Pharmacokinetics of flavoxate in rats.

The plasma levels, the urinary excretion and the biliary excretion of piperidinoethyl-3-methylflavone-8-carboxylate (flavoxate, F) were studied in rats after i.v. and after oral administration. Parallel experiments were made with 3-methyl-flavone-8-carboxylic acid (M), the main metabolite of F. The substances are found in blood and are excreted in the urine and in the bile. The quantities excreted in the urine after oral administration are similar to those excreted after i.v. administration, showing that the enteric availability of the drugs is almost complete. The end product in urine and in bile is represented by a substance which yields M after a strong acid hydrolysis. There are marked pharmacokinetic differences between F and M, probably related to their physical properties.