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