Pharmacokinetics of 5-fluorouracil administered orally, by rapid intravenous and by slow infusion.

Pharmacokinetic studies of 5-fluorouracil (5-FUra) were performed on 18 patients divided into three groups: seven patients were given 5-FUra i.v. by rapid injection; five patients received the drug p.o.; and six patients were treated by continuous i.v. infusion for 96 hr. The results showed rapid i.v. injection to be manifested by high early levels of drug achieved both in plasma and bone marrow with a rapid fall afterwards. Administration of 5-FUra p.o. gave rise to erratic plasma values due to greater variability in absorption, whereas 96-hr i.v. infusions showed constant levels of the drug in plasma and significantly (50- to 1000-fold) less 5-FUra in bone marrow. The main difference observed between rapid injection and slow infusion in the kinetics of the drug was the very high level of 5-FUra reached by rapid injection in plasma and bone marrow, which was of short duration (min) when compared to the low sustained levels observed during infusion. This route-dependent pharmacokinetic profile is consistent with the reported absence of myelosuppression in prolonged infusion and may be related to the resultant lower levels of 5-FUra achieved in bone marrow.