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REGENXBIO's Regulatory Update on RGX-121 for MPS II

On August 24, 2026, REGENXBIO Inc. (Nasdaq: RGNX) announced significant news regarding its investigational gene therapy, RGX-121, which is aimed at treating Mucopolysaccharidosis type II (MPS II), commonly referred to as Hunter Syndrome. This therapy has drawn attention as the FDA has placed a clinical hold on RGX-121 due to certain findings observed in the CAMPSIITE® study.

Clinical Hold Overview

The hold was implemented following the discovery of asymptomatic anomalies found in spine MRI scans of five participants in the trial. According to Curran Simpson, REGENXBIO’s President and CEO, the company does not anticipate resubmitting the Biologics License Application (BLA) for RGX-121 in the near future.

Simpson emphasized that they believe these MRI findings are unique to their Hunter Syndrome program and necessitate more extensive follow-ups and data analyses to fully evaluate the benefit-risk profile of RGX-121. Currently, all five participants in question are reported to be doing well, showing clinical stability and improvement in neurobehavioral assessments.

Importance of Findings

The unusual MRI findings, which either point to small nodules or cystic masses, were revealed during an enhanced monitoring program REGENXBIO had put in place a few months ago in response to earlier clinical holds related to another therapy, RGX-111. This strategic implementation included both brain and spine MRI monitoring for participants who had been treated with RGX-121 approximately three to six years previously.

Importantly, the radiologists involved have deemed these MRI findings to be non-serious and likely benign. While no brain nodules were visible on MRI scans, the clinical significance and underlying prevalence of these spine findings in MPS II patients are yet unknown since spine imaging is not standard practice for this rare disease.

Expert Opinions

Dr. Roberto Giugliani, a prominent expert in genetics from Brazil, pointed out the multi-faceted neurodevelopmental and systemic impacts that boys with neuronopathic MPS II face. He mentioned that while data on the natural history of this ultra-rare disease is limited, findings like the ones observed could be intrinsic to how Hunter Syndrome affects the body. Dr. Giugliani expressed satisfaction that the patients involved in this study are doing well and remain asymptomatic.

Next Steps for REGENXBIO

REGENXBIO and its partner NS Pharma are currently evaluating additional imaging data and long-term follow-up metrics while also awaiting feedback from the FDA, including the full clinical hold letter. This information will be taken into consideration as the company decides on the next steps for RGX-121.

About MPS II

MPS II is an X-linked recessive disorder, mainly affecting males, caused by a deficiency in the lysosomal enzyme iduronate-2-sulfatase (I2S). This deficiency leads to a harmful build-up of glycosaminoglycans (GAGs) in various tissues, contributing to severe dysfunction in cells, tissues, and organs, including the central nervous system. Worldwide, approximately 2,000 patients are diagnosed with MPS II, with around 500 new cases annually. Most affected boys exhibit severe symptoms, with early developmental milestones often attained, but noticeable developmental delays emerge within the next two years.

About RGX-121

RGX-121 is a cutting-edge gene therapy intended to deliver the IDS gene directly into the central nervous system, potentially providing a lasting solution to the deficiency of the I2S protein. Several designations, such as Orphan Drug Product, Rare Pediatric Disease, Fast Track, and Regenerative Medicine Advanced Therapy (RMAT), have been awarded to RGX-121 by the FDA. It stands poised to be a significant advancement in treatment options for boys suffering from MPS II.

Company Background

Founded in 2009, REGENXBIO is committed to harnessing the transformative power of gene therapy to improve lives. With a robust late-stage pipeline, the company aims to address rare and retinal diseases and continues to push the boundaries of AAV gene therapy. REGENXBIO's initiatives have led to thousands of successful treatments, including those involving the noteworthy ZOLGENSMA® therapy. For further details regarding their innovative approaches, one can visit www.REGENXBIO.com.