[Antitumor activity and function of S-1, a new oral tegafur-based formulation].

TS-1 (S-1), developed by the scientific theory of both potentiating antitumor activity of 5-fluorouracil (5-FU) and reducing gastrointestinal toxicity induced by 5-FU, is a new oral formulation consisting of 1 M tegafur, 0.4 M gimeracil and 1 M oteracil potassium. We investigated the antitumor efficacy of S-1 alone and in combination with other cytotoxic anticancer drugs using subcutaneously or orthotopically implanted murine and human tumors in rodents. As a single agent, S-1 showed higher antitumor activity with its low intestinal toxicity compared to continuous venous infusion 5-FU, the most effective dosing method of 5-FU, and/or to clinically available oral fluoropyrimidines such as UFT, doxyfluridine and capecitabine on various murine tumors and human tumor xenografts. Especially, it was noteworthy that S-1 as a DPD-inhibitory fluoropyrimidine markedly affected human tumor xenografts with high expression levels of DPD on which other fluoropyrimidines showed a low antitumor activity. In combination with other anticancer drugs such as CPT-11 and taxanes, S-1 exercised synergistic antitumor efficacy not only on 5-FU-sensitive tumors with low expression levels of thymidylate synthase (TS) but also on 5-FU-resistant tumors with originally higher and/or elevated levels of TS expression. As one of the reasonable mechanism of antitumor synergism by the combination, CPT-11 and taxanes were found to reduce the expression of TS in human tumor resistant to 5-FU with high expression TS levels. Throughout our preclinical antitumor studies of S-1, alone and/or in combination with other anticancer drugs, it would be expected to contribute greatly to the treatment of cancer patients.