Comparison of Pharmacokinetics of 5-fluorouracil and 5-fluorouracil with Concurrent Thymidine Infusions in a Phase I Trial

The serum half-life of 5-fluorouracil (5-FUra) in humans is best described as a biexponential decay function, with t1/2 alpha = 7.8 +/- 2.6 (S.E.) min and t1/2 beta = 36.8 +/- 13.5 min during initial courses of this drug alone. Pharmacokinetics of 5-FUra during courses of daily therapy (for 5 days) revealed prolongation of t1/2 in both components of the decay curve, which has not been previously reported. Despite the efficacy of thymidine (dThd) given as a continous i.v. infusion of 8 g/sq m/day in prevention of high-dose methotrexate toxicity, continuous infusion of dThd at this dose does not prevent the toxicity of 5-FUra orreverse inhibition of DNA and RNA synthesis by 5-fura. On the contrary, continuous infusion of dThd appears to increase the toxicity of 5-FUra during continuous dThd infusion revealed prolongation of the 5-FUra t1/2 which remained stable through the course of 5 days of 5-FUra with dThd. This protracted t1/2 is believed to account at least in part for the increased toxicity of 5-FUra with dThd. Dose-limiting mucositis, myelosuppression, and gastrointestinal toxicity were observed at 5-FUra doses ranging from one-half to two-thirds the customarily tolerated dose of 5-FUra alone in similar courses of daily bolus therapy (for 5 days).