Barrier filtration
One potential source of viral contamination is from raw materials used
in cell culture such as media or nutrient and glucose feeds.
Biologically derived raw materials are at highest risk for containing
viral contaminants. However, some feeds at high or low pH values that do
not allow for viral propagation and, as such they may not require risk
mitigation for viruses. To avoid potential viral contamination of cell
culture components, viral filters, high-temperature short time (HTST)
treatment, acid treatment, detergent treatment, ultraviolet
inactivation, and gamma irradiation can be implemented. In theory these
methods could be implemented by media vendors but the practicalities of
shipping and receiving large volumes of liquid media makes
implementation on site by the manufacturer more realistic. Viral
filtration can be suitable for various manufacturing processing,
including facilities that implement single-use, disposable
manufacturing, including for newer products like cell and gene therapy.
The advantage of viral filtration is that no new components are added to
the media and there is minimal risk of damaging or precipitating growth
factors or nutrients. Filters have recently been launched specifically
for use in filtering cell culture media. Other filters typically used in
downstream processing have been marketed by the vendors for use in
filtering cell culture media as summarized in Table 5 below.
There are several challenges with implementing viral filtration of these
raw materials. It is challenging to get complete assurance of viral
removal as this would require viral filtration of all media used for
cell line development, banking, and all cell culture scale up steps
before the production reactor. Use of a viral filter may add capital
cost, floorspace requirements and operational complexity. Just as with
traditional viral filtration methods, the pre-use flushing and integrity
testing of the filters must be considered. Clogging by some media
components (e.g. shear protectants, anti-foam, etc.) should be avoided
by heat treating them separately
instead.(Liu, Carroll et al. 2000,
Wolfgang, Mitterer et al. 2013,
Carbrello, Perreault et al. 2017)
Another potential challenge in implementing viral filters as a barrier
method is the potential interactions between media components (i.e.
shear protectants like Pluronic F-68, block copolymers, antifoam,
non-ionic surfactants, IgF, insulin, metals, glucose, amino acids,
vitamins, lipids, fatty acids, trace elements, and antibiotics) and the
viral filters. There must be assurance that any filter
extractables/leachables do not affect the cell culture process or
product quality or that critical media components are not retained by
the filter. The impact of media viral filtration may require evaluation
of cell culture bioreactors versus controls in scale-down studies.
Available literature does not describe methods of demonstrating viral
clearance capabilities of these filters
(Liu, Carroll et al. 2000,
Kleindienst and Manzke 2016), therefore,
demonstration of the filters viral retention performance is currently
mostly studied by the vendor to support their products. Some media
filters have been shown to provide virus and bacteria retention
(Carbrello, Perreault et al. 2017,
MerckMillipore 2017).