Denali Therapeutics announced on Mar. 26 that the U.S. Food and Drug Administration (FDA) has granted accelerated approval to its enzyme replacement therapy, Avlayah, for the treatment of Hunter syndrome.
The approval marks a significant development for patients with Hunter syndrome, a rare genetic disease that primarily affects boys and leads to cognitive delays, behavioral issues, and hearing loss. There are around 500 patients in the United States with this condition.
According to Denali, Avlayah is the first new medicine approved for Hunter syndrome in nearly two decades and is also "the first FDA-approved biotherapeutic that uses the transferrin receptor to cross the blood-brain barrier." Tracy Beth Høeg, acting director of the Center for Drug Evaluation and Research at FDA, said Avlayah is "the first product approved to address neurologic complications of Hunter Syndrome." The drug’s full approval will depend on results from a confirmatory trial designed to verify its clinical benefits.
Avlayah supplies patients with enough IDS enzyme using Denali’s proprietary TransportVehicle platform. This technology enables delivery across the blood-brain barrier into the central nervous system—a feature that distinguishes it from Takeda's Elaprase, another enzyme replacement therapy approved in 2006. Phase 1/2 data showed Avlayah led to a 91% reduction in heparan sulfate levels in cerebrospinal fluid after treatment; at 24 weeks, most treated patients had levels similar to those without Hunter syndrome.
Analysts have described this regulatory milestone as a positive signal for both Denali and rare disease research more broadly. Stifel analysts called it a “welcome positive” while William Blair analysts said it “provides a key validation for the company’s differentiated TransportVehicle chemistry.”
The FDA's decision comes after recent challenges faced by other companies developing treatments for rare diseases. In February, REGENXBIO's gene therapy candidate was rejected by regulators due to concerns over study design and biomarker validity; REGENXBIO has stated plans to request further discussions with FDA regarding potential resubmission.
