Impact of excipient choice on the aerodynamic performance of inhalable spray-freeze-dried powders.

Spray-freeze-drying (SFD) is a process in which a solution is dispersed into a freezing medium and dried by sublimation, resulting in lyophilized powders with spherical particles. This study aims at screening and evaluating the impact of the excipient choice and spray solution characteristics in SFD on the physico-chemical characteristics of lyospheres and rate their suitability for producing pulmonary applicable powders using a novel SFD method. A monodisperse droplet-stream was injected into a vortex of cold gas for the production of inhalable, uniform spherical lyophilisates with a narrow particle size distribution. Model solutions containing graded contents (0.3%, 1.0%, and 3.0% w/v) of common bulk-forming excipients like mannitol, lactose, poylvinylpyrrolidone (PVP), maltodextrin or hydroyxpropyl methylcellulose (HPMC) and their blends were dispersed using a single 20 μm pinhole diaphragm. Powders were analyzed regarding their geometric particle size, apparent density, mechanical stability and aerodynamic performance. The diameter of the frozen droplets partially correlated with the Ohnesorge number of the spray solutions. The lyosphere powders had median geometric particle diameters ranging from 20 µm to 81 µm. Some powders showed signs of particle shrinkage during the drying step and diameters were reduced down to 30% of their initial size. The apparent particle densities ranged from 0.009 g/cm to 0.087 g/cm. The mechanical stability of the lyospheres depended on the constituents and concentration of the initial spray solution. Mannitol/maltodextrin formulations yielded large porous particles with promising performance in the Next-Generation-Impactor, emitted fractions between 92 and 98% (w/w) and fine-particle-fractions of over 55% (w/w). According to our first steps towards formulations for free-flowing inhalable spray freeze-dried powders the impact of excipient choice on the SFD process is significant and based on the current findings we consider mannitol or mannitol/maltodextrin as best performing formulations.