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On June 24, 2025, Avidity Biosciences, alongside Dr. Nicholas Johnson, presented the much‑anticipated topline results from the FORTITUDE Phase 1/2 study of del‑brax (also known as delpacibart braxlosiran), during an FSHD University webinar. This clinical trial represents a significant step forward in targeting the root cause of facioscapulohumeral muscular dystrophy (FSHD) — and the initial findings are promising.
The trial enrolled adults with FSHD in dosing cohorts of 2 mg/kg and 4 mg/kg of del‑brax, administered intravenously every six weeks at first, then every 13 weeks. The topline safety data were encouraging: no serious adverse events related to the drug were reported, and no participants discontinued treatment due to side effects. Importantly, most reported adverse events were mild to moderate (e.g., fatigue, lower hemoglobin, occasional cold sores), and serious events that did occur (like fractures) were deemed unrelated.
A major highlight was the identification and monitoring of KHDC1L, an RNA-based biomarker regulated by DUX4, the pathological gene in FSHD. Baseline levels of KHDC1L were significantly elevated—around 6‑ to 9‑fold higher in FSHD patients than in healthy controls. After treatment, KHDC1L levels dropped rapidly and consistently in participants receiving del‑brax compared to those on placebo, demonstrating drug engagement with the root cause of the disease. Alongside, creatine kinase—a marker of muscle damage—also showed a consistent decline in treated participants.
Although the trial wasn’t powered to prove efficacy, exploratory measures showed promising trends. Del‑brax treated participants exhibited improvements—relative to placebo—in mobility tests like the 10‑Meter Walk‑Run, Timed-Up‑and‑Go, and quantitative muscle strength testing. Upper limb function measured by Reachable Workspace also trended favorably. Patient-reported outcomes, including overall quality of life indicators, showed improvements aligned with those functional gains.
In conjunction with the topline data, Avidity announced that the FDA has opened an accelerated approval pathway for del‑brax, and they have initiated the global Phase 3 FORWARD trial involving approximately 200 participants across North America, Europe, and Japan. This trial will formally assess efficacy over 18 months, focusing on functional mobility and muscle strength.
The FORTITUDE results and FDA’s fast-track endorsement position del‑brax as a leading candidate for FSHD treatment. The ongoing FORWARD Phase 3 trial, expected to complete in 2026–27, will be critical in determining whether these early promise and signals translate into a clinically meaningful therapy.
For patients and families, this brings cautious optimism: the first therapy directly targeting DUX4 has shown initial safety, molecular activity, and encouraging functional signals. As the phase 3 trial progresses, we remain hopeful that del‑brax could become the first approved treatment for FSHD.
