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Few anti-influenza drugs are licensed in the United States for the prevention and therapy of influenza A and B virus infections. This shortage, coupled with continuously emerging drug resistance, as detected through a global surveillance network, seriously limits our anti-influenza armamentarium. Combination therapy appears to offer several advantages over traditional monotherapy in not only delaying development of resistance but also potentially enhancing single antiviral activity. In the present study, we evaluated the antiviral drug susceptibilities of fourteen pandemic influenza A (H1N1) virus isolates in MDCK cells. In addition, we evaluated favipiravir (T-705), an investigational drug with a broad antiviral spectrum and a unique mode of action, alone and in dual combination with the neuraminidase inhibitors (NAIs) oseltamivir, peramivir, or zanamivir, against oseltamivir-sensitive pandemic influenza A/California/07/2009 (H1N1) and oseltamivir-resistant A/Hong Kong/2369/2009 (H1N1) virus. Mean inhibitory values showed that the tested virus isolates remained sensitive to commonly used antiviral drugs, with the exception of the Hong Kong virus isolate. Drug dose-response curves confirmed complete drug resistance to oseltamivir, partial sensitivity to peramivir, and retained susceptibility to zanamivir and favipiravir against the A/Hong Kong/2369/2009 virus. Three-dimensional analysis of drug interactions using the MacSynergy II program indicated an overall synergistic interaction when favipiravir was combined with the NAIs against the oseltamivir-sensitive influenza virus, and an additive effect against the oseltamivir-resistant virus. Although the clinical relevance of these drug combinations remains to be evaluated, results obtained from this study support the use of combination therapy with favipiravir and NAIs for treatment of human influenza virus infections. [ABSTRACT FROM AUTHOR]