C-Methylated Flavonoids from <i>Cleistocalyx operculatus</i> and Their Inhibitory Effects on Novel Influenza A (H1N1) Neuraminidase

As part of an ongoing study focused on the discovery of anti-influenza agents from plants, four new (<b>1</b>−<b>4</b>) and 10 known (<b>5</b>−<b>14</b>) C-methylated flavonoids were isolated from a methanol extract of <i>Cleistocalyx operculatus</i> buds using an influenza H1N1 neuraminidase inhibition assay. Compounds <b>4</b>, <b>7</b>, <b>8</b>, and <b>14</b>, with a chalcone skeleton, showed significant inhibitory effects on the viral neuraminidases from two influenza viral strains, H1N1 and H9N2. Compound <b>4</b> showed the strongest inhibitory activity against the neuraminidases from novel influenza H1N1 (WT) and oseltamivir-resistant novel H1N1 (H274Y mutant) expressed in 293T cells with IC<sub>50</sub> values of 8.15 ± 1.05 and 3.31 ± 1.34 μM, respectively. Compounds <b>4</b>, <b>7</b>, <b>8</b>, and <b>14</b> behaved as noncompetitive inhibitors in the kinetic studies. These results indicate that C-methylated flavonoids from <i>C. operculatus</i> have the potential to be developed as neuraminidase inhibitors for novel influenza H1N1.