Sequence dependent exposure of mammary carcinoma cells to Taxotere and the MEK1/2 inhibitor U0126 causes enhanced cell killing in vitro.

Taxol (paclitaxel) and Taxotere (docetaxel) are considered as two of the most important anti-cancer chemotherapy drugs. The cytotoxic action of these drugs has been linked to their ability to inhibit microtubule depolymerization, causing growth arrest and subsequent cell death. Studies by a number of laboratories have also linked suppression of mitogen activated protein kinase (MAPK) signaling to enhanced Taxol toxicity. The present study examined the interactions of the semi-synthetic taxane Taxotere with MEK1/2 inhibitors in epithelial tumor cells. Concurrent treatment of MDA-MB-231 mammary and DU145 prostate carcinoma cells with Taxotere and MEK1/2 inhibitor resulted in protection from the anti-proliferative effects of Taxotere in MTT assays. In contrast, in MCF-7 mammary cells, concurrent Taxotere and MEK1/2 inhibitor treatment weakly enhanced the anti-proliferative effects of the taxane. Sequential treatment of MDA-MB-231 and MCF-7 cells with Taxotere followed by MEK1/2 inhibitor also enhanced the anti-proliferative effects of the taxane in MTT assays. However, no enhancement was observed in DU145 or PC-3 cells. Colony formation assays, including isobologram analyses, provided a more definitive demonstration that MCF-7 and MDA-MB-231 cells were sensitized to the toxic effects of Taxotere by U0126. Similar data were observed using Laulimalide, which binds to tubulin at a different site to Taxotere. The enhancement in Taxotere anti-proliferative effects by U0126 correlated with increased cell killing, 48-72h after treatment of cells that was blocked by inhibition of caspase 9, but not caspase 8, function. This observation was associated with prolonged suppression of ERK1/2 and AKT activity, without alteration in either p38 or JNK1/2 activity. Collectively these findings demonstrate that sequential administration of Taxotere followed by MEK1/2 inhibition can lead to increased cell death and loss of reproductive capacity in some, but not all, human tumor cells.