Amorphous and Crystalline Particulates: Challenges and Perspectives in Drug Delivery.

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.

Effect of drug-polymer interactions on the aqueous solubility of milled solid dispersions.

The role of molecular interactions in ball milled solid dispersions in determining the aqueous solubility of the poorly water-soluble drug, griseofulvin (GF) has been examined. Ball milled solid dispersions of GF and hydroxypropylmethylcellulose acetate succinate (HPMCAS) and GF and polyvinylpyrrolidone (PVP) were prepared and characterized by laser diffraction, scanning electron microscopy and X-ray powder diffraction and the aqueous saturation solubility measured and analyzed using one way ANOVA. The results showed that solid dispersions of GF and HPMCAS possessed an aqueous GF saturation solubility of about ten times higher than the GF solubility achieved from PVP-based solid dispersions. Furthermore, although the aqueous solubility of GF did not vary with the milling conditions used to prepare the solid dispersions with PVP, significant changes in solubility were observed upon changing the milling conditions for preparation of the GF/HPMCAS solid dispersions. Surprisingly, the GF/HPMCAS solid dispersion prepared using spray drying exhibited a significantly lower aqueous solubility than those prepared by bead milling despite their smaller particle size and GF being fully in its amorphous form. It is thought that the higher surface energy of the spray-dried solid dispersions negatively affected the aqueous solubility of GF. In conclusion, the results suggest that the molecular interactions occurring between GF and HPMCAS affect the aqueous solubility of GF and that the molecular interactions appear to remain in the liquid state. In contrast no molecular interactions were evident in the GF/PVP solid dispersions.