Effect of uridine coadministration on 5'-deoxy-5-fluorouridine disposition in rats

Uridine (UR) inhibits the metabolic activation of 5'-deoxy-5-fluorouridine (dFUR) to 5-fluorouracil (FU) by the intestinal pyrimidine nucleoside phosphorylases and could potentially reduce its intestinal toxicity. This study examined the effect of UR coadministration on the absorption and disposition of an oral dose of dFUR. Rats were given dFUR alone (500 mg kg-1) and dFUR (300 mg kg-1) plus UR (4.5 g kg-1) in a random crossover experiment. Simultaneous injection of a tracer dose of [6-3H]dFUR was used to asses the total body clearance (Cl) of dFUR. The absorption of UR was rapid and variable. The UR dose produced a maximal blood concentration of 80 micrograms/ml for UR and 100 micrograms/ml for its metabolite uracil (U). The absorption of dFUR was slower than UR, as indicated by its later time of maximal concentration. UR did not alter the Cl of dFUR, but reduced the absorption rate of dFUR from the gastrointestinal tract and significantly reduced the absolute oral bioavailability of dFUR from 55.2% to 33.4%. The effects of UR coadministration on the dFUR metabolite FU were opposite to those on dFUR; the FU availability was increased sixfold, and the elimination of FU was reduced. Based on the known competition between pyrimidine bases for their saturable metabolic enzymes, the increase in FU availability by UR coadministration was likely due to a competitive inhibition of FU metabolism by U. This study established the complex pharmacokinetic interactions between dFUR and UR and between their metabolites, which may be important in the modulation of dFUR activity by UR.