Mannitol is popular as a crystalline bulking agent in freeze-drying, but tends to form different crystalline modifications which may negatively impact storage stability. Mannitol-based pharmaceutical formulations usually contain additional formulation additives which may alter the extent of crystallization and the modification of mannitol. In this study, the impact of pure sucrose, trehalose, polysorbate 80, and citric acid as well as multi-component systems of these additives, on mannitol crystallization was investigated. Pure mannitol and 10 formulations with excipients were lyophilized using identical freeze-drying conditions. The product cakes from center and edge vials were characterized using Karl Fischer residual-moisture analysis, X-ray powder diffraction, and differential scanning calorimetry. The study revealed substantial differences in residual moisture content and significant changes in crystallinity induced by even small amounts of additives, especially in the case of sucrose.
Mannitol is the most commonly used bulking agent in freeze-drying formulation design. The benefit of using mannitol is that it crystallizes during freezing and permits drying processes at higher product temperatures, and thus with higher sublimation rates relative to purely amorphous systems (1). Mannitol, however, is known to form different crystalline modifications which compromises reproducibility of product characteristics and storage stability due to phase transformations (2, 3). The different modifications have been described extensively in the literature (4, 5). Because freeze-dried mannitol-based pharmaceuticals typically contain other excipients to fulfill the stabilization needs of the active pharmaceutical ingredient, the extent of crystallization and modification may be different than for pure mannitol (6).This study investigates the influence of frequently used additives on the crystallization of mannitol. The additives used were: sucrose and trehalose, both widely used as lyoprotectants to stabilize proteins or peptides during drying; polysorbate 80, a surfactant frequently added to avoid surface denaturation of proteins during freezing; and citric acid, a popular buffer system (1).
Materials and methods
Residual moisture measurements. Residual moisture was determined by coulometric Karl-Fischer titration using a water vaporizer (Mitsubishi Chemical Company CA-06 with VA-06, Tokyo, Japan). Approximately 35–90 mg of the solid samples were transferred to the vaporizer unit where the sample was heated to 140° C. Extra dry nitrogen was used as a carrier to transfer the moisture into the measurement cell.
Differential scanning calorimetry. The authors used a Mettler Toledo 822 DSC with STARe SW 9.01 software (Mettler Toledo, Switzerland) for thermal analysis. Approximately 20 mg of sample was weighed into a 40 μL aluminum pan and compressed in a dry-nitrogen atmosphere. Pans were then sealed hermetically and transferred into the DSC cell. Two different methods were applied. First, all formulations were scanned with a ramp rate of 10 °C/min from 0 °C–180 °C to reveal potential glass transitions, melting events, or recrystallization. A lower ramp rate (2.5 °C/min) was used for some formulations showing inconclusive results at fast ramp rates.