Evaluation of the optimal dosage of S-1 in adjuvant SOX chemotherapy for gastric cancer

Gastric cancer (GC) is the second leading cause of cancer-related mortality worldwide. The usual treatment of GC consists of surgery with additional adjuvant chemotherapy. In the present study, the feasibility and safety of adjuvant S-1 plus oxaliplatin (SOX) chemotherapy for patients with GC and the optimal dosage of S-1 were determined. Eligible patients were randomly assigned to either arm A (30 cases) receiving 70 mg/m2 S-1 (in two seperate half doses) daily or arm B (30 cases) receiving 80 mg/m2 S-1 (in two seperate half doses) daily. The S-1 was administered twice daily for 14 days followed by a 7-day rest period for the third week. A total of 130 mg/m2 oxaliplatin was administered on day 1 every 3 weeks for each arm. The cumulative rates of the relative total administration dose of S-1 at 100% in the 6th treatment course was 71.4% [95% confidence interval (CI), 56.5-90.3%] in arm A, which was significantly higher than 21.4% (95% CI, 10.5-43.6%) in arm B (P=0.001). The most common grade 3/4 toxicities were neutropenia (19.6%), thrombocytopenia (19.6%) and vomiting (16.1%). Grade 3/4 thrombocytopenia was observed in 7.1% of patients in arm A and in 32.1% of patients in arm B (P=0.019). With regard to the adverse events induced by S-1 administration, the incidence of diarrhea (3.6 vs. 42.9%; P<0.001) was significantly higher in arm B than in arm A, as anticipated. Collectively, adjuvant SOX therapy for GC is feasible and safe, and when combined with 130 mg/m2 oxaliplatin, 70 mg/m2/day S-1 appears to the optimal dose.