Rockville, Md., Jan 28, 2026, 10:02 (EST)
- The U.S. FDA has placed clinical holds on Regenxbio’s gene therapy trials for RGX-111 and RGX-121, targeting rare childhood disorders.
- The company reported that a brain tumor was discovered in one participant of the RGX-111 trial and is investigating if the case is linked to the treatment.
- Shares fell roughly 20% during morning trading, following an even steeper decline before the market opened
The U.S. Food and Drug Administration has placed a clinical hold on Regenxbio’s experimental gene therapies RGX-111 and RGX-121 after a brain tumor was found in a child receiving treatment, the company announced Wednesday. Shares of Regenxbio dropped about 20% to $10.71 in morning trading, following a roughly 30% dive before the market opened. The company noted it has not yet received the FDA’s complete clinical hold letter. 1
Regenxbio’s two mucopolysaccharidosis (MPS) programs, targeting Hurler and Hunter syndromes, have hit a regulatory snag, with a clinical hold now in place. This comes just weeks before an FDA decision on RGX-121, expected Feb. 8, Fierce Biotech reported. The timing complicates matters, as a delay could push back what investors had counted on as an imminent regulatory milestone. 2
Leerink Partners analyst Mani Foroohar described the pause as “a confusing and unexpected update,” highlighting growing concerns over “FDA unpredictability” in gene therapy reviews. He suggested the recent sell-off might be an overreaction, noting that Regenxbio’s Duchenne muscular dystrophy program remains the key value driver and could challenge Sarepta Therapeutics’ approved Elevidys. 3
Regenxbio reported that a tumor showed up during a routine brain MRI in a five-year-old who had received RGX-111 roughly four years ago and was asymptomatic. Early genetic tests on the removed tumor revealed an adeno-associated virus (AAV) vector integration event tied to overexpression of PLAG1, a proto-oncogene known to promote cancer when activated. However, the company has not confirmed a direct causal link. CEO Curran Simpson expressed surprise at the FDA’s move to pause their RGX-121 program, emphasizing that “patient safety is our top priority.” Regenxbio also noted no tumors have been found in nine other individuals treated with RGX-111 or among 32 patients given RGX-121. 4
A securities filing revealed that the FDA has put the company’s investigational new drug applications for RGX-111 and RGX-121 on clinical hold. This affects a Phase I/II trial for RGX-111 and a Phase I/II/III trial for RGX-121. 5
Hurler and Hunter syndromes are rare genetic disorders linked to cognitive decline and heart issues, frequently proving deadly. Regenxbio is developing gene therapies to replace the faulty genes in patients suffering from these diseases. 6
Both programs rely on AAV as their delivery method—a reengineered virus that transports therapeutic genes into cells. The company flagged an integration signal indicating the vector’s DNA might have inserted itself into the patient’s genome, a red flag for regulators since this can sometimes interfere with normal cell growth control.
Regenxbio’s pipeline extends beyond Duchenne and an eye-disease gene therapy project with AbbVie, also covering MPS programs partnered with Nippon Shinyaku. The company noted that thousands of patients have been treated using its AAV platform, including those on Novartis’ Zolgensma. 7
The FDA hasn’t specified how long the holds might last. If they find a causal link, it could mean extended follow-up, more tests, or alterations to the studies. Even if the cause remains unclear, a lengthy pause would delay progress in diseases where patients’ conditions can deteriorate and treatment choices are scarce.
