Finding biomarkers that capture neuroinflammation is critical to the continued use of gene therapies carried on an adeno-associated virus (AAV) as a transport agent, according to a recent review.
AAV-based gene therapies are of growing importance to treating neurodegenerative and neuromuscular disorders like spinal muscular atrophy (SMA). But as a virus — even a harmless one — an AAV’s use can cause neuroinflammatory reactions that are of considerable clinical concern and weigh on a gene therapy’s safety and efficacy.
The review, “Management of Neuroinflammatory Responses to AAV-Mediated Gene Therapies for Neurodegenerative Diseases,” was published in the peer-reviewed journal MDPI.
In general, AAV-based therapies work by using a virus, engineered to not cause any disease, to infect a patient’s cells so to deliver a working copy of a gene that is either mutated or missing to a cell.
Zolgensma, marketed by Novartis, is an approved AAV-based gene therapy for SMA that delivers a functional copy of the SMN1 gene to motor neurons. SMN1 normally provides a protein that is needed for motor neuron survival and muscular health. But in people with SMA, a mutation causes little or no protein to be made from this gene, leading to motor neuron and muscle loss.
Despite advances in the design and application of AAV-based therapies, and evidence that the central nervous system (CNS, brain and spinal cord) is rather protected from immune responses, preclinical studies in large animal models continue to show markers of neuroinflammation (inflammation of the CNS) and CNS disease with their use.
How much of a risk this poses to patients remains unknown, as AAV-related neuroinflammation is only beginning to emerge as an important topic. Clinical trials to date have not been designed to emphasize understanding the cell types and mechanisms involved in neuroinflammation, or to collect biomarkers of such inflammation, the review study notes.
Its researchers argue that existing techniques, such as magnetic resonance imaging (MRI) and blood and spinal fluid samples, could all be used to test for a wider range of potential neuroinflammatory markers.
Reliable biomarkers will help scientists better assess how different AAV delivery methods and molecular components affect the inflammatory response.
AAV-based therapies are delivered either systemically (into blood circulation) or directly to the CNS.
Systemic delivery, in which large amounts of viral particles flood the body in the hope that a therapeutic number of them will act where needed, remains the most common delivery method with shown safety.
But its high doses may trigger exaggerated immune responses.
Targeted AAV delivery to the CNS by direct injection to the brain or spinal canal allows for clinically relevant efficacy at lower doses. But some animal studies have shown evidence of immune reactions.
Zolgensma, for example, is approved for systemic administration (IV injection, directly into a vein) in SMA children up to age 2. Its use with intrathecal delivery (IT, through the spinal canal) is being studied for older patients in an open-label Phase 1/2 clinical trial (NCT03381729) called STRONG.
Other studies, however, report finding no evidence of neuroinflammation, which underscores the need for further comparison of experimental designs and of biomarker selection decisions across studies of this kind.
The study’s researchers point toward vector design as another way to control AAV-mediated neuroinflammation.
Similar to the popular children’s Lego toy system, AAVs are composed of molecular building blocks that can, to a certain extent, be exchanged to fit a given need.
They also suggest that more can be done to select the least immunogenic (prone to generate an immune response) combinations, and without sacrificing how efficiently and widely an AAV carries a therapy throughout the body.
TLR9 shows promise as a therapeutic target. However, much of the work done to understand how it functions has occurred outside the CNS, and its precise role in AAV-related neuroinflammation remains poorly understood.
Unwanted effects of TLR9 activation that have been seen in some animal studies highlight the risks involved in triggering this cellular defense mechanism, and signal caution with future AAV-based therapies.
Finally, immunosuppressants and other treatments that lower an immune response might complement CNS-targeting gene therapies. More research is needed to identify the most effective compounds and potential approaches.
Understanding the body’s response to AAV-based gene therapies will help researchers understand the full potential that they offer in treating neurodegenerative diseases. Including biomarkers that can evaluate neuroinflammation at key time points may also be important to more fully understanding “the impact immune reactions can have on treatment safety and efficacy.”
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