On March 24, 2026, the FDA granted accelerated approval to Avlayah (tividenofusp alfa-eknm) for Hunter syndrome, also known as MPS II. Denali Therapeutics developed the drug. It is the first enzyme replacement therapy for Hunter syndrome that can cross the blood-brain barrier and reach the central nervous system.
If your child has Hunter syndrome, or if you're a caregiver for someone who does, you probably saw the headline. Maybe a friend sent it. Maybe a Facebook group lit up. After the initial rush of hope came a quieter feeling. What now? What do I actually do with this information?
That's what this post is for. Not just the science behind Avlayah, though we'll cover that. The real focus here is the practical steps families should take when a new treatment gets approved for a condition they've been fighting for years.
What Is Avlayah and How Does It Treat Hunter Syndrome?
Hunter syndrome is a rare genetic disorder where the body can't produce enough of an enzyme called iduronate-2-sulfatase (I2S). Without it, complex sugars called glycosaminoglycans (GAGs) build up inside cells and cause progressive damage to organs, bones, airways, and, in about two-thirds of patients, the brain.
The only approved treatment before Avlayah was Elaprase (idursulfase), which came on the market in 2006. Elaprase helps with the peripheral symptoms. Joint stiffness, liver and spleen enlargement, breathing difficulties. But it cannot cross the blood-brain barrier. For children with the severe, neuronopathic form of Hunter syndrome, the most devastating part of the disease kept progressing untouched.
Avlayah changes that. The drug uses a Transport Vehicle platform developed by Denali that binds to the transferrin receptor on blood vessel walls in the brain. This receptor normally shuttles iron into the brain. Avlayah essentially hitches a ride, carrying the I2S enzyme across the barrier and into the central nervous system where it's needed most.
In a Phase 1/2 clinical trial published in The New England Journal of Medicine in January 2026, 47 patients received Avlayah by weekly IV infusion. CSF heparan sulfate, a direct marker of disease activity in the brain, dropped by 91% at 24 weeks. 93% of treated patients reached heparan sulfate levels within the normal range (Muenzer et al., NEJM 2026). These numbers represent something families have never had before. Measurable evidence that a drug is reaching and clearing storage material from the brain.
Steps for Families After the Hunter Syndrome FDA Approval
This is the part that gets lost in the press releases. An FDA approval doesn't automatically mean your doctor calls you the next morning with a prescription. There are steps between "the FDA approved this" and "my child is receiving this treatment," and nobody hands you a checklist. So here's one.
1. Call your child's metabolic specialist about Avlayah
Don't wait for them to call you. Specialists managing rare disease patients are often juggling hundreds of families, and they may not proactively reach out about every new approval. Call the office, reference the Avlayah approval specifically, and ask for an appointment to discuss whether your child is a candidate. If you don't currently have a metabolic specialist, this is the time to get one. Your pediatrician can refer you to the nearest metabolic or genetics center, and the National MPS Society maintains a list of physicians experienced with MPS II.
2. Understand what "accelerated approval" means
Avlayah was granted accelerated approval, which the FDA uses when a drug treats a serious condition and shows results on a surrogate endpoint (in this case, CSF heparan sulfate reduction) that is reasonably likely to predict clinical benefit. The drug is approved and available. But Denali is required to run a confirmatory trial, called COMPASS, to verify that the biomarker improvements translate into real-world clinical outcomes like preserved cognitive function and reduced neurodevelopmental decline. If the confirmatory trial fails, the FDA could theoretically withdraw the approval.
3. Ask about eligibility and timing
Avlayah is approved for pediatric patients weighing at least 5 kg (about 11 lbs) who have not yet developed advanced neurologic impairment from MPS II. That second criterion matters enormously. The drug works best when started early, before irreversible brain damage has occurred. If your child was recently diagnosed through newborn screening or early genetic testing, the window for maximum benefit may be open right now. If your child is older and already showing significant cognitive decline, the conversation with your doctor will be different, and it's still worth having.
4. Talk to insurance about Hunter syndrome treatment costs
Enzyme replacement therapies for rare diseases are among the most expensive medications in the world. Elaprase costs roughly $300,000 to $500,000 per year depending on the patient's weight. Avlayah pricing hasn't been widely published yet, but families should expect a similar range or higher. Contact your insurance company and ask specifically about coverage for Avlayah (generic name tividenofusp alfa-eknm). Request a prior authorization. If you're denied, ask your doctor's office to submit a peer-to-peer review. Denali Therapeutics may also have a patient assistance program or copay support. Check their website or ask your specialty pharmacy.
5. Ask about MPS II clinical trials, including COMPASS
Even though Avlayah is commercially available, the COMPASS Phase 2/3 trial is still enrolling. For some families, participating in COMPASS could offer additional monitoring, more frequent assessments, and structured follow-up that goes beyond what routine clinical care provides. It also contributes data that will help secure full traditional approval. Ask your specialist whether your child might be eligible and whether trial participation would offer advantages over standard commercial access.
6. Connect with Hunter syndrome support groups and advocacy
The National MPS Society (mpssociety.org) is the primary advocacy organization for all MPS conditions in the United States. They run family conferences, maintain a physician directory, and provide case management support for families dealing with insurance barriers. Project Alive (projectalive.org) focuses specifically on Hunter syndrome and funds research. Both organizations were actively involved in the Avlayah development process and can help families understand what the approval means in practical terms.
These organizations also maintain private Facebook groups and online forums where families share real experiences with treatments, insurance fights, infusion logistics, and school accommodations. The knowledge in those communities is irreplaceable. A parent who has been managing weekly Elaprase infusions for 8 years knows things that no clinical trial publication will tell you.
Other MPS II Treatments and Hunter Syndrome Clinical Trials
Avlayah isn't the only new approach to Hunter syndrome. Understanding the full picture helps families make informed decisions about treatment and trial participation.
Pabinafusp alfa, developed by JCR Pharmaceuticals, was approved in Japan in 2021 and uses a similar transferrin receptor approach to deliver I2S across the blood-brain barrier. A global Phase 3 trial is currently enrolling, and if successful, it could offer families a second brain-penetrant ERT option.
Gene therapy remains the most ambitious frontier. RGX-121, an AAV-based gene therapy from REGENXBIO, received a Complete Response Letter from the FDA in February 2026, meaning the agency requested additional data before it would consider approval. That doesn't mean the program is dead. It means there's more work to do. Separately, a team at the University of Manchester treated its first patient, a young boy named Oliver Chu, with a stem cell gene therapy approach in February 2025. That trial is in its earliest stages.
MPS II Newborn Screening and Early Hunter Syndrome Diagnosis
Hunter syndrome affects an estimated 1 in 100,000 to 1 in 170,000 male live births. The severe neuronopathic form accounts for roughly two-thirds of cases. Most children are diagnosed between ages 2 and 4, often after parents notice developmental delays, recurrent ear infections, coarse facial features, or an enlarged abdomen.
Illinois became the first state to add MPS II to its newborn screening panel in late 2017. Missouri followed in 2018. As of 2023, only those 2 states had full population-based screening, though a handful of others are piloting or considering it. Newborn screening for MPS II matters more now than ever because Avlayah's eligibility requires that treatment begin before advanced neurologic damage. A child diagnosed at 6 months through newborn screening has a fundamentally different treatment window than a child diagnosed at age 4 after missing developmental milestones.
If you live in a state that doesn't yet screen for MPS II and your family has a history of Hunter syndrome, talk to your pediatrician about genetic testing early. The earlier you know, the more options you have.
The Emotional Part Nobody Puts in the Press Release
Finding out that a new treatment exists for your child's condition is not a simple feeling. It's hope tangled with fear. What if we can't access it? What if it doesn't work for our kid? What if we started too late? Some families have lost children to Hunter syndrome while waiting for a treatment like this. For them, the news carries a grief that coexists with gratitude that other families may not have to face the same outcome.
If you're a caregiver reading this and feeling overwhelmed, that's normal. You don't have to figure everything out today. Start with one step. Call the specialist. Or call the National MPS Society at 919-806-0101 and tell them what you're dealing with. They've helped thousands of families through exactly this kind of moment.
“The approval of a new therapy is not the end of the journey. For most families, it's the beginning of a new set of decisions, conversations, and logistics. But for the first time in 20 years, Hunter syndrome families have a real choice to make. That alone is worth something.”
Trial Friend editorial team
Hunter Syndrome Treatment Resources and Next Steps
Talk to your metabolic specialist or geneticist about Avlayah eligibility. Contact insurance about prior authorization and coverage. Reach out to the National MPS Society (mpssociety.org) or Project Alive (projectalive.org) for support. Ask about the COMPASS confirmatory trial if your child qualifies. If your state offers MPS II newborn screening, make sure your pediatrician is aware. If it doesn't, consider genetic testing if there's a family history.