Apoptosis induced by DM4 and/or AVE9633 in presence or absence of Zosuquidar in HL60, K562 and their variant P-gp expressing cells. Cells were treated with DM4 or AVE9633 at 20 nM or/and 40 nM in the presence or absence of Zosuquidar (0.3 μM) for 48 h, then stained with Annexin V/propidium iodide for flow cytometry. All experiments were done in triplicate. A: HL60 (grey) and HL60/DNR (black). B: K562 (blank), K562/HHT40 (stripe), K562/HHT90 (grey) and K562/Dox (black).