Del-brax for facioscapulohumeral muscular dystrophy

Last updated Oct. 16, 2025, by Marisa Wexler, MS
Fact-checked by José Lopes, PhD

What is del-brax for facioscapulohumeral muscular dystrophy?

Del-brax is an RNA-based therapy being developed for individuals with facioscapulohumeral muscular dystrophy (FSHD), sometimes abbreviated as FMD or FSH. Treatment is given by infusion into the vein, and targets the abnormal buildup of a protein called DUX4 in muscle, which drives disease symptoms. The therapy is now being tested in clinical trials.

In FSHD, mutations in the DUX4 gene lead to the production of the DUX4 protein in cells where it normally isn’t. This is thought to cause cellular toxicity that leads to the disease’s symptoms. Del-brax is designed to reduce the production of the DUX4 protein by targeting the gene’s messenger RNA, an intermediary molecule made when the gene is read to make protein. The therapy contains an antibody that targets muscle cells to deliver a small interfering RNA molecule targeting the DUX4 gene messenger RNA.

Therapy snapshot

How will del-brax be administered?

In clinical studies, del-brax is being given intravenously, or by infusion into the bloodstream. In an ongoing Phase 3 clinical trial, infusions are given every six weeks.

Del-brax in clinical trials

Del-brax is being tested in a Phase 1/2 clinical trial called FORTITUDE (NCT05747924). This three-part study mainly is evaluating safety and the treatment’s effect on disease-related biomarkers. Participants are adults with FSHD, randomly assigned to receive del-brax or a placebo.

• Interim data have indicated that del-brax treatment improved measures of muscle strength, motor function, and life quality after four months. It also reduced markers of muscle damage and abnormal DUX4 protein activity. Treatment was overall well tolerated, with no serious side effects reported.

Avidity is hoping to use biomarker data from FORTITUDE to apply for accelerated approval of del-brax in the U.S. Accelerated approval is a form of conditional approval in which the U.S. Food and Drug Administration (FDA) allows a therapy to be brought to market based on early clinical evidence that it is likely effective, while still requiring drug developers to conduct additional tests to prove clinical benefit.

Patients who complete the placebo-controlled study can enter an extension study (NCT06547216), in which all participants are given long-term treatment with del-brax.

In addition, a Phase 3 trial dubbed FORTITUDE-3 (NCT07038200) is further testing the safety and efficacy of del-brax. This study is expected to enroll approximately 200 FSHD patients ages 16 to 70. Participants will be randomly assigned to infusions of del-brax or a placebo over about 1.5 years. The main goal is to evaluate the effect of treatment on a measure of muscle strength called the Quantitative Muscle Testing composite score.

Del-brax side effects

Available data from the Phase 1/2 FORTITUDE clinical trial indicate that del-brax side effects may include fatigue, cold sores, and low counts of hemoglobin, which is the protein used to carry oxygen through the bloodstream.

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