Effect of sipjeondaebo-tang on the pharmacokinetics of S-1, an anticancer agent, in rats evaluated by population pharmacokinetic modeling

S-1 (TS-1^fi) is an oral fluoropyrimidine anticancer agent containing tegafur, oteracil, and gimeracil. Sipjeondaebo-tang (SDT) is a traditional oriental herbal medicine that has potential to alleviate chemotherapy-related adverse effects. The aim of the present study was to evaluate the effect of SDT on the pharmacokinetics of S-1. Sprague-Dawley rats were pretreated with a single dose or repeated doses of SDT for seven consecutive days (1200 mg/kg/day). After the completion of pretreatment with SDT, S-1 was orally administered and plasma concentrations of tegafur, its active metabolite 5-FU, and gimeracil were determined by liquid chromatography-tandem mass spectrometry (LC/MS/MS). A population pharmacokinetic model was developed to evaluate the effect of SDT on pharmacokinetics of tegafur and 5-FU. Although a single dose of SDT did not have any significant effect, the absorption rate of tegafur decreased, and the plasma levels of 5-FU reduced significantly in rats pretreated with SDT for seven days in parallel to the decreased gimeracil concentrations. Population pharmacokinetic modeling also showed the enhanced elimination of 5-FU in the SDT-pretreated group. Repeated doses of SDT may inhibit the absorption of gimeracil, an inhibitor of 5-FU metabolism, resulting in enhanced elimination of 5-FU and decrease its plasma level.