• The FDA cited uncertainty regarding the study’s eligibility criteria to define a population with neuronopathic disease versus attenuated disease as one of the reasons for its denial. 
• The FDA also pointed to how closely the natural history external control matched the trial’s patient population.
• Last month, the FDA placed a clinical hold on two of the company’s experimental gene therapy programs for the treatment of rare childhood diseases, including RGX-121.

Shares of REGENXBIO Inc. (RGNX) plummeted about 19% after-hours on Monday after the U.S. Food and Drug Administration (FDA) denied approval for its gene therapy in treating Hunter syndrome, an ultra-rare neurodegenerative disease.

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The company said on Monday that the FDA outlined several reasons for not approving the gene therapy in a Complete Response Letter (CRL).

REGENXBIO was seeking approval for RGX-121 (clemidsogene lanparvovec) in the treatment of Mucopolysaccharidosis II, an ultra-rare neurodegenerative disease also known as Hunter syndrome.

The FDA had previously accepted the RGX-121 under an accelerated approval pathway in May 2025.

Reasons For Denial

In the CRL dated Feb. 7, 2026, the FDA said that the previous agreement to the study protocol had been in principle.

The U.S. federal agency cited uncertainty regarding the study’s eligibility criteria to define a population with neuronopathic disease versus attenuated disease as one of the reasons for its denial.

The FDA also pointed to how closely the natural history external control matched the trial’s patient population and agreed that CSF HS D2S6 is an appropriate surrogate marker that is reasonably likely to predict real clinical benefit.

"This decision is devastating for the families of boys living with this progressive, life-threatening disease," said Curran Simpson, President and CEO of REGENXBIO.

"We are concerned about FDA's feedback regarding the overall development path and evaluation of the data in the context of the urgent need for this irreversible ultra-rare disease. We remain confident in the quality and volume of evidence demonstrating the long-term potential of RGX-121 to positively change the trajectory of Hunter syndrome," Simpson added.

Last month, the FDA placed a clinical hold on two of the company’s experimental gene therapy programs for the treatment of rare childhood diseases, including RGX-121.

Path Forward

The CRL listed several potential paths forward, including a new study, treating additional patients, conducting longer-term follow-up, and using an untreated control arm, the company said.

The company said it is planning to meet with the FDA to address the complete response letter, gather expert input and longer-term clinical data to better define the eligible patient population, and work toward resubmitting its application as quickly as possible.

Hunter syndrome is a rare genetic disorder that mostly affects males and causes complex sugars to build up in cells, which can impact physical and/or cognitive development.

How Did Stocktwits Users React?

On Stocktwits, retail sentiment around RGNX shares was in the ‘extremely bearish’ territory over the past 24 hours amid ‘extremely high’ message volumes.

One bearish user said it was a blessing to be able to exit with only a 28% haircut, after two failed drugs.

Shares of RGNX have gained over 30% in the past year.

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