Effect of topoisomerase inhibitors on mammosphere-derived cells for 24 h. MCF7 cells were grown in non-adherent conditions. After 7 days mammospheres were dissociated for single cells and plated in 96-well plates at a density of 5,000 cells/well, in the presence of 1% serum. Cells were treated with various concentrations of etoposide (A), CPT (B) or TPT (C) for 24 h. Cell viability was examined by the neutral red cytotoxicity assay and growth inhibition was calculated, compared to the control of each cell type. Results represent means ± SE of 3 experiments. Statistical significance was determined by the student’s t-test analysis: *p < 0.05; **p < 0.01.