Modulating the Physical Form of Mannitol Crystallizing in Frozen Solutions: The Role of Cosolute and Processing

Mannitol is widely employed as a bulking agent in lyophilized formulations. Our goal was to evaluate the role of noncrystallizing cosolutes in inhibiting mannitol crystallization and preventing the formation of mannitol hemihydrate (MHH) in frozen solutions. The individual influence of two common stabilizers (sucrose and trehalose) and three model proteins (lysozyme, bovine serum albumin, and immunoglobulin G) on the crystallization behavior of mannitol was investigated by differential scanning calorimetry (DSC) and X-ray diffractometry (XRD). Sugars exerted a more pronounced crystallization inhibitory effect than proteins. In the presence of sugars, mannitol predominantly crystallized as MHH while the proteins facilitated the crystallization of δ-mannitol. Annealing the frozen solutions at −25 °C favored MHH crystallization. A higher annealing temperature of −10 °C accelerated mannitol crystallization and promoted the formation of the anhydrous δ-polymorph. The crystallization inhibitory effect of proteins was surmounted with annealing, while at a high sugar concentration, a substantial fraction of mannitol was retained amorphous even after annealing.