A Study to Compare Eribulin Mesylate + Pertuzumab + Trastuzumab With Paclitaxel or Docetaxel + Pertuzumab + Trastuzumab

Inclusion criteria

• Patients with breast cancer that is confirmed histologically or cytologically
• Patients who are confirmed to be HER2 positive for the primary or a metastatic lesion at a participating medical institution
• Patients with no medical history of treatment for advanced/recurrent cancer using a regimen of drugs including chemotherapeutics
• >=6 months have passed since perioperative treatment with anticancer agents
• Presence of a measurable lesion not required
• Female aged 20-70 years old at the time of consent acquisition
• Baseline left ventricular ejection fraction (LVEF) measured by ECHO or MUGA of >=50%
• Eastern Cooperative Oncology Group (ECOG) Performance Status (PS) of 0-1
• Patients who have maintained major organ functions, meeting all of the following criteria on a test within 28 days before enrollment. If there are multiple test results during this period, that obtained immediately before enrollment should be adopted.

(1) Neutrophil count: >=1,500/mm3 (2) Platelet count: >=100,000/mm 3 (3) Hemoglobin: >=9.0 g/dL (4) Total bilirubin: <=1.5 mg/dL (5) AST (GOT), ALT (GPT): <=2.5 times the ULN (<=5 times in those with liver metastasis) (6) Serum creatinine: <=1.5 mg/dL 10) Patients with a life expectancy of at least 6 months 11) Patient who submits written consent herself after receiving sufficient explanation about this study 12) Patients who can undergo QOL investigation

Exclusion criteria

• Patients planning to undergo radical surgery if they respond to a treatment
• Patients who have non-hematological adverse events assessed as Grade >=3 in the Common Terminology Criteria for Adverse Events ver. 4.0 in the Japanese JCOG version (CTCAE v4.0-JCOG) at the time of enrollment
• Patients who have symptomatic metastases to the central nervous system or whose symptoms are hard to control
• Patients who have active double cancer
• Patients who have poorly controlled hypertension, or unstable angina
• Patients who have a past history of congestive heart failure assessed as Class ll or higher in the New York Heart Association (NYHA) classification, or clinically significant arrhythmia requiring treatment
• Patients with a past history of myocardial infarction within 6 months before enrollment
• Patients who are expected to undergo major surgical treatment or who had severe injury within 28 days before enrollment, or who require major surgical treatment during the study treatment period
• Patients with interstitial pneumonia which is symptomatic or requires treatment
• Pregnant women, those with a positive pregnancy test, and lactating women
• Patients with active systemic infection (including HCV and HBV), or who are found to be HIV-positive
• Patients with hypersensitivity against pertuzumab and trastuzumab
• Patients whom the investigator consider unable or unwilling to follow the protocol requirements