POL6326 will be given by i.v. infusion over 2 hours. Treatment will occur on days prior to, on the day of and on days after treatment with eribulin. Different doses and dosing frequencies will be investigated

Inclusion Criteria:

• Histologically confirmed invasive cancer of the breast.
• Presence of at least one measurable lesion per RECIST 1.1 criteria
• Stage IV disease by AJCC criteria (7th edition).
• HER2 negative (IHC 0,1 or FISH HER2:CEP17 ratio < 2.0)
• Must have had treatment with at least 2 but no more than 3 previous regimens in the metastatic setting. Previous treatment must have included an anthracycline and taxane in either the adjuvant or metastatic setting.
• At least 21 days from the completion of any previous cytotoxic chemotherapy or biological therapy at time of initiation of POL6326.
• ECOG performance status < 2

Exclusion Criteria:

• Previously received eribulin.
• Peripheral neuropathy > Grade 2.
• Receipt of any other investigational agent within the 28 days prior to Day 1.
• Receipt of colony stimulating factor filgrastim, pegfilgrastim, or sargramostim within 14 days prior to Day 1.
• Radiation therapy within the 14 days prior to Day 1.
• Severe concurrent illness or psychiatric illness/social situation that would limit compliance with study requirements.
• History of other malignancy ≤ 5 years previous with the exception of basal cell or squamous cell carcinoma of the skin which were treated with local resection only or carcinoma in situ of the cervix.
• Pregnant or breastfeeding.
• Known HIV positivity on combination antiretroviral therapy; these patients are at increased risk of lethal infections when treated with marrow-suppressive therapy