This expansion of a phase 1 study assessed the safety and efficacy of E7389-LF in patients with human epidermal growth factor receptor type 2-negative metastatic breast cancer.... Objective response rate was 35.7% (95% confidence interval [CI]: 18.6-55.9) for all patients and 42.9% (95% CI: 21.8-66.0) for patients with HR+ BC. Median progression-free survival was 5.7 months (95% CI: 3.9-8.3)....E7389-LF was tolerable and favourable antitumour activity was observed, particularly in patients with HR+ BC.