FDA Places Clinical Hold on Regenxbio’s AAV Gene Therapy Programs

ROCKVILLE, MD —January 28, 2026— Regenxbio shares plunged more than 20% on Wednesday following an FDA order to halt dosing in two of its lead AAV gene therapy programs for ultra-rare pediatric diseases. The clinical hold affects RGX-111 for MPS I (Hurler syndrome) and RGX-121 for MPS II (Hunter syndrome), triggered by a potential safety signal involving a brain tumor in a young patient.

The safety event occurred in a five-year-old child who had received the RGX-111 AAV therapy four years prior. During routine imaging, an asymptomatic intraventricular CNS tumor was detected. Preliminary molecular analysis indicated evidence of AAV vector genome integration associated with the overexpression of PLAG1, a known proto-oncogene. While the child reportedly continues to show “positive developmental advancements,” the FDA extended the hold to the RGX-121 AAV program as well, citing similarities in the NAV AAV9 vector design and patient populations.

Regenxbio CEO Curran Simpson expressed surprise at the broad hold, noting that RGX-121 has a clean safety profile in over 30 patients, some monitored for nearly seven years. RGX-111 utilizes an AAV9 vector to deliver the IDUA gene to the central nervous system, whereas RGX-121 delivers the IDS gene. This news comes at a critical juncture for RGX-121, which has a pending FDA marketing application with a fast-approaching decision date of February 8.

Industry analysts suggested the market’s sharp reaction reflects growing concerns over AAV safety and “FDA unpredictability,” though some view the stock drop as an overcorrection. While Regenxbio investigates the “single, inconclusive incident,” the company remains focused on its broader pipeline, including AAV therapies for Duchenne muscular dystrophy. To date, no similar tumor events have been observed in any other patients treated with these AAV candidates.