Effect of PSP, CPT and PSP+CPT treatments on HL-60 apoptosis. HL-60 cells were treated with PSP (25 μg/ml) for 72 hour before CPT (1 μM) was added. CPT was added at 72 hours and samples were collected after five hours of incubation. Cells were identified and quantified using annexin V/PI flow cytometric assay. R3 represents viable cells (PI-ve, annexin V-ve); R4 represents early apoptotic cells (PI-ve, annexin V+ve) and R2 represents late apoptotic cells (PI+ve, annexin V+ve). Values are expressed as means (SD) (n = 4). * P < 0.001, vs. CPT.