Four courses versus eight courses of adjuvant S-1 for patients with stage II gastric cancer (JCOG1104 [OPAS-1]): an open-label, phase 3, non-inferiority, randomised trial.

Background: Postoperative adjuvant chemotherapy with S-1 for 1 year (corresponding to eight courses) is standard care for stage II gastric cancer. Whether the duration of S-1 could be shortened to 6 months (corresponding to four courses) without worsening survival is unclear. The aim of this study was to investigate the non-inferiority of four courses of S-1 compared with eight courses of S-1 for patients with stage II gastric cancer.

Methods: We did a phase 3, open-label, randomised controlled, non-inferiority trial at 59 hospitals in Japan. Patients aged 20-80 years with stage II adenocarcinoma of the stomach were randomly assigned (1:1) to eight courses or four courses of S-1. Randomisation was done by the Japan Clinical Oncology Group Data Center website, using a minimisation method with a random component using institution, stage (IIA vs IIB), age (<70 years vs ≥70 years), and mode of operation (open gastrectomy with bursectomy vs open gastrectomy without bursectomy vs laparoscopic gastrectomy) as adjustment factors. One course was 80 mg/day per m of S-1 administered for 4 weeks followed by a rest for 2 weeks. The primary endpoint was relapse-free survival, analysed by intention to treat, with a non-inferiority margin for the hazard ratio (HR) set at 1·37. This study is registered at UMIN-Clinical Trial Registry, number UMIN000007306.

Findings: Between Feb 16, 2012, and March 19, 2017, 590 patients were enrolled (295 per group). 528 (89%) patients were analysed at the first planned interim analysis in March, 2017, at which time the point estimate of HR for the four-course group compared with the eight-course group was 2·52 (95% CI 1·11-5·77), which exceeded 1·37 and met the prespecified criteria for early termination. Predictive probability for showing non-inferiority at the final analysis was calculated to be 2·9%. The study was stopped for futility. Updated 3-year relapse-free survival analysed in May, 2017, was 93·1% (95% CI 87·8-96·1) for the eight-course group and 89·8% (84·2-93·5) for the four-course group (HR 1·84, 95% CI 0·93-3·63). The most common grade 3-4 adverse event was neutropenia, observed in 46 (16%) patients in the eight-course group and 51 (17%) patients in the four-course group.

Interpretation: S-1 for 1 year should remain as standard adjuvant chemotherapy for stage II gastric cancer.

Funding: Japan Agency for Medical Research and Development; the Ministry of Health, Labour and Welfare of Japan; the National Cancer Center Research and Development Fund, Japan.