RNA Medicine Reaches a Disease No Drug Has Ever Touched

Ionis wins Priority Review for zilganersen, targeting the first-ever treatment for a rare, fatal brain disease

9 Apr 2026

For patients with Alexander disease, there has never been a medicine to slow the illness, let alone stop it. A mutation in the GFAP gene causes a toxic protein to accumulate in brain cells, steadily stripping people, mostly children, of movement, speech, and independence. Roughly one in two million people carry the diagnosis. Until now, none has had access to a pharmacological treatment.

That may change. The Food and Drug Administration has accepted a priority review application for zilganersen, a therapy developed by Ionis Pharmaceuticals that works by suppressing abnormal GFAP protein production at its molecular source. The agency has set a target decision date of 22nd September 2026.

The clinical evidence is modest but consistent. In a pivotal trial of 54 participants across 13 sites in eight countries, patients receiving zilganersen walked 33% faster than controls after 61 weeks. Measures of daily function, communication, and seizure activity also favoured the therapy. The FDA had already granted the drug breakthrough therapy, orphan drug, and rare paediatric disease designations; the European Medicines Agency has granted orphan status as well.

New data from the trial are expected at the American Academy of Neurology's annual meeting in Chicago in April 2026, ahead of the regulatory decision.

If zilganersen clears the FDA's bar, it would be the first disease-modifying therapy for Alexander disease. The milestone would also mark Ionis's first independent commercial launch in neurology, capping more than three decades of work on RNA-targeted medicine. Whether the approval holds and the drug reaches patients at meaningful scale are questions the September deadline will not fully resolve.