Background:Eribulin mesylate is a non-taxane microtubule inhibitor which can be used after anthracycline and taxane treatment in patients with metastatic breast cancer (MBC). The purpose of this study was to investigate the efficacy and safety of eribulin monotherapy in heavily pretreated patients with MBC.Methods:In this study, a total of 45 eligible patients with MBC who received eribulin in HCG Cancer Speciality Center from November 2014 to March 2016 were prospectively analyzed. Breslow (generalized Wilcoxon) survival analysis was carried out for progression-free survival and for overall survival. Patients were excluded if they had not taken treatment for 3 cycles and defaulted/expired during the treatment.Results:In this study, median age of patients was 52 years. A total of 27 (60%) patients had estrogen receptor and progesterone receptor (PR) positive primary tumors, whereas HER2 was overexpressed or amplified in 7 (15.6%); a triple negative subtype was recorded in 13 patients (28.9%). Regarding toxicity, 30 patients (66.67%) tolerated treatment well and 3 patients (6.67%) got anemia, 6 patients (13.3%) experienced neutropenia, and 7 (15.62%) patients had neurological toxicity. About 14 (31.1%) patients showed PR, 12 (26.7%) patients had stable disease (SD), whereas 19 (42.25%) patients showed progression disease (PD). Response evaluation at 6 cycles was possible in 18 patients and revealed that 4 (22.5%) patients showed PR, 10 (55.5%) patients had SD, whereas 4 (22.2%) patients had PD. Progression-free survival of the overall study population was 3.95 months.Conclusions:Eribulin mesylate is efficacious and tolerable chemotherapy as second- and third-line treatment options for MBC.