This is a Phase 2, multicenter, single-arm, feasibility study evaluating eribulin in combination with capecitabine as an adjuvant chemotherapy regimen in approximately 65 subjects with early-stage (I-II), human epidermal growth factor receptor 2 (HER2)- normal, estrogen receptor (ER)-positive breast cancer.

Inclusion Criteria:

1. Male subjects aged greater than or equal to 18 years and female subjects who must be postmenopausal (at least 12 months consecutive amenorrheic or have had a bilateral oophorectomy or, if they have had a hysterectomy but with ovaries intact, then females must be age 55 or older and with postmenopausal follicle-stimulating hormone [FSH] levels).

2. Subject is a candidate for chemotherapy in the adjuvant setting.

   • Adjuvant therapy must begin within 84 days of the final surgical procedure for breast cancer.

3. Histologically confirmed Stage I to II invasive breast cancer. Subjects may have more than one synchronous primary breast tumor.

4. Receptor Status:

   • HER2-normal as determined by a negative fluorescence in situ hybridization (FISH) result or 0 to 1+ by immunohistochemistry (IHC) staining result
   • ER-positive, node-negative or ER-positive Grade 1 or 2 node-positive breast cancer

5. ECOG performance status of 0 or 1

6. Adequate renal function as evidenced by serum creatinine less than or equal to 1.5 mg/dL or calculated creatinine clearance greater than or equal to 50 mL/min per the Cockcroft and Gault formula

7. Adequate bone marrow function as evidenced by ANC greater than or equal to 1.5 x 10^9/L, hemoglobin greater than or equal to 10.0 g/dL, and platelet count greater than or equal to 100 x 10^9/L

8. Adequate liver function as evidenced by bilirubin less than or equal to 1.5 times the upper limits of normal (ULN) and alkaline phosphatase, alanine aminotransferase (ALT), and aspartate aminotransferase (AST) less than or equal to 3 x ULN

9. Male subjects must have had a successful vasectomy (confirmed azoospermia), or their female partners must not be of childbearing potential, or male subjects must agree to use and have their female partners use a highly effective method of contraception (e.g., total abstinence, an intrauterine device, a double-barrier method [such as condom plus diaphragm with spermicide] throughout the entire study period and for 30 days after study drug discontinuation..

10. Voluntary agreement to provide written informed consent and willingness and ability to comply with all aspects of the protocol

Exclusion Criteria:

1. Stage III and IV invasive breast cancer
2. Prior chemotherapy, radiation therapy, immunotherapy or biotherapy for current breast cancer
3. Nonmalignant systemic disease (cardiovascular, renal, hepatic, etc) that would preclude any of the study therapy drugs
4. Subjects with a concurrently active second malignancy other than adequately treated nonmelanoma skin cancers or in situ cervical cancer
5. Subjects with pre-existing neuropathy greater than Grade 2
6. Subjects with known positive human immunodeficiency virus (HIV) status
7. Females of childbearing potential. Females will be considered to be of childbearing potential unless they are postmenopausal (at least 12 months consecutive amenorrheic or have had a bilateral oophorectomy or, if they have had a hysterectomy but with ovaries intact, then females must be age 55 or older and with postmenopausal FSH levels).
8. Subjects with current gastrointestinal disease or other condition resulting in an inability to take or absorb oral medications
9. Subjects with known allergy or hypersensitivity to eribulin mesylate or its excipients, or to fluoropyrimidine therapy (with or without documented dihydropyrimidine dehydrogenase [DPD] deficiency)
10. A clinically significant electrocardiogram (ECG) abnormality, including a marked baseline prolongation of QT/QTc interval (time between the start of the Q wave and the end of the T wave/QT interval corrected for heart rate) (e.g., repeated demonstration of a QTc interval greater than 500 ms)
11. Any medical or other condition which, in the opinion of the investigator, would preclude participation in a clinical trial