The combinatorial activities of oseltamivir and molnupiravir against influenza virus infections in vitro and in vivo.

Oseltamivir, a neuraminidase inhibitor, is widely used in the clinic for treating influenza virus infections. However, suboptimal efficacy and risk of drug resistance development remain major challenges. Molnupiravir, a ribonucleoside analog, was originally developed to treat influenza, but was repurposed and first approved for treating COVID-19 in 2021. Considering their complementary mode-of-actions, this study aimed to investigate the combinatorial activities of oseltamivir and molnupiravir against influenza virus infections. In cell culture models, we found that β-d-N4-hydroxycytidine (NHC), the active form of molnupiravir, exerted more potent antiviral activities against influenza A and B viruses, when compared to oseltamivir treatment. Combination of NHC with oseltamivir exhibited a synergistic antiviral effect against the influenza A/Puerto Rico/8/34 H1N1 strain, but not the influenza B/Washington/02/2019 strain. In a mouse model infected with the PR/8 virus strain, treatment with molnupiravir alone or in combination with oseltamivir effectively attenuated lung injury and reduced viral load in the tissue. Taken together, molnupiravir can be explored in combination with oseltamivir to treat influenza, especially for patients infected with the oseltamivir-resistant strains, whereas further research is warranted.