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).