The affects of GEM metabolites on Cmax and AUC of plasma 5-FU after S-1 administration may be little lower than expected based on the presence of CDHP in plasma. The above-mentioned mechanism may explain our results that PK parameters of plasma 5-FU

after S-1 administration did not differ with and without GEM administration. Moreover, no enhancement of 5-FU systemic exposure after S-1 administration in the presence of GEM may be an advantage in reducing the frequency of adverse events [16]. The synergistic effects of S-1 and GEM may be explained by the following mechanism occurring in tumor cells. S-1 is converted into 5-FU. An active metabolite of 5-FU is fluorodeoxyuridine monophosphate (FdUMP), which inhibits DNA synthesis by forming of ternary complex with 5,10-methylene

tetrahydrofolate and thymidylate synthase. GEM inhibits ribonucleotid reductase, a key enzyme in the salvage pathway of pyrimidine LY2835219 biosynthesis. Consequently, GEM reduces the synthesis of deoxyuridine monophosphate, a major competitor of FdUMP, resulting enhancement of 5-FU cytotoxicity [17]. Another potential mechanism is that 5-FU leads to an increase in cell surface human equilibrative nucleoside transporter 1 (hENT1) [18, 19]. The most active GEM uptake is via hENT1. Thus, increased hENT1 expression by 5-FU may augment GEM cytotoxicity by increasing GEM concentrations in tumor cells. In conclusion, the present study obtained by the limited number of patients demonstrated the selleck combination chemotherapy of S-1 with GEM did not affect the PK of each drug. As GANT61 concentration S-1 combined with GEM may be a promising regimen, further investigations should be carried out to elucidate the synergistic mechanisms between the two drugs. References 1. Moore MJ, Goldstein

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