Ionis's Zilganersen Becomes First Drug to Show Disease-Modifying Effect in Alexander Disease as FDA Sets September Decision
An antisense drug stabilized walking speed in a 54-patient pivotal trial; the FDA accepted Ionis's NDA for Priority Review with a target action date of September 22, 2026.
Editor's Note ·
- Clarification:
- The article attributes the sentence about secondary and exploratory endpoints ("Results across key secondary and exploratory endpoints evaluating adaptive function, communication, gastrointestinal symptoms, sleep and seizures also consistently favored zilganersen") to Ionis's AAN 2026 release. That verbatim wording actually appears in Ionis's FDA Priority Review acceptance release, which is also among the article's cited sources. The AAN release lists the corresponding endpoints as "communication, swallowing, gastrointestinal and autonomic symptoms." The claim is accurate and sourced; only the link target is imprecise.
Overview
An investigational antisense medicine from Ionis Pharmaceuticals has become the first drug to show a disease-modifying effect in Alexander disease, a rare and often fatal neurological disorder with no approved treatment. The U.S. Food and Drug Administration has accepted the company’s New Drug Application for the medicine, zilganersen, for Priority Review and set a target action date of September 22, 2026, according to BioSpace.
The data underpinning the filing come from a pivotal study in which zilganersen stabilized patients’ walking speed, a measure of the progressive loss of mobility that defines the disease. Ionis described the topline results as “the first time an investigational medicine has shown a positive disease-modifying impact in AxD,” according to BioSpace.
What We Know
Alexander disease, abbreviated AxD, is “a rare, progressive and often fatal neurological disease that occurs in approximately 1 per 1 to 3 million people worldwide and affects a type of cell in the brain called astrocytes,” according to BioSpace. It is generally characterized by progressive neurological deterioration resulting in loss of functional mobility and loss of independence, BioSpace reported. The condition is caused by disease-causing variants in the GFAP gene, which drive the accumulation of excess glial fibrillary acidic protein (GFAP).
Zilganersen is an antisense oligonucleotide — an RNA-targeted medicine designed to inhibit production of the excess GFAP that accumulates because of those variants, BioSpace reported. The drug is built to bind to the messenger RNA made from the GFAP gene and prevent it from being turned into protein, FierceBiotech reported.
The pivotal study (NCT04849741) enrolled 54 participants aged 1.5 to 53 years across 13 sites in eight countries, according to BioSpace. Participants were randomized in a 2:1 ratio to receive zilganersen or control for a 60-week double-blind treatment period, with two dose cohorts of 25 mg and 50 mg dosed every 12 weeks, BioSpace reported.
On the trial’s primary endpoint, zilganersen 50 mg “demonstrated statistically significant and clinically meaningful stabilization on the primary endpoint of gait speed as assessed by the 10-Meter Walk Test (10MWT) compared to control at week 61 (mean difference 33.3%, p=0.0412),” according to BioSpace.
Data presented at the American Academy of Neurology 2026 Annual Meeting added a biomarker result: zilganersen reduced plasma GFAP levels by 33.6% at Week 61 compared to control, with a nominal p-value of 0.003, according to BioSpace. Results across key secondary and exploratory endpoints evaluating adaptive function, communication, gastrointestinal symptoms, sleep and seizures also consistently favored zilganersen, BioSpace reported. In a patient-reported measure, 21% of treated participants reported feeling “much better” versus 0% on control, according to BioSpace.
The FDA has granted zilganersen Breakthrough Therapy, Orphan Drug and Rare Pediatric Disease designations, BioSpace reported. “If approved, zilganersen will be the first and only treatment for Alexander disease, marking a breakthrough for patients,” said Brett Monia, Ph.D., chief executive officer of Ionis, according to BioSpace.
Amy Waldman, M.D., a pediatric neurologist and the study’s lead investigator, said the data “mark a meaningful step forward for families who have waited so long for innovation in Alexander disease,” according to BioSpace.
What We Don’t Know
The FDA’s Priority Review acceptance does not guarantee approval; the agency’s decision is expected by the September 22, 2026 target action date, according to BioSpace. The published topline and conference data describe stabilization of gait speed over the 60-week double-blind period, but long-term durability beyond the trial window and the drug’s effect across the full age range of patients are not detailed in the company’s disclosures. Pricing, and whether Ionis will market the drug itself, have not been announced.
Analysis
The result extends Ionis’s long focus on antisense medicines for rare neurological conditions and reflects a broader pattern in rare-disease drug development, where validated biomarkers and small, single-arm or modestly sized controlled trials can support regulatory filings for conditions with no existing therapy. The GFAP reduction reported at AAN provides a mechanistic readout that aligns with the drug’s design, while the walk-test endpoint ties the molecular effect to a functional outcome that matters to patients. With a September decision pending, zilganersen would, if cleared, give clinicians their first targeted option for a disease that has until now been managed only supportively.