RESUMEN
To improve oral absorption of sanfetrinem, a broad-spectrum, beta-lactamase-stable antibiotic, two different ester prodrugs have been selected. Both prodrugs proved to be readily hydrolyzed after absorption before reaching the systemic circulation. The objective of this study was to evaluate the role of intestinal and liver metabolism in the conversion of the two prodrugs into the active compound. In vitro experiments were performed in different rat tissues involved in the absorption process. Moreover data obtained with in vitro experiments have been integrated with data obtained in vivo using a surgically prepared rat model which allows for the measurement of the amount of intact prodrug that overcomes the intestinal mucosa and its presence in the portal vein. Both prodrugs proved to be readily cleaved by jejunum and liver microsomes. The rates of ester hydrolysis with these two tissues were 10- to 30-fold higher than those calculated in intestinal juice at pH 7.4 and about 100-fold higher than in buffer at pH 5.5. These data suggest that both the intestinal wall and liver could play an important role in the conversion of the two prodrugs in active parent compound. In the in vivo experiment, relative to sanfetrinem levels, very low concentrations of intact esters were measured in the portal vein blood, indicating that the two prodrugs are nearly completely hydrolyzed to the active drug by the intestinal wall. In conclusion this study demonstrated that the intestinal epithelium plays a major role in the conversion of the two prodrugs into sanfetrinem. The liver, despite its high esterase activity seems to be only marginally involved.