Looking to the future, what trends are we likely to see? Given the high level of interest in antibodies and the number of monoclonals entering clinical trials, mammalian cell culture will remain a key technology for the foreseeable future.
We will also continue to see improvements to the technology both in terms of expression systems and culture processes. There is, for example, strong interest in improving the inherent properties of cells used for production. Examples include the CHOK1SV derivative of the CHO cell, adapted to grow in suspension in chemically defined medium.8
With increasing knowledge of those aspects of cell biology that influence the behavior of cells in a production process we are likely to observe an increasing use of cell-engineering approaches to improve the properties of cells.9 This includes properties that enhance the cell’s productive capabilities and others that improve product characteristics.
For example, an engineered variant of CHO has been created in which the gene for a-1, 6-fucosyl transferase (FUT8) has been knocked out.10 This variant is unable to add fucose to glycan structures on proteins and it turns out that this can improve the ADCC effector function of antibodies (a property that is potentially useful in a number of therapeutic applications).
While mammalian cell culture continues to be important, there is also an upturn in interest in microbial expression systems driven by product trends and progress in technology. On the product front there are, for example, new generations of engineered binding molecules based on antibody fragments and other protein scaffolds that are significantly smaller than antibodies and are produced in microbial culture.11, 12 Two antibody fragments, produced in E.coli, are already on the market: Cimzia® and Lucentis®.
Microbial expression technology has also progressed. A wider range of micro-organisms are now available, including engineered strains, that provide opportunities to select a host best suited to a given protein. For example, 2009 saw the first approval of a product (ecallantide) made in Pichia pastoris.2
A broader choice of expression technology also provides somewhat more flexibility in process design, e.g., whether the protein will be secreted or produced intracellularly.
There are significant advantages in the use of microbial systems: shorter process development timescales, less complex processes, and reduced risk of viral contamination. The drawbacks of microbial culture, which were observed 30 years ago, i.e., the difficulty of making large complex molecules like antibodies and the inability of microorganisms to provide human glycan structures on glycoproteins, remain challenging but are gradually being overcome. Whole antibodies have been expressed in E. coli13 and yeast, and Pichia pastoris has been glycoengineered to produce human glycan structures.14
In summary, the last 30 years have seen extraordinary progress in the field of recombinant expression. There is no reason to expect the next three decades to be any less exciting. Novel protein therapies and a continuing drive to improve process economics, shorten development timelines, and enhance product quality will ensure a continuing flow of advances in protein-expression technology.