Objective:Eribulin mesylate (EM) is a fully synthetic macrocyclic ketone analogue of the marine natural product halichondrin. EM has been reported to be active in metastatic breast cancer. In this paper, we report efficacy and safety of data of EM in a retrospective, real-world series of patients with poor prognosis, hormone-refractory, or triple-negative metastatic breast cancer.Materials and Methods:The analysis was carried out at 4 interrelated oncology centers. EM was delivered at the dose of 1.4 mg/m2 in 100 mL of normal saline over 2 to 5 minutes on days 1 and 8 every 21 days. EM was continued until disease progression or unacceptable toxicity. Side effects were reported every cycle as per standard clinical practice and graded according to NCI-CTCAE, version 4.0. Time-to-progression and overall survival were reported.Results:In this series of 90 patients the overall response rate was 22%, and 21% and 23% in the hormonal-resistant group and the triple-negative one, respectively. Stable disease was recorded in 24%, 21%, and 27%, respectively, in the whole series, the hormonal-resistant group, and the triple-negative one, respectively. Time-to-progression was 3.5 months (range, 1 to 22 mo) in the whole series and 3.0 months (range, 1 to 14.7 mo) and 3.4 months (range, 2.2 to 16.2 mo) in the hormonal-resistant group and the triple-negative one, respectively. Overall survival reached a median of 11.4 months.Conclusions:This multicenter study, albeit retrospective, demonstrates the activity of this combination as third-line chemotherapy option in a challenging clinical setting such as triple-negative or hormone-resistant patients with breast cancer progressing after several lines of hormonal manipulations.