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