近幾年胃癌的發生率在西方國家雖然有逐年下降的趨勢，但是在台灣仍位居第四大癌症死因。將近40~50 %的病人診斷時已是無法根治切除的、局部進行期（locally advanced）或轉移性的末期胃癌。化學治療能有意義的延長進行期或轉移性胃癌之存活期及改善生活品質，僅有少數化療藥品能緩解胃癌惡化，最近幾年，許多新藥也加入胃癌治療的行列，包括taxanes、topoisomerase I抑制劑與口服的fluoropyrimidine [如UFT（tegafur + uracil）、TS-1（tegafur + gimeracil +oteracil）、capecitabine]等。
TS-1為一種新型口服5-FU的複方化療藥物，其中包含三種不同藥理作用的藥品：tegafur、gimeracil與 oteracil，它們分別以莫耳數比1：0.4：1組合而成。其中tegafur（FT）為5-FU的前趨藥品，口服吸收後在肝臟代謝為5-FU。Gimeracil為dihydropyrimidine dehydrogenase（DPD）抑制劑，與5-FU併用時能延長5-FU在血液與腫瘤的維持濃度。Oteracil經由抑制消化道上的orotate phosphoribosyl transferase，減少5-FU磷酸化所造成的胃腸毒性，同時增加5-FU在腫瘤細胞的濃度。日本學者以晚期胃癌病人所做的研究，口服S-1，一日兩次持續28天，休息14天的療程，總反應率為44 %~53.6 %，中位存活時間為244天，一年存活率為37 %。S-1已核准在日本上市使用於治療胃癌與結腸直腸癌，歐美的研究結果反應率較日本的研究為低，其腹瀉與手足症候群的發生率較高，但骨髓抑制作用比較低。造成此種差異的因素目前並不清楚，可能是藥品的劑量、療程不同或是種族遺傳差異所致。口服的藥品增加治療的便利性與改善病人的生活品質，為腫瘤治療一項重大的進展，可作為針劑fluoropyrimidine之外的另一種選擇。

Although the incidence of gastric cancer has steadily declined in western countries, it is the fourth leading cause of death from cancer in Taiwan. Approximately 40 to 50 % of patients presented with unresectable, locally advanced or metastatic disease when gastric cancer were diagnosed. For patients with advanced or metastatic gastric cancer, systemic chemotherapy can improve overall survival and quality of life. Only a small number of chemotherapeutic agents provide palliation. Recently, a number of novel compounds have been studied in gastric cancer, including taxanes, topoisomerase I inhibitor, and oral fluoropyrimidine [UFT (tegafur with uracil), TS-1, capecitabine].
TS-1 is a novel combination of tegafur with two 5-FU modulating substances, gimeracil and oteracil, at a molar ratio of 1:0.4:1. Tegafur is converted to 5-FU by metabolism in the liver. Gimeracil inhibits the degradation of 5-FU by inhibition of dihydropyrimidine dehydrogenase. When combined with 5-FU, this results in the prolonged maintenance of 5-FU concentration in the plasma and tumor. Oteracil decreases the phosphorylation of 5-FU in the gastrointestinal tract by inhibiting orotate phosphoribosyl transferase. This result reduced gastrointestinal toxicity. The overall response rate was 44.6 %. Median survival time and 1-year survival rate were 244 days and 37 %, respectively. The drug has been approved in Japan for the treatment of gastric and colorectal cancer. However, studies in Europe and USA have been inconclusive and reported significant side effects. More diarrhea and hand-foot syndrome and less myelotoxicity were reported compared with Japanese studies. Oral chemotherapeutic agents permit treatment on an outpatient basis, providing an alternative choice other than parenteral fluoropyrimidine.