Hydroxypropyl methylcellulose as a novel tool for isothermal solution crystallization of micronized paracetamol

Pulmonary inhalation is increasingly being selected as a preferred route for the delivery of both small and large drug macromolecules for the treatment of a range of pathologies. The direct crystallization of micronized powders, in particular, paracetamol, remains difficult, as it requires the ability to work in high solution supersaturations where agglomeration, wall crusting, and heterogeneous nucleation hinder the control of crystal size and crystal size distribution. Polymer additives are recognized to help drive the production of a given polymorph or controlling crystal shape by means of adsorption on the crystal surface. With the aim of exploiting the polymer-control nucleation and growth of crystals for enhanced direct crystallization of micronized powders, batch cooling crystallization of paracetamol in water was carried out in the presence of 0.1-0.8% w/w hydroxypropyl methylcellulose (HPMC). In the presence of polymer, the onset of nucleation was delayed and extended beyond the cooling time of the solution, resulting in an isothermal cooling crystallization and the production of micronized paracetamol with a mean crystal size D50, in the range of 15-20 μm and an improved crystal size distribution. Equally, the rate generation of solution cloudiness was reduced by over 3-fold for the highest HPMC concentration tested, with no detectable impact on final product yield. The mechanisms for nucleation delay and growth inhibition by HPMC is unknown; however, a modification of crystals shape observed upon the addition of HPMC to the solution suggested it might be related to mass transfer limitations and intermolecular hydrogen bonding between the large HPMC and the small drug molecules. This technique can potentially be used for direct crystallization of other micronized drugs. © 2014 American Chemical Society.