Understanding the mechanism of copurification of “difficult to remove” host cell proteins in rituximab biosimilar products

Host cell proteins (HCPs) are considered a critical quality attribute and are linked to safety and efficacy of biotherapeutic products. Researchers have identified 10 HCPs in Chinese hamster ovary (CHO) that exhibit common characteristics of product association, coelution, and age-dependent expression and therefore are "difficult to remove" during downstream purification. These include cathepsin D, clusterin, galectin-3-binding protein, G-protein coupled receptor 56, lipoprotein lipase, metalloproteinase inhibitor, nidogen-1 secreted protein acidic and rich in cysteine (SPARC), sulfated glycoprotein, and insulin-like growth factor-2 RNA-binding protein. While the levels of HCPs in the investigated biosimilars were within the acceptable range of <100 ppm, certain "difficult to remove" HCPs were found in the biosimilar samples. This article aims to elucidate the underlying interactions between these "difficult to remove" HCPs and the mAb product. Surface study of rituximab exhibited unstable discontinuous patches of residues on the protein surface that have high propensity to get buried and lower the solvent exposed area. The higher order structure and the receptor binding were not affected, except for one of the biosimilars, owing to extremely low-HCP levels in its final drug product. Finally, based on the combined experimental and computational data from this study, a probable mechanism of retention for the 10 HCPs is proposed. The results presented here can guide downstream process design or avenues for protein engineering during product discovery to achieve more effective removal of the impurities.