Characterization of Particles in Protein Solutions: Reaching the Limits of Current Technologies

Recent publications have emphasized the lack of characterization methods available for protein particles in a size range comprised between 0.1 and 10 μm and the potential risk of immunogenicity associated with such particles. In the present paper, we have investigated the performance of light obscuration, flow microscopy, and Coulter counter instruments for particle counting and sizing in protein formulations. We focused on particles 2–10 μm in diameter and studied the effect of silicon oil droplets originating from the barrel of pre-filled syringes, as well as the effect of high protein concentrations (up to 150 mg/ml) on the accuracy of particle characterization. Silicon oil was demonstrated to contribute significantly to the particle counts observed in pre-filled syringes. Inconsistent results were observed between different protein concentrations in the range 7.5–150 mg/ml for particles