The most advanced experimental drug for the most common form of adult muscular dystrophy has failed to beat placebo on its main measure, leaving thousands of American families in the same position they were in a week ago, with no approved treatment for the disease itself.
Novartis announced on Sept. 8 that its global Phase 3 HARBOR study of del-desiran in myotonic dystrophy type 1 did not show a statistically significant improvement over placebo on video hand opening time, a measure of how quickly a person can release a grip. The company said it saw clinical activity in secondary endpoints and exploratory analyses, and will review the full dataset and talk to regulators about a path forward.
For families, the immediate meaning is narrow. Nobody was taking this drug outside a trial, so no prescription is disrupted. What changed is the timeline. A therapy many patients expected to reach the market within a couple of years is now, at best, delayed and unresolved.
Inside the HARBOR Study and the Measure It Missed
HARBOR enrolled 159 people aged 16 to 65 with myotonic dystrophy type 1 across nine countries. Participants were randomized to receive an intravenous infusion of del-desiran or placebo every eight weeks for seven doses, with a final assessment at week 54. Alongside the hand opening measure, the study tracked hand grip strength, quantitative muscle testing, activities of daily living and a 10 meter walk and run test.
The primary endpoint was unusual. Video hand opening time is a newer measure built to capture myotonia, the delayed muscle relaxation that makes it hard to let go of a doorknob, a steering wheel or a handshake. It is clinically meaningful but relatively untested as a regulatory endpoint, which is part of why the result is being read carefully rather than as a verdict on the drug. One analyst quoted in trade coverage pointed to variability in the hand relaxation test as a possible factor.
In its statement, Novartis said safety findings were generally consistent with previously reported data. Shreeram Aradhye, the company's president of development and chief medical officer, said developing therapies for a disease this complex remains difficult and that setbacks are part of scientific progress.
Del-desiran is an antibody oligonucleotide conjugate, pairing an antibody that binds transferrin receptor 1 on muscle cells with a small interfering RNA designed to break down the toxic DMPK messenger RNA that drives the disease. It has received orphan drug, fast track, and breakthrough therapy designations from the FDA. Novartis acquired it through its purchase of Avidity Biosciences, a deal valued at about 12 billion dollars.
A Condition That Reaches Well Beyond Muscle
Myotonic dystrophy type 1 is caused by an expanded stretch of CTG repeats in the DMPK gene and is inherited in an autosomal dominant pattern, so a parent with the condition has a 50 percent chance of passing it to each child. The National Human Genome Research Institute describes it as a multisystem disorder rather than a muscle disease alone.
That distinction matters. Beyond muscle weakness and myotonia, the condition can involve cardiac conduction problems, breathing difficulties during sleep, early cataracts, insulin resistance, gastrointestinal symptoms, excessive daytime sleepiness and cognitive effects. Cardiac and respiratory complications are the leading causes of death, which is why cardiology and pulmonary follow-up matter as much as anything done for the muscles.
How many Americans are affected is uncertain. Published prevalence estimates have long run between five and 20 cases per 100,000 people, but a study published in Neurology that screened de-identified newborn blood spots from New York State found CTG repeat expansions in DMPK up to five times more common than those estimates suggested, at roughly one in 2,100 births. The authors concluded the disease is likely underdiagnosed, and diagnostic delays measured in years are common.
Repeat length shapes severity. Larger expansions are generally associated with earlier onset and a more severe course, and the largest expansions are linked to childhood and congenital forms. Repeats also tend to lengthen when passed between generations.
Consequences for Other Programs Chasing the Same Target
The result moved beyond one company. Reuters reported that Novartis shares fell about 9 percent in Switzerland, among the worst trading days in the company's history, and that shares of Dyne Therapeutics and Sarepta Therapeutics fell 30 percent and 15.5 percent in U.S. premarket trading. BioPharma Dive reported that Dyne was down 22 percent and Novartis more than 13 percent later in the session. The differing figures reflect premarket versus intraday trading and separate listings.
The selloff spread because these programs share design features. Sarepta's candidate is also a small interfering RNA aimed at DMPK RNA, using a different targeting approach. Dyne's therapy uses a similar muscle targeting antibody strategy and overlapping functional endpoints. A miss on one raises questions about the endpoints and the platform, not only the molecule.
None of that tells patients whether the underlying approach works. An earlier phase 1/2 study showed the drug reached skeletal muscle and reduced disease-associated DMPK RNA. Novartis reported activity on secondary measures in HARBOR, leaving open the possibility that the trial measured the wrong thing rather than that the drug did nothing. That will not be settled until the full dataset is presented or published, and Novartis has not said when.
The company said its other two antibody oligonucleotide conjugate programs continue. It has filed delpacibart zotadirsen for accelerated approval in a subset of Duchenne muscular dystrophy and received priority review, and it plans to meet with the FDA about a facioscapulohumeral muscular dystrophy candidate. It also reaffirmed its five year sales growth guidance through 2030.
Where Families Can Turn While the Data Is Reviewed
This result does not change current clinical care. People with the condition should continue cardiac monitoring, including periodic ECGs, sleep and respiratory assessment, eye examinations and diabetes screening, and should keep taking any medications prescribed for symptoms such as myotonia or daytime sleepiness.
Anesthesia deserves particular attention. People with myotonic dystrophy face elevated risk of respiratory and cardiac complications during and after surgery, including procedures as routine as a colonoscopy. Patient organizations publish anesthesia guidance meant to be handed directly to a surgical team, and families are advised to carry it to pre-operative appointments.
Families interested in research can search current studies on ClinicalTrials.gov, including an open label extension for HARBOR participants, and can find disease information and care resources through the Muscular Dystrophy Association. Genetic counseling is worth discussing for anyone with a family history, particularly around family planning.
What comes next is a regulatory conversation rather than a public one. Novartis said it will engage with health authorities after evaluating the full HARBOR dataset. Until then, the drug's future is undetermined, and the disease still has no approved treatment.
Key Questions Answered
What did Novartis announce? The Phase 3 HARBOR study of del-desiran in myotonic dystrophy type 1 did not show a statistically significant improvement over placebo on its primary endpoint, video hand opening time. The company reported activity on some secondary and exploratory measures.
Does this affect anyone currently taking a medication? No. Del-desiran is investigational and was available only through clinical trials. No approved treatment for the underlying disease is being withdrawn, because none exists.
Is the drug finished? Novartis has not said so. It is evaluating the full dataset and will engage with health authorities about a possible development path. No timeline has been announced.
What is myotonic dystrophy type 1? An inherited multisystem disorder caused by an expanded CTG repeat in the DMPK gene. It causes muscle weakness and delayed muscle relaxation, and can also affect the heart, breathing, eyes, digestion, metabolism, and cognition.
How common is it? Published estimates have ranged from five to 20 cases per 100,000 people, though a New York newborn screening analysis found DMPK expansions up to five times more common than that, suggesting substantial underdiagnosis.
Why did other companies' shares fall? Dyne Therapeutics and Sarepta Therapeutics are developing candidates for the same disease using related approaches and overlapping functional endpoints, so a failure in one program raised questions about the others.
What should patients and families do now? Continue routine cardiac, respiratory, eye, and metabolic monitoring, keep anesthesia guidance available for any procedure, and speak with a neuromuscular specialist about current or upcoming trials.
