For families with a son suffering from Hunter syndrome, progressive brain deterioration long meant the end of any hope for therapy. Treatment for physical symptoms has existed since 2006. Against cognitive decline, however, there was no effective option for twenty years until the US Food and Drug Administration (FDA) approved the first medication targeting exactly that on March 25, 2026: Avlayah from Denali Therapeutics.
What is Hunter Syndrome?
Hunter syndrome, medically known as mucopolysaccharidosis type II (MPS II), is a rare genetic metabolic disorder affecting almost exclusively boys. The cause is the absence of the enzyme iduronate-2-sulfatase. Without this enzyme, glycosaminoglycans accumulate in cells throughout the body, including the brain. The consequences include coarse facial features, enlarged organs, bone damage, and heart problems. In the most severe form, children develop progressive cognitive impairment, losing language and motor skills. According to several epidemiological studies, the disorder occurs in approximately one in 100,000 to 170,000 newborns. An estimated 500 people with the condition live in the United States.
Enzyme replacement therapy idursulfase (brand name Elaprase, manufacturer Takeda) has been approved since 2006 and controls physical symptoms. The central nervous system remains unreachable with this approach: idursulfase does not cross the blood-brain barrier. Cognitive decline proceeded unimpeded over two decades.
What Makes Avlayah Different
Denali Therapeutics designed Avlayah (active ingredient: tividenofusp alfa) to cross the blood-brain barrier. The active substance is coupled to a transport protein that uses an active uptake mechanism of the brain. In the phase 1/2 clinical trial, 47 pediatric patients aged 0.3 to 13 years received the medication as weekly intravenous infusions. According to FDA information in the approval announcement of March 25, 2026, heparan sulfate levels in cerebrospinal fluid, the central biomarker for brain accumulation, decreased by an average of 91 percent. Ninety-three percent of patients reached normal values by week 24. Stabilizations and improvements were documented in cognitive behavior, hearing, and speech development.
The FDA used accelerated approval procedures since no alternative therapies for neurologic manifestations existed. Full confirmation depends on the ongoing phase 2/3 COMPASS trial, which according to the company is 95 percent enrolled.
What Critics Point Out
The approval carries restrictions that patient advocates view as problematic. The FDA approved Avlayah exclusively for children up to age 16. Adolescents over 16 with the condition who did not participate in childhood studies lack regular access. Speaking with StatNews, patient representatives called this age cap a two-tiered system within the same disease.
There is also the price. Avlayah costs approximately 5,200 US dollars per dose, according to the manufacturer. For a 10-kilogram child, this totals around 270,000 US dollars annually; for a 30-kilogram child, approximately 811,000 US dollars yearly. For comparison, Elaprase, the previous therapy, already costs around 500,000 US dollars annually, making it among the world's most expensive medications.
The frequency of adverse effects is another limitation: all 47 study participants experienced at least one adverse event during the safety phase, mostly infusion-related reactions such as fever and hives. Fifty-seven percent required dose reduction. Avlayah carries a Boxed Warning for allergic reactions including anaphylaxis and can only be administered under medical supervision.
In Comparison
The approval fits into a series of first therapies for severe pediatric inherited disorders that lacked any treatment options for decades.
Spinal muscular atrophy type 1 (SMA): Before 2016, children with this most severe form typically died before age two or were permanently ventilator-dependent. With FDA approval of nusinersen (Spinraza) in December 2016 and gene therapy onasemnogen abeparvovec (Zolgensma) in 2019, the picture fundamentally changed. Children treated early today achieve motor milestones previously unthinkable: sitting, standing, and in some cases walking.
Pompe disease: Before alglucosidase alfa (Myozyme) approval in 2006, children with the infantile form of this lysosomal storage disease typically died within the first year of life from heart failure. Enzyme replacement therapy significantly extended survival and improved respiratory function. Hunter syndrome itself received its first approved treatment with idursulfase in 2006 for body symptoms. Twenty years later comes the first option for the brain.
Three Hurdles to Broad Implementation
Three questions will determine how many affected families actually benefit from the approval.
First, the COMPASS trial must confirm efficacy. FDA accelerated approval requires that Denali demonstrates clinical benefit within agreed timeframes, not just biomarker normalization. Without such proof, approval could be withdrawn.
Second, the age limit must be politically reviewed. Patient advocates are pushing for expanded indication for adults or at least the possibility of continued therapy for children who grew up with the treatment. Denali announced patient assistance programs, but these are not structural solutions.
Third, the insurance landscape decides. With an annual cost up to 811,000 US dollars and a complex prior authorization process, physicians spend on average 13 hours weekly on insurance inquiries, according to medical associations. For children for whom the treatment works, every delay means irreversible neurological damage.