First FDA-Approved Therapy for Ultra-Rare Alexander Disease Targets Root Cause

Alexander disease, which causes progressive neurological decline and muscle weakness, now has its first FDA-approved therapy. Ionis Pharmaceuticals developed this genetic medicine to target the root cause of this life-threatening orphan disorder.

The FDA’s late Thursday decision approved zilganersen for treating both children and adults with Alexander disease. Marketed under the brand name Zanvastro, this product represents a pivotal element of the Carlsbad, California-based biotech’s evolving commercial strategy.

Alexander disease arises from mutations in the gene responsible for glial fibrillary acidic protein (GFAP). While normal GFAP supports astrocytes throughout the central nervous system, mutated forms accumulate, causing nervous system damage that leads to seizures, developmental issues, muscle weakness, and elevated intracranial pressure. This ultra-rare condition affects approximately one in every one million to three million people globally.

Zanvastro is an antisense oligonucleotide that binds to and degrades pre-messenger RNA for GFAP, thereby reducing the synthesis of the disease-causing protein. Administered via intrathecal injection, the dosing interval is every three months.

Approval follows pivotal trial results reported nearly a year ago, where 49 participants were assessed via a 10-meter walk test. At week 61, patients receiving the drug demonstrated statistically significant and clinically meaningful stabilization of gait speed, whereas the control group showed deterioration. Blood tests confirmed the drug engaged its target and modulated the disease’s underlying cause, while improving motor function. Adverse reactions were mild or moderate, with vomiting, back pain, and cough being the most common.

Ionis estimates 300 U.S. patients have Alexander disease, according to Chief Global Product Strategy Officer Kyle Jenne during a Friday conference call. The company is moving Zanvastro into distribution channels, and the product will be available shortly with a list price of $285,000 per dose. With quarterly dosing, this equates to $1.14 million annually before insurance rebates or discounts.

Ionis has prior neuroscience experience through the spinal muscular atrophy drug Spinraza and Qalsody, which treats a rare form of amyotrophic lateral sclerosis; both are marketed by Biogen under a partnership.

Ionis has shifted away from partnering out internally developed assets, choosing to handle commercialization independently. Tryngolza, approved for familial chylomicronemia in 2024 and severe hypertriglyceridemia in June, was the first product in this strategy, followed by Dawnzera, approved last summer for hereditary angioedema. Zanvastro marks the first neurology medicine Ionis will commercialize on its own, while the pipeline’s most advanced candidate is obudanersen, an antisense oligonucleotide in Phase 3 for Angelman syndrome, with data expected in the second half of 2027.

While Ionis retains U.S. commercialization rights, it remains open to international partnerships. In June, Recordati licensed ex-U.S. rights to the Alexander disease drug for $30 million upfront plus royalties, assuming responsibility for regulatory submissions in other markets.

FDA approval of Zanvastro included a priority review voucher, a fast pass for future rare pediatric disease drug candidates. While many biotechs sell these to raise capital, Ionis can leverage its financial strength—reported cash of $2.1 billion at the end of June—to decide the voucher’s fate. CEO Brett Monia noted the company is weighing options, acknowledging the voucher could apply to pipeline assets, which currently span eight medicines in clinical development.

“That’s priority now and that pipeline is going to grow,” Monia said. “We’re expecting additional neurology medicines to enter the clinic in the near future.”

Image: koto_feja, Getty Images

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