Protein particles: what we know and what we do not know

All therapeutic protein products contain intrinsic particles formed by the aggregation of protein monomers. There is growing interest in understanding particles in biopharmaceutical products, fostered on one hand by significant advancements in particle analysis and on the other hand by concerns about potential impact of particles on product quality and safety. With currently available methods, particles in therapeutic proteins can be counted, sized, and characterized in a rudimentary way over a broad size range (from 10s of nanometers to 100 s of micrometers). Here, we review the known attributes of common protein particles, and then discuss the gaps in our current knowledge. The capabilities, limitations, and opportunities for improvement of common particle counting and characterization methods are listed. We conclude that further analytical progress is needed to better classify and characterize the diversity of particles encountered in therapeutic proteins, which may vary in the degree of protein unfolding, the inclusion of nonprotein nucleation centers, and aggregate morphology. Very little is known about the potential correlation between specific particle attributes and increased immunogenicity. In this environment of uncertainty, a deeper understanding about specific particle attributes and potentially increased immunogenicity is greatly needed and will likely be an area of future intensive research.