Bovine serum has long been preferred as a culture supplement for mammalian cells because of its low concentrations of immunoglobulins,
which can complicate downstream processing. It also contains mitogens and growth factors that promote cell growth. But adding
bovine sera can increase costs and impede scale-up. For one thing, lots can vary in composition and quality so that cell lines
can be sensitive to some lots and not to others. Availability is sometimes a problem, because of the products' traditional
ties to the cattle industry. Another source of concern since the late 1990s has been the risk of "mad cow disease" or bovine
spongiform encephalopathy (BSE). The prion believed to cause both that disease in cattle and Creutzfeld-Jacob disease in humans
can be present in bovine serum, and it is virtually impossible to detect and remove.
To ease downstream processing of recombinant products, many companies have begun to use serum-free and protein-free media
to reduce costs and increase safety in animal cell culture. The main focus in media formulation is consistency in production
and performance of large lots, which can be more carefully monitored with serum-free media. But serum-free media may have
some serum-sourced ingredients. By contrast, protein-free media are chemically synthesized and produced and thus well defined
in composition. Both serum-free and protein-free media are manufactured with certain cell lines in mind, each providing just
the nutrients a given cell line needs. Obviously, such media are more expensive. But they may make up for that cost by facilitating
downstream purification. Selection of TSE-free culture medium components is also important in microbial fermentations.
Some fermentation processes work well in the laboratory, only to fail at the industrial scale. Such failure can be attributed
to many causes. For one thing, scaleup involves increasing bioreactor size as well as production capacity, but equipment might
not function at the same efficiency on a larger scale. Scaleup affects other factors as well. Optimizing output means maximizing
cell mass production and product formation. Most often, the parameters of oxygen-in and waste-out will change at higher scales.
Perhaps the required raw materials are too expensive. Perhaps cells or proteins aggregate in large numbers.
Biotechnologists face many challenges when taking their processes from the laboratory to the manufacturing scale. Large-scale
fermentation often gives lower yields than laboratory experiments would suggest. The goal of any commercial fermentation process
is to achieve high productivities, product yields, and product concentrations. Large bioreactors are expensive, so bioprocess
scientists must estimate their ultimate scale (and future needs) early. Here are some questions involved in scaling up a fermentation
Regarding the product. What amount will be required, and what size vessel will produce that amount? How big and how stable is the protein molecule?
How will temperature and cell proteases affect it?
Regarding the cell line. What are its characteristics (such as genetic stability and anchorage dependence)? How fragile are the cells? What are their
kinetics of growth and product formation? Does the product accumulate inside the cells or is it excreted to the culture medium?
What will the product yield be? What levels of containment are required, and what safety issues are involved?
Regarding the fermentation vessel. From what materials (glass, plastic, or stainless steel) will it be constructed? How will media be mixed without damaging
fragile cells? Will control, operation, and sterility maintenance of the reactor be difficult? What are the fixed capital
costs (reactor and instrumentation) and the running costs (media, disposables, labor, maintenance, downtime, and so on) for
different types of equipment? Will the process be batch, fed batch, perfusion, or continuous culture?
Cost. Can the process produce an acceptable amount of purified recombinant protein to justify all the money put into it? Or will
it be so expensive that no patient will be able to afford the drug?
Viral contamination. Mammalian cell lines are prone to contamination because of the complexity and duration of their culture (one to two weeks
per batch, many months for continuous culture). They host many of the same adventitious agents that humans do.