BioPharm: Selection of media is also important to a successful cell-culture process. What are the key considerations when choosing cell
culture media? Can you discuss approaches to developing or selecting media?
Ng: The cell-culture medium serves two purposes in cell culture: it functions as the medium (with a buffering capacity) where
the cells grow, and it provides nutrients for cell growth and production. As mentioned previously, each process is unique.
The selection of the media depends on the process, business, or regulatory constraints. In some cases, these constraints arise
downstream of the cell culture process. When selecting for culture media, a process developer should always consider the business
and regulatory needs, the presence of unstable materials, lot-to-lot variation, media compatibility with the production system,
available grades of the culture media, ease of formulation, and the impact on downstream purification.
With advances in culture media development, several protein-free chemically defined (CD) media are readily available for off-the-shelf
use. The use of these protein-free CD media helps in several aspects of development: they reduce regulatory risk from animal-derived
components and lot-to-lot variation, and they increase the developability of culture media (because all of the components
are known). In addition, they can reduce cost in some cases.
The approach of cell culture media development can generally be divided into two main groups: a top-down approach and bottom-up
approach. In the top-down approach, a developer seeks an effective basal media or additive that can improve protein production
without the need to know each of the components or how the components work. This top-down approach is usually conducted through
a series of mixture experiments as well as through properly designed experiments to screen different additives. In contrast,
the bottom-up approach relies heavily on understanding at the molecular, cellular, and process levels. In this approach, a
measurable amount of additives are added because a particular component is either depleted or serves key functions at the
molecular or cellular level. One way to find out the depleted component in the bottom-up approach is to analyze the spent
media from the cell culture. Once identified, the depleted components can be added back to the media through front loading
in the basal media, or feed enhancements can be introduced to the culture at different culture duration times. The recent
advancement of "omics"—metabolomics, proteomics, and genomics—has shed light on the identification of key components in culture
media, as well.
BioPharm: What factors should be considered when choosing a bioreactor system?
Ng: Once the cell line and culture media have been identified, a process can be further developed through various bioreactor culture
systems. There are two main categories in selecting a bioreactor system: bioreactor operating mode and scale-independent,
as well as scale-dependent, operating parameters.
Additionally, there are several operational modes that can be considered when developing a cell culture process: the fed-batch
process, perfusion process, perfusion with cell bleeding, and a chemostat process. Although a fed-batch process is frequently
used in the current biopharmaceutical industry, the long residence time of products in the bioreactor at an elevated temperature
might not be desirable for certain proteins. For such proteins, perfusion-based or continuous cultures (where products are
continuously being removed) are favored. Again, each process offers its own advantages and disadvantages and needs to be properly
assessed.
The selection of scale-independent parameters also depends on properties of the proteins of interest and the growth characteristics
of the cells being cultured. During the various process development stages, these parameters are characterized to maximize
the process productivity and robustness. One caveat to the development is that improving productivity should not compromise
the quality attributes of the product. The effects of these scale-independent parameters on product quality attributes should
be critically assessed during development. In practice, the selection of relevant product quality attributes can make a difference
in development efforts. Similarly, proper risk assessment tools should be used and well documented to guide the development
efforts in achieving the desired end points.
Scale-dependent parameters are usually addressed during the latter stage of development. The selection of these parameters
can affect process scalability. Several considerations for scale-independent parameters include bioreactor scales and dimensions,
and other engineering considerations of the bioreactor design, such as bioreactor dimensions, sparger size, rate, and location;
impeller type, size, position, and agitation rate; and other considerations, including pumping direction, working volume,
nutrient injection point, base injection point, and heat transfer.
In most cases, the selection of one parameter or operational condition can affect the other parameters.
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