FDA Halts Regenxbio’s Hunter Syndrome Gene Therapy Again Over Spine Scan Findings

The FDA has again halted testing of a Regenxbio gene therapy — this time RGX-121 for Hunter syndrome (MPS II) — after safety concerns emerged from patient spinal scans.

What was found

The agency placed a clinical hold after spine MRI abnormalities were identified in trial participants. According to the company, masses described as “likely” benign were found on the spines of five patients treated three to six years ago.

Those patients remained asymptomatic, with stable or improved neurocognitive assessments, and no brain masses were detected. Regenxbio emphasised there is “no clinical or pathological evidence to confirm the nature or causation of the spine MRI findings.”

Why the delay before detection matters

The three-to-six-year interval is the most consequential detail, and it illustrates a structural problem with gene therapy oversight.

These are permanent, one-time interventions. Once genetic material is delivered, it cannot be withdrawn if a problem emerges later, and the relevant risks may take years to become visible.

Conventional drug safety monitoring assumes a temporal relationship close enough to notice — a reaction within hours or days, a laboratory abnormality within weeks. A finding appearing on a scan half a decade after a single administration falls outside how safety surveillance is normally structured, and it was only detected because these trials involve long-term imaging follow-up.

Why the FDA acted despite no symptoms

The patients are well, the masses are described as likely benign, and causation is unestablished — which might seem thin grounds for stopping a programme.

The reasoning is about what cannot be excluded. Gene therapy delivered by viral vectors raises a specific theoretical concern: genetic material integrating into the genome near a gene controlling cell growth could, in principle, contribute to abnormal proliferation. That is the mechanism behind serious adverse events in earlier gene therapy history, and it is precisely the sort of thing that presents as masses years later.

With asymptomatic patients and no tissue diagnosis, the agency cannot determine whether it is looking at incidental findings or the early manifestation of exactly that risk. Pausing enrolment while that is investigated is the conservative choice, particularly in a paediatric population.

What Hunter syndrome is

MPS II is a rare inherited disorder in which a missing enzyme allows complex sugar molecules to accumulate in cells throughout the body, causing progressive damage to multiple organs and, in the severe form, the brain.

Enzyme replacement therapy exists and does not meaningfully cross the blood-brain barrier, so it does little for the neurological decline that is the most devastating feature. That is the gap RGX-121 was designed to fill — delivering the genetic instructions directly to the central nervous system.

Which is why the finding is painful. Children with severe Hunter syndrome face progressive neurological deterioration with no effective treatment, and the risk calculation for their families is unlike that for patients with alternatives.

The second setback this year

This is the second clinical hold on a Regenxbio programme in 2026. The first, in January, followed a brain-cancer case in a patient receiving a similar therapy for a different rare disease; the FDA rejected the related application in February.

Regenxbio said it no longer plans to file a resubmission “in the near term,” and its shares fell more than 25%. The company is working with partner NS Pharma to evaluate the data and incorporate FDA feedback.

Why a pattern is more concerning than an event

A single unexplained finding in one programme is the ordinary background of drug development. Two safety signals within a year, both involving abnormal growths, in programmes using similar underlying technology, is a different situation.

It raises the possibility that the issue lies with the platform — the vector, the delivery approach, or something about how the genetic material behaves after administration — rather than with either individual product. Investors reading it that way explains a 25% decline on news about a single programme.

The broader problem this illustrates

Gene therapies for rare diseases can be transformative, and they raise long-term safety questions that take years to surface.

The difficulty is structural: approval decisions must be made on follow-up shorter than the period over which the relevant risks appear. A therapy approved on two-year data may reveal a problem at year five, in patients who cannot undo the treatment.

Repeated holds are a reminder of how carefully regulators and developers must weigh durable benefit against uncertain, delayed risk — especially in children with few other options, where the pressure to accept uncertainty is greatest and the consequences last longest. Regulatory news, not medical advice.