Background: The potential of S-1 for the treatment of metastatic renal cell carcinoma (mRCC) has been shown in two phase II studies. We aimed to assess the safety, tolerance, pharmacokinetics and clinical activity of S-1 combined with sorafenib in patients with mRCC. Patients and methods: In this multicenter, single-arm, open-label, phase I/II study of S-1 plus sorafenib, we recruited patients with clear-cell or papillary renal cell carcinoma who had received a maximum of one prior cytokine-based regimen. The phase I primary end points were the maximum tolerated dose (MTD) and recommended dose (RD). S-1 was administered orally at 60, 80, 100 or 120 mg/day on days 1-28 of a 42-day cycle in combination with sorafenib (400 or 800 mg/day), given daily with dose adjustment. In phase II, the primary end point was to assess the overall response rate (ORR) at the RD. Results: Nine patients were enrolled into phase I and 21 (including 6 patients who received the RD in the phase I portion) were enrolled into phase II. In the phase I portion, the MTD could not be determined, and the RD was defined as S-1 80 mg/m2/day on days 1-28 + sorafenib 800 mg/day on days 1-42. In the phase II portion, 21 patients were fully assessable for efficacy and safety. The confirmed ORR was 52% [95% confidence interval (CI) 29.8-74.3], including one complete response (5%) and 10 partial responses (48%). The median progression-free survival was 9.9 (95% CI 6.5-17.1) months. The most frequently reported treatment-related adverse event for all grades was hand-foot skin reaction (100%). The major reasons for dose reduction were hand-foot skin reaction (38%) and rash (14%). Conclusion: Combination therapy with S-1 plus sorafenib is effective and tolerable for patients with mRCC. However, skin events management is important in S-1 plus sorafenib combination therapy.
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