(484) 588-5461
info-usa@progen.com

The Road To Even Safer AAV Gene Therapies

Date: November 2024

Although AAVs are among the safest gene therapy vectors, they do pose a few risks that can still be reduced.


One of the main advantages of using recombinant adeno-associated viruses (rAAVs) as vectors for in vivo gene therapy and a key reason for their growing popularity in clinical studies is that they are very safe. rAAVs are non-pathogenic, non-replicating, can transduce a wide variety of tissues, and have long-term efficacy, continuing to express their transgene for a long time after vector injection.

There are currently eight FDA- and EMA-approved gene therapies on the market that use rAAVs as vector, including Zolgensma, for treating spinal muscular atrophy, and Hemgenix, for treating Hemophilia B. In addition, many clinical trials – although still in their early stages – indicate that a number of other AAV gene therapies are likely also safe. Despite this, rAAVs have posed a few key safety concerns that are important to understand.

Immunogenicity

Although rAAVs generally are weakly immunogenic, a few patients in clinical trials have had severe immune responses to AAV gene therapies. These events, which took place during early clinical phases when the optimal dosing of AAVs was still being assessed, occurred when macrophages recognized the viral peptides that make up the capsids of rAAVs, triggering an immune response. Unfortunately, the underlying diseases that lead patients to be treated by gene therapies also tend to make patients more susceptible to potentially develop unwanted immune responses from the treatment.

To decrease the likelihood of these unwanted immune responses, ideally, patients would have the fewest number of rAAV vectors in their bodies to produce the desired therapeutic outcome. When rAAVs are produced, although most consist of capsids containing the vector genome, unfortunately, some capsids are empty or contain only partial vector or host cell genomes. These empty and partial capsids can be problematic because although they do not provide any therapeutic benefit to the patient, they do have the potential to trigger an immune response.

One way to minimize the possibility of an immune response to AAV gene therapy is to test patient sera for neutralizing antibodies. PROGEN is currently developing a powerful new serological assay that could be used to help determine which patients are more likely to safely receive AAV gene therapy.

Oncogenicity

Another beneficial characteristic of rAAVs that contributes to their safety is that they generally do not integrate into the host genome. Because they have had their replication gene removed when being engineered to serve as gene therapy vectors, they usually persist in host cells episomally. And although the FDA noted in a 2021 briefing document that there are no known instances of rAAV vectors causing tumors in humans or nonrodent species, the oncogenicity of rAAVs remains a potential safety concern for humans.

Possible ways that rAAVs could induce the formation of tumors include the silencing of tumor suppressor genes, the activation of oncogenes, and through random integration. In addition, DNA impurities – non-vector sequences that may be packaged along with the vector genome when rAAV vectors are assembled – may increase the risk of oncogenicity.

There are a number of approaches that can be taken to reduce these risks. Carefully selecting a serotype and promoter can decrease the possibility of tumor formation, while improving manufacturing methods and the vector design can reduce DNA impurities. In addition, a more comprehensive characterization of capsids by next-generation sequencing and quantifying partially filled capsids, which may contain host cell DNA, would further reduce the risk of oncogenicity. 

Ultimately, characterizing these oncogenic factors for each AAV gene therapy before they enter clinical trials can reduce the risk of tumor formation in patients, enabling them to benefit as much as possible from the desired therapeutic effect.

Toxicity

While various AAV serotypes differ in their tissue tropism, many have some tropism for the liver. So when AAV gene therapies are administered intravenously, most circulate through the liver before being delivered to their target tissue. In the rare cases where the liver is unable to manage the viral dosage, patients have developed elevated liver enzymes, drug-induced liver injury, and very infrequently, liver failure and death.

In order to reduce the risk of hepatotoxicity, more evidence-based patient selection measures specific to disease and vector dose could be implemented, based on clinical data. Another way to reduce this risk is to enhance tissue tropism through vector engineering. Better understanding the underlying immunological causes of hepatotoxicity is essential for ensuring that AAV gene therapy is safe for even more patients.


 

Return to Blog Center 


Interested in learning more about AAVs? 

Comparability Amid Change 

The transfer from early development to clinical grade manufacturing often entails changes in the manufacturing process of therapeutical AAV products. It is fundamental for AAV producers to collect extensive enough data along the way to meet the comparability requirements of regulatory agencies such as the FDA.

 

Read more