4.3. Cell Viability Assay

Cell viability was measured by the MTT cytotoxicity assay [33]. A549 and NCI-H460 cells were seeded into 96-well plates at a density of 5000–10,000 cells per well, with three replicate wells per group. When the cells of each well reached 70–80% confluent, the indicated concentrations of arenobufagin were added. After treatment for 24 and 48 h, the cells were incubated with 10 μL of MTT (5 mg/mL) for 2–4 h at 37 °C. Then, the medium was discarded and 150 μL of DMSO was added to each well to dissolve formazan for measurement. The optical density (OD) was measured at an absorbance wavelength of 490 nm, using a microplate reader (Thermo Fisher, Waltham, MA, USA).