Downstream Processing or Purification of Recombinant Protein in Pharmaceutical Scale

Regardless of the type and configuration of cell and bioreactor combination used for recombinant product preparation, the cells are harvested at their optimal growth and viability to ensure the highest yield per unit of cell culture. The harvested biomass, which can be in the form of cell media or cell fractions, containing the putative product is subjected to a streamlined purification process tailored to the large-scale preparation of recombinant proteins. In scaling up the purification scheme (or stream) of recombinant proteins, many of the laboratory-scale preparation procedures such as centrifuging several thousand liters of cell suspension to collect a cell pellet become too expensive, if not impractical. For most proteins, the final product, designed for parenteral injection, must meet purity and sterility standards and be below maximally acceptable cellular or microbial contamination [i.e., less than 0.5 endotoxin (a component of bacterial cell wall) unit per ml of sterile solution for an injectable dosage form], as defined by the FDA. Many of these issues are considered in developing downstream or process-stream procedures for the purification of recombinant proteins.

The objective of developing a downstream purification process is to isolate the biotechnology products, employing the fewest possible steps with the simplest purification technology to achieve the required purity. The downstream process must provide reproducible purity, within a given economic framework. The lability or stability of the protein will vary with the microenvironment (i.e., bacterial, yeast, or mammalian cell lysates versus culture media or broths harvested and stored as starting materials). Minimizing the downstream process and avoiding expensive and time-consuming centrifugation, a production scientist can reduce substantially the processing time required for large-scale protein purification.

Large-scale purification methods for small molecules and short polypeptides are well established and published in the chemical engineering literature. Interested readers should refer to them for details. Although we will focus on downstream purification processes relevant to the production of recombinant products, bear in mind that many of these purification pro-

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