Long-Term Slowing of Progression in Huntington's Disease with Pridopidine Treatment

In a two-year follow-up of the PROOF-HD trial, continuous pridopidine treatment in participants remaining off antidopaminergic medications was associated with a significant slowing of Huntington's disease progression across multiple functional and cognitive outcomes compared to natural-history trajectories.

The Gist

Imagine Huntington's Disease (HD) as a relentless, steep downhill slope that a person is forced to walk down every day. Without any help, this slope gets steeper and steeper, causing people to lose their balance (motor control), their ability to think clearly (cognition), and their independence (function). For a long time, doctors have had no way to build a railing or slow down the descent; they could only watch and help people as they fell.

This paper is about a new attempt to build that railing using a medicine called Pridopidine.

The Initial Hurdle: The "Noisy" Crowd

In a previous big test (the PROOF-HD trial), the medicine didn't seem to work for everyone at first glance. Why? Because the group of people being tested was like a crowded room where some people were wearing heavy, noisy backpacks (other medications called antidopaminergic drugs, or ADMs). These backpacks were so loud and heavy that they drowned out the quiet, helpful signal of the new medicine.

However, researchers noticed something interesting: the people who took off those noisy backpacks (stopping the other meds) seemed to be walking more steadily than those who didn't.

The New Experiment: A Two-Year Race

To see if this "backpack-free" group was really getting ahead, the researchers ran a longer test. They followed 90 people who took Pridopidine consistently for two years and kept their "backpacks" off.

But how do you know if they are actually slowing down the descent? You can't just look at them in a vacuum. You need a reference point.

• The Reference Point: The researchers used data from thousands of other HD patients in large databases (ENROLL-HD and TRACK-HD) to create a "virtual average" of what usually happens. Think of this as a ghostly race of people walking down the same steep slope without any special help. This represents the "natural history" of the disease—how fast the slope usually gets steeper.

The Results: Slowing the Descent

When they compared the Pridopidine group to this "ghostly race," the results were encouraging:

• The Analogy of the Treadmill: Imagine the disease is a treadmill speeding up. Without medicine, the speed doubles every year. With Pridopidine, the treadmill is still moving, but it's moving much slower.
• The Numbers: The study found that Pridopidine slowed the progression of the disease by 39% to 88% across different areas like thinking, walking, and daily living skills.
  • In simple terms: If the "ghostly race" would have lost 10 points of ability in two years, the Pridopidine group only lost 1 to 6 points. They stayed much closer to where they started.

Even when they looked at specific motor skills (like how smoothly someone moves their hands), the medicine showed a massive 77.8% slowing of the decline compared to the expected natural path.

The Bottom Line

Think of this study as finding a speed bump on that steep downhill slope. It doesn't stop the slide completely, and it doesn't turn the hill into a flat road. But for the people who took the medicine consistently and avoided conflicting drugs, the slide became much more manageable.

In everyday language:
For two years, people with Huntington's Disease who took Pridopidine and avoided certain other medications didn't just "hold on"; they actually slowed down the disease's attack significantly compared to what is normally expected. It suggests that with the right conditions, this drug might help people stay independent and functional for longer than they otherwise would have.

Technical Summary

1. Problem Statement

Huntington's Disease (HD) is a neurodegenerative disorder characterized by progressive deterioration in motor control, cognition, and functional capacity. Currently, there are no approved therapies proven to slow the disease's progression.

The PROOF-HD Phase 3 trial previously investigated pridopidine (a sigma-1 receptor agonist) but failed to meet its primary or key secondary endpoints in the overall study population. However, a post-hoc observation suggested that participants who remained off antidopaminergic medications (ADMs) during the double-blind phase showed clinical benefits compared to placebo. The critical knowledge gap addressed in this study is whether these benefits persist over a longer duration (two years) and how they compare against the expected natural history of the disease, which was not fully established in the initial trial.

