Modelling of ftorafur and 5-fluorouracil pharmacokinetics following oral UFT administration. A population study in 30 patients with advanced breast cancer

The pharmacokinetics of ftorafur, 5-fluorouracil (5FU) and uracil were investigated in order to built a population pharmacokinetic model for the anticancer drug UFT, administered with leucovorin and vinorelbine.

A total of 31 patients with metastatic breast cancer were treated with escalating oral doses of UFT (300 to 500 mg per day) plus leucovorin (90 mg per day) in combination with intravenous vinorelbine (15 to 25 mg/m²). Concentration-time data were obtained on days 1, 8, 15 and 21 of cycle 1.

Of the 31 patients treated, 30 were available for the pharmacokinetic analysis. Ftorafur, 5FU and uracil appeared rapidly in plasma and showed large interpatient variations. Ftorafur concentrations were higher than those of 5FU and uracil. AUC significantly increased between day 1, and days 8, 15 and 21. Ftorafur C_max and AUC values were proportional to UFT dose, whereas C_max and AUC values of 5FU and uracil were not linearly related to UFT dose. The pharmacokinetics of ftorafur were ascribed to a two-compartment open model in which 5FU was produced from the central compartment. The absorption and exponential distribution rate constants were assumed equal. The effect of uracil on 5FU elimination was straightforward, since no reasonable curve-fitting could be obtained for 5FU data when this covariate was not taken into account. The uracil concentration inducing a 50% reduction in 5FU elimination was 2.67 μmol.l⁻¹. This result confirms the important role played by uracil as a competitive inhibitor of 5FU catabolism.

A pharmacokinetic model for ftorafur and 5FU was developed and should be useful to further study drug interactions and establish dosing guidelines.