Solvent effects on physicochemical behavior of estradiols recrystallized for transdermal delivery

Previously we have recrystallized estradiol with various organic solvents and investigated solvate molecules within estradiol crystals by using CP/MAS solid-state NMR. To investigate the effect of recrystallization solvents on the physicochemical properties of recrystallized estradiol, four different crystal habits of estradiol were recrystallized and their physicochemical properties were characterized by optical microscopy, solubility, and FT-IR measurements. Various crystal habits in size and shape were produced by the interaction between the estradiol and different solvents. Although the estradiol crystal habits prepared from ethanol and methanol had larger particle size, they were more soluble in PBS than those recrystallized from isopropanol and acetone. In spite of the low solubilities, the estradiols prepared from isopropanol and acetone were released in PBS and permeated through the hairless mouse skin similar to the others. Thus, although microscopic observation of recrystallized estradiols revealed that the estradiol had different crystal habits, the release and permeation properties of different estradiol crystals might be independent on the solvate molecules associated with the solvent used for recrystallization.