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REGENXBIO Faces Regulatory Challenges on RGX-121 for MPS II

Rockville, MD – February 9, 2026 – Today, REGENXBIO Inc. (NASDAQ: RGNX) announced that the U.S. Food and Drug Administration (FDA) has issued a Complete Response Letter (CRL) concerning its Biologics License Application (BLA) for RGX-121, a gene therapy aimed at treating Mucopolysaccharidosis II (MPS II), commonly referred to as Hunter syndrome. This condition is a rare neurodegenerative disease that severely impacts boys, leading to progressive cognitive decline.

In an earlier development, the FDA had accepted the RGX-121 BLA via an accelerated approval pathway in May 2025. However, the recent CRL details multiple concerns that prevented the FDA from granting approval. These include issues around the eligibility criteria of the study population and the relevancy of the biomarkers used as surrogate endpoints to predict clinical benefit.

Curran Simpson, President and CEO of REGENXBIO, conveyed the distress felt by families affected by this life-threatening condition. “This decision is devastating for the families of boys living with this progressive, life-threatening disease,” he stated. Simpson emphasized the critical need for effective treatments for this ultra-rare condition and remains confident in the evidence supporting RGX-121’s potential to alter the course of Hunter syndrome.

The CRL suggests several paths forward for REGENXBIO, including the possibility of conducting another clinical study, extending patient treatment duration, or implementing an additional control arm in their research. These options, however, pose significant challenges, especially given the ultra-rare nature of MPS II.

Historically, REGENXBIO has collaborated actively with the FDA throughout the BLA process, believing they had sufficiently addressed the points raised regarding RGX-121. Expert analyses and reviews of clinical data were also conducted, affirming the treatment's promise. Despite these efforts, the FDA concluded that the presented data did not substantiate substantial evidence of RGX-121's effectiveness.

Moving forward, REGENXBIO intends to schedule a Type A meeting with the FDA to clarify the concerns noted in the CRL and to strategize on how to resubmit the BLA with more comprehensive data. They aim to bolster evidence that clearly delineates the patient population suffering from the neuronopathic form of MPS II and to present additional long-term clinical observations.

Joseph Muenzer, M.D., Ph.D., a leading expert in MPS II, expressed disappointment over the current situation while maintaining hope for RGX-121’s future. He noted that the irreversible brain damage caused by MPS II is well-documented, leading to early mortality among affected individuals without appropriate intervention. New innovations, particularly gene therapy, represent a beacon of hope for many families caught in this distressing scenario.

Terri Klein, President and CEO of the National MPS Society, highlighted the urgency for new treatment options, stressing that families have been waiting for two decades. Klein urged the FDA to expedite the drug development process for ultra-rare diseases, advocating for a quicker path to market for critical therapies like RGX-121.

About RGX-121

RGX-121 (clemidsogene lanparvovec) is an innovative gene therapy designed to address the deficiency of the iduronate-2-sulfatase (IDS) enzyme in the CNS, aiming to reverse the debilitating effects of MPS II. This one-time treatment endeavors to provide a lasting source of IDS, thereby correcting the underlying disorder.

The application for RGX-121 has been bolstered by positive data regarding biomarkers, safety, and functionality gathered from trials. The therapy has also been recognized by regulatory bodies, receiving designations such as Orphan Drug Product and Rare Pediatric Disease status from the FDA.

The need for effective treatments remains pressing, as MPS II continues to inflict significant suffering on patients and their families. REGENXBIO is determined to push forward in exploring new avenues to realize the promise of RGX-121.