Anthracycline-containing regimens or taxane versus S-1 as first-line chemotherapy for metastatic breast cancer

Background We have previously demonstrated S-1 is non-inferior to taxane with respect to overall survival as first-line chemotherapy for HER2-negative metastatic breast cancer. We aimed to confirm whether S-1 is also non-inferior to anthracycline-containing regimens in the same setting. Methods We conducted an open-label, non-inferiority, Phase 3 study. Individuals who had HER2-negative metastatic breast cancer, had received no chemotherapy for advanced disease and had endocrine therapy resistance, were randomly assigned to the anthracycline-containing regimens or S-1. The primary endpoint was overall survival. A pre-planned combined analysis of our two Phase 3 studies was also carried out. Results We enrolled 230 patients (anthracycline, n = 115; S-1, n = 115). Median overall survival was 30.1 months (95% CI 24.9-35.8) with the S-1 group and 33.7 months (95% CI 25.5-36.9) with the anthracycline group. The HR for the anthracycline group was 1.09 (95% CI 0.80-1.48). The combined analysis constituted 814 patients (395 assigned to standard treatment (anthracycline or taxane); 419 assigned to S-1). Median overall survival was 36.3 months in the standard treatment group and 32.7 months in the S-1 group. S-1 was non-inferior to standard treatment in terms of overall survival (HR 1.06 (95% CI 0.90-1.25); P non-inferiority = 0.0062). Conclusions S-1 could be considered a new treatment option for first-line chemotherapy for patients with HER2-negative metastatic breast cancer.

Automated summary

The study demonstrates that S-1 is non-inferior to anthracycline-containing regimens as first-line chemotherapy for patients with HER2-negative metastatic breast cancer. S-1 may be considered a new treatment option in this setting.