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Dynamic vapour sorption of freeze-dried pharmaceuticals

22 October 2015  •  Author(s): Claudia Kunz and Henning Gieseler, Department of Pharmaceutics, University of Erlangen

Freeze drying is gaining in importance as the number of biopharmaceuticals that are unstable in a solution increases. According to recent reports, a growth of 10% may be expected for freeze-dried products in the next 10 years. The technique offers the opportunity to gently dry temperature-sensitive drugs such as proteins or peptides. Since freeze drying is a rather expensive drying technology, formulation and process optimisation strongly focus on the design of a robust and fast cycle. Interestingly, partially crystalline systems offer advantages with regard to processability as well as product appearance.

Dynamic vapour sorption of freeze-dried pharmaceuticals

Both formulation and process parameters affect the degree of crystallinity of excipients. For example, a high total or relative concentration of the bulking agent facilitates its crystallisation. Meanwhile, the freezing step is important, since freezing conditions strongly influence the crystallisation tendency of formulation compounds. A slow freezing rate and/or exposure to a temperature below the freezing point but well above the glass transition temperature of the maximally freeze-concentrated solute, Tg′, support crystallisation. In addition, during secondary drying, crystallisation may be finalised for several excipients, such as mannitol, and hydrates can be eliminated.

Accurate determination of the degree of crystallinity is imperative in drawing a reliable conclusion about process and formulation performance. In the field of freeze-dried products, differential scanning calorimetry (DSC) and X-ray powder diffraction (XRPD) are established methods for this purpose. However, dynamic vapour sorption (DVS) offers a new opportunity to measure the degree of crystallinity of the formulation components in a more representative way…

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