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On March 25, 2026, the U.S. Food and Drug Administration granted accelerated approval to Avlayah (tividenofusp alfa-eknm), making it the first therapy specifically approved to treat the neurologic manifestations of Hunter syndrome. Developed by Denali Therapeutics, Avlayah is also the first FDA-approved biologic engineered to cross the blood-brain barrier using transferrin receptor-mediated transport, a milestone with implications well beyond this single rare disease.
Hunter syndrome, formally known as mucopolysaccharidosis type II (MPS II), is a rare X-linked lysosomal storage disorder affecting approximately 500 people in the United States, almost exclusively males. The condition results from a deficiency of the enzyme iduronate-2-sulfatase (IDS), leading to the accumulation of glycosaminoglycans (GAGs), particularly heparan sulfate, across tissues, including the brain. Roughly two-thirds of affected individuals experience progressive neurocognitive decline, the feature most responsible for disability and reduced life expectancy. Until now, the only available enzyme replacement therapy, Takeda's Elaprase (approved in 2006), has addressed physical symptoms but has not crossed the blood-brain barrier to reach the central nervous system.
Avlayah combines the IDS enzyme with Denali's proprietary TransportVehicle platform, which binds to the transferrin receptor on brain endothelial cells and uses receptor-mediated transcytosis to deliver the enzyme into the CNS. The approval was supported by Phase 1/2 data published in the January 1, 2026 issue of The New England Journal of Medicine. In that open-label study of 47 pediatric patients, Avlayah reduced cerebrospinal fluid heparan sulfate by 91% at 24 weeks, with 93% of measured patients achieving levels within the normal range. The therapy also received Breakthrough Therapy, Fast Track, Priority Review, and Orphan Drug designations.
Acting CDER Director Tracy Beth Hoeg, MD, PhD, noted that the accelerated approval was based on cerebrospinal fluid heparan sulfate reduction as a surrogate endpoint reasonably likely to predict clinical benefit. Denali is conducting the Phase 2/3 COMPASS trial, now more than 95% enrolled, to generate confirmatory data. Continued approval may be contingent on verification of clinical benefit in that study.
The approval carries broader significance for the managed care community. Avlayah's list price is $5,200 per 150 mg single-use vial, administered as a weekly infusion, with costs varying by patient weight. The drug represents the first commercial validation of the TransportVehicle platform, and analysts have projected peak U.S. sales exceeding $250 million. The labeling includes a boxed warning for severe allergic reactions, including anaphylaxis, and the FDA recommends monitoring hemoglobin, kidney function, and urine protein levels. For payers managing rare disease formularies, the approval introduces a new cost center and a potential shift in standard of care for a population with limited therapeutic options and high unmet neurologic need.
