FDA Approves Ionis's Zanvastro (Zilganersen) as First-Ever Disease-Modifying Therapy for Alexander Disease
On September 3, 2026, the U.S. Food and Drug Administration (FDA) approved Ionis Pharmaceuticals' Zanvastro (zilganersen) for the treatment of Alexander disease (AxD) in pediatric and adult patients. Zanvastro is the first and only disease-modifying therapy approved for this ultra-rare, progressive, and often fatal neurological disorder, which has historically had no targeted treatment options.
In connection with the approval, the FDA awarded Ionis a Rare Pediatric Disease Priority Review Voucher (PRV).1 Ionis has priced Zanvastro at $285,000 per dose, with the drug administered quarterly via intrathecal (IT) injection.
Clinical Trial Evidence
The FDA approval was based on positive results from a pivotal 60-week, randomized, double-blind, placebo-controlled study of Zanvastro in patients with Alexander disease.
- Primary Endpoint: For patients aged 5 years and older, Zanvastro (50 mg quarterly) demonstrated statistically significant and clinically meaningful stabilization of gait speed as measured by the 10-Meter Walk Test (10MWT). Treated patients maintained stable gait speed, while those in the control group experienced a 33.3% decline (least square mean difference of 33.3%, p=0.041).
- Pediatric Subgroup: For patients aged 2 to 4 years, Zanvastro demonstrated significant improvement in gross motor function compared to control, as assessed by the Gross Motor Function Measure-88 (GMFM-88).
- Safety Profile: Zanvastro demonstrated a favorable safety and tolerability profile, with most adverse events being mild or moderate in severity.
Mechanism of Action
Alexander disease is caused by mutations in the glial fibrillary acidic protein (GFAP) gene that lead to the overproduction and toxic accumulation of GFAP in astrocytes, damaging neurons and myelin over time. Zanvastro is an RNA-targeted antisense oligonucleotide (ASO) designed to bind to GFAP messenger RNA (mRNA) and induce its degradation, thereby reducing the production of toxic GFAP protein at its genetic source.
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An instance of Rare disease pipelines only accelerate through early FDA alignment and breakthrough designations. — Ionis leveraged the FDA's special pediatric priority designation to successfully accelerate and clear its novel rare neurodegenerative disease pipeline asset. ↩︎