Solid State Forms of Theophylline: Presenting a New Anhydrous Polymorph

A previously unreported anhydrous polymorph (Form IV) of theophylline has been crystallized and characterized. Solubility determination, crystallization, and equilibration experiments indicated that Form IV is the most thermodynamically stable anhydrous polymorph. The hydration behavior of theophylline was investigated by three different methods: solubility, crystallization, and slurry experiments. The value of water activity at which the monohydrate form of theophylline becomes the most thermodynamically stable was investigated, and these results help to explain differences reported in the literature. Using solvent mixtures with a range of water activity values, it was demonstrated that the monohydrate, Form M, is produced from mixtures with a w ≥ 0.70 at 25 °C.