Plasma levels of 5-fluorouracil after oral and intravenous administration in cancer patients.

1. Plasma levels of 5-fluorouracil (5FU) have been determined in eleven cancer patients after 0.5 g and 1.0 g intravenous doses, and in one patient after paired 1.0 g oral and intravenous doses. 2. The plasma half-life after the 0.5 g intravenous dose was relatively constant, irrespective of the stage and spread of the disease. 3. Plasma kinetics of the drug were dose dependent. Doubling of the intravenous dose produced a 1.5-fold increase in plasma half life, a two-fold increase in initial plasma drug concentration, and a three-fold increase in area under the concentration/time curve. 4. In one patient receiving paired 1.0 g intravenous and oral doses nine weeks apart, an increase in the bioavailability of the drug coincided with a marked clinical regression in palpable intra-abdominal metastases. 5. The significance of measuring plasma drug kinetics and their relationship to drug efficacy and toxicity are discussed.