Evaluation of the potential of air jet milling of solid protein-poly(acrylate) complexes for microparticle preparation

It was the aim of this study to evaluate the potential of air jet milling for the preparation of protein-loaded microparticles in industrial quantities. The model protein horseradish peroxidase was incorporated via co-precipitation in carbomer (NaC934P) (1:100) and a poly(methacrylate) (Eudragit®L100-55) (1:100) used as carrier matrix. Co-precipitation of the model protein and each polymer in aqueous solution was achieved either by a pH-shift or by the addition of various non-solvents. Dried protein/polymer complexes (desiccator under vacuumization at 4 °C with silica blue gel) were ground with an air jet mill and resulting microparticles were investigated regarding protein load, remaining protein activity, size distribution and shape. Results of this study showed that the polymer used and the method of co-precipitation has a great impact on protein load. Using carbomer a maximum protein load of 60±1% was achieved, whereas in case of Eudragit® L100-55 the maximum was 78±5% (means±SD; n=3–4). Using petroleum ether, isopropanol or tetrahydrofurane as non-solvents led to significantly higher protein loads than a pH-shift from 7 to 5, 4 and 3.5, respectively. Determination of the remaining protein activity after milling showed, that the grinding air pressure (GAP) has a major impact on protein stability. In case of Eudragit®L100-55 at a GAP of 4.5 bar peroxidase activity was almost completely lost, whereas 42±1% loss in activity was determined at a GAP of 2.5 bar. The mean particle size of protein/carbomer and protein/poly(methacrylate) particles was determined to be 3.6–5.2 and 4.5–8.7 μm at a GAP of 2.5 bar and 2.7–3.1 and 2.4–3.1 μm at a GAP of 4.5 bar, respectively. Generally, 90% of all particles were in the range of 3–16 μm. All particles were of spherical shape exhibiting a non-porous surface.According to these results, air jet milling seems to represent a novel method for the large-scale production of protein drug loaded microparticles.