5-Fluorouracil concentrations in rat plasma, parotid saliva, and bile and protein binding in rat plasma

The pharmacokinetics of 5-fluorouracil were studied over a 60-min period in rats that received 12.5, 25.0, and 50.0 mg/kg iv. The plasma concentration-time relationship and the detectability in bile and parotid saliva (a route of elimination heretofore given little or no attention) were examined. Protein binding of 5-fluorouracil at concentrations chosen to approximate those found in plasma was determined by equilibrium dialysis. Bile-plasma and parotid saliva-plasma concentration ratios were calculated. 5-Fluorouracil concentrations were quantitated by high-performance liquid chromatography. Plasma concentrations at all doses studied appeared to rapidly decline. The half-life, however, at the 50.0-mg/kg dose (27 min) was significantly longer (p less than 0.025) than the corresponding half-life at the 25.0-mg/kg dose (22 min). This may be attributed to an easily saturable hepatic degradation. Although an observed decline in bile-plasma and parotid saliva-plasma concentration ratios at higher doses may represent saturation of these excretary routes, the small amounts of 5-fluorouracil detected in bile and parotid saliva probably contribute negligibly to the elimination of the total drug equivalents administered. Parotid saliva-plasma concentration ratios were not useful in predicting plasma protein binding as determined by equilibrium dialysis. Excretion of intravenously administered 5-fluorouracil in saliva, however, exposes the upper GI tract to this agent and may play a part in causing stomatitis in patients receiving the drug by this route.