First Brain-Crossing Therapy Approved for Children With Hunter Syndrome

Hunter syndrome, also called mucopolysaccharidosis type II (MPS II), is a rare inherited disorder in which the body lacks an enzyme needed to break down certain complex sugars. They build up in cells throughout the body — and crucially in the brain — causing progressive physical and neurological decline. On 25 March 2026, the US Food and Drug Administration granted accelerated approval to Avlayah (tividenofusp alfa-eknm), the first therapy designed to reach the brain to treat the disease.

The challenge with Hunter syndrome has long been the blood-brain barrier, a protective wall that keeps most medicines out of the brain. Earlier enzyme-replacement therapies could ease the disease's effects on the body but could not address the neurological damage. Avlayah, made by Denali Therapeutics, fuses the missing enzyme to a "transport vehicle" that latches onto a receptor on the barrier, ferrying the enzyme across so it can work in the brain as well as the body. The FDA described it as the first biologic specifically designed to cross the blood-brain barrier and reach the whole body.

“They build up in cells throughout the body — and crucially in the brain — causing progressive physical and neurological decline.”

The trial results behind the approval were striking. In a Phase 1/2 study of 47 children — from three months to 13 years old, with a median age of five — Avlayah produced a 91% reduction in cerebrospinal fluid heparan sulfate, a key marker of the disease's activity in the brain, by week 24. In 41 of 44 treated patients (93%), that marker fell into the range seen in people without Hunter syndrome. "This approval reflects the determination and partnership of the MPS community," said Denali CEO Ryan Watts.

Important context tempers the celebration. Hunter syndrome is very rare — affecting roughly 500 people in the United States — and the approval is accelerated, meaning continued approval depends on a confirmatory trial verifying real clinical benefit beyond the biomarker. The therapy is for children treated before advanced neurological impairment sets in. Even so, for families facing a relentless disease that robs children of their abilities, the first treatment that can reach the brain offers something they have never had before: a genuine chance to change the course of the disease.