Dr Clive Glover, director, Strategy, Pall Corporation discusses recent industry developments surrounding cell and gene therapies and why safety is a key priority for this treatment area.
Gene therapy
In January 2020, the US Food and Drug Administration (FDA) published a series of seven guidances regarding cell and gene therapy. These have not come too soon, given some much-publicised regulatory rejections. For example, the FDA turning down the Biologics License Application of BioMarin valoctocogene roxaparvovec for severe haemophilia A. Following relatively recent European regulatory revisions, the FDA guidances, as usual, apply to the manufacture as well as the development and clinical trials of drugs. A major challenge of cell and gene therapy developers is balancing clinical considerations with logistical demands such as manufacture. The evolution of a robust manufacturing process that complies with the FDA guidance is reliant in part on the technical innovations by companies like Pall, a global supplier of filtration, separations, and purification products.
In the case of high-dose adeno-associated virus (AAV) gene therapies, which have been associated with hepatic and systemic inflammatory toxicities, there has been discussion on shifting focus from finding safer ways of delivering high doses of AAVs to re-engineering AAVs to eliminate entirely the need for high doses. This can be done by processes such as directed evolution, allowing them to better target the required tissue. If the therapy is more tissue-specific, not as high a dose would be needed. However, this can have a dramatic effect on manufacturing capacity with manufacturing titres dropping by as much as 100-fold with AAVs adapted to hybrid or artificial serotypes. Thus, the drug development focus diverts from clinical to manufacturing effectiveness.
Drug makers utilise different approaches to produce recombinant AAVs (rAAVs). Most clinical-stage investigational AAV gene therapies are produced by transient transfection in human cells. However, scaling up these processes for mass production has proved difficult as maintaining batch to batch consistency is challenging, especially at volumes higher than 500 litres. Therefore, developers are investigating alternative means, such using insects with baculovirus. Indeed, BioMarin is using such a system to manufacture their investigational gene therapy for haemophilia. This allowed them to boost the quantity of viral vector per batch, as well as the batch volume, easily achieving 2000 litres per batch.
But there are potential downsides. AAVs produced by different systems may behave differently in the clinic. Several functionally relevant AAV characteristics differed between rAAVs produced using transiently transfected human cells and those produced by live baculovirus infected insect cells. These included post-translational modifications such as acetylation, glycosylation, methylation, and phosphorylation potentially lowering the potency of the virus in vivo. In addition, the low-level methylation of rAAV genomes occurred at different sites depending on the system employed. Another factor to consider is the need to completely eliminate any baculoviruses from the final drug, to prevent serious harm to patients. Moreover, while production may be more effective, a higher therapeutic dose may be needed. Further, the additional manufacturing steps may increase process complexity.
Another important consideration is that of adventitious viruses, which can infect the producer cells. Such viruses must also be eliminated from the drug product. Although no patient has yet contracted an infection from a virus contaminating a monoclonal antibody or recombinant protein drug, this is because of robust processes put in place to ensure unwanted viruses are eliminated. But these same processes would not be feasible for gene and cell therapies that incorporate AAVs and lentiviruses because the desirable and indeed necessary AAVs and lentiviruses would also be removed or destroyed. Therefore, drug developers are looking to viral filters for virus control during the manufacturing process.
In biotechnology and plasma processes, viruses are robustly removed by virus filters comprised of polymeric membrane barriers that retain virus particles based on size. AAVs can be separated from larger adventitious agents that are over 30 nm in size by introducing virus filters to viral vector production processes. It should be noted though that filter throughput and performance is impacted by multiple integrated elements, including viral load, protein concentration, foulants, pressure, operating flux, ionic strength, and process interruptions. These conditions must therefore be considered to ensure the an appropriate virus filter is chosen. The addition of viral filters both upstream and downstream would contribute to a virus-control strategy that as effective as that of antibody and recombinant protein drugs.
In general, gene and cell therapies were initially approved because they were aimed at treating patients with rare and severe – or often fatal – diseases. Due to the comparatively high level of risk and harms from these diseases, patients were generally more willing to accept riskier medicines. Nowadays though, gene and cell therapies are being developed for more common diseases, which may already have viable treatment options, like haemophilia. That is why innovations are required for the manufacture of gene and cell therapies, which need to be rapidly scalable while continuing to prioritise patient safety.