Electron microscope images of double mesoporous core-shell silica nanospheres (DMCSSs) before (A) and after (B) the venom loading. Cell viability was assessed using the MTT assay. RPMI 8226 cells (C), U266 cells (D) and normal PBMCs (E) were treated overnight with different concentrations (0, 1, 5, 10, 25, 50, 100 and 1000 ng/ml) of NP (open circles), WEV (gray triangles) or WEV+NP (closed black squares). RPMI 8226 cells (F), U266 cells (G) and normal PBMCs (H) were treated with 25 ng/ml of NP (open circles), 25 ng/ml of WEV (gray triangles) or 10 ng/ml of WEV+NP (closed black squares) for different incubation times (0, 1, 2, 6, 12, 24, 36 and 48 h). The data collected from independent experiments (n = 5) are shown, and the results are expressed as the mean percentage of viable cells ± SEM.