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Background S-033447, an active form of orally available prodrug S-033188, is a novel small molecule inhibitor of cap-dependent endonuclease that is essential for influenza virus transcription and replication. In this study, we evaluated the inhibitory effect of S-033188 in combination with neuraminidase inhibitors on the replication of influenza A/H1N1 virus in cultured cells. Methods The inhibitory effects of S-033447 in combination with NA inhibitors on the cytopathic effect of A/PR/8/34 strain in Madin–Darby canine kidney cells cultured for 2 days were tested and EC50 were determined. The combination index (CI), which were obtained when S-033188 and NA inhibitor were added at the closest ratio of each EC50 value, were used for the evaluation of these combinational effects (Table 1). CI values were calculated by the Chou and Talalay method, in which combinational effect were determined according to the criteria as follows: synergistic if CI ≤ 0.8, additive if 0.8 < CI < 1.2, and antagonistic if CI ≥ 1.2. CI = (DA/A + B)/DA + (DB/A + B)/DB + (DA/A + B × DB/A + B)/(DA × DB) DA: the EC50 of S-033447 DB: the EC50 of NA inhibitor DA/A + B: the concentration of S-033447 giving 50% inhibition in combination with NA inhibitor at the closest ratio of each EC50 value DB/A + B: the concentration of NA inhibitor giving 50% inhibition in combination with S-033447 at the closest ratio of each EC50 value Results All CI values were lower than 0.8, under the condition that both S-033447 and NA inhibitor (oseltamivir acid, zanamivir hydrate, laninamivir, or peramivir trihydrate) were added at the closest ratio of each EC50 value (Table 1). Conclusion S-033447 in combination with oseltamivir acid, zanamivir hydrate, laninamivir, or peramivir trihydrate synergistically inhibited the replication of influenza A/H1N1 virus in MDCK cells. Table 1. Combination effect of S-033447 and NA inhibitor in MDCK cells infected with A/PR/8/34 strain Substance A Substance B DA (nmol/L) DB (nmol/L) DA/A+B (nmol/L) DB/A+B (nmol/L) CI Combination effect S-033447 oseltamivir acid 4.51 3171.97 1.17 586.94 0.49 synergistic S-033447 zanamivir hydrate 4.49 1565.38 1.22 305.99 0.52 synergistic S-033447 laninamivir 4.52 212.74 1.12 56.02 0.58 synergistic S-033447 peramivir trihydrate 4.41 213.77 1.13 56.66 0.59 synergistic Disclosures All authors: No reported disclosures.

open forum infectious diseases

Synergistic Antiviral Activity of S-033188/S-033447, a Novel Inhibitor of Influenza Virus Cap-Dependent Endonuclease, in Combination with Neuraminidase Inhibitors In Vitro