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The purpose of this study was to evaluate the effect of taste-masking excipients on in vitro and in vivo performance of a leuprolide metered-dose inhaler (MDI) suspension formulation. Taste-masking excipients (aspartame and menthol) were added to a leuprolide suspension MDI formulation. The leuprolide MDI formulation with the taste-masking excipients was characterized in terms of milling time, particle size distribution, dose delivery and uniformity, and drug absorption in dogs. The data were compared with a formula that did not contain taste-masking excipients. It was found that the longer milling time for the leuprolide suspension with the taste-masking excipients was required to obtain a similar particle size distribution compared with the formula without taste-masking excipients using a fluid energy mill. Although measurable differences in mass median aerodynamic diameter (MMAD) and geometric standard deviation (GSD) were not observed between the two formulations, the percent of particles < or = 5 microns and the actuator retention for the formula with the taste-masking excipients were significantly different from the formula without taste-masking excipients using the Marple-Miller cascade impactor. Taste-masking excipients did not show a significant effect on valve delivery and through-can dose uniformity. However, the mean ex-actuator dose was 150.4 mg for the formula with the taste-masking excipients and 162.2 mg for the reference formula, respectively, indicating a significant difference. In tracheostomized dogs, both formulations showed comparable pharmacokinetic parameters including Cmax, Tmax, AUC0-12 and bioavailability (F%), indicating that the taste-masking excipients do not have an effect on lung absorption of leuprolide acetate. Therefore, inclusion of taste-masking excipients in the leuprolide MDI suspension formulation showed a significant impact on drug micronization, exactuator dose, and particle deposition pattern. Mechanistically, the unfavorable performance of leuprolide MDI in the presence of taste-masking excipients could be due to modification of the properties of the suspension itself and alteration of propellant evaporation following actuation.

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.