Pretreatment with S-1, an Oral Derivative of 5-fluorouracil, Enhances Gemcitabine Effects in Pancreatic Cancer Xenografts

Background: The systemic administration of gemcitabine (GEM) has been accepted as a standard treatment for patients with advanced pancreatic cancer. The major mediator of cellular uptake of GEM is the human equilibrative nucleoside transporter 1 (hENT1) whose expression is up-regulated by thymidylate synthase inhibitors, such as 5-fluorouracil (5-FU). S-1 is a novel oral derivative of the 5-FU prodrug tegafur combined with two modulators. Recent clinical trials have reported the promising effect of S-1 in pancreatic cancer. The purpose of this study was to evaluate the relationship between different schedules and the effects of GEM/S-1 combination therapy on pancreatic cancer xenograft models.

Materials And Methods: Human pancreatic tumor xenografts were prepared by subcutaneous implantation of MiaPaCa-2 into nude mice. Expression of hENT1 was determined by quantitative RT-PCR. GEM cellular uptake was determined using [3H] GEM.

Results: Significant increases in hENT1 expression and GEM cellular uptake were observed after S-1 treatment. Six different treatment schedules (no treatment, single agent of GEM or S-1, combination treatment with GEM either before, simultaneously or following administration of S-1) were compared. Significant tumor growth inhibition was observed in the mice treated with S-1 followed by GEM compared to either untreated mice or the mice treated with the other schedules.

Conclusion: Based on the effects of S-1 on the uptake of GEM, S-1 should be used before GEM treatment. The GEM/S-1 combination therapy in patients with pancreatic cancer may be promising and should be tested in clinical trials.