Preparation and physicochemical characterization of supercritically dried insulin-loaded microparticles for pulmonary delivery

In the search for non-invasive delivery options for the increasing number of therapeutic proteins, pulmonary administration is an attractive route. Supercritical fluid (SCF) drying processes offer the possibility to produce dry protein formulations suitable for inhalation. In this study, insulin-loaded microparticles suitable for pulmonary administration were prepared and characterized. N-Trimethyl chitosan (TMC), a polymeric mucoadhesive absorption enhancer and dextran, a non-permeation enhancer, were used as carriers for insulin. The particles were prepared by spraying an acidic water/DMSO solution of insulin and polymer into supercritical carbon dioxide. The mean size of the particles was 6–10 μm (laser diffraction analysis) and their volume median aerodynamic diameter ca. 4 μm (time-of-flight analysis). The particles had a water content of ca. 4% (w/w) (Karl–Fischer), and neither collapsed nor aggregated after preparation and storage. In the freshly prepared dried insulin powders, no insulin degradation products were detected by HPLC and GPC. Moreover, the secondary and tertiary structures of insulin as determined by circular dichroism and fluorescence spectroscopy were preserved in all formulations. After one-year storage at 4 °C, the particle characteristics were maintained and the insulin structure was largely preserved in the TMC powders. In conclusion, SCF drying is a promising, protein-friendly technique for the preparation of inhalable insulin-loaded particles.