REGENXBIO Inc. (RGNX), yesterday announced that the U.S. FDA has issued a Complete Response Letter (CRL) for its Biologics License Application for RGX-121 (clemidsogene lanparvovec), a one-time gene therapy candidate for the treatment of Mucopolysaccharidosis II, also known as Hunter syndrome.
The BLA for RGX-121 had been accepted under the accelerated approval pathway in May 2025. In the CRL dated February 7, 2026, the FDA stated that although it agreed with the study protocol in principle, it identified several issues preventing approval. These included uncertainty around the eligibility criteria used to define the neuronopathic patient population, concerns regarding the comparability of the natural-history external control group, and questions about the suitability of CSF HS D2S6 as a surrogate endpoint reasonably likely to predict clinical benefit.
The agency outlined several potential paths forward, such as conducting a new study, treating additional patients with longer-term follow-up, or using an untreated control arm-approaches the company noted would be challenging in an ultra-rare disease like MPS II.
REGENXBIO said it plans to request a Type A meeting with the FDA to discuss the CRL and its strategy for resubmitting the BLA. The company intends to provide additional evidence from global MPS II experts to further clarify the neuronopathic population and supplement the application with longer-term clinical data.
RGX-121 is designed to deliver the IDS gene directly to the central nervous system, potentially enabling long-term production of the iduronate-2-sulfatase enzyme beyond the blood-brain barrier.
The BLA submission was supported by biomarker, functional, and safety data from the CAMPSIITE I/II/III trial, with follow-up out to 12 months. The therapy has received multiple FDA designations, including Orphan Drug, Rare Pediatric Disease, Fast Track, and RMAT.
MPS II is a rare, X-linked recessive disorder caused by a deficiency of the I2S enzyme, leading to the accumulation of glycosaminoglycans and progressive multi-organ dysfunction, including severe neurological decline in its neuronopathic form. There remains a significant unmet need for treatments that address the neurological manifestations of the disease.
RGNX has traded between $5.03 and $16.19 over the past year. The stock closed yesterday's trading at $10.31, down 0.19% and fell further in the pre-market to $9.01, down 12.61%.
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