AAV-based gene therapies hold promise for treating CNS conditions

Meaningful progress is being made in the development of gene therapies for rare neurological diseases caused by single gene mutations.

However, most patients affected by central nervous system (CNS) disorders — including Alzheimer’s disease, Parkinson’s disease and epilepsy — do not have obvious causative gene mutations. Instead, these conditions are multifactorial and involve intricate molecular pathways. Thus, approaches to gene therapy must be designed to effectively address the underlying mechanisms of disease, rather than to deliver healthy copies of a mutated gene.

While multiple viral vectors are being investigated for in vivo CNS gene therapies — including adeno-associated virus (AAV), lentivirus (LV) and even rabies — AAV-based therapeutics have been the most successful to date.

Deborah Phippard, PhD

AAV-mediated gene therapies in development for Parkinson’s disease aim to deliver dopamine or dopamine-synthesizing enzymes. Those in development for focal epilepsies focus on reversing increases in network excitability by delivering genes encoding neuro-modulatory peptides, neurotrophic factors, enzymes and potassium channels. Therapies involving ex vivo modification of cells by AAV or other modalities followed by transfusion into the patient are outside the scope of this article but also offer promising avenues of research.

In this article, we explore challenges to development of effective in vivo AAV-based gene therapies for complex neurological diseases, with a focus on assessing, minimizing and mitigating immunogenicity.

Understanding the challenges

Advancements in technology, improved understanding of neurobiology and ongoing research have contributed to significant strides in CNS gene therapy development, but a number of challenges remain.

Crossing the blood-brain barrier (BBB). The impermeability of the BBB creates an obstacle for systemic delivery of gene therapy, although recent research has demonstrated the ability of certain engineered AAV capsids to cross the BBB in animal models (Deverman BE, et al.; Flytzanis NC, et al).

Optimizing transduction efficiency. Transduction efficiency varies widely depending on target cell type, especially between dividing and non-dividing cells. AAVs are the gold standard for use with non-dividing cells. Transduction efficiency is influenced by AAV serotype, promoter type and mode of administration to the target cell.

Targeting specific cell types. Cell-type specific gene transfer may be important for allowing genetic modification of only a subset of affected cells within the CNS to avoid unintended effects on neighboring cells or systems. AAV serotypes differ in their neuronal tropism, making serotype an important consideration in CNS gene therapy design.

Developing delivery methods. Safely and effectively introducing genes into the brain is a significant hurdle, requiring careful design of viral vectors, lipid nanoparticles and other delivery systems. Direct injection into the brain, spinal cord or nerve parenchyma has the advantage of targeting specific neuronal subpopulations while limiting off-target neuronal transduction, but will be less accessible for larger patient populations. Less invasive, indirect injections result in wider distribution localized to more superficial levels within tissue.

Achieving long-term expression. Safe, stable and sustained expression is crucial for ensuring that introduced genes continue to produce the desired therapeutic effect over the long term. Viral vectors can elicit an immune response that may neutralize the vector and limit transgene delivery and expression, making a thorough assessment of immunogenicity a prerequisite for clinical applications.

Adapted from Yang TY, et al. Mol Ther Methods Clin Dev. 2022;doi:10.1016/j.omtm.2022.07.018.

Assessing immunogenicity

AAV-based gene therapies have the potential and versatility to deliver breakthrough treatments for CNS diseases. AAVs are small, non-enveloped viruses that depend on other viruses for replication. While wild type AAVs frequently infect humans, they generally induce only a mild immune response and are not known to cause any disease. However, given that more than 50% of the general population has some degree of preexisting immunity to serotypes of AAV, a comprehensive evaluation of immunogenicity is critical (Boutin S, et al).

Both preexisting host immunity and de novo host immunity to AAV vectors — and the transgene product— can be immunological barriers to safe, effective gene therapies. Due to the risk associated with preexisting antibodies, the FDA recommends that developers “strongly consider contemporaneous development of a companion diagnostic to detect antibodies” to the gene therapy product, especially if preexisting immunity is used as an exclusion criterion. It is important to keep in mind that the FDA will very carefully consider both safety risk and assay performance. Depending on multiple variables, the FDA may very well consider the AAV gene therapy to pose significant risk and, therefore, require an investigational device exemption (IDE) for the immunogenicity assay. Thus, it is critical to not only prepare and submit for a significant risk determination (SRD), but also design and perform all the studies required to validate an assay under an IDE.

For neurological diseases where gene therapy is locally administered to an immune-privileged site, the risk of immunogenicity may be lower than with systemic administration. Nevertheless, it is still advisable to perform an SRD to avoid any unexpected surprises from a regulatory agency later in development that could significantly add to timelines.

Importantly, animal models do not accurately predict immunogenicity, incidence or severity in humans, likely due to species-related differences in immune cell functions.

Beyond the intrinsic properties of an AAV-based gene therapy product, there are also treatment-, manufacturing- and patient-related factors that contribute to immunogenicity. This leads to several schools of thought. On the one hand, it may accelerate clinical development to use the same route of administration, device and manufacturing process already used in investigational new drug-enabling and clinical studies. On the other hand, many companies with exciting new adaptations and advances have shown significant advantages in animal models with the potential to move the field forward.

In our experience working with innovative gene therapy developers, we have found that it is imperative to develop a bioanalytical strategy to measure, monitor and characterize both pre-existing immunity and host immune responses during clinical studies of AAV-based therapies.

It is important to consider the right sampling time points, blood volumes and sample matrices for each of these assays. For gene therapies that are administered locally, as may often be the case with CNS conditions, it may be necessary to correlate or extrapolate data obtained from the systemic circulation if local sampling is not feasible. If an assay is to be used for patient selection, the assay will need to be certified to Clinical Laboratory Improvement Amendments or, in the case of a significant risk determination, under an IDE.

AAV is the most prominent vector for gene delivery in CNS and most clinical trials.

To expand the application of AAV-based gene therapy from monogenic diseases to more prevalent neurological disorders, it is critical to understand the heterogeneous phenotypic spectrum of each disease, elucidate the molecular pathways involved, and design and effectively deliver therapies with an immunogenic profile that enables efficient transduction and long-term expression.

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Deborah Phippard, PhD, is chief scientific officer at Precision for Medicine.