Host Cell Proteins downstream processing

Several key issues have to be addressed in the downstream processing of biopharmaceuticals regardless of the expression system. The removal of host cell proteins and nucleic acids, as well as other product- or process-related or adventitious contaminants, is laid down in the regulations and will not differ between the individual expression hosts. The identity, activity and stability of the end product has to be demonstrated regardless of the production system. The need for pharmaceutical quality assurance, validation of processes, analytical methods and cleaning procedures are essentially the same. 

Process-related impurities encompass those that are derived from the manufacturing process, that is, cell substrates (e.g., host cell proteins, host cell DNA), cell culture (e.g., inducers, antibiotics, or media components), or downstream processing. Product-related impurities (e.g., precursors and certain degradation products) are molecular variants arising during manufacture and/or storage that do not have properties comparable with those of the desired product with respect to activity, efficacy, and safety. 

Process-related impurities encompass aU possible material that is used during manufacture and which might still be present in the final product These include cell components (e.g., host cell protein, DNA and RNA) and components of the cell culture medium (e.g., antibiotics, inducers, media). In addition, possible downstream-derived components (e.g., enzymes, (bio)-chemical reagents, inorganic salts and solvents) must be considered. Any adventitiously introduced material which is not part of the manufacturing process of either DS or DP is considered as a contaminant. For viral products and processes, special attention is paid to endogenous or adventitious viruses, which should ideally not be present or at least be removed/inac-tivated by the manufacturing process. 

The separation tasks for ion-exchange chromatography differ mainly by the type of expression system used. The most abundant systems are mammalian cells, microbial cells (yeast), and the bacterial organism E. coli. All expression systems have a common feature that they not only generate the target protein but also process related impurities, such as host cell proteins, DNA, viruses, endotoxins. In addition the downstream process has to isolate the active form of the target molecule from all its derivatives, for example, oxidated, deamidated, acetylated forms, dimers, aggregates, and unfolded proteins. 

Shukla, A. A., Jiang, C., Ma, J., Rubacha, M., Flansburg, L. and Lee, S. S. Demonstration of robust host-cell protein clearance in biopharmaceutical downstream process. Biotechnol. Prog. 24 615-622, 2008. 

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