LAUSR

ABSTRACT Crystalline nanodispersions of naproxen stabilized with hydroxypropyl cellulose were prepared at different pHs both below and above the pKa of the drug. After spray drying together with lactose, the measured particle size of the rehydrated dispersions using laser light scattering/Mie analysis was found to depend strongly on the pH of the original, processed nanodispersion. The pH‐dependent solubility of this weak acid resulted in formation of a dissolved fraction of drug in the nanodispersions, this being higher as the pH increases. There was therefore a loss of crystalline naproxen during processing at higher pH, with fewer and/or smaller nanoparticles in the nanodispersion after nanomilling. After spray drying, the dissolved naproxen fraction formed an amorphous solid, as shown by differential scanning calorimetry and X‐ray powder diffraction. The amorphous fraction re‐dissolves on re‐hydration. An improved disaggregation and smaller particle size of the nanoparticles at higher pH was observed because of the lower amount of crystalline nanoparticles now present. If the pH of the nanodispersion during processing is kept below the pKa for a weak acid, then a high yield of nanoparticles should be expected with little formation of an amorphous phase of the drug.