High-resolution chromatography normally yields a protein that is 98-99 per cent pure. The next phase of downstream processing entails formulation into final product format. This generally involves:
• Addition of various excipients (substances other than the active ingredient(s) which, for example, stabilize the final product or enhance the characteristics of the final product in some other way).
• Filtration of the final product through a 0.22 ^m absolute filter in order to generate sterile product, followed by its aseptic filling into final product containers.
• Freeze-drying (lyophilization) if the product is to be marketed in a powdered format.
The decision to market the product in liquid or powder form is often dictated by how stable the protein is in solution. This, in turn, must be determined experimentally, as there is no way to predict the outcome for any particular protein. Some proteins may remain stable for months (or even years) in solution, particularly if stabilizing excipients are added and the solution is refrigerated. Other proteins, particularly when purified, may retain biological activity for only a matter of hours or days when in aqueous solution.
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