By Elias Schisgall

Shares of Regenxbio plunged after the Food and Drug Administration placed clinical holds on two drug development programs after a five-year-old participant in one study developed an intraventricular CNS tumor.

The stock fell 33% to $9.02 in pre-market trading Wednesday after closing Tuesday at $13.41. At close, the stock was up 76% over the past 12 months.

Regenxbio said Wednesday that the FDA's decision followed preliminary analysis of the tumor, which was observed in a participant in the company's phase 1/2 study of RGX-111 to treat Hurler syndrome, a genetic disease.

The FDA also put a hold on RGX-121 to treat Hunter syndrome, a similar genetic disease to Hurler syndrome, citing similarities between the products, study populations, and shared risks.

President and Chief Executive Officer Curran Simpson said the company was surprised by the FDA's decision to pause the RGX-121 program.

"These are separate therapies, and the positive safety profile of RGX-121 in more than 30 patients treated, including those dosed nearly seven years ago, remains unchanged," Simpson said. "Patient safety is our top priority, and we, our investigators, and the patient community remain confident in the benefit-risk ratio of RGX-121 and are highly encouraged by the meaningful efficacy profile demonstrated in the pivotal trial."

The five-year-old participant who developed the tumor continues to be asymptomatic with positive developmental advancements, Regenxbio said. It said it was still investigating whether the tumor was drug-related.

The company said it is still awaiting the full clinical hold letter from the FDA.

Write to Elias Schisgall at elias.schisgall@wsj.com

(END) Dow Jones Newswires

January 28, 2026 09:08 ET (14:08 GMT)

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