Optimizing T-705 (favipiravir) treatment of severe influenza B virus infection in the immunocompromised mouse model.

Background: Influenza B virus infections remain insufficiently studied and antiviral management in immunocompromised patients is not well defined. The treatment regimens for these high-risk patients, which have elevated risk of severe disease-associated complications, require optimization and can be partly addressed via animal models.

Methods: We examined the efficacy of monotherapy with the RNA-dependent RNA polymerase inhibitor T-705 (favipiravir) in protecting genetically modified, permanently immunocompromised BALB scid mice against lethal infection with B/Brisbane/60/2008 (BR/08) virus. Beginning at 24 h post-infection, BALB scid mice received oral T-705 twice daily (10, 50 or 250 mg/kg/day) for 5 or 10 days.

Results: T-705 had a dose-dependent effect on survival after BR/08 challenge, resulting in 100% protection at the highest dosages. With the 5 day regimens, dosages of 50 or 250 mg/kg/day reduced the peak lung viral titres within the treatment window, but could not efficiently clear the virus after completion of treatment. With the 10 day regimens, dosages of 50 or 250 mg/kg/day significantly suppressed virus replication in the lungs, particularly at 45 days post-infection, limiting viral spread and pulmonary pathology. No T-705 regimen decreased virus growth in the nasal turbinates of mice, which potentially contributed to the viral dynamics in the lungs. The susceptibility of influenza B viruses isolated from T-705-treated mice remained comparable to that of viruses from untreated control animals.

Conclusions: T-705 treatment is efficacious against lethal challenge with BR/08 virus in immunocompromised mice. The antiviral benefit was greatest when longer T-705 treatment was combined with higher dosages.