Protein binding of flavoxate and of 3-methylflavone-8-carboxylic acid.

The protein binding properties of piperidino-3-methylflavone-8-carboxylate (flavoxate, F) and of its main metabolite, i.e. 3-methylflavone-8-carboxylic acid (M), were investigated by dialysis-equilibrium technique. During the dialysis, F hydrolyzes to a notable extent into M. Therefore a true equilibrium of F could not be obtained. With this limitation, the results show that F has a small protein affinity. The protein binding properties of M were separately investigated with bovine albumin, human albumin and with human, rat, rabbit and dog plasma. The Freundlich's isotherms, which describe the affinities of albumin to M, were calculated. They show that unconjugated M, possibly present in plasma, is by 99.5% in a protein-bound form.