Combinations of Favipiravir with Doxycycline, Azithromycin or Ivermectin Exert Synergistic Effects Against Influenza A H3N2 Virus Replication

Influenza A viruses constantly threaten the global population, with seasonal outbreaks occurring in different parts of the world, including avian influenza. Severe influenza A virus infections are strongly associated with the cytokine storm, which can contribute significantly to morbidity and even mortality. The virulence and high mutability of these viruses necessitate more effective treatment strategies and regimens to manage patients, especially those with a severe disease. Favipiravir is an antiviral agent approved in Japan for treating influenza virus strains resistant to the current antivirals. The objective of this study is to investigate the combination treatment of Favipiravir paired with selected repurposed drugs to determine the effectiveness of these combinations against influenza A virus replication as well as their effects on cytokine expression. Specific combinations of Favipiravir with Doxycycline, Azithromycin or Ivermectin were identified to be highly synergistic and effective in inhibiting live virus titers of an influenza H3N2 clinical strain by 4 log₁₀. Furthermore, combinations of Favipiravir with Doxycycline or Azithromycin also exhibited immunomodulatory effects on pro-inflammatory cytokines by strongly reducing the relative mRNA expression of <i>IFN-γ</i>, <i>IL-6</i>, <i>TNF-α</i> and <i>IL-1β</i>. Notably, monotherapy with Andrographolide also completely inhibited influenza virus titers by 4 log₁₀. Specific combinations of Favipiravir with Artesunate or Andrographolide revealed additive effects by inhibiting influenza virus titers by about 2 or 1.5 log₁₀, respectively. Our findings indicate that specific drug combinations show promising efficacy and potential in the treatment of influenza and warrant further studies using influenza models of human cell, tissue and animal infection.