The activity and safety of eribulin mesylate (E7389), a nontaxane microtubule dynamics inhibitor with a novel mechanism of action, were evaluated in patients with locally advanced or metastatic breast cancer previously treated with an anthracycline, taxane, and capecitabine. Eligible patients in this single-arm, open-label phase II study received eribulin mesylate (1.4 mg/m(2)) administered as a 2- to 5-minute intravenous infusion on days 1 and 8 of a 21-day cycle. The primary end point was objective response rate (ORR) assessed by independent review. Of 299 enrolled patients who had received a median of four prior chemotherapy regimens, 291 received eribulin (for a median of four cycles). Of these, 269 patients met key inclusion criteria for the primary efficacy analysis. The primary end point of ORR by independent review was 9.3% (95% CI, 6.1% to 13.4%; all partial responses PRs), the stable disease (SD) rate was 46.5%, and clinical benefit rate (complete response + PR + SD > or = 6 months) was 17.1%. The investigator-reported ORR was 14.1% (95% CI, 10.2% to 18.9%). Median duration of response was 4.1 months, and progression-free survival was 2.6 months. Median overall survival was 10.4 months. The most common treatment-related grade 3 or 4 toxicities were neutropenia (54%; febrile neutropenia, 5.5%), leukopenia (14%), and asthenia/fatigue (10%; no grade 4); grade 3 neuropathy occurred in 6.9% of patients (no grade 4). Eribulin demonstrated antitumor activity in extensively pretreated patients who had previously received an anthracycline, taxane, and capecitabine, with a manageable tolerability profile.