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Gemcitabine monotherapy or gemcitabine-containing combination chemotherapy is the standard first-line therapy for advanced pancreatic cancer. After disease progression, there is no standard regimen available. In a previous phase II trial, S-1 has been reported to show considerable efficacy, achieving a response rate of 37.5% in chemo-naïve patients with pancreatic cancer. This study evaluated the efficacy and toxicity of S-1 in patients with gemcitabine-refractory metastatic pancreatic cancer. Eligibility criteria were histologically proven pancreatic adenocarcinoma with confirmation of progressive disease while receiving gemcitabine-based first-line chemotherapy, 20–74 years of age, Karnofsky performance status of 80–100 points, with measurable metastatic lesions, adequate hematological, renal and liver functions, and written informed consent. S-1 was administered orally at 40 mg/m2 twice daily for 28 days with a rest period of 14 days as one course. Administration was repeated until the appearance of disease progression or unacceptable toxicity. The primary endpoint of this study was an objective response, and secondary endpoints included toxicity, progression-free survival (PFS) and overall survival, as well as clinical benefit response in symptomatic patients. Forty patients from two institutions were enrolled between September 2004 and November 2005. The most common adverse reactions were fatigue and anorexia, although most of those adverse reactions were tolerable and reversible. One patient developed grade 3 pneumonitis without neutropenia and recovered with appropriate antibiotic treatment. Although no complete response was seen, partial response was obtained in six patients (15, 95% confidence interval, 3.9–26%). Stable disease was noted in 17 patients (43%), and progressive disease in 15 patients (38%). Out of 19 evaluable patients, a clinical benefit response was observed in four patients (21%). The median PFS was 2.0 months, and the median survival time was 4.5 months with a 1-year survival rate of 14.1%. S-1 as monotherapy had marginal anti-tumor activity with tolerable toxicity in patients with gemcitabine refractory metastatic pancreatic cancer. [ABSTRACT FROM AUTHOR]