Effect of crystal habit on the dissolution behaviour of simvastatin crystals and its relationship to crystallization solvent properties.

Simvastatin crystals, having same crystal structure but different types of habits and hence different intrinsic dissolution rate, were prepared by recrystallization from solvents selected according to their polarity index. Scanning electron microscopy, laser diffraction, image analysis, X-ray powder diffractometry, Fourier transform infrared spectroscopy and differential scanning calorimetry were used to investigate the physicochemical characteristics of the prepared crystals. The isolated crystals exhibited different crystal habits but possessed the same internal crystal structure. In this study the comparative intrinsic dissolution behaviour of the simvastatin crystals with different types of habits was studied and explained by surface energy and correlated to different solvent systems that were used for crystallization. In our work we diminished the influence of all other physical parameters that could influence the dissolution rate, e.g. particle size, specific surface area and polymorphism in order to focus the study onto the impact of crystal shape itself on the dissolution rate of simvastatin crystals. Rod shaped crystals isolated from more hydrophilic solvent mixture dissolved faster than plate-like crystals obtained from solvent mixture with lower polarity index. We correlated this fact to the different growth rate of the individual faces which resulted in different relative size of the individual crystal faces exposed to the dissolution medium as well as the chemical nature of those faces which in turn influenced the wettability and subsequent dissolution of the active pharmaceutical ingredient.