OBJECTIVES: Baloxavir marboxil (formerly S-033188) is a first-in-class, orally 
available, cap-dependent endonuclease inhibitor licensed in Japan and the USA 
for the treatment of influenza virus infection. We evaluated the efficacy of 
delayed oral treatment with baloxavir marboxil in combination with a 
neuraminidase inhibitor in a mouse model of lethal influenza virus infection.
METHODS: The inhibitory potency of baloxavir acid (the active form of baloxavir 
marboxil) in combination with neuraminidase inhibitors was tested in vitro. The 
therapeutic effects of baloxavir marboxil and oseltamivir phosphate, or 
combinations thereof, were evaluated in mice lethally infected with influenza 
virus A/PR/8/34; treatments started 96 h post-infection.
RESULTS: Combinations of baloxavir acid and neuraminidase inhibitor exhibited 
synergistic potency against viral replication by means of inhibition of 
cytopathic effects in vitro. In mice, baloxavir marboxil monotherapy (15 or 
50 mg/kg twice daily) significantly and dose-dependently reduced virus titre 
24 h after administration and completely prevented mortality, whereas 
oseltamivir phosphate treatments were not as effective. In this model, a 
suboptimal dose of baloxavir marboxil (0.5 mg/kg twice daily) in combination 
with oseltamivir phosphate provided additional efficacy compared with 
monotherapy in terms of virus-induced mortality, elevation of cytokine/chemokine 
levels and pathological changes in the lung.
CONCLUSIONS: Baloxavir marboxil monotherapy with 96 h-delayed oral dosing 
achieved drastic reductions in virus titre, inflammatory response and mortality 
in a mouse model. Combination treatment with baloxavir acid and oseltamivir acid 
in vitro and baloxavir marboxil and oseltamivir phosphate in mice produced 
synergistic responses against influenza virus infections, suggesting that 
treating humans with the combination may be beneficial.