Historic FDA Decision Expected on Hunter Syndrome Gene Therapy
The U.S. Food and Drug Administration is scheduled to announce its decision on April 5, 2026, regarding RGX-121, a one-time gene therapy developed by REGENXBIO Inc. that would become the first approved treatment specifically designed to address the root cause of Hunter syndrome, also known as mucopolysaccharidosis type II (MPS II).
Hunter syndrome is a rare X-linked genetic disorder caused by a deficiency of the enzyme iduronate-2-sulfatase (I2S). Without this enzyme, complex sugars accumulate in cells throughout the body, causing progressive damage to the brain, heart, airways, liver, spleen, bones, and joints. The condition primarily affects males and has an estimated prevalence of 1 in 162,000 live births.
How RGX-121 Works
Unlike existing enzyme replacement therapies (ERT) that require weekly intravenous infusions and cannot cross the blood-brain barrier, RGX-121 is delivered as a single intrathecal injection directly into the cerebrospinal fluid. The therapy uses an adeno-associated virus (AAV9) vector to deliver a functional copy of the IDS gene to cells in the central nervous system.
“This is not just a new drug. It is a fundamentally different approach. For the first time, we have the potential to address the neurological devastation of Hunter syndrome, which current treatments simply cannot reach.” — Dr. Kenneth Marshall, REGENXBIO Chief Executive Officer
Clinical Trial Results
The FDAs decision is based on data from the Phase I/II and Phase II/III clinical trials, which enrolled 42 patients aged 4 months to 5 years. Key findings include:
- Cognitive development: 85% of treated patients showed stabilized or improved neurodevelopmental scores at 24 months
- CSF biomarkers: Average 73% reduction in heparan sulfate levels in cerebrospinal fluid
- Liver and spleen size: Significant reduction in organ volume in 90% of patients
- Safety: No serious treatment-related adverse events; most common side effects were transient mild fever and irritability
The Advisory Committee Vote
In March, the FDAs Cellular, Tissue, and Gene Therapies Advisory Committee voted 14-1 in favor of recommending approval, with committee members expressing strong support for the therapys potential to change the natural course of the disease.
What Approval Would Mean
For the approximately 2,000 Americans living with Hunter syndrome, FDA approval of RGX-121 would represent a watershed moment. Current standard of care involves weekly enzyme replacement infusions costing approximately $300,000 to $500,000 per year and do not address the neurological component of the disease.
REGENXBIO has not disclosed pricing for RGX-121 but has indicated it will be in line with other approved gene therapies, which have ranged from $2.1 million to $3.5 million for a one-time treatment. Health economists argue that a one-time curative therapy may actually reduce lifetime treatment costs compared to decades of weekly ERT infusions.
The Broader Rare Disease Landscape
The Hunter syndrome decision comes amid a broader wave of gene therapy approvals for rare diseases. The FDA has approved gene therapies for spinal muscular atrophy, hemophilia A and B, sickle cell disease, and several other conditions in recent years. Analysts at Evaluate Pharma estimate the gene therapy market will reach $32 billion globally by 2028.
Patient advocacy groups, including the National MPS Society and Project Alive, have been vocal in their support for RGX-121s approval. A decision is expected by 5:00 PM ET today, with REGENXBIO planning to host an investor call shortly after the announcement.
