Culture media development continues to be an area that offers the potential to dramatically improve the productivity of biopharmaceutical manufacturing processes. Having a well-defined media formulation optimized for maximum protein production can significantly improve product titers, thereby reducing costs and improving efficiency. In this article, we discuss the historical aspects of media development from the initial dominance of serum as a supplement in cell-culture media to the current trend toward serum-free, chemically defined, protein-free media customized for particular cell lines. We will also examine the advantages of developing a robust serum-free cell-culture medium containing defined animal-free protein supplements.
The first application of cell culture technology was for vaccine production in the 1950s. Traditionally, embryonated chicken eggs were used for vaccine production. Because of increased demand for vaccines, there was a need for new technologies that could produce large quantities. The first mammalian cells used to generate a commercial product were monkey kidney cells, which were used to produce the polio vaccine.1 These were an attached cell line and required serum for growth. Further advances occurred during this time with the commercial production of an inactivated foot-and-mouth disease vaccine in baby hamster kidney (BHK) cells. This cell line was cultured in Eagle's medium, supplemented with 5% adult bovine serum up to a scale of 5,000 L.2 Eagle's minimal essential medium, the first basal synthetic medium, contained a mixture of amino acids, vitamins, cofactors, carbohydrates, and salts to support cell growth. However, supplementation was also required, generally by addition of animal serum. Another major advancement in the use of mammalian cell culture for the production of biologics for human use came with the US Food and Drug Administration's acceptance of the use of continuous cell lines. Continuous cells lines such as Chinese hamster ovary (CHO) and myeloma cells (NSO and SP2/0) meant cells could be cultured in suspension and were readily scalable, allowing for increased product yields.
Serum as a Supplement
Bovine serum has been the supplement of choice during the development of general cell culture. Serum has been shown to provide all the essential nutrients for cell growth and productivity, including macromolecular proteins, low-molecular-weight nutrients, anti-oxidants, and carrier proteins for water-insoluble components, and is known to contain anti-apoptotic factors.3 Serum also contains a high concentration of albumin, which can protect cells from stress factors generated under bioreactor conditions such as pH nutrient fluctuations and shear forces, as well as a number of transport functions.4 Although serum delivered many benefits for the industry, serum also has important drawbacks, such as batch-to-batch variability, leading to a lack of process and product consistency, complex downstream processing to remove serum contaminants from the final product, and fluctuating costs. Even more important, the transmissible spongiform encephalopathies (TSEs) and other adventitious agents in bovine serum resulted in a strong regulatory drive for the biopharmaceutical industry to eliminate serum and other animal-derived components from the manufacturing process.5 A 1999 report by Wessman and Levings indicated that as much as 20–50% of commercial fetal bovine serum was virus-positive.6 These regulatory constraints led to the development of defined media supplements and the industry's move away from the use of serum to a serum-free medium.