In vitro studies on the hydrolysis of frusemide in gastrointestinal juices.

To elucidate a possible explanation for a relatively low bioavailability, the hydrolysis of frusemide in gastrointestinal juices was studied in vitro at concentrations likely to be present in vivo. Between 1.0 and 4.4% of the frusemide molecules were hydrolyzed to 4-chloro-N-furfuryl-5-sulphamoyl-antranilic acid (CSA) after 1 h at 37 degrees C in gastric juices. The rate of hydrolysis was inversely connected to pH. No fall in frusemide concentration was observed and no CSA was found in duodenal juices after 4 h at 37 degrees C. In three buffer solutions with the same pH as three gastric juices frusemide was hydrolyzed to CSA at a lower rate than in the gastric juices (pH 1.2, P less than 0.15;pH 1.4, P less than 0.001; pH 1.6, P less than 0.001). The solubility of frusemide was significantly higher in gastric juice from two fasting subjects (83-104 mg l-1) than in buffer solutions with the same pH (52-58 mg 1-1). The solubility of frusemide was significantly increased (by 40-50%) in gastric juice obtained after pentagastrin stimulation compared with its solubility in the mixed gastric secretion obtained after fasting. The binding of frusemide to macromolecules was 28.0 +/- 9.7% in ventricular secretion after fasting while it was 1.4 +/- 2.6% in the fluid obtained after pentagastrin stimulation. It is concluded that a hydrolysis of frusemide in the stomach prior to absorption cannot explain the relatively low bioavailability of the drug observed after oral intake.