The influence of excipients on the stability of the moisture sensitive drugs aspirin and niacinamide: comparison of tablets containing lactose monohydrate with tablets containing anhydrous lactose.

The purpose of this study is to test the hypothesis that in tablet formulations, moisture-sensitive drugs formulated with lactose monohydrate have the same stability as formulations containing anhydrous lactose, and to characterize the kinetics of niacinamide degradation in the solid state. Aspirin and niacinamide decomposition were used as indicators of stability. Aspirin and niacinamide tablets containing either lactose monohydrate or anhydrous lactose were separately investigated at different temperatures and relative humidities; the stability tests were done at 25 degrees C--60% RH, 40 degrees C--80% RH, 60 degrees C--60% RH, 60 degrees C--80% RH, and 80 degrees C--80% RH. Official U.S. Pharmacopeia methods were used for the aspirin and niacinamide assays. Statistical analysis showed that tablets containing lactose monohydrate have the same stability as tablets containing anhydrous lactose, which means that even though water is present in the crystal structure, the bound water does not influence the reaction rate. In addition, niacinamide degradation in the solid-state can be described by a third order rate equation.