Clinical benefit of S-1 in metastatic breast cancer

Cytotoxic agents such as anthracycline or taxanes have provided a good clinical response for breast cancer patients, although they have failed to prolong the survival rate and to improve the quality of life (QOL) of these patients. On the other hand, cytostatic agents such as 5-fluorouracil (5-FU) have to be focused to accommodate a long term progression with respect to efficacy and the patients' QOL improvement. S-1 was a newly developed and orally administered fluorinated pyrimidine containing 1 M tegafur (FT) and two classes of a modulator, 5-chloro-2,4-dihydroxypyrimidine (CDHP) and potassium oxonate (Oxo) at a molar ratio of FT : CDHP : Oxo= 1 : 0.4 : 1. Specific dose limiting factors such as neutropenia, diarrhea and stomatitis have been observed in a previous phase I study. Two phase II studies of 4 weeks treatment of S-1 (1 M tegafur-0.4 M gimestat-1 M otastat potassium) for the advanced or metastatic breast cancer patients were carried out in Japan. Among 108 evaluable patients for response, there were 10 complete response (CR) and 35 partial response (PR) with an overall response rate of 41.7% (95% confidence interval, 32.3-51.5%). The incidence of toxicity (> or = grade 3) was as followed: neutropenia 9.3%, anemia 0.9%, stomatitis 1.9% and nausea/vomiting 0.9%. The median follow-up period for patients was 802 days. S-1 will be the new promising oral agent like a capecitabine which has been widely used as a third-line chemotherapy for the heavily treated breast cancer patients.