Application of slurry bridging experiments at controlled water activities to predict the solid-state conversion between anhydrous and hydrated forms using theophylline as a model drug

Abstract

The role of water activity ( a w), relative humidity (RH) and temperature on the hydration state of theophylline has been investigated. Slurry bridging experiments at controlled water activities, using powder X-ray diffraction (PXRD) and thermogravimetric analysis (TGA) to characterise the solid phase, established that the hydrate is the thermodynamically stable form of theophylline at a w≥0.5 at 4 °C, a w≥0.64 at 30 °C, and a w≥0.76 at 40 °C. These data were used to produce a phase stability diagram for anhydrous/hydrate theophylline versus temperature. Anhydrous theophylline was spray dried in an attempt to reduce crystallinity. The spray dried theophylline was stored at a range of temperatures (4–40 °C) and humidities (22–89% RH). Samples were analysed at 3, 6, 9, 26 and 52 weeks using TGA and at the 26 and 52 weeks by PXRD. The solid state stability of the spray dried theophylline closely correlated to the phase stability diagram produced using the slurry bridging experiments. The data suggest that the slurry bridging technique at controlled water activities provides an accurate method of rapidly predicting the physically stable form in anhydrous/hydrate systems.