Three leukemic cell lines show different sensitivity to NSC606985-induced apoptosis. (A) NB4, U937, and K562 cells were treated with 50 nM NSC606985 for 24 and 48 hours. Growth-inhibiting rate (top) and cell viability (bottom) were determined. The values are the mean + SD of a triplicate experiment. (B) K562 cells were incubated with the indicated concentrations of NSC606985 for 24 and 48 hours, and the growth inhibition rates were determined. The values are the mean + SD of a triplicate experiment. (C) U937 (top row) and K562 cells (bottom row) were treated with or without 50 nM and 100 nM NSC606985, respectively, for 48 hours. Cytomorphology was examined with Wright staining. Images were viewed using an Olympus BX60 microscope equipped with a 100 ×/1.3 objective lens, and were acquired through a SPOT RT camera and SPOT software. For K562 cells, the treatment with etoposide at 100 μM for 24 hours was used as positive control. (D) K562 cells were incubated with the indicated concentrations (nM) of NSC606985 or with 100 μM etoposide for 48 hours. DNA samples from these treated cells were electrophoresed in a 2% agarose gel. (E) NB4 and U937 cells were treated with NSC606985 at the indicated concentrations for 48 hours, and DNA samples from the cells were electrophoresed in a 2% agarose gel. All experiments were repeated 4 times with similar results.