| The chemical reactivity and physical stability of model-lyophilized systems were investigated. The extent of ionization of several acid-base indicators in amorphous sucrose and trehalose matrices were measured by diffuse reflectance UV-Vis spectroscopy, from which the Hammett acidity functions were calculated. The acidity of the solid lyophile can show pronounced differences from the solution pH prior to lyophilization. The Hammett acidity functions depend on the type of buffer used (citrate, phosphate, malate, succinate, tartarate, and glutarate), water content, and indicator concentration. Using PVP and dextran as model matrices, the rates of sucrose inversion reaction were correlated to the Hammett acidity function in the amorphous lyophiles. While the molecular mobility measured by microcalorimetry were comparable in the two systems, the significantly higher inversion rates in dextran systems were attributed to its lower acidity. Our next objective was to identify processing conditions and formulation variables so as to primary dry at temperatures substantially above the collapse temperature. The water-rich sections of the water-glycine-raffinose and water-glycine-trehalose solid-liquid state diagrams were investigated by differential scanning calorimetry and X-ray powder diffractometry to determine the extent of glycine crystallization. On subsequent freeze-drying, the crystalline to amorphous solute ratios were identified, above which primary drying was carried out at least 20°C higher than the collapse temperature. When dried under these conditions, there was significant retention in the activity of the model protein, lactate dehydrogenase. Finally, in a different model system, the effect of annealing on raffinose crystallization was investigated. The phase separation due to annealing-induced crystallization led to significant loss of protein activity during freeze-drying. Formulation variables and processing conditions were important attributes for a successful freeze-drying cycle. |
