Pure insulin highly respirable powders for inhalation

The aim of the present research was to investigate the possibility to obtain by spray drying an insulin pulmonary powder respirable and stable at room temperature without the use of excipients. Several insulin spray-dried powders were prepared with or without the addition of excipients (mannitol, bovine serum albumin, aspartic acid) from water dispersions or from acidic aqueous solutions. Each formulation was characterized using laser diffraction, scanning electron microscopy and in vitro aerosol performance with a Turbospin DPI device. Stability was assessed by the quantification of impurities with a molecular mass greater than that of insulin (HMWP) and related proteins (A21+ORP). Insulin powders prepared without excipients from an acid solution showed a shrivelled, raisin-like shape of non-aggregated microparticles and a high respirability (FPF>65%). The optimal result with respect to respirability and stability was reached when the pH of the insulin acetic acid solution to spray dry was adjusted at pH 3.6 with ammonium hydroxide. The median volume diameter of the obtained powder was 4.04 μm, insulin content 95%, emitted dose of 89.5%, MMAD 1.79 μm and fine particle fraction of 83.6%. This powder was stable at room temperature over a period of eighteen months with respect to the content of A21+ORP. As far as the HMWP content was concerned, the powder complied with the specification limits for a period of five months. The insulin acetic powder opens up the possibility of a more effective pulmonary therapy less dependent on refrigerated storage.