The objective of this study was to investigate the influence of digestive gastrointestinal absorption function on the pharmacokinetics of the orally-administered anticancer drug, Tegafur-gimestat-otastat potassium (TS-1), by measuring the plasma 5-fluorouracil (5-FU) concentration using stable isotope breath tests.

Twenty-nine patients with progressive/recurrent digestive organ cancer were enrolled for this pharmacokinetic study, and blood samples were obtained from each patient. The area under-the-time-concentration curve between 0 and 480 min (AUC0-480 min), time-of-drug concentration peak (T(max)), maximum drug concentration (C(max)) and the half-life period (t(1/2)) of 5-FU were investigated. Simultaneously, a continuous (13)C-acetate breath test was performed for each patient. The parameters measured with the breath test were the area under the (13)CO(2) excretion rate curve between 0-4 h (AUC(0-4h)), peak (13)CO(2) value and elimination rate constant (K(el)) value.

The AUC(0-8h) and C(max) of 5-FU were significantly correlated with K(el) (p=0.012 and p=0.024, respectively), and the 5-FU C(max) value was significantly correlated with the peak value of (13)CO(2) (p=0.037). Multivariate regression analysis also found the C(max) of 5-FU to be associated with K(el) (p=0.0118). The C(max) and AUC(0-8h) of 5-FU were also significantly correlated (p<0.0001).

The results of this study suggest that gastrointestinal absorption is closely-related to plasma 5-FU concentration after oral administration of TS-1.