Gene Therapy: Viral Vector Technology Innovators Prepare for Take-Off
Written by: Justin Wilson at Withers & Rogers.
With more gene therapy clinical trials securing authorisation and getting underway globally, innovators are focusing on developing viral vector carrier technologies that are safer and easier to manufacture at scale, cost efficiently.
Viral vectors, such as retrovirus, adenovirus and adeno-associated virus (AAV), can be used to insert genetic information into the target cells of patients, including those with rare genetic diseases, potentially curing them with a single treatment. Despite considerable interest in this area of research science since the early 90s, regulatory approval for new gene therapies has taken time to achieve, mainly due to setbacks in early clinical trials including several cases of genotoxicity.
To date, just 12 gene therapies have been approved by the European Medicines Agency (EMA) for use in Europe, and of these, only eight still have a valid marketing authorisation. These include therapies for Haemophilia A and B – Roctavian and Hemgenixrespectively, one for spinal muscular atrophy, known as Zolgensma, and Upstaza for the treatment of aromatic L‑amino acid decarboxylase (AADC) deficiency. Earlier this year, the UK’s Medicines and Healthcare Products Regulatory Agency (MHRA) was the first regulatory authority globally to approve Casgevy – a gene therapy developed using CRISPR gene-editing technology - for the treatment of sickle cell disease and transfusion-dependent beta thalassaemia.
The most widely used viral vector carrier technology to emerge to date is AAV, mainly due to its safety benefits as it is not pathogenic, but also due to its low immunogenicity and long-term gene expression. However, as with any other naturally occurring virus, there is a risk that patients may have pre-existing immunity to AAV, with neutralising antibodies, as a result of prior exposure to the wild-type virus. These antibodies can potentially inhibit the transduction of target cells by AAV vectors, thus impeding successful gene transfer, and may have potential safety consequences. This issue could also significantly affect efficacy in cases where it might be necessary to readminister a gene therapy. To address this issue, some innovators have been looking for alternative viral vector carrier technologies, which are less likely to generate an immune response.
One approach that has been tried to mitigate the effect of pre-existing immunity is to use AAV vectors based on non-human species. For example, AAVrh10 and AAVrh74 are AAV vectors isolated from rhesus macaques to which there is a lower prevalence of pre-existing immunity in the human population. To take this approach to the next level, there are a number of companies designing novel AAV capsids that show reduced immunogenicity, and combining these with other approaches to dampen any immune response in the patient.
For Example, Selecta Biosciences (now Cartesian Therapeutics following a merger) has developed patented methods (e.g. WO 2020/223205 and WO 2023/064350) for administering viral vectors and synthetic nanocarriers attached to an immunosuppressant to reduce unwanted immune responses and enhance gene expression. The patented method describes the recommended composition of each dose to be administered, according to a specific schedule. In particular, this method can be used on subjects with pre-existing immunity against a viral antigen of the viral vector.
Other innovators have been exploring ways to improve the efficacy of AAV as a therapy for rare genetic diseases. Specifically, they have been looking for ways to adapt AAV for use in different areas of the body. For example, it has been shown that AAV serotype 1 is particularly effective in the treatment of diseases affecting muscle or liver tissue, whereas AAV serotype 9 is better suited to the treatment of diseases affecting brain or lung tissue. Modification of the various AAV serotypes allows more targeted delivery of the gene therapy. For example, Dyno Therapeutics has developed their bCap 1 AAV vector which crosses the blood-brain barrier after intravenous injection to target cells throughout the CNS, and their eCap 1 AAV vector which more efficiently transduces cells of the retina after intravitreal injection.
One limitation to AAV vectors is the amount of genetic material that can be loaded into them is limited to about 4.7 kb which means that it is difficult to deliver large gene products. To overcome this issue, two approaches have been developed. The first is to use a split vector approach in which the large gene product is split over two or three vectors. It has been found that the full length gene can be reassembled in the target cells to allow expression of the gene. Alternatively, it may be possible to develop a micro-gene, which is short enough to allow the gene product to fit into a single vector but is still at least partially functional to be efficacious. Both these approaches have been used in the treatment of Duchenne Muscular Dystrophy in which the gene associated with the condition is about 2,200 kb in length, the largest known human gene. For example, see WO 2023/004125 and WO 2021/108755.
Another key challenge for scientists involved in the development of gene therapies is the ability to manufacture viral vectors at scale and cost-effectively. To address this problem, some innovators have been looking for ways to streamline the production process. For example, Oxford Biomedica has developed a modified viral vector production system (see WO 2019/175600) designed to simplify manufacturing. In this system, a hydrolytic nuclease is expressed in the production cell and secreted into the cell culture during lentiviral vector production. This causes the degradation of unwanted or residual nucleic acid and eliminates the need for a separate processing step. Other innovators have focused on technologies capable of developing stable cell lines to tackle the problem of batch variability in viral vector manufacturing.
The increased prospect of securing regulatory approval means that the flow of investment into cell and gene therapy has grown significantly. As the patent landscape grows, it is increasingly important that innovators protect their technologies at an early stage and seek advice to avoid the risk of patent infringement.
Realising the potential of gene therapy will involve bringing together various strands of research and development activity – everything from identifying new viral vectors, to immune modulation techniques and streamlined production methods. Owners of patented technologies in any of these fields will have an opportunity to lead the way in finding cures for some of the world’s rarest, life-limiting diseases.