May 26, 2026

Dear Duchenne Community:

Today we are pleased to announce the submission of a Biologics License Application (BLA) to the U.S. Food and Drug Administration (FDA) for zeleciment rostudirsen (z-rostudirsen, also known as DYNE-251) for the treatment of individuals with Duchenne muscular dystrophy (DMD) amenable to exon 51 skipping.

The BLA includes data from the positive Phase 1/2 DELIVER clinical trial. Based on these data, we are seeking Accelerated Approval and have requested Priority Review for the BLA, which, if granted, would shorten the review process from 10 months to 6 months following the FDA’s 60-day filing review period. We continue to expect a potential U.S. launch of z-rostudirsen in Q1 2027, assuming the FDA grants Priority Review and approval is received on the anticipated timeline. Our full press release can be found here.

Additionally, last week we announced the initiation of the Phase 3 confirmatory FORZETTO trial of z-rostudirsen.

Information about the FORZETTO trial can be found below. Our full press release can be found here.

About the Phase 3 FORZETTO Trial

• FORZETTO is a global, randomized, placebo-controlled, double-blind, confirmatory Phase 3 trial designed to assess the efficacy, safety, and tolerability of z-rostudirsen administered intravenously to ambulatory male participants with DMD amenable to exon 51 skipping.

• The trial will enroll approximately 90 participants 4 to 18 years of age who will be randomized 1:1 to receive z-rostudirsen 20 mg/kg or placebo every four weeks (Q4W). The first U.S. trial site is now activated and open to enrollment, and we expect to open additional sites in the United States over the next few months, followed by sites outside of the United States.

• The primary endpoint is the change from baseline in rise from floor (RFF) velocity, also referred to as time to rise (TTR), at Week 73. RFF velocity is a clinically meaningful measure of muscle strength and motor coordination commonly assessed in DMD clinical trials.

• Secondary endpoints include changes from baseline in stride velocity 95th percentile (SV95C), North Star Ambulatory Assessment (NSAA) total score, 10-meter walk/run (10MWR) velocity, four-stair climb (4SC) velocity, and forced vital capacity percent predicted (FVC%p) as well as additional functional and patient-reported outcome measures. Following the 72-week double-blind placebo-controlled treatment period, participants will be eligible to enroll in a 96-week open-label long-term extension.

Dyne has aligned with the U.S. Food and Drug Administration (FDA) on the design and protocol for the FORZETTO Phase 3 study. FORZETTO is designed to confirm earlier findings and support the potential transition from Accelerated Approval to traditional approval in the United States, as well as to support regulatory submissions outside the U.S.

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

The FORZETTO Phase 3 study design was informed by meaningful input from the Duchenne patient community. In coordination with global Duchenne patient organizations, we are committed to maintaining transparent communication and providing regular updates to keep the community informed of our regulatory and clinical progress.

With gratitude,

The Dyne Therapeutics Team

* Z-rostudirsen 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- rostudirsen have not been established.