Flavoxate and 3-methylflavone-8-carboxylic acid. Assay methods in blood and urine, plasma-red cells repartition and stability.

The following assay methods for pharmacokinetic studies on flavoxate (F) and on its main metabolite, i.e. 3-methylflavone-8-carboxylic acid (A), are described. 1. Spectrophotometry for the assay of F and of A in plasma, 2. TLC-Spectrodensitometry and GLC for the assay of A in urine after acid hydrolysis, 3. TLC-Spectrodensitometry for determining the F : A ratio in plasma or in urine. It was found that F hydrolyzes into A. This process depends on the pH and on the medium. In water, at pH 5.0, F is stable, while in phosphate buffer at pH 7.4 the semi-hydrolysis time is 60 min. In a solution with bovine serum albumin, in rat, rabbit, dog or human plasma the semi-hydrolysis times are between 5 and 60 min. Finally the plasma-red cells repartitions of F and of A were studied in vitro in rat, rabbit, dog and human blood and found between 0.8 and 2.0 for F and between 2.1 and 4.6 for A.