REGENXBIO, the Rockville, Maryland-based gene therapy company, announced that the FDA has placed a clinical hold on RGX-121, its investigational one-time gene therapy for MPS II, also known as Hunter Syndrome, after asymptomatic spine MRI findings turned up in five participants from the company’s CAMPSIITE study. The company said it does not expect to resubmit RGX-121’s Biologics License Application in the near term as a result. CEO Curran Simpson addressed the finding directly: “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.” Shares fell sharply in premarket trading the day of the announcement, with reported declines ranging from roughly 22% to as much as 26% depending on the source and exact timing.
What the MRI Findings Actually Showed
The five participants, treated with RGX-121 between three and six years earlier, showed small nodules or cystic masses on spine MRI scans. Investigators characterized the findings as nonserious and likely benign, and REGENXBIO says all five patients continue to do well clinically, showing overall stability to improvement on neurocognitive and neurobehavioral assessments, with no findings identified in the brain. Outside commentary offered some context for why this might not be alarming on its own: geneticist Roberto Giugliani noted that findings like these may be inherent to how Hunter Syndrome itself affects the body over time, separate from any effect of the gene therapy. Even so, the company and its development partner, NS Pharma, are now evaluating additional patient imaging and say they’ll factor FDA feedback into future decisions about RGX-121’s path forward.
A Program That’s Already Had a Rough 2026
This isn’t RGX-121’s first regulatory setback this year. Back in January 2026, the FDA placed both RGX-121 and its sister program RGX-111 on clinical hold after a central nervous system tumor turned up in a patient treated with RGX-111, a different gene therapy; the FDA extended that hold to RGX-121 as well given similarities between the two programs and their study populations, even though the safety signal itself appeared only in the RGX-111 trial. The FDA rejected RGX-121’s BLA outright in February 2026, initially demanding a larger patient population and an untreated control arm before it would reconsider; the partial clinical hold itself wasn’t formally lifted until April 30, 2026, weeks after that rejection. The agency reversed course in June 2026, agreeing to review a resubmission based on REGENXBIO’s existing study data without those added requirements, clearing a path toward a planned third-quarter refiling. This new clinical hold now derails that timeline entirely, on a different safety signal than the one that triggered January’s hold.
What Happens Next for REGENXBIO
With RGX-121 now on hold indefinitely, REGENXBIO’s near-term catalysts shift elsewhere: the company says it remains focused on submitting a BLA for its Duchenne muscular dystrophy program this quarter and expects wet age-related macular degeneration data in the fourth quarter. REGENXBIO is headquartered in Rockville, Maryland, within the BioHealth Capital Region, and this setback lands at a moment when the company had been working to demonstrate renewed regulatory momentum after February’s rejection. Whether this latest hold proves to be a temporary detour or a more lasting setback for RGX-121 will depend heavily on what additional imaging and longer-term follow-up data show, and on how the FDA ultimately interprets findings both the company and outside experts are currently describing as likely unrelated to the therapy’s core safety profile.
