Background: Eribulin mesylate is currently indicated as a sequential monotherapy to be administered after two chemotherapeutic regimens, including anthracycline and taxane treatments, for treatment of metastatic breast cancer. This open-label, multicenter phase II study was designed to evaluate the efficacy and safety of eribulin as a first- or second-line treatment for patients with metastatic breast cancer. Methods: The primary objective was to determine the overall response rate. Secondary objectives were to evaluate progression-free survival and the safety profile. Patients were scheduled to receive eribulin mesylate 1.4 mg/m 2 intravenously on days 1 and 8 of a 21-day cycle. Patients received the study treatment unless disease progression, unacceptable toxicity, or a request to discontinue from the patient and/or investigator eventuated. Results: Between December 2012 and September 2015, 32 patients with metastatic breast cancer were enrolled at 10 participating clinical institutions in Japan, and toxicity and response rates were evaluated. The overall response rate was 43.8% (95% confidence interval [CI] 26.5-61.0). The clinical benefit and tumor control rates were 56.3% (95% CI 39.0-73.5) and 78.1% (95% CI 63.8-92.5), respectively. Median progression-free survival was 8.3 months (95% CI 7.1-9.4). A subgroup analysis did not identify any factors affecting the efficacy of eribulin. The most common adverse events were neutropenia (71.9%), alopecia (68.7%), and peripheral neuropathy (46.9%). As a first- or second-line therapy, eribulin showed sufficient efficacy for metastatic breast cancer compared with taxane and capecitabine treatment in previous clinical trials. The safety profile of eribulin was acceptable. Conclusions: Eribulin may be another option for first-line chemotherapeutic regimens for metastatic breast cancer.
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