Chemical stability of amorphous materials: specific and general media effects in the role of water in the degradation of freeze-dried zoniporide.

The objective of the present work was to determine whether hydrolysis in a model lyophile was influenced by general media effects with water-changing properties of the medium or via a specific mechanism of water as a reactant. Four formulations of zoniporide and sucrose (1:10) were prepared with variable amounts of sorbitol [0%-25% (w/v) of total solids). These formulations were then equilibrated at 6% and 11% relative humidity using saturated salt solutions. The lyophile cakes were analyzed by differential scanning calorimetery (DSC), (isothermal microcalorimetry (IMC), solid- state nuclear magnetic resonance (ssNMR) spectroscopy, and ultraviolet-visible diffuse reflectance (DFR) spectroscopy. DSC and IMC were used to assess the global molecular mobility. ssNMR relaxation times were measured to access local mobility. The DFR was used to determine the solid-state acidity expressed as the Hammett acidity function. Stability of samples was evaluated at 40°C by monitoring potency and purity by high-performance liquid chromatography (HPLC). Results were interpreted in terms of the various roles of water: media effect, plasticization, polarity, and reactant. The kinetics of hydrolysis was observed to be correlated with either/both specific "chemical" effects, that is, water reactant as well as media effect, specifically global molecular mobility of the matrix. Increase in reaction rate with increase in water content is not linear and is a weaker dependence than in some hydrolytic reactions in organic solvents. A moderate amount of an inert plasticizer, sorbitol, conferred additional stabilization, possibly by restricting the amplitude and frequency of fast motions that are on a small length scale.