Month: April 2026

What happened
On March 25, 2026, the FDA approved Avlayah (tividenofusp alfa-eknm) for the treatment of neurologic manifestations of Hunter syndrome (MPS II) in pediatric patients weighing at least 5 kg. It is the first new FDA-approved treatment for this condition in nearly 20 years and the first FDA-approved biologic designed to cross the blood-brain barrier.

The problem Avlayah solves
Hunter syndrome affects approximately 500 people in the U.S., almost exclusively males, and is caused by a deficiency of the enzyme iduronate-2-sulfatase (IDS) that leads to toxic glycosaminoglycan accumulation throughout the body and brain.

The only existing treatment, Elaprase (idursulfase, Takeda), has been available since 2006. It reduces urinary glycosaminoglycan levels and improves walking ability and organ volumes, but it does not cross the blood-brain barrier. Cognitive decline, behavioral symptoms, and hearing loss have remained untreated.

Avlayah fuses the IDS enzyme to Denali’s proprietary TransportVehicle platform, which binds to the transferrin receptor and delivers IDS to peripheral tissues and the CNS through receptor-mediated transcytosis across the blood-brain barrier. It is administered intravenously once weekly at 15 mg/kg.

Clinical data
In a Phase I/II study of 47 pediatric patients, the 44 with measurements at week 24 showed a 91% average decrease in CSF heparan sulfate. At baseline, none had CSF HS below the upper limit of normal; at week 24, 93% did. These are biomarker results under the accelerated approval pathway. Continued approval is contingent on the ongoing Phase II/III COMPASS trial, which is more than 95% enrolled. The labeling includes a boxed warning for allergic reactions, including anaphylaxis.

Regulatory context
Weeks before this approval, the FDA rejected Regenxbio’s RGX-121, a competing gene therapy for Hunter syndrome, citing issues with the clinical trial design and the use of heparan sulfate as a surrogate endpoint. The fact that FDA accepted the same surrogate for Avlayah with a different data package is notable. Leerink Partners noted that the acceptance is encouraging for the broader rare disease landscape. The FDA also granted Denali a Rare Pediatric Disease Priority Review Voucher.

Pricing
Avlayah’s wholesale acquisition cost is $5,200 per 150 mg vial, dosed weekly at 15 mg/kg. Denali estimates annual costs of approximately $270,000 for a 10 kg infant and approximately $811,000 for a 30 kg child. Elaprase carries estimated annual costs in the $375,000 to $600,000 range depending on weight.

The Anton Take
Two things stand out. First, the science: getting large molecules across the blood-brain barrier has been a central challenge in drug development for decades. Avlayah is the first approved product to do it via a transferrin receptor transport mechanism. That is a proof of concept for a platform that could extend across lysosomal storage disorders and potentially into neurodegenerative diseases.

Second, the regulatory signal. In a period of high-profile rare disease rejections, this accelerated approval on a biomarker surrogate from a small single-arm study stands out. The contrast with the Regenxbio rejection weeks earlier shows FDA is differentiating between data packages, not shutting the door on surrogate endpoints. The COMPASS trial readout will determine whether these signals translate into clinical benefit and whether Avlayah converts to full approval.