The purpose of this study is to evaluate survival, response rate, safety and tolerability of YM155 given in combination with docetaxel as first-line treatment in subjects with human epidermal growth factor 2 non-overexpressing (HER2 negative) metastatic breast cancer.

This is an outpatient study. All subjects enrolled in this study will receive a combined regimen of YM155 and docetaxel or docetaxel alone given during 21 day cycles. Each subject will be assessed at the end of each cycle to determine if the subject can continue to the next cycle. Each subject assigned to receive YM155 in combination with docetaxel will be eligible to continue receiving the combination regimen in this study until one of the discontinuation criteria is met.

If a subject discontinues treatment with at least stable disease (SD) that subject will complete follow-up visits every 12 weeks for 2 years or until initiating another systemic anti-breast cancer treatment, exhibiting progressive disease (PD), or death.

Each subject will be contacted by the study site every 12 weeks for survival following the End of Treatment Visit. The contacts will continue until death or for no more than 2 years.

Inclusion Criteria:

• Histologically- or cytologically-proven adenocarcinoma of the breast that is HER2 negative. Subjects with hormone receptor positive or negative status are eligible. Additionally, subjects with triple negative status (meaning estrogen receptor negative, progesterone receptor negative and HER2 negative) are eligible
• No prior chemotherapy regimen for metastatic breast cancer
• Eastern Cooperative Oncology Group (ECOG) performance status ≤ 1 at the Baseline Visit
• The subject's life expectancy is estimated to be > 12 weeks at the Baseline Visit
• The subject must be non-pregnant and non-lactating. All sexually active subjects of childbearing potential must agree to use an adequate method of contraception throughout the study period

Exclusion Criteria:

• Hypersensitivity to docetaxel or polysorbate 80
• Neuropathy ≥ Grade 2 at the Baseline Visit
• Known brain or leptomeningeal metastasis as assessed through medical history review and physical examination
• The subject has known Human Immunodeficiency Virus (HIV), Hepatitis B surface Antigen or hepatitis C antibody