Docetaxel and interferon-beta (IFN-beta) were tested alone and in combination for their antiproliferative activity against the estradiol receptor (ER) positive MCF-7 and the ER negative MDA-MB231 breast cancer cell lines. Cell growth inhibition was determined after a 3 day incubation by the sulforhodamine B assay. The antiproliferative effects of the drug combinations were analysed using Berenbaum's hyperplane theorem to determine additive, synergistic and antagonistic effects. Docetaxel was found to be equally effective in inhibiting cell growth of both cell lines. On the other hand MCF-7 cells were more sensitive to the antiproliferative activity of IFN-beta than MDA-MB231 cells. At low docetaxel:IFN-beta molar concentration ratios a synergistic interaction was observed for MCF-7 cells, whereas an additive interaction was found for MDA-TMB231 cells. Higher molar ratios resulted in additive interactions on MCF-7 and antagonistic interactions on MDA-MB231 cells. MCF-7 cells seem to be more sensitive to treatment by the combination of docetaxel and IFN-beta than the MDA-MB231 cells. Therefore an ER positive breast carcinoma may possibly profit by the combination of docetaxel and IFN-beta, but further studies are necessary to clarify the therapeutic usefulness and optimal scheduling of the drug combination.

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