FDA places clinical hold on Regenxbio gene therapy after MRI abnormalities

The FDA has placed a clinical hold on Regenxbio's experimental gene therapy for Hunter Syndrome, RGX-121, after spinal MRI scans revealed asymptomatic, likely benign masses in five trial participants. This safety halt represents a major regulatory setback, prompting Regenxbio to delay its planned near-term application resubmission. Following the announcement on August 24, 2026, Regenxbio's shares plunged by over 24% as the company and partner NS Pharma pause to evaluate the imaging data.

FDA clinical hold on RGX-121

• ▪Following the announcement of the FDA clinical hold on August 24, 2026, Regenxbio shares fell by more than 24% to 25% in early trