected 80-100% of mice from death and reduced lung consolidation - ribavirin was similarly effective at 20, 40, and 80 mg/kg/day. When treatments were initiated after virus challenge, delaying treatment with oseltamivir even 1 day caused it to be ineffective. Ribavirin prevented mortality by 50-80% when treatments were delayed 1-4 days after infection. The combination of the two drugs (oseltamivir at 20 mg/kg/day and ribavirin at 40 mg/kg/day) was no better than ribavirin alone. In contrast to what we observed with influenza A virus infections, oseltamivir and ribavirin showed similar dose-related antiviral activities against influenza B virus infections. The compounds both significantly increased survival when treatments started up to 4 days after infection, but ribavirin was more active than oseltamivir (50-80% survival compared to 30-40% survival, respectively, when starting treatments on days 2-4 after infection). By varying the doses of each drug that were used in combination (oseltamivir at 1.25, 2.5 and 5 mg/kg/day; ribavirin at 5, 10 and 20 mg/kg/day) certain dosage combinations were superior to either compound used alone as assessed by decreased mortality, lung virus titre, lung score and lung weight parameters. These activities differed from published results with older, more established virus strains as oseltamivir was less effective and ribavirin was more active than previously reported.