Toward an understanding of the factors influencing anhydrate-to-hydrate transformation kinetics in aqueous environments

Approximately one-third of all drugs can form hydrates, and therefore an in-depth understanding of the factors affecting anhydrate-to-hydrate transformation kinetics in a variety of environments is of interest. In this work, real-time analytical methods, in the form of Raman spectroscopy and optical microscopy, were used to establish representative transformation profiles and growth kinetics for hydrate formation of several drug substances. By comparing and contrasting the behavior of these different hydrates in terms of their specific properties such as solubility, dissolution rate, and growth rate of hydrate phase, and the influence of external factors such as seeding and agitation conditions, an increased understanding of hydrate formation has been achieved. The overall transformation rate from anhydrous to hydrate form was found to be not only a function of compound-specific properties, but was also strongly affected by surface properties and external factors such as the presence of seeds and the degree of shear forces.