Background: As a single agent, eribulin is an active agent among MBC pts. In the first-line setting, pertuzumab improves progression free and overall survival when added to trastuzumab and chemotherapy (CT); however there are limited data in the second line or beyond. The primary aim of this study was to evaluate the activity of eribulin in combination with trastuzumab and pertuzumab in pts with HER2+ MBC whose cancers are refractory to trastuzumab containing regimens. Safety and toxicity were examined and correlative studies are also planned. Methods: Single center, single-arm, phase II, 2 cohorts study planned to enroll up to 81 pts. The studied combination would be of clinical interest if the true overall response rate (ORR) was 40% among those without prior pertuzumab exposure (Cohort A) and 30% among those with prior pertuzumab exposure (Cohort B). The trial was stopped prematurely due to low accrual. Prior to the start of the phase II study, a run-in established dose of eribulin with standard dose of trastuzumab and pertuzumab (1.4mg/m2 days 1, 8 /21 days). Results: Of 32 pts enrolled, 6 were in the run-in, 19 in Cohort A and 7 in Cohort B. Most pts were heavily pre-treated (median [range] lines of CT for metastatic disease, run-in: 4[2-11], cohort A: 2.5 [0-7], cohort B: 3[1-8]). In cohort A, 5 pts had partial responses (ORR = 26% [95% CI 9-51%]), 1 (5%) experienced stable disease (SD) lasting ≥ 6 months and 11 (58%) had a SD lasting < 6 months. In cohort B, the ORR was 0% [95% CI 0-41%]), 1(14%) experienced SD lasting ≥ 6 months and 5 (75%) had a SD lasting < 6 months. The combination was well tolerated, most adverse events (AE) were mild to moderate. Hematologic toxicity was the most frequent grade 3-4 AE. Analyses looking at tissue genomic markers and cell free DNA will be presented. Conclusions: We observed modest activity of the treatment combination in those without prior pertuzumab exposure but limited activity in unselected pts with HER2+ MBC. The study was limited by small sample size. Correlative studies may help us to understand resistance and response to eribulin with anti-HER2 therapy and the interplay between CT and targeted therapy. Clinical trial information: NCT01912963