2. Methodology

This study represents a long-term follow-up analysis (up to Week 104) of the PROOF-HD trial data, focusing on a specific subgroup and utilizing external historical controls.

• Study Population: The analysis focused on 90 participants who received continuous pridopidine at a dose of 45 mg twice daily and remained off ADMs throughout both the double-blind phase and the open-label extension period.
• Comparator Construction: Since a concurrent placebo group for the full two-year duration was not the primary focus of this specific analysis, the researchers constructed external comparators using data from two large natural-history cohorts:
  • ENROLL-HD
  • TRACK-HD
  • Method: Propensity-score weighting was applied to these external datasets to create a synthetic control group that matched the baseline characteristics of the pridopidine-treated participants.
• Statistical Analysis:
  • Model: Mixed-effects models for repeated measures (MMRM) were used to estimate least-squares mean changes from baseline to Week 104.
  • Outcomes: The study evaluated multiple clinical endpoints, including the composite Unified Huntington's Disease Rating Scale (cUHDRS), Total Functional Capacity (TFC), Symbol Digit Modalities Test (SDMT), Total Motor Score (TMS), and quantitative motor performance (Q-Motor FT-IOI).
  • Metric: The primary metric of success was the percent slowing of disease progression relative to the weighted natural-history comparators.

3. Key Contributions

• Longitudinal Validation: This study provides the first evidence of pridopidine's efficacy over a two-year continuous treatment period, moving beyond the shorter duration of the initial Phase 3 trial.
• Refined Subgroup Identification: It validates the hypothesis that the therapeutic signal of pridopidine is most pronounced in patients who avoid concomitant antidopaminergic medications, which may have previously masked the drug's effect in the broader population.
• Advanced Comparative Methodology: The study demonstrates the utility of propensity-score weighted external controls (from ENROLL-HD and TRACK-HD) to evaluate long-term disease-modifying effects in the absence of a long-term concurrent placebo arm.
• Multi-Domain Efficacy: It establishes that pridopidine may slow progression across diverse domains: motor function, cognitive performance, and functional capacity.

4. Results

At the 104-week (two-year) mark, continuous pridopidine treatment was associated with statistically significant slowing of disease progression compared to the natural-history trajectories.

• Comparison with ENROLL-HD: Participants showed a slowing of progression ranging from 39.5% to 88.3% across the following measures:
  • cUHDRS (Composite UHDRS)
  • TFC (Total Functional Capacity)
  • SWR (Stroop Word Reading)
  • SDMT (Symbol Digit Modalities Test)
  • TMS (Total Motor Score)
• Comparison with TRACK-HD: Similar patterns were observed, with slowing ranging from 48.5% to 81.5% across the same measures.
• Quantitative Motor Performance: Specifically, the Q-Motor FT-IOI (a measure of fine motor control) showed a 77.8% slowing of progression.
• Exploratory Analyses: Analyses that included participants who took concomitant ADMs showed similar directional trends, though the primary signal remained strongest in the ADM-free cohort.

5. Significance

This study offers a pivotal shift in the understanding of pridopidine's potential for treating Huntington's Disease. While the initial Phase 3 trial was considered a "miss" for the general population, this long-term analysis suggests that pridopidine acts as a disease-modifying agent for a specific, well-defined patient population (those off ADMs).

The findings suggest that:

1. Duration Matters: The therapeutic benefit of pridopidine may require longer-term exposure to become clinically apparent.
2. Concomitant Medication is Critical: The interaction between pridopidine and antidopaminergic drugs is likely antagonistic, suggesting that future trial designs or clinical guidelines must strictly manage concomitant ADM use to detect efficacy.
3. Clinical Hope: The observed 40–88% slowing of progression across multiple domains represents a substantial clinical benefit, potentially delaying the loss of independence and motor function in HD patients.

These results support the continued investigation of pridopidine as a potential disease-modifying therapy, provided that patient selection criteria regarding concomitant medications are strictly adhered to.