Phase I and pharmacologic study of 72-hour infused 5-fluorouracil in man

The relationships between the administered dose, clearance, and the toxicity spectrum of 5-fluorouracil (5-FU) administered as 72-hour constant infusion have been studied in 21 patients with advanced cancer. This was done as a pilot study for possible future combination using 5-FU as a radiosensitizer. Individual patients tolerated up to 65 mg/kg/24 hours, but serious toxicity appeared once as low as 35 mg/kg. Limiting toxicity proved to be "mixed" with upper intestinal symptoms (nausea and vomiting), stomatitis, and central nervous system signs all occurring in various patients. Central nervous system effects (both cerebellar and vomiting) proved as troublesome as stomatitis. There was only a general link between the administered dose and the subsequent toxicity grade, but a reasonably quantitative relationship emerged when the serum 5-Fu levels obtained and the degree of patient toxicity were compared. The clearance of 5-FU was confirmed to be nonlinear over the entire dose range studied (25-65 mg/kg/24 hours), consistent with a two-compartment model of drug metabolism. One compartment appears to be systemic (extra-hepatic) metabolism (probably anabolic removal) which is saturated at just below 15 mg/kg/day. Doses above that level lead to drug accumulation. No steady state was reached, contrary to previous reports. At the higher infusion rates, clearance progressively approaches that predicted by the assumption that the second compartment is splanchnic blood flow and catabolism. While 5-FU can be administered as a 72-hour infusion as one possible schedule for use as a single agent or for combined modality studies, CNS effects are quite troublesome in comparison to longer infusions to toxicity.