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Minipig Study Tests Gene Therapy for Huntington's Disease

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Huntington's disease is a progressive neurodegenerative disorder that results from a mutation in the huntingtin gene, HTT. Currently, there is no cure for the disease.  

As we learn more about the underlying pathophysiology of Huntington's disease, researchers continue to focus on the development of treatments that target the specific cause, rather than simply alleviating the disease symptoms. Gene therapy is one example of such therapeutic, and research in this area continues to grow, both in academia and industry.

A new body of work published in Science Translational Medicine outlines the preclinical testing of a microRNA-based gene therapy for Huntington's disease – AMT-130 – in minipigs.

In simplistic terms, the therapy aims to prevent the production of the toxic mutant version of the HTT protein that causes Huntington's disease. The research data from minipigs showed that intracerebral delivery of the therapy into the striatum resulted in widespread distribution and reduced mutant HTT protein for up to 12 months post injection.

Technology Networks spoke with Astrid Vallès, associate director in translational biology at uniQure, and Melvin Evers, senior director of preclinical biology at uniQure to learn more about the preclinical study, the gene therapy landscape and the next steps in the development of AMT-130.  

Q: Why are current treatment options for Huntington's disease limited, and why is a gene therapy approach favorable?

Huntington’s disease is an autosomal dominant neurodegenerative disorder, caused by mutations (increased polyglutamine tract) in the huntingtin – HTT – gene. Current treatment options are indeed limited and only symptomatic, but fortunately, many promising therapies are in development as there is increased knowledge of the disease mechanisms. One of the most promising approach is to lower the expression of (mutant) HTT. With gene therapy, we can specifically and effectively reduce the expression of the disease-causative gene. Next to being very long lasting (a single administration of gene therapy can be effective for years), the big advantage of gene therapy is that it has the potential to be disease modifying.

Q: What are some of the key considerations when developing and testing a gene therapy for a neurodegenerative disorder?

Key considerations are biodistribution, safety, efficacy and biomarkers. For biodistribution, it is important to establish the brain regions where the gene therapy needs to be effective. In the case of Huntington’s disease, the first affected areas are caudate and putamen (deep brain regions), while other brain areas (like cortical regions) are affected later in the disease. For this reason, targeting both deep brain regions and cortical regions is considered key for disease modification.

For safety, it important that the approach has no detrimental effects. Many preclinical studies are done to assess the potential toxicology of the gene therapy product, before going into humans.

The gene therapy must have demonstrated efficacy for it to be successful. We evaluate this first in preclinical disease models, by showing that there is the desired biological response and disease modification in these models. In humans, the use of biomarkers (measured in blood, cerebrospinal fluid (CSF) or through MR-imaging methods) is key to help us predict clinical efficacy in the patients, especially for neurodegenerative disorders with slow disease progression.

Q: Can you talk about how the gene therapy works to treat Huntington's disease?

Our gene therapy for Huntington’s disease is named AMT-130. It consists of a capsid, an inactivated virus called adeno-associated virus, or AAV. Inside the AAV a piece of DNA is encapsulated. After direct administration into the brain tissue, the AAV is taken up by brain cells and transported to the cell nucleus. In the nucleus, the AAV opens up and the piece of DNA is released and stays in the nucleus. This piece of DNA encodes a therapeutic transgene called a microRNA. This microRNA has been engineered to specifically target the HTT messenger RNA.

The microRNA technology used is called miQURE and the specific microRNA is called miHTT. Upon binding of miHTT to HTT messenger RNA, the huntingtin messenger RNA is broken down, thus preventing the production of the toxic mutant HTT protein that causes Huntington’s disease.

Q: Why was the study conducted in minipigs as opposed to other in vivo models, such as mice?

We have conducted several preclinical studies to test the biodistribution, safety and efficacy of our therapy. Other in vivo models (mice and rats) were used in previous studies, tailored to show mechanism of action of the drug (that is, does the drug work as predicted), and proof-of-concept (i.e., does the drug modify the disease-like phenotype in this model?).

When it comes to biodistribution (does the drug reach the right brain area?), it is very important to test this in a larger brain. This is because, very often, approaches that work in small animals do not work in larger species. If in this case the approach is effective in a larger brain, it gives much more confidence that it may work in humans.

Q: The study found "strong, sustained, and brain-widespread vector distribution, human HTT protein lowering, and associated biomarker changes". Can you talk about some of the key findings on the success of the therapy in more detail?

The minipig model we used for the study is very valuable, because it expresses a fragment of mutant HTT as transgene. This allowed us to (i) test our gene therapy in a large brain, (ii) evaluate how efficient it is across the many brain regions and (iii) establish how long-lasting the effects of a single administration of our gene therapy was.

We observed a very efficacious reduction of mutant HTT in all brain regions that are relevant for the disease for up to one year after administering the therapy. We also evaluated the potential of candidate biomarkers to follow-up efficacy of the therapy. This is important, as in humans, it will not be possible to directly measure the effects in the brain, and efficacy biomarkers (surrogate measures of efficacy) are needed. Among others, we measured miHTT (the active molecule of our gene therapy) and mutant HTT in CSF.

Our conclusion is that, to follow-up efficacy of AMT-130, miHTT in CSF has greater potential value than mutant HTT in CSF (as the latter underestimates the effects seen in deep regions of the brain, which are the regions AMT-130 predominantly targets and are most relevant for the disease). This will be valuable information to aid interpretation of the biomarkers measured in our clinical trial.

Q: The study involves a surgical protocol for gene therapy delivery. How will this be adapted for clinical translation?

We have committed substantial effort to mirroring the clinical procedure in the large animals. While setting up the procedure, we have learned and adapted a lot of the knowledge from deep brain stimulation surgeries.

As in the clinical setting, the Huntington’s disease minipigs were placed in the magnetic resonance imaging (MRI) to visualize the brain and plan the surgical catheter implementation. After this planning phase, the catheters were placed, fixed in a surgical suite. Subsequently, the subjects were again placed in the MRI and the catheter connected to a pump to infuse AMT-130. This infusion is done under MRI guidance – thus the filling of the brain structures (the striatum) is tracked live. After administration, the catheters are removed, and subjects returned for a quick recover.

Q: For our readers that may be unfamiliar, what are some of the drawbacks associated with gene therapy, and how have you considered this in the development of this therapeutic?

The main advantage of gene therapy using AAVs is its durable effect after a one-time treatment. This durability is also the major drawback.

Once administered there is no option to modulate the expression of the therapeutic transgene. Also, due to the antibodies that will be generated against the AAV capsid, re-administration is not feasible yet. Thus, you have one shot to give the patient the most optimal dose for life. Therefore, extensive preclinical efficacy and safety studies have been performed, also in large animals, to get better understanding on the long-term efficacy and safety of the AMT-130 gene therapy to come up with a clinical dose that is considered to be both safe and efficacious.

Q: The development and manufacturing of gene therapies can carry increased costs compared to traditional therapies. How will you ensure that, if made clinically available, this therapy is accessible?

There are no current treatments for Huntington’s disease and the need for a therapy should one be approved by regulators will have profound impact on patients and their families. uniQure has formed important relationships and collaborations with patient advocacy and academic groups who are committed to finding a treatment option for this devastating disease. We would build on these relationships to partner our efforts to create accessibility to meet demand, and this also could include collaboration with other health care providers in the biotech and pharmaceutical industries. We are early in the clinical development of AMT-130 and look forward to pursuing patient access strategies as we move forward over the next few years.

Q: What are your next steps?

We are continuing with our Phase I/II clinical trial of AMT-130, which is a randomized, sham controlled, double-blinded study to explore the safety, tolerability, and proof of concept of AMT-130 in patients with early manifest Huntington’s disease. The study, which includes two dose cohorts, will randomize a total of 26 patients to either treatment with AMT-130 or an imitation surgical procedure.

The first dose cohort includes 10 patients, of which six patients received treatment with AMT-130 and four patients received imitation surgery. The second dose cohort is planned to include 16 patients, of which 10 patients will receive treatment with AMT-130 and six patients will receive imitation surgery. The trial consists of a blinded 12-month study period followed by unblinded long-term follow up for five years after administration of AMT-130.

Patients receive a single administration of AMT-130 through MRI-guided, convection-enhanced stereotactic neurosurgical delivery directly into the striatum (caudate and putamen). The planned Phase Ib/II study of AMT-130 will be conducted in Europe and is expected to begin enrolling patients in the second half of 2021. This open-label study will enrol 15 patients with early manifest Huntington’s disease across two dose cohorts.

Together with the U.S. study, the European study is intended to establish safety, proof of concept, and the optimal dose of AMT-130 to take forward into Phase III development or into a confirmatory study should an accelerated registration pathway be feasible. 

Melvin M. Evers and Astrid Vallès were speaking to Molly Campbell, Science Writer for Technology Networks.