Phase II study of trastuzumab in combination with S-1 and cisplatin in the first-line treatment of human epidermal growth factor receptor HER2-positive advanced gastric cancer

Purpose: The use of trastuzumab, a monoclonal antibody targeting the HER2 protein, in combination with 5-fluorouracil/platinum-based chemotherapy improves survival in patients with HER2-positive advanced gastric cancer. In addition, TS-one (S-1)/platinum is also used as a standard of care in Asian countries. However, little is known about the combination of S-1/cisplatin chemotherapy and trastuzumab in patients with HER2-positive advanced gastric/gastroesophageal junction (GEJ) cancer. Methods: We conducted a single-arm, two-stage, open-label, multicenter phase II study. Trastuzumab was administered intravenously on day 1 of the first cycle at 8 mg/kg and 6 mg/kg on day 1 of subsequent cycles. Cisplatin was administered intravenously at 60 mg/m² on day 1 of each cycle after trastuzumab. S-1 was administered orally [based on body surface area (BSA)] twice a day for 14 days in a 3-weekly cycle. Patients with BSA of <1.25 received a total of 80 mg of S-1, those with BSA ≥1.5 received 120 mg, and the remaining received 100 mg daily in two divided doses. Results: All evaluable patients experienced tumor reduction during the trial. The primary end point (overall survival rate) was 59.3 %, with a clinical benefit rate of 66.7 %. Median progression-free survival was 7.4 months; 62.6 % patients were free from disease progression at 6 months. Median overall survival was 14.6 months, and the median time to treatment failure was 6.0 months. Conclusion: The combination of trastuzumab with S-1 and cisplatin demonstrated good activity, was generally well tolerated, and is a feasible treatment option in the first-line treatment of HER2-positive advanced gastric/GEJ cancers.