Phase I and pharmacologic study of oral fluorouracil on a chronic daily schedule in combination with the dihydropyrimidine dehydrogenase inactivator eniluracil

Purpose: To determine the maximum-tolerated dose (MTD), toxicities, and pharmacokinetics of oral fluorouracil (5-FU) administered twice daily in combination with oral eniluracil, an inactivator of dihydropyrimidine dehydrogenase, administered for 28 days every 35 days. Patients and Methods: Oral 5-FU 1.35 mg/m² twice daily was administered with oral eniluracil 10 mg daily for 14 to 28 days, followed by a 1-week rest period. Eniluracil was started 1 day before 5-FU. Patients then received escalated doses of oral 5- FU 1.35 to 1.8 mg/m² twice daily with an increased dose of eniluracil 10 mg twice daily for 28 days. A reduced dose of 5-FU 1.0 mg/m² with eniluracil 20 mg twice daily was evaluated. Results: Thirty-six patients with solid malignancies were enrolled onto the study. Diarrhea was the principal dose- limiting toxicity of oral 5-FU and eniluracil given on this chronic schedule. The recommended phase II dose is 5-FU 1.0 mg/m² twice daily with eniluracil 20 mg twice daily. Mean (SD) values for terminal half-life, apparent volume of distribution, and systemic clearance of 4.5 hours (0.83 hours), 19 L/m² (3.0 L/m²), and 51 mL/min/m² (13 mL/min/m²), respectively. An average of 77% of 5-FU was excreted unchanged in urine after 28 days of treatment. The mean (range) 5-FU C(SS,min) values achieved at the 1.0 mg/m² dose level were 22 ng/mL (8 to 38 ng/mL). Conclusion: Chronic oral administration of 5-FU with oral eniluracil is tolerable and produces 5-FU steady-state concentrations similar to those achieved with protracted intravenous administration of 5-FU on clinically relevant dose schedules. Eniluracil provides an attractive means of administering 5-FU on protracted schedules. (C) 2000 American Society of Clinical Oncology.