Crystalline and amorphous dispersions have been the focus of academic and industrial research due to
their potential role in formulating poorly water-soluble drugs. This review looks at the progress made starting
with crystalline carriers in the form of eutectics moving towards more complex crystalline mixtures. It also covers
using glassy polymers to maintain the drug as amorphous exhibiting higher energy and entropy. However, the
amorphous form tends to recrystallize on storage, which limits the benefits of this approach. Specific interactions
between the drug and the polymer may retard this spontaneous conversion of the amorphous drug. Some studies
have shown that it is possible to maintain the drug in the amorphous form for extended periods of time. For the
drug and the polymer to form a stable mixture they have to be miscible on a molecular basis. Another form of
solid dispersions is pharmaceutical co-crystals, for which research has focused on understanding the chemistry,
crystal engineering and physico-chemical properties. USFDA has issued a guidance in April 2013 suggesting that
the co-crystals as a pharmaceutical product may be a reality; but just not yet! While some of the research is still
oriented towards application of these carriers, understanding the mechanism by which drug-carrier miscibility
occurs is also covered. Within this context is the use of thermodynamic models such as Flory-Huggins model
with some examples of studies used to predict miscibility.
Keywords: Solid dispersions, milling, surfactants, saturation solubility, thermal analysis, co-crystals.
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