Impact of freezing procedure and annealing on the physico-chemical properties and the formation of mannitol hydrate in mannitol–sucrose–NaCl formulations

The goal was to investigate the impact of NaCl on the physico-chemical properties of mannitol–sucrose formulations during freezing and drying, with special focus on mannitol hydrate formation. Differential scanning calorimetry (DSC) and low-temperature X-ray powder diffraction (LTXRD) were used to study the frozen solutions. After lyophilization the products were analyzed with DSC, temperature-modulated DSC (TMDSC), X-ray powder diffraction (XRD) and Karl-Fischer titration. DSC showed an inhibition of mannitol crystallization by sucrose and NaCl during freezing. The glass transitions of the maximally freeze-concentrated solutions (Tg′) were lowered by both mannitol and NaCl. By the application of an annealing step during lyophilization mannitol crystallinity could be increased. However, lyophilization with an annealing step promoted the formation of mannitol hydrate, which is known to undergo conversion into the anhydrous polymorphs of mannitol upon storage. LTXRD revealed that mannitol hydrate was formed at temperatures below −30 °C, but not at −27 °C. The tendency that mannitol hydrate is predominantly formed at lower temperature was confirmed by XRD of lyophilized products, produced at different annealing temperatures. For the development of lyophilization cycles the lowered Tg′, as well as the tendency to mannitol hydrate formation predominantly at lower temperature needs to be considered.