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Characterizing amorphous materials with gravimetric vapour sorption techniques: even small amounts of amorphous materials can have a significant effect on the drug product. Are gravimetric vapour sorption techniques an effective solution to characterize amorphous materials?

Article Excerpt
Amorphous materials in pharmaceutical formulations yield complex and challenging problems concerning the performance, processing and storage of these products. (1) The presence of amorphous materials can be wanted or unwanted, depending on the desired or undesired unique properties of the amorphous state. Additionally, during the processing of pharmaceutical solids (e.g., milling, spray drying, tablet compaction, wet granulation and lyophilization), various degrees of disorder in the form of crystal defects and/or amorphous regions may be generated. Even relatively low levels of amorphous material (<10%) may have a detrimental impact on the stability, manufacturability and dissolution characteristics of the formulated drug product. Amorphous materials are inherently metastable and tend to revert to a more thermodynamically stable, crystalline form. As this instability has a potentially negative impact on storage and drug potency, it is important to quantify and control the amorphous content of pharmaceutically relevant materials. For these reasons, investigating the amount and stability of amorphous material is critical.

Gravimetric vapour sorption methods have been widely used to study amorphous materials; for example, several different gravimetric vapour sorption techniques have been developed to quantify amorphous contents below 1%. (2-5) Also, gravimetric vapour sorption has been used to investigate moisture-induced phase changes, (6-8) as well as vapour-induced crystallization kinetics. (9-13) This paper outlines several recent applications of gravimetric vapour sorption techniques to characterize amorphous materials.

Instrumentation and methods

Gravimetric vapour sorption instruments are well-established methods for determining vapour sorption isotherms. These instruments measure the sample mass change as the vapour environment surrounding the sample is altered. An increase in mass is typically associated with vapour sorption, while a decrease is caused by vapour desorption. The vapour concentration...

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