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Manufacture Advanced Medicine

Let’s Focus on Filtration

Gene therapies are an exciting new class of treatment that alters the genetic composition of cells to correct disease-causing mutations – offering patients new therapies for previously untreatable genetic diseases. In 2017, the FDA approved its first gene therapy; at the time of writing, there are 17 approved cell and gene therapies, with many more on the way (1).

Although the pace of discovery is promising, it is only half the battle. Without robust manufacturing processes, these therapies will remain out of reach for the vast majority of patients. Viral vectors, used to transport therapeutic genetic material into cells, must be effectively processed and purified for clinical use. Manufacturing viral vectors for rare diseases already poses challenges, even if the number of patients requiring treatment – and therefore the volume of viral vectors – is low. Therapies for more common diseases such as Alzheimer’s, which impacts at least 50 million people worldwide, would be even more challenging to manufacture (2).

Fortunately, the industrialization of gene therapy manufacture has improved in recent years, with large-scale production becoming a reality. But, we must approach gene therapy manufacture with the same commitment to safety as that of other biopharma products. In the case of monoclonal antibodies (mAbs) and recombinant proteins, virus safety countermeasures have been so robust that there has not been a single case of someone contracting a viral infection from a contaminated recombinant product. The question is whether this level of safety can also be assured for gene therapies – and what lessons the advanced medicine field can learn from mAbs.

For recombinant and mAb manufacturing, the most common approach is to select and test incoming raw materials. Components such as cell culture media are treated to mitigate the risk of adventitious agents using techniques such as gamma irradiation or high temperature before transfer to the bioreactor. In the downstream process, virus safety is achieved by validating several viral reduction and inactivation steps, both dedicated like virus filtration and low pH inactivation, as well as non-dedicated, like chromatography steps that primarily serve for purification purposes. The downstream viral safety measures have proven ample to keep patients safe, while the upstream measures have mainly contributed to safeguarding continuity of production by reducing bioreactor infection frequency with its potentially devastating operational and financial consequences. Lately virus filtration in the upstream is also considered as an additional measure to further reduce the risk of downtime, especially in single use facilities without access to high temperature equipment and in flexible multiproduct factories, that can’t afford missing deadlines and milestones due to a potential bioreactor infection.

When manufacturing cell and gene therapies, there are fewer options to prevent virus infection or inactivate viruses downstream because such measures could damage or remove the product itself. This is especially true for therapies that use lentivirus (LV) vectors, which are more fragile than adeno-associated virus (AAV). AAV can be treated “harshly” (with a detergent, pH treatment, and a coarse >20 nm virus filter to remove larger viruses) and still maintain a good yield. LV is larger and these treatments would irreparably remove or damage the viral vector itself. But there is still the need to remove a wide range of adventitious agents that could potentially ingress adherent and suspension-based viral vector cell culture. Therefore, manufacturers should look at ensuring a maximum log reduction value (LRV) safety barrier for upstream processing.

Are viral filters the answer? Some manufacturers don’t currently use them in the upstream process. This is partly because robust downstream virus clearance historically provided ample patient safety for mAbs and recombinants, and upstream virus filters were in many cases not deemed financially viable to cover the risk of bioreactor infection. In gene therapy, these downstream “safety nets” are only partially in place, so there is a real risk that a bioreactor infection upstream could directly impact patients.

With safety and quality increasingly prioritized in gene therapy manufacturing and fewer options downstream, we believe more companies will look to virus filtration of cell culture media. Viral filters can robustly remove viruses from biotech and plasma processes using a polymeric membrane barrier to retain virus particles based on size. Similarly, AAV can be separated from larger adventitious agents in the downstream with a course >20 nm virus filter. The integral performance criterion is the LRV and throughput over time. These specifications can be altered and influenced by various interrelating aspects such as viral load, protein concentration, presence of foulants, pressure, operating flux, ionic strength, and process interruptions. Choosing and validating a virus filter appropriate for the expected range of conditions is therefore imperative.

We’re seeing an increasing number of manufacturers turning to these products as the maturing field addresses the risks associated with gene therapy manufacturing. When the first gene therapies came onto the market, patients were willing to accept associated risks if they had a chance of curing their once incurable and often fatal genetic condition. Now, as the industry invests in appropriate adventitious agent control with a high focus on safety, patients will not have to take this risk.

As industrial scale gene therapies are being developed, it’s up to us as an industry to make sure we produce them in the safest way possible, to help as many patients as we can.

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  1. FDA, “Approved Cellular and Gene Therapy Products” (2020). Available at:
  2. Alzheimer’s Disease International, “World Alzheimer Report” (2018). Available at:
About the Authors
Morven McAlister

Senior Director of Regulatory and Validation Consultancy at Pall Biotech.

Aernout Martens

Global Product Manager Virus Filtration at Pall Biotech.

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