The main purpose of this study is to determine if AZD8931 can improve the efficacy of standard chemotherapy for the treatment of advanced breast cancer. This study will be conducted in 2 parts: the first part (phase I) will determine a dose of AZD8931 that can be safely administered with paclitaxel chemotherapy. The second part (phase II) will determine the efficacy and safety of AZD8931 in combination with paclitaxel chemotherapy in breast cancer.