Comparative pharmacokinetic study of continuous venous infusion fluorouracil and oral fluorouracil with eniluracil in patients with advanced solid tumors

Purpose: To compare the pharmacokinetics of continuous venous infusion (CVI) fluorouracil (5-FU) with that of oral eniluracil/5-FU and to describe toxicities and clinical activity of prolonged oral administration of eniluracil/5-FU. Patients and Methods: A randomized, open-label, cross-over study compared CVI 5-FU to an oral 5-FU/eniluracil combination. Seventeen patients (arm A) were randomly assigned to receive eniluracil/5-FU combination tablets (10:1 mg/m² BID for 7 days) during the first study period, followed by 5-FU (300 mg/m² CVI for 7 days) during period 2, with a 14-day washout between periods. Sixteen patients (arm B) received treatment in the opposite sequence. In period 3, all patients received eniluracil/5-FU tablets BID for 28 days. Plasma levels of 5-FU during CVI and oral administration were analyzed in periods 1 and 2. Dihydropyrimidine dehydrogenase (DPD) activity was determined by measuring plasma uracil, urinary α-fluoro-β-alanine, and peripheral-blood mononuclear cell (PBMC) DPD activity. Results: There were no grade 3 or 4 toxicities in either arm. Partial responses were observed in three patients. Another three patients had stable disease for ≥ 3 months. Eniluracil and 5-FU pharmacokinetics were similar to those observed in previous studies and were unaffected by administration sequence. The mean ± SD steady-state plasma concentration (C_p) and area under the curve (AUC)_144-168h for CVI 5-FU (104 ± 45 ng/mL and 2,350 ± 826 ng·h/mL, respectively) were three-fold greater than those for oral 5-FU (38.1 ± 7.7 ng/mL and 722 ± 182 ng·h/mL, respectively [P < .00001]). Individual 5-FU concentrations during CVI were highly variable, whereas those after eniluracil/5-FU were very reproducible. DPD activity in PBMCs before each study period was normal. Conclusion: Both CVI 5-FU and oral eniluracil/5-FU were well tolerated, with moderate activity in these heavily pretreated patients. However, 5-FU steady-state C_p and AUCs achieved with oral eniluracil/5-FU were significantly less than with CVI 5-FU.