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Understanding the Tendency of Amorphous Solid Dispersions to Undergo Amorphous–Amorphous Phase Separation in the Presence of Absorbed Moisture

Formulation of an amorphous solid dispersion (ASD) is one of the methods commonly considered to increase the bioavailability of a poorly water-soluble small-molecule active pharmaceutical ingredient (API). However, many factors have to be considered in designing an API–polymer system, including any potential changes to the physical stability of the API. In this study, the tendency of ASD systems containing a poorly water-soluble API and a polymer to undergo amorphous–amorphous phase separation was evaluated following exposure to moisture at increasing relative humidity. Infrared spectroscopy was used as the primary method to investigate the phase behavior of the systems. In general, it was observed that stronger drug–polymer interactions, low-ASD hygroscopicity, and a less hydrophobic API led to the formation of systems resistant to moisture-induced amorphous–amorphous phase separation. Orthogonal partial least squares analysis provided further insight into the systems, confirming the importance of the aforementioned properties. In order to design a more physically stable ASD that is resistant to moisture-induced amorphous–amorphous phase separation, it is important to consider the interplay between these properties.