Characterization of the Sucrose/Glycine/ Water System by Differential Scanning Calorimetry and Freeze-Drying Microscopy

Abstract

The objective of this study was to characterize the thermal properties of systems containing various ratios of amorphous and crystalline components using both differential scanning calorimetry (DSC) and freeze-drying microscopy. The glycine/sucrose system was used as a model system, since it is routinely used in protein formulations. DSC analysis revealed that the addition of glycine to sucrose solutions resulted in a decrease in the glass transition (T'g) of the system. The T'g of a pure sucrose solution (7% w/v) decreased from -32.3 to -51.5 degrees C for a mixture containing a sucrose/glycine ratio of 2:5. The glass transition of the sucrose/glycine mixture decreased linearly as more glycine was added to the system. This decrease in glass transition resulted in severe collapse during freeze-drying of these mixtures above T'g. However, collapse was not observed during freeze-drying if the DSC thermogram of the sucrose/glycine mixture exhibited a transition resulting from recrystallization of the amorphous glycine. Mixtures having a sucrose/glycine ratio of 3:4 and 2:5 had a glass transition of -48 degrees C and -51.5 degrees C, respectively. Despite their low glass transition temperatures, these samples freeze-dried readily at a product temperature > T'g using a fast freeze-drying cycle (primary drying at a shelf temperature of +20 degrees C and chamber pressure of 100 mTorr) without any sign of collapse. The crystallization of the amorphous glycine from the frozen mixture of sucrose and glycine provided support during freeze-drying which prevented the macroscopic collapse of the final product. Freeze-drying microscopy visually revealed the crystallization and allowed for prediction of cake quality upon lyophilization. Although the freeze-drying microscope is not as sensitive as the DSC in detecting all transitions (it cannot detect a glass transition), it clarifies the interpretation of DSC, and together they provide valuable information regarding the relevance of each of the transitions to the final freeze-dried product elegance.

Determination of the Influence of Primary Drying Rates on the Microscale Structural Attributes and Physicochemical Properties of Protein Containing Lyophilized Products

“…The T g ' of sucrose is reported as À328C. 22 However, it has been reported that adding increasing amounts of protein to most formulations leads to a higher T c and T g ', 1,21 and, importantly in the context of this work, the difference between T g ' and T c (T c ¼ the temperature at which the freeze-drying front completely collapses) 12 can increase at increasing higher protein loadings. 1,18,23 The T g ' and T c of the sucrose-containing protein formulations in this study is À33 and À288C for the 5 mg mL À1 BSA formulation and À26 and À98C for the 50 mg mL À1 BSA formulation respectively.…”

Infrared spectroscopic studies of protein formulations containing glycine

“…18 As pointed out earlier, analysis of lyophiles of pure glycine, as well as sucrose-glycine mixtures by XRD, confirmed that b-glycine was the only crystalline polymorph present. a-Glycine and g-glycine were absent in all the samples.…”

Freeze-Dry Microscopy of Protein/Sugar Mixtures: Drying Behavior, Interpretation of Collapse Temperatures and a Comparison to Corresponding Glass Transition Data

“…In a few cases, both T c and T 0 g values for excipients or a final formulation are reported. 9,31 An important observation for freezedrying cycle design is that collapse temperatures have been reported 2-58C above the corresponding T 0 g for various systems; 19 in short, the two methods do not necessarily measure the same property, and the difference is not negligible. In addition, information about the temperature ''tolerance'' of a given product to product temperature excursions exceeding the predefined critical temperature is helpful for robustness testing in process design, but such data are typically not available in the literature.…”