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The purpose of this study was to evaluate in vitro enzymatic degradation and protection of leuprolide acetate in the mucosal homogenates of rat small intestine. When leuprolide was incubated at 37 degrees C with the homogenates, it was degraded quickly. The apparent Michaelis-Menten constant, K(m), and the maximal reaction velocity, Vmax, for leuprolide were 898 mM and 3.4 nmol/min/mg protein, respectively. At least four metabolites of leuprolide were observed in HPLC chromatograms, which were related to cleavages by some serine proteases. In the presence of protease inhibitors in the incubation medium, degradation of leuprolide was significantly suppressed by antipain and 3,4-dichloroisocoumarin (DCI), whereas bestatin and p-hydroxymercuribenzoic acid (PCMB) showed weaker protection than antipain and DCI, and alpha 2-macroglobulin (MG) exhibited no protection. When a w/o/w emulsion formulation was used, rapid degradation of the drug in intestinal homogenates was also inhibited. Therefore, the present study with representative protease inhibitors and a w/o/w formulation revealed that the enzymatic degradation of leuprolide is preventable in the rat intestinal mucosal homogenates.