Paclitaxel/Taxol prevents microtubule disassembly, resulting in cell cycle arrest and apoptosis. It is used clinically as a frontline therapy for ovarian tumors and has shown increasing efficacy in the treatment of breast cancer. The most encouraging data were obtained with a combination therapy using paclitaxel and anti-Her2/neu antibody. However, improvements in combination therapy with paclitaxel/Taxol for the treatment of breast cancer are still needed. Our past effort has focused on the effects of paclitaxel on gene expression signaling and apoptosis in epithelial cancer cells. We first found that paclitaxel induces the JNK/SAPK kinase system in breast carcinomas, and this activation is involved in paclitaxel-induced apoptosis. In contrast paclitaxel also activates MEK1 kinase leading to an elevation of ERK1 and ERK2 activity. MEK1 members are known to be critical for cell growth and proliferation in a number of systems. By using a combination of MEK inhibitors and paclitaxel, we were able to obtain greatly enhanced tumor killing by using suboptimal levels of the two drugs. Apoptosis in the presence of both drugs is five to tenfold more than with each drug alone. Additionally, we have also initiated an analysis of a derivative of epothilone B, dEpoB, which has similar effects as paclitaxel. We found that dEpoB also exhibits enhanced tumor killing activity in the presence of MEK inhibitor. DEpoB has the advantage over paclitaxel in that it is not transported by the multi-drug resistant transporter system, which allows many tumors to acquire chemotherapy-resistance. Thus, dEpoB may be effective where paclitaxel is not. Used in a clinical setting, these combinations may lower toxicity and enhance tumor killing. Accordingly, the four aims of this project are: (1) To determine if paclitaxel and an MEK inhibitor can cause enhanced tumor elimination in a preclinical model. (2) To similarly study the combined effects of dEpoB and an MEK inhibitor. (3) To use Affymetrix analysis and microchip arrays to identify genes that are altered by the use of these drug combinations. (4) In the event that Specific Aim (1) and/or Specific Aim (2) show efficacy in the preclinical models, we propose to initiate a clinical trial to test the effects of combination therapy studied in Specific Aims 1 and/or 2.