Must-Knows from the RGX-121 Hold

Regenxbio has disclosed that the FDA placed a clinical hold on RGX-121 (clemidsogene lanparvovec), its investigational gene therapy for Mucopolysaccharidosis type II (MPS II), also known as Hunter Syndrome.¹ The hold follows the identification of asymptomatic spine MRI findings in five participants enrolled in the company's CAMPSIITE study. As a result, Regenxbio does not expect to resubmit its Biologics License Application for the therapy in the near term.

The findings emerged through an expanded MRI monitoring protocol that Regenxbio implemented several months ago, following an earlier clinical hold tied to a related program, RGX-111.¹ That enhanced surveillance, which added spine imaging to standard brain MRI monitoring, identified either a small nodule or a small cystic mass in the spine scans of five participants who had received RGX-121 via intracisternal or intraventricular administration between three and six years earlier. No brain nodules or masses were detected in any participant.

Investigators have characterized the spine findings as nonserious, and radiologists involved in the program believe they are likely benign.¹ All five affected participants continue to perform well clinically, showing overall stability or improvement on neurocognitive and neurobehavioral assessments. Importantly, spine MRI is not a standard component of MPS clinical monitoring, so the underlying prevalence of this type of finding in this patient population remains unknown. For now, investigators plan to continue observing the affected patients through periodic imaging rather than pursuing additional intervention.

Why Does this Finding in An Ultra-Rare Disease Program Carry Broader Significance?

The update illustrates how postmarket-style vigilance is increasingly being built into gene therapy programs well ahead of approval.¹ The expanded monitoring plan that surfaced these findings wasn't triggered by a new adverse event in this specific study; it was a proactive measure adopted after a hold affecting a related program. That decision, and its consequences, underscore how CNS-directed AAV gene therapies are being held to increasingly rigorous, long-tail imaging surveillance standards, even years after dosing. Roberto Giugliani, MD, PhD, the Department of Genetics, UFRGS, stated in a press release,¹ "Boys with neuronopathic MPS II experience a multitude of neurodevelopmental and systemic effects. While imaging natural history is limited for this ultra-rare disease, I believe that asymptomatic, likely benign findings like these may be inherent to the impact of Hunter Syndrome throughout the body."

REGENXBIO's response signals an effort to contain the implications to this specific program rather than its broader platform. Curran Simpson, President and CEO, Regenxbio, stated in the press release,¹ "We believe these findings are unique and limited to our Hunter Syndrome program, and require longer-term follow-up and additional data analysis to assess the benefit-risk profile of RGX-121.”

For teams managing CNS-delivered gene therapies, the episode is a reminder that long-term imaging surveillance protocols, and how quickly they're expanded across a portfolio when one program raises questions, are becoming a meaningful part of risk management even for products that have already progressed deep into late-stage development.¹

What Data Gaps Does the Long-Term Follow-Up Study Aim to Close?

The companion long-term follow-up study, enrolling up to 54 prior RGX-121 recipients, tracks safety and a wide biomarker and neurodevelopmental panel, including plasma and cerebrospinal fluid iduronate-2-sulfatase, urine and CSF glycosaminoglycan levels, and four separate developmental assessment scales, for up to five years post-dosing.² That breadth reflects a program still accumulating evidence rather than one anchored to a single validated surrogate. For regulators weighing accelerated approval, a wide net of exploratory endpoints without a firmly established primary measure can read as unresolved evidentiary strength, a dynamic consistent with the FDA's stated concerns about surrogate validation in its prior response to the application.

References

1. REGENXBIO Inc. REGENXBIO announces regulatory update for RGX-121 for mucopolysaccharidosis type II (MPS II). Press Release. August 24, 2026. https://regenxbio.gcs-web.com/news-releases/news-release-details/regenxbio-announces-regulatory-update-rgx-121-mps-ii
2. ClinicalTrials.gov. Long-term follow-up for RGX-121. ClinicalTrials.gov identifier: NCT04597385. https://clinicaltrials.gov/study/NCT04597385