Freeze-drying of isomalt diastereomer mixtures and their effects on protein stabilization during freeze-drying

The number of protein-based pharmaceuticals has rapidly increased in the last decade and their share of new active pharmaceutical ingredients is high. Because proteins as molecules are prone to stability issues, some protein pharmaceuticals must be formulated as solid dosage forms if their stability in aqueous solutions is not sufficient. Freeze-drying is one of the major technologies used to produce solid protein pharmaceuticals but it also subjects proteins to many degradative stresses. Therefore, stabilizing excipients must be incorporated in the formulations. The aim of this thesis was to study isomalt as a novel excipient for freeze-dried formulations. Isomalt is a non-reducing sugar alcohol currently used as an excipient in commercially available solid oral dosage forms. Isomalt was freeze-dried in four different diastereomer compositions and its solid state characteristics were extensively analyzed with diffractometric, thermal and vibrational spectroscopic methods, coupled with multivariate analysis and quantum chemical calculations. After freeze-drying, the stability of amorphous isomalt was evaluated to observe any changes occurring in solid state properties during storage in the presence of moisture. The protein-stabilizing effects of isomalt were studied during freeze-drying and subsequent storage with lactate dehydrogenase (LDH). Sucrose, a standard freeze-drying excipient, served as a comparison to isomalt in this study. Protein stability was analyzed using protein activity tests and circular dichroism and in addition to that, the physical stability of the samples was also analyzed. The complementary analytical techniques showed that plain isomalt was successfully transformed from a crystalline to amorphous form with freeze-drying and three out of the four diastereomer compositions studied remained stable during storage. During storage at the highest relative humidity condition, one of the diastereomer compositions showed signs of physical instability. All in all, isomalt was shown to be a suitable excipient for freeze-drying as its stability in the amorphous form proved to be good. When isomalt and sucrose were freeze-dried with LDH, the results showed that LDH activity was almost fully retained after freeze-drying with sucrose, whereas isomalt performed inferiorly as a stabilizing excipient. During storage however, the protein-stabilizing effects of isomalt were noticeable, as only moderate loss in protein activity occurred, whereas samples containing sucrose lost most of their LDH activity. In summary, isomalt was shown to be a suitable excipient for freeze-dried formulations based on its physical properties. Isomalt had also some potential as a protein-stabilizing excipient during freeze-drying and subsequent storage. Future studies with different proteins are required to evaluate the potential of isomalt as a novel cryo- and lyostabilizing excipient in lyophilized protein formulations