The Influence of Mannitol Hemihydrate on the Secondary Drying Dynamics of a Protein Formulation: A Case Study

The objective of this research was to study the atypical secondary drying dynamics observed during the freeze-drying of a formulation consisting of mannitol, disaccharide, and sodium chloride, where “bursts” of water vapor release were observed during secondary drying as detected by comparative pressure measurement. “Thief” samples were removed at the end of primary drying and during secondary drying as the shelf temperature was increased in a stepwise fashion. These samples were examined by X-ray powder diffraction and thermal analysis. From the X-ray powder diffraction data, we determined that mannitol crystallized predominantly as its hemihydrate. The physical state of mannitol changed from the hemihydrate form to anhydrous forms during secondary drying. Investigation of the effect of excipients on mannitol crystallization demonstrated that sodium chloride (at 225 mM, 1.3% w/v) had the greatest influence on hemihydrate crystallization, followed by trehalose and sucrose. However, only negligible hemihydrate formation was observed when mannitol was freeze-dried either by itself or in the presence of 150 mM sodium chloride and no hemihydrate in the presence of 75 mM sodium chloride. In general, a combination of a disaccharide and sodium chloride promoted the hemihydrate formation to a greater extent than the individual components. Comparative pressure measurement was demonstrated to be an effective tool to monitor mannitol hemihydrate dehydration during secondary drying.