Host cell proteins (HCPs) are a significant class of process-related impurities commonly associated with the manufacturing of biopharmaceuticals. However, due to the increased use of crude enzymes as biocatalysts for modern organic synthesis, HCPs can also be introduced as a new class of impurities in chemical drugs. In both cases, residual HCPs need to be adequately removed to ensure product purity, quality, and patient safety. Although a lot of attentions have been focused on defining a universally acceptable limit for such impurities, the risks associated with residual HCPs on product quality, safety, and efficacy often need to be determined on a case-by-case basis taken into consideration of residual HCP profile in the product, the dose, dosage form, and administration route etc. Here we describe the unique challenges for residual HCP control presented by the biocatalytic synthesis of a Merck investigational stimulator of interferon genes protein (STING) agonist, MK-1454, which is a cyclic dinucleotide synthesized using E. coli cell lysate overexpressing cyclic GMP-AMP synthase (cGAS) as a biocatalyst. In this study, a holistic characterization of residual protein impurities using a variety of analytical tools, together with in silico immunogenicity prediction of identified proteins, facilitated risk assessment and guided process development to achieve adequate removal of residual protein impurities in MK-1454 API.

Host cell proteins (HCPs) are process-related impurities that may co-purify with biopharmaceutical drug products. Within this class of impurities there are some that are more problematic. These problematic HCPs can be considered high-risk and can include those that are immunogenic, biologically active, or enzymatically active with the potential to degrade either product molecules or excipients used in formulation, and often are difficult-to-purify. Why should the biopharmaceutical industry worry about these high-risk host cell proteins? What approach could be taken to understand the origin of this co-purification and to deal with these high-risk HCPs? To answer these questions, the BioPhorum Development Group (BPDG) HCP Workstream initiated a collaboration among its 26-company team with the goal of industry alignment around high-risk HCPs. A sub team was formed, in which the members performed literature searches and discussed the information available around this topic. A survey to the BPDG HCP Workstream team members led to team discussions and insights into a list of frequently seen problematic HCPs. These HCPs were further classified based on their potential impact into different risk categories that could be beneficial to the biopharmaceutical industry for targeted monitoring of those HCP impurities in CHO-produced biologics to minimize risk to product quality, safety, and efficacy.