Preparation of high concentration protein powder suspensions by milling of lyophilizates

Pharmaceutical formulations utilizing protein drugs as powders can be used as drug delivery systems in various ways. Besides powders for inhalation, another promising approach is their use as suspensions in non-aqueous liquids for subcutaneous administration providing high protein stability and good injectability. In this study protein powder suspensions were prepared using a swing-mill. Milling of lyophilizates containing a model monoclonal antibody in presence of the suspension vehicle was compared to cryogenic dry milling. Wet media milling led to injectable suspensions, but resulted in monomer loss and increase in protein aggregation. When the lyophilizates were cryogenic dry ball milled less aggregation and monomer loss were detected. Differences related to protein integrity were found for different process parameters, which were successfully optimized. If not cooled with liquid nitrogen, dry milling resulted in increased damage to the mAb. The type of polyol stabilizer, as well as the protein to stabilizer ratio, did not affect the preservation of protein integrity. As finding the right milling duration is time and resource intensive, a correlation between lyophilizate cake hardness and milling duration was established. Based on this approach high concentration lyophilizates were successfully micronized. Suspensions of cryogenic milled powders lead to clogging of 25G needles, which could be prevented by an additional sieving step. Depending on the suspension vehicle, low viscosity formulations (<10 mPa·s) even at high concentrations (≥100 mg/ml protein concentration) were obtained featuring good injectability.