Classic isobologram at IC50: rapamycin plus CN-A in MCF-7 cells. Isoboles for the combination of rapamycin with CN-A that were isoeffective (IC50) for inhibition of proliferation of MCF-7 cells. The dashed line indicates the zero interaction of the isobole. Cells (3 × 104 cells/ml) were treated with rapamycin and CN-A for 5 days. Values are the means of three separate experiments. CN-A, cotylenin A; IC50, the concentration of the drug required for 50 % inhibition of cell growth.