RGX-121 Treatment Shows Promising Results for MPS II Patients Through 1-Year Follow-Up.

REGENXBIO announced positive data from the CAMPSIITE trial of RGX-121 for MPS II treatment. The one-time treatment significantly reduced CSF levels of HS D2S6, a key biomarker, by >80% and sustained it through 1 year. Patients demonstrated continued skill acquisition or stability through 1 year, with a strong correlation to neurodevelopmental outcomes. If approved, RGX-121 would be the first commercially-available therapy to directly address the genetic cause of Hunter syndrome.

Rockville, Md., September 5, 2025 — REGENXBIO Inc. (Nasdaq: RGNX) announced positive 12-month pivotal data from the Phase I/II/III CAMPSIITE® trial of clemidsogene lanparvovec (RGX-121) for the treatment of Mucopolysaccharidosis Type II (MPS II), also known as Hunter syndrome. The data, presented at the International Congress of Inborn Errors of Metabolism (ICIEM) 2025, demonstrate the long-term potential of RGX-121 to improve outcomes for patients with this rare, X-linked recessive disease.

The trial results showed a 80% median reduction in cerebrospinal fluid (CSF) levels of heparan sulfate (HS) D2S6, a key biomarker of MPS II brain disease, sustained through 1 year. This reduction is consistent with previously reported topline pivotal results, which met the primary endpoint of CSF HS D2S6 reduction at week 16 with statistical significance (p 0.0001). The new data also indicate a strong correlation between measured CSF HS D2S6 levels at week 16 and neurocognitive outcomes at one year, supporting the use of this biomarker as a surrogate endpoint under the accelerated approval pathway.

Pivotal participants demonstrated neurodevelopmental skill acquisition or stability on all sub-scales of the Bayley Scales of Infant and Toddler Development, 3rd Edition (BSID-III) at one year. The FDA has acknowledged these positive results and the strong correlation between biomarker levels and neurodevelopmental outcomes, which could support a potential accelerated approval for RGX-121 early next year.

RGX-121 is a one-time AAV therapeutic designed to deliver the iduronate-2-sulfatase (IDS) gene to the central nervous system (CNS), providing a permanent source of secreted iduronate-2-sulfatase (I2S) protein beyond the blood-brain barrier. If approved, RGX-121 would be the first and only potential one-time, commercially-available therapy designed to directly address the underlying genetic cause of Hunter syndrome.

The FDA completed a pre-license inspection and bioresearch monitoring information inspection for the RGX-121 BLA in August 2025, with no safety-related concerns raised. The FDA is expected to make a decision on the application by February 8, 2026.

References:
[1] https://www.morningstar.com/news/pr-newswire/20250905ph66175/regenxbio-presents-positive-twelve-month-pivotal-data-from-phase-iiiiii-campsiite-trial-of-rgx-121-for-treatment-of-mps-ii