Influence of Drug Brittleness, Nanomilling Time, and Freeze-Drying on the Crystallinity of Poorly Water-Soluble Drugs and Its Implications for Solubility Enhancement.

The aim of this study was to assess whether wet bead milling of dexamethasone and tacrolimus suspensions leads to a lower degree of crystallinity of nanocrystals, and if the degree of crystallinity affects the drug solubility, in addition to particle size. Powder X-ray diffraction (XRD) was used to determine the degree of crystallinity of the particles, which decreased during milling until reaching a plateau: the particles had ∼79% degree of crystallinity after 5 h milling. Different milling times were required for the two drugs in order to reach their plateaux, 2 h for dexamethasone and 3 h for tacrolimus. These results could be explained with the brittleness of the drugs. Dexamethasone was more brittle than tacrolimus, with an apparent elastic modulus of 16 GPa compared to ∼12 GPa of tacrolimus. Freeze-drying the nanosuspensions resulted in a reduction in the degree of crystallinity to ∼35% for dexamethasone and to ∼45% for tacrolimus in comparison to non-freeze-dried particles. Solubility studies were performed with a Sirius® inForm based on in situ UV/VIS spectroscopy. The reduced degree of crystallinity of nanocrystals after milling was responsible, in addition to the nanoparticle size, for the solubility increase. Indeed, while the smallest particle size (394 nm for dexamethasone and 240 nm for tacrolimus) did not always result in the highest increase in solubility (factor of 1.04 for dexamethasone and 1.3 with tacrolimus), the smallest degree of crystallinity was always characteristic of the maximum solubility obtained (factor of 1.15 for dexamethasone and 1.7 for tacrolimus).