1034 Background: Eribulin is a structurally simplified analog of halichondrin B, which inhibits microtubule dynamics via a novel mechanism characterized by suppression of microtubule growth, lack of effect on microtubule depolymerization, and sequestration of tubulin into nonfunctional aggregates. This study was designed to assess the activity and tolerance of eribulin in chemotherapy refractory patients with advanced breast cancer. Methods: Eribulin was evaluated in a single-arm Phase II trial in female patients with refractory breast cancer, ECOG performance status of 0–1, measurable disease, and neuropathy ≤ Grade 2. Patients received ≥ 1 prior chemotherapy regimen, including an anthracycline and a taxane. Eribulin was administered as a 2–5 min IV bolus of 1.4 mg/m2 on Days 1, 8, and 15 of a 28-Day cycle (Group 1). The schedule was modified to Days 1 and 8 of a 21-Day cycle (Group 2), because of dose delays. The primary efficacy endpoint was ORR according to RECIST criteria based upon independent review (IR) of tumor assessment. Results: Of 104 patients enrolled, 103 received eribulin treatment: 70 in Group 1, 33 in Group 2. Median age was 55 yrs (range 32–84). Patients had received a median of 4 prior chemotherapy regimens (range 1–11). Sixty-one percent of tumors were ER+, 14% Her2/neu 3+, and 29% were triple (ER, PR, Her-2) negative. The incidence of dose interruption, delay, or omission during Cycle 1 was 63% (Group 1) and 18% (Group 2). The most common drug related toxicities were neutropenia (75%, Grades 3: 31%, Grade 4: 30%, febrile neutropenia: 3.9%), fatigue (52%, Grade 3: 2.9%, no Grade 4), alopecia (Grade 1/2: 41%), nausea (37%, Grade 3: 1%, no Grade 4), and anemia (36%, Grade 3: 1%, no Grade 4). Peripheral neuropathy occurred in 34% of patients (Grade 3: 3.9%, no Grade 4). Best overall response rate (all PR) by IR was 14.5% and 15.2% in Groups 1 and 2, respectively; the combined ORR was 14.7% (95 % CI: 9–23%). Median PFS was 85 days, and the 6 mo PFS rate was 31%. Conclusions: Eribulin given as a 2–5 min IV infusion on Days 1, 8 of a 21-Day cycle or Days 1, 8, 15 of a 28-Day cycle exhibited a 15% PR rate by IR and a low incidence of Grade 3 neuropathy in this heavily chemotherapy pretreated population. The most common toxicity was neutropenia. The 21-Day schedule had an acceptable toxicity profile. No significant financial relationships to disclose.