Beyond Myotonia: What DM1 Patients and Physicians Really Need from a Novel Therapy | REACH MarketVue®

A Disease Defined by What It Touches

DM1 is caused by an abnormal expansion of CTG trinucleotide repeats in the DMPK gene, which produces toxic RNA that accumulates in cells and disrupts splicing across virtually every organ system. The result is a disorder that simultaneously affects skeletal muscle, cardiac muscle, the brain, and smooth muscle - and does so with striking variability from one patient to the next.

In clinical practice, this heterogeneity creates real challenges. Neurologists describe patients who present primarily with cardiac arrhythmias while remaining ambulatory, and others who experience significant muscle weakness with cognitive deficits. No single severity measure captures the full picture, and physicians are left managing a mosaic of symptoms across multiple specialties.

What Patients and Physicians Prioritize

Skeletal Muscle Weakness: The Most Visible Burden

When KOLs were asked what a novel DM1 therapy most needs to address, skeletal muscle weakness emerged as the top priority - and notably, it was consistently ranked above myotonia itself. Physicians explained that while myotonia is the most clinically apparent and universally present feature of the disease, it is weakness that most limits patients’ functional independence. The inability to climb stairs, hold a job, or remain ambulatory has a more direct impact on quality of life than hand stiffness that can be at least partially managed with symptomatic agents like mexiletine.

Physicians also drew an important distinction between the recoverable and the irreversible. Once skeletal muscle tissue undergoes significant fibrotic replacement, the potential for functional recovery is substantially diminished - a dynamic that makes the timing of therapeutic intervention a critical variable. This is why KOLs are particularly attentive to whether emerging therapies can not only slow progression but achieve meaningful functional improvement in patients who are still ambulatory.

CNS Symptoms: Underrecognized and Undertreated

Perhaps the most consistent theme across KOL interviews was the importance - and frequent underestimation - of CNS involvement in DM1. Fatigue and excessive daytime sleepiness are among the most burdensome symptoms patients report, but they are multifactorial in origin: driven simultaneously by central dysregulation of the sleep-wake cycle, sleep apnea, and the secondary effects of progressive muscle weakness and respiratory compromise.

Cognitive impairment is particularly notable because patients themselves often cannot articulate or self-identify its progression - a characteristic that makes it challenging to capture through standard patient-reported outcome measures. Neurologists noted that behavioral changes and reliability issues (such as missing appointments) are often the first signals of cognitive decline in their practice, rather than patient complaints. Despite this, physicians who treat patients with DM1 are unequivocal: a therapy that reaches the CNS and demonstrates measurable benefit on fatigue, sleep, or cognition will have a meaningful commercial advantage over one that does not.

Cardiac Manifestations: A Major Cause of Premature Mortality

Although cardiac symptoms were not included in the FDA’s Voice of the Patient survey for DM1, KOLs were emphatic about their clinical significance. Arrhythmias, including heart block and atrial fibrillation, affect a majority of patients over the disease course and represent the second leading cause of premature death in DM1 after respiratory failure.

From a therapeutic standpoint, cardiologists see real potential for DMPK-modulating therapies to correct the mis-splicing of the cardiac sodium channel, which is believed to drive a particularly dangerous arrhythmia phenotype associated with sudden death. Even partial splicing correction in cardiac tissue could translate to meaningful reductions in arrhythmia burden, a claim supported by preclinical data.

GI and Smooth Muscle: A Less Visible but Real Burden

Gastrointestinal dysfunction - including constipation, diarrhea, dysphagia, and gastroparesis - affects a meaningful proportion of DM1 patients and is driven by smooth muscle involvement. While GI symptoms ranked lower than skeletal muscle and CNS concerns in terms of therapeutic priority, KOLs acknowledge that for some patients they are the primary source of disability. The absence of validated outcome measures for GI function in DM1 currently limits both the assessment of disease burden and the ability to claim therapeutic benefit in this domain.

What Differentiation Looks Like in This Landscape

The emerging therapeutic landscape for DM1 is generating real excitement, but also real scrutiny. Oligonucleotide approaches from Dyne Therapeutics, Avidity Biosciences, and PepGen are demonstrating clear proof-of-concept on biomarkers and early functional endpoints.

For a novel therapy to stand out, KOLs described a set of differentiation criteria that track closely with the tissue priorities outlined above. Depth of knockdown matters: the 30-40% DMPK knockdown achieved by leading oligonucleotide programs is viewed as promising but not sufficient for robust phenotypic correction, and physicians would like to see near-complete knockdown from next-generation approaches. Breadth of tissue penetration is equally critical. Therapies that address skeletal muscle alone are seen as a meaningful but incomplete advance; the strongest competitive position will belong to approaches that can credibly demonstrate CNS benefit.

Bottom Line

The conversation around therapeutic choice, KOLs emphasized, will ultimately be individual: different patients will weight muscle function, cognitive preservation, and administration burden differently, and the market is likely to accommodate multiple differentiated entrants. The full strategic analysis - including detailed assessment of efficacy expectations by tissue type and KOL perspectives on product positioning - is available in the complete MarketVue® DM1 report.