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HD InsightsPerspective: Closing Thoughts on GENERATION-HD

Closing Thoughts on GENERATION-HD
By Daniel Claassen, MD

Daniel Claassen, MD is an Associate Professor of Neurology at Vanderbilt University and Editor of HD Insights.

The disappointing news of 2 halted programs reverberated around the HD community this past month. Tominseren, and WVE-120101/2 were proposed as antisense oligomer (ASO) treatments for HD patients. These treatments do not presently appear viable options for therapies, but for different reasons. It’s difficult to express the profound hype and the hope that accompanied these trials. ASO treatments appear promising for neurologic disease: Nusinersen was approved for the treatment of spinal muscular atrophy, and the videos of children who can now walk are miraculous. I was at a European HD program, and when watching these videos, I remember a comment from the audience microphone, “can we imagine a world where HD patients will have a treatment like this?” Fast forward to 2021, and the mood is…flat.

GENRERATION-HD was a massive undertaking. Over 800 HD patients were dosed with Tominersen. The global scope of this trial, speed at which it enrolled, and effort to methodically follow the protocol was simply astonishing. Participants were dosed initially every 4 or 8 weeks, then later the schedule was delayed to every 8 or 16 weeks. There was a lot of enthusiasm about this program, despite the sacrifice involved. The criteria ensured a motor manifest population, rather independent in ADLs, and without substantial comorbidities. For the Wave program, dosing was every 4 weeks, with similar clinical criteria for inclusion, but was a dose finding and safety study.

Phase 1 data from tominersen showed clear reductions in mutant Huntington protein, in a dose dependent level. This non-selective allele approach would also theoretically reduce wild-type Htt expression. (Remember, there are 2 alleles, a mutant expanded CAG-repeat and a wild-type allele). Several early concerns regarding elevation of neurofilament light chain, and increased ventricular size were noted, but the treatment seemed to ‘hit the target.’ The Wave program was much earlier, and took a different approach, using a single nucleotide polymorphism targeted approach to reduce only the mutant huntington (Htt) protein, leaving the wild type protein alone. Side by side, the pros and cons of selective or non-selective approach appear to have encouraged a vigorous debate regarding the role of wild type Htt in neurodevelopment, response to stress, and safety. Several recent reports have emphasized the potential deleterious effects of wild-type Htt reductions, but overall these programs have forced the field to consider what role, if any, wild type Htt has in humans. This has been a welcome discussion in the field, and one that will continue long after these trials. It appears that Wave has developed a promising method to assess wild-type Htt, and we hope that Roche and others will test this method on their stored samples.

The endpoints of GENERATION-HD deserve consideration. It is clear that regulatory agencies across the globe cannot agree what endpoint is considered useful for HD. The FDA only allowed the Total Functional Capacity (TFC) scale, a crude marker of ADLs in persons with HD. In Europe, acceptance of a composite endpoint which includes objective motor (Unified Huntington Disease Rating scale), cognitive, and TFC measurements were accepted as a primary endpoint. It is fair to say that coming to a ‘global’ agreement on endpoints is crucial for rare diseases. However, it’s easier to agree on an endpoint like COVID-19 infection, but harder for one like disease progression in a neurodegenerative disorder. It does not help patients to have bickering between countries on which endpoint to pursue. Hopefully, we will see regulatory bodies work in tandem, and not in opposition, for drug development. It’s a tall order, but would make for better science and an assessment of treatments.

On the topic of endpoints, there is a movement in neurodegeneration to find better, reliable endpoints, that are quantifiable, and not subject to noise. The promotion of quantitative digital biomarkers appears to be a way forward. GENERATION-HD employed a secondary outcome measure assessing digital outcomes. As a global community, we would anticipate data from this study to be publicly available for use in developing better biomarkers of disease progression in HD. There is always a tension with private companies collecting data, but allowing for algorithm development, hypothesis testing, and analytic approaches to this vast data set will only help the field.


Where Do We Go From Here?

Given the potential issues with outcomes, treatment side effects, and patient inclusion, it is not fair to say we should abandon mutant Htt protein reductions. There is no reason, at present, to believe that the pathophysiologic progression of HD is not driven by mutant Htt protein expression. Undoubtedly, a deep dive into the data from GENRERATION will allow us to look at patient subtypes, clinical safety, variation in treatment responses, longitudinal clinical progression, and sub-analysis on imaging, digital, and other secondary biomarkers. These will be important and help us move forward. For Wave, it appears that there are continued advances in the SNP targeted therapy, with promise of an improved allele-specific ASO in development. This approach is touted to provide improved drug distribution and efficacy.

It is worth reminding the community of current and future studies in HD. Uniqure is testing the hypothesis that AAV-mediated gene therapy can reduce mutant Htt, and is currently in the midst of a Phase 1 sham-controlled study. Several other companies, Voyager and Spark, are pursuing similar approaches. These are invasive treatments, requiring surgical delivery of the AAV virus to the striatum. There are a number of potential impediments to scalability of this method, but the potential benefit of ‘direct-to-striatum’ delivery appears promising.

Aside from these studies, there are others that are of interest. One of the more compelling ideas is the concept of somatic instability in HD (increasing CAG repeat number over time). This is now an important treatment target, and companies like Triplet Therapeutics are pursuing therapies that may stabilize this instability. Oral medications offer an important convenience for patients. Here, novel approaches including RNA splicing (PTC Therapeutics), and repurposing of Branaplam (Novartis), are in early stages of development. These offer additional opportunities to assess how reductions in mutant Htt will potentially alter the disease course in HD.

At the end of the day, clinicians, and patients, are sailing together. The grief, frustration, disappointment, and dread of a failed trial has left many of us searching for what gives us hope. Many of my patients have reminded me that our hope is not in the immediate promise of treatments, but in things like family, faith, nature, and in the support of the HD community. I always find that my HD patients remind me of what is important, and they exemplify resilience in the face of adversity. We will continue to serve this population with steadfast resolve.