Regenxbio Inc. (NASDAQ:RGNX) suffered another setback in its effort to bring RGX-121 to patients with Hunter syndrome. The FDA placed the experimental gene therapy on clinical hold after spinal scans identified abnormalities in five treated participants, and Reuters reported that Regenxbio shares fell more than 24% in premarket trading following the announcement.
The findings add a new safety question to an already difficult regulatory path. However, the five patients remained asymptomatic, while investigators classified the abnormalities as non-serious and radiologists considered them likely benign. Investors must now determine whether the hold represents a manageable delay or another serious obstacle for the program.
Bull Case
The most encouraging detail is that Regenxbio said all five patients remained asymptomatic, adding that the participants remained clinically stable or demonstrated improvements in cognitive and behavioral assessments. The abnormalities were considered non-serious by investigators, and radiologists believed they were likely benign, with the spinal scans showing a small lump or fluid-filled mass in patients who received RGX-121 approximately three to six years earlier. These observations do not eliminate the potential safety concern, but they suggest the imaging findings have not been accompanied by an evident deterioration in the patients’ clinical condition.
RGX-121 also addresses a serious inherited disorder with significant unmet need. Hunter syndrome, also known as MPS II, can progressively damage the brain as well as other organs and tissues. Regenxbio designed RGX-121 as a one-time treatment that uses an AAV9 vector, which is a harmless virus, to deliver the gene required to produce the enzyme missing in people with the condition.
Current treatments illustrate why a successful one-time therapy could still be meaningful. Takeda Pharmaceutical’s Elaprase requires weekly infusions and treats the physical effects of Hunter syndrome. Denali Therapeutics’ Avlayah, approved in March, treats neurological symptoms in certain children. RGX-121’s potential value will ultimately depend on whether Regenxbio can demonstrate an acceptable safety profile and provide evidence sufficient for FDA approval. The company and its partner, NS Pharma, are not making an immediate decision about the program. They plan to examine additional imaging and longer-term follow-up data and await the FDA’s full clinical-hold letter before determining the next steps.
Bear Case
The timing of the findings creates a significant concern, as the abnormalities were detected approximately three to six years after treatment, and that raises questions about the longer-term monitoring that may be required for RGX-121. Even if the findings prove benign, Regenxbio must address the FDA’s concerns before the hold can be lifted and the affected trial activities can resume.
This is also not RGX-121’s first regulatory setback. The therapy was previously affected by an FDA clinical hold involving two Regenxbio gene-therapy programs. Earlier in 2026, the regulator declined to approve RGX-121 because of concerns about the trial design and the evidence supporting the application. The latest hold further delays an already uncertain approval process. Regenxbio said it does not expect to resubmit its application in the near term, making a near-term resubmission unlikely. The company must now address both the FDA’s earlier concerns about its evidence and the newly identified imaging abnormalities.
The treatment landscape has, meanwhile, continued to advance. Elaprase remains available for physical symptoms, while Avlayah provides a recently approved option for neurological symptoms in certain pediatric patients. These alternatives could make further delays more consequential for RGX-121’s eventual commercial position.
Conclusion
The absence of symptoms and investigators’ assessment that the findings were non-serious provide some reassurance, but they do not resolve the regulatory risk, and RGX-121 now faces safety questions on top of earlier concerns about its clinical evidence.
For investors, the next major development will be the FDA’s full clinical-hold letter and Regenxbio’s review of longer-term imaging data. Until those details arrive, RGX-121’s future, as well as the timeline for any renewed approval effort, remains highly uncertain.
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This article is originally published at Insider Monkey.
