3.5. Synergistic effect of compound 9 on NA inhibitory activity of oseltamivir
The noncompetitive mechanism of these compounds on NA prompted an investigation into the inhibitory effect of the combination of oseltamivir, a known competitive inhibitor, together with compound 9. As a result, the NA inhibitory activity of oseltamivir was increased considerably in the presence of compound 9 (at 1 μg/ml or 3.76 μM) (Fig. 5). The oseltamivir activity was increased on H9N2 and H1N1 with IC50 values from 6.77; 48.57 to 1.58; 18.21 ng/ml, respectively. These results suggest that oseltamivir and compound 9 inhibit synergistically the NA activity by binding of compound 9 to different sites of oseltamivir-bound enzyme.