2.2. Cell Cytotoxicity Assay

The cytotoxicity effect of resistomycin on cancer cells and normal cells was assessed by MTT (3-(4,5-dimethylthiazol)-2,5-diphenyltetrazolium bromide) assay (Sigma-Aldrich Corp., St. Louis, MO, USA). Cells were treated with various concentrations of resistomycin in the absence or presence of 2.5 or 5 μmol/L SB203580 (Beyotime Institute of Biotechnology, Shanghai, China) pretreatment for 1 h. The negative control was treated with the same volumes of DMSO. MTT was added to each well of the plate after 24–72 h of continuous culture. About 3.5–4 h later the medium was sucked away and 150 μL of DMSO was added to each well. Then, the absorbance in the plate was measured at 570 nm and the half-maximal inhibitory concentration (IC₅₀) values were calculated from the plotted results, using the negative control as 100% cell survival.