The effect of process variables on the degradation and physical properties of spray dried insulin intended for inhalation

The aim of this study was to investigate the effect of process variables on the degradation and physical properties of spray dried insulin intended for inhalation. A 2 4 full factorial experimentally designed study was performed to investigate the influence of the following independent spray drying variables: feed flow rate, nozzle gas flow rate, inlet air temperature and aspirator capacity (drying gas flow rate). Human insulin (biosynthetic and Ph.Eur. quality) was dissolved in distilled water to concentrations of 5 mg/ml. The solutions were spray dried in a Mini Spray Dryer Büchi and the dry powders produced were characterized by high performance liquid chromatography, size exclusion chromatography, laser diffraction, thermo gravimetric analysis, scanning electron microscopy and weighing. The degradation of insulin was found to be affected mainly by the process variables that determine the outlet air temperature, i.e.: inlet air temperature, aspirator capacity and feed flow rate. The outlet air temperature should be kept below 120 °C to avoid degradation. A statistical optimization of the spray drying variables was performed, and found to recommend an experiment with an outlet air temperature of 61±4 °C. This experiment ought to generate a yield of 54±7% by weight of particles with a mass median diameter 2.9±0.4 μm, moisture content 3.9±0.5% by weight, content of high molecular weight proteins 0.3±0.1% by area, A-21 desamido insulin 0.3±0.05% by area and other insulin related compounds 0.3±0.1% by area.