What It Will Take to Modify the Course of Huntington’s Disease | REACH MarketVue®

REACH Market Research conducted primary research with movement disorder neurologists and neuropsychiatrists practicing in the United States, Germany, and the United Kingdom for MarketVue® (U.S.) Huntington’s Disease. The following synthesizes KOL perspectives on what it will take to succeed in this space.

The Need for a DMT and Why Chorea Treatment Alone Falls Short

Current symptomatic therapies for Huntington’s disease address, at best, the surface of a deeply heterogeneous disease. VMAT2 inhibitors and antipsychotics can meaningfully reduce chorea, and SSRIs offer some relief for depression and anxiety. But these treatments do not impact the behavioral symptoms such as anger, impulse control and apathy that most disrupt daily functioning and caregiver relationships, and the cognitive decline that ultimately drives disability. The available toolkit was shaped around the most visible symptom rather than the most burdensome one.

Against this backdrop, physicians describe the need for disease modification in explicitly functional terms. A therapy that slows neurodegeneration across cognitive, behavioral and motor domains through demonstrating a clinically meaningful change on the cUHDRS and total functional capacity (TFC) scales is what the field is waiting for. Biomarker reduction, including neurofilament light chain and huntingtin protein lowering, is viewed as supportive evidence, but KOLs are clear that clinical endpoints must anchor any approval claim. The minimally clinically important difference on the composite UHDRS is well characterized and physicians would like to see a disease modifying therapy clear it unambiguously.

How High the Demand Is for a DMT

One of the clearest signals that support the extent to which patient demand for disease-modifying therapy overrides concerns about treatment burden is the story of uniQure’s AMT-130 which requires an eight- to ten-hour stereotactic brain surgery, a procedure that would give most pause. Yet across centers, physicians report that most eligible patients would proceed without hesitation, and that ineligible patients are already asking about off-label access. The community’s response to the AMT-130 readout which has generated enormous buzz and demand from patients reflects how long this population has been waiting.

The willingness to participate in tominsersen’s GENERATIONHD2 trial by Roche tells a similar story. Tominersen’s first trial GENERATIONHD1 failed to show benefit in the primary analysis and produced safety signals at higher doses, yet enrollment in the redesigned Phase 2 study, targeting earlier-stage patients at a lower, less frequent intrathecal dose, has proceeded and is fully enrolled. Physicians acknowledge that an intrathecal regimen administered every four months carries real patient burden but in the absence of alternatives, the administration schedule does not suppress demand.

Three Mechanistic Hypotheses

KOL feedback on the pipeline converges around three distinct disease-modifying strategies: reduce expression of the huntingtin (HTT) protein, block drivers of somatic CAG repeat expansion, or provide neuroprotection through downstream targets. Enthusiasm is not evenly distributed.

HTT Lowering: The Most Plausible But Technically Challenging

Physicians broadly view huntingtin-lowering approaches - whether delivered via AAV, antisense oligonucleotide, siRNA, or small molecule - as the most directly disease-modifying strategy. The logic is straightforward: the disease is caused by a genetic mutation which creates abnormal protein prone to misfolding therefore reducing expression of HTT should slow disease progression. That said there are challenges that complicate translation from mechanism to clinical effect. The question of whether allele selectivity (i.e., lower only mutant HTT not all forms) is required remains unresolved. Can a non-selective approach such as tominersen or HTT small molecules including SKY-0515 and votoplam balance efficacy and safety with careful dosing calibration or will safety signals emerge with these approaches since wild-type HTT is essential for adult neuronal function, and complete knockout in animal models is lethal?

Somatic Expansion: A Growing Target with Unanswered Safety Questions

Alternative approaches such as targeting the drivers of somatic CAG repeat expansion has grown substantially in recent years. The observation that neurons, particularly striatal neurons, accumulate additional CAG repeats over time helps explain both the region-specific vulnerability of HD and much of the individual variability in disease onset that germline repeat length alone cannot account for. MSH3 is the primary target in this space, with several preclinical programs working toward clinical entry. KOLs see this as a mechanistically distinct and potentially complementary approach to HTT lowering rather than a replacement for it. The key uncertainty is safety: targeting DNA repair machinery carries inherent risks around genomic instability and cancer that will need to be carefully characterized before broad clinical use.

Neuroprotection: Possible Use Adjunctively But Unlikely a Standalone Option

Neuroprotective approaches, most prominently sigma-1 receptor agonism via pridopidine, are viewed with more skepticism, primarily because they act downstream of the causative mutation rather than addressing it directly. Physicians do not dismiss neuroprotection entirely; the argument that it could serve as an adjunct to other disease modifying mechanisms is credible. But as a standalone strategy, the mechanism’s lack of HD specificity have tempered enthusiasm. The field is watching whether Prilenia’s confirmatory trial design can produce a clean primary endpoint result in the defined population.

Trial Design: What Two High-Profile Setbacks Have Taught the Field

Two of the most closely watched trials in Huntington’s disease history have now produced instructive failures. The lessons are specific enough to shape the design requirements for anything that follows.

Tominersen: The Cost of Enrolling Patients Too Late

GENERATION HD1 enrolled manifest HD patients across a broad range of disease stages and found dose-dependent worsening in the treatment arm with declining rating scale scores at higher doses. A rigorous post-hoc analysis identified a signal of benefit in a subgroup of earlier-stage patients within the original 781 patient cohort; a finding compelling enough to support the redesigned GENERATION HD2, which focuses on prodromal and early manifest disease at a lower, less frequent dose. The prevailing interpretation is that HTT lowering may be most effective and least likely to cause harm when the neurodegenerative process has not yet advanced significantly. Intervening earlier limits both the ceiling on benefit and the exposure to risk from suppressing wild-type huntingtin in neurons that still depend on it.

AMT-130: The Regulatory Risk of Relying on Natural History Control

AMT-130’s Phase 1/2 data showed a 75% slowing of composite UHDRS progression and stable neurofilament light chain in the high-dose cohort at three years which has been hailed as the first statistically significant disease-modifying signal in HD history. But the FDA’s subsequent flip-flopping on whether to require a RCT before approval has caused a significant delay in getting the therapy available to patients. With only 12 patients on the high dose and a propensity-matched external control drawn from the ENROLL-HD registry, the data may be vulnerable to selection bias and placebo effect. Critically, striatal volume biomarker data were not available for the natural history control patients, meaning the external ENROLL-HD control group may have been systematically different from the treated patients on a metric with prognostic significance.

The combined lesson from both trials points toward two key design requirements: selection for early-stage patients with the highest chance of benefit and inclusion of appropriate comparator to eliminate any ambiguity in the results. However, a randomized comparator comes at a price: long-term sham controls are ethically fraught when the administration burden is high, as with neurosurgery or IT injections, yet regulators and payers have signaled that selection of an appropriate control arm will be key for submission and reimbursement. The ENROLL-HD registry has indicated the intention to collect biomarker data including CSF, blood and MRI as part of the HDClarity and imageClarity substudies; however, the timing of when these datasets will become available is unclear.

The Bottom Line

Huntington’s disease presents a clear genetic target, a motivated and accessible patient community, and a pipeline built on increasingly sophisticated mechanisms. The path to approval is harder than the early AMT-130 signal suggested but it is now better defined. The questions that remain - which mechanisms will produce the necessary effect size in the right patient population and how trial design should evolve - are examined in depth in the MarketVue® (U.S.) Huntington’s Disease report.