Cells (1 × 103 per well) were seeded into 96-well plates. After overnight incubation, cells were treated with different concentrations of DFMO (0, 0.25, 0.5, 1, and 1.5 mM), and incubated for 24, 48, or 72 h. Then 20 μl Cell Titer 96 Aqueous One Solution (Promega Corporation, Madison, WI) were added and cells incubated for another 1 h. Absorbance was read at 492 nm.