Ionis Reports Positive Phase 1‑3 Data for Zilganersen in Alexander Disease
Ionis Pharmaceuticals disclosed that its investigational RNA‑targeted therapy zilganersen met the primary endpoint in a pivotal Phase 1‑3 trial for Alexander disease, showing a 33.3% stabilization of gait speed in patients aged ≥5 years. The results, presented at the 2026 American Academy of Neurology meeting, also delivered statistically significant improvements on secondary motor and patient‑reported outcomes, and the drug remains under FDA Priority Review with a PDUFA deadline of September 22, 2026.
Key trial outcomes
Ionis presented data from a global, multicenter, randomized, double‑blind study (NCT04849741) that enrolled 53‑54 participants ranging from 1.5 to 53 years across 13 sites in eight countries. Patients were randomized 2:1 to receive zilganersen (25 mg or 50 mg) or pooled control for a 60‑week double‑blind period, followed by open‑label extensions.
* Primary endpoint: In the ≥5‑year cohort, zilganersen achieved a 33.3% stabilization of gait speed on the 10‑Meter Walk Test (10MWT) at Week 61 versus control (p = 0.041).
* Secondary motor endpoint: Children 2‑4 years old demonstrated a 22.9‑point gain on the Gross Motor Function Measure‑88 (GMFM‑88) compared with control (p = 0.034).
* Patient‑ and clinician‑reported outcomes: All key secondary measures—including Most Bothersome Symptom, Patient Global Impression of Severity, Patient Global Impression of Change, and Clinician Global Impression of Change—favored zilganersen.
* Exploratory biomarker: Plasma glial fibrillary acidic protein (GFAP) fell 33.6% at Week 61 (p = 0.003), suggesting a disease‑modifying effect.
Safety profile
Adverse events were predominantly mild to moderate. Serious treatment‑emergent adverse events occurred in 37.5% of zilganersen‑treated participants versus 47.1% in the control arm, underscoring a favorable safety signal.
Regulatory status
Zilganersen is currently under FDA Priority Review with a target action date of September 22, 2026. The therapy holds Breakthrough Therapy, Orphan Drug, and Rare Pediatric Disease designations in the United States and an Orphan Drug designation from the European Medicines Agency.
Market reaction
Ionis shares edged higher following the filing, up 0.13% to $74.88 in a relatively flat market (S&P 500 +1.03%). The modest price movement reflects investor optimism tempered by the early‑stage nature of the data and the rarity of Alexander disease, which affects roughly 1 per 1‑3 million people worldwide.
Strategic implications
If approved, zilganersen would become Ionis’s first disease‑modifying therapy for a rare neurodegenerative disorder, complementing its existing neurology portfolio that includes Spinraza, Wainua, and Qalsody. The company highlighted the result as a milestone toward expanding its RNA‑targeted pipeline and delivering value to patients with high unmet need.
Forward‑looking statements in the filing caution that regulatory approval and commercial outcomes remain subject to uncertainty.
Key Takeaways
- Phase 1‑3 trial met primary endpoint: 33.3% gait‑speed stabilization in patients ≥5 years (p=0.041).
- Secondary motor improvement of 22.9 points on GMFM‑88 in children 2‑4 years (p=0.034).
- Exploratory biomarker showed 33.6% reduction in plasma GFAP (p=0.003).
- Safety favorable; serious adverse events lower in zilganersen arms (37.5% vs 47.1%).
- Zilganersen under FDA Priority Review; PDUFA decision expected September 22, 2026.