Effect of pristine MSNs, RES-loaded MSNs (0–700 μg/mL), and bulk RES (0–700 μM) on cell viability of MNT-1 and A375 melanoma cells by the 3-(4,5 dimethyl-2-thiazolyl)-2,5-diphenyl tetrazolium bromide (MTT) assay at 24, 48, and 72 h. Results are expressed as mean ± SD. * indicates significant differences between control at p < 0.05 for 24 h; # indicates significant differences between control at p < 0.05 for 48 h; and + indicates significant differences between control at p < 0.05 for 72 h. (a) MNT-1 cells exposed to RES-loaded MSNs; (b) A375 cells exposed to RES-loaded MSNs; (c) MNT-1 cells exposed to pristine MSNs; (d) A375 cells exposed to pristine MSNs; (e) MNT-1 cells exposed to bulk RES and (f) A375 cells exposed to bulk RES.