April 16, 2026

Dear Myotonic Dystrophy Community:

With two myotonic dystrophy type 1 (DM1) clinical trials ongoing, we are sharing information to ensure that the community is informed about opportunities to participate in studies investigating the potential of zeleciment basivarsen (z-basivarsen, also known as DYNE-101) to be a safe and effective treatment for DM1.

In March, during the Muscular Dystrophy Association Conference 2026, we announced the initiation of HARMONIA, our global Phase 3 clinical trial of z-basivarsen in individuals with myotonic dystrophy type 1 (DM1).

Information on this trial can be found below and is now available on clinicaltrials.gov as well. Over the next few months, we expect to open additional sites to provide more opportunities for enrollment both within and outside of the United States. The press release announcing the HARMONIA trial can be found here.

Additionally, we continue to enroll our Phase 1/2 ACHIEVE clinical trial. Information about ACHIEVE can be found here.

About the HARMONIA Trial:

HARMONIA is a global, randomized, placebo-controlled, double-blind, confirmatory Phase 3 clinical trial evaluating the efficacy, safety and tolerability of zeleciment basivarsen (z-basivarsen, also known as DYNE-101) in people living with myotonic dystrophy type 1 (DM1).

• The trial will enroll approximately 150 participants age 16 and older who will be randomized 1:1 to receive 6.8 mg/kg of z-basivarsen or placebo every eight weeks (Q8W).

• The primary goal of the study is understanding the change from baseline in the five times sit to stand (5xSTS) test at week 49. The 5xSTS test is a reliable and responsive measure that reflects key areas of DM1 impairment, including lower extremity strength, balance and trunk strength, which are critical to performing daily activities.

• Additional assessments in the study include video hand opening time, strength testing, the 10-Meter Walk/Run test, the Myotonic Dystrophy Health Index, and other patient- and clinician-reported outcomes.

• The trial also includes a broad set of exploratory endpoints designed to assess multiple domains of DM1 central nervous system (CNS) impact.

• Following the 48-week double-blind placebo-controlled treatment period, participants will be eligible to enroll in a 24-week long-term extension.

• Study treatment and all study tests and procedures are provided at no cost to study participants. Participants will be reimbursed for approved travel-related expenses. Services to assist with arranging any travel, hotel, and meals during study visits may be available to participants at no cost.

• To request more information, please email clinicaltrials@dyne-tx.com

Your insights have shaped our work, and we are deeply grateful to the trial participants and families whose dedication continues to move DM1 research forward with real hope for the future.

With gratitude,

The Dyne Therapeutics Team

* Z-basivarsen is an investigational therapeutic and has not been approved by the FDA or any other regulatory authority for commercial use, and the safety and efficacy of z-basivarsen have not been established.