Phase II Study of S-1 plus Trastuzumab for HER2-positive Metastatic Breast Cancer (GBCCSG-01)

Treatment strategies for patients with human epidermal growth factor 2 (HER2)-positive metastatic breast cancer (MBC) have significantly progressed. The use of trastuzumab, a monoclonal antibody targeting the HER2 (human epidermal growth factor 2) protein, in combination with chemotherapy improves survival in patients with HER2-positive breast cancer. S-1, an oral combination of fluorouracil derivatives, is widely used in Japan and is more convenient than intravenous drugs. However, little is known about the combination of S-1 and trastuzumab in patients with HER2-positive MBC.

We conducted a single-arm, open-label, multicenter prospective phase II study to evaluate the efficacy of an S-1 plus trastuzumab regimen for HER2-positive MBC. S-1 was administered orally [80-120 mg, based on body surface area (BSA)] twice a day for 14 consecutive days in a 3-week cycle. Patients with BSA of <1.25 m² received a total of 80 mg of S-1, those with BSA ≥1.5 m² received 120 mg, and the remaining received 100 mg daily in two divided doses. Trastuzumab was administered intravenously at 8 mg/kg on day 1 of the first cycle and at 6 mg/kg on day 1 of subsequent cycles, i.e., every 3 weeks.

Between December 2008 and March 2013, 10 patients were enrolled and received a median of 17 (range=3-76) cycles of treatment. Overall response and clinical benefit rates were 60.0% and 90.0%, respectively. Progression-free survival was 15.8 (95% confidence interval=9.4-29.6) months and overall survival was 45.5 (95% confidence interval=37.1-62.2) months. Grade 3/4 adverse events included were neutropenia and hyperglycemia in one patient each (10.0%). There was no clinically significant cardiotoxicity.

The combination of S-1 and trastuzumab was tolerable and had excellent efficacy with good response and disease control in this study. S-1 plus anti-HER2 therapy is a feasible treatment option for HER2-positive MBC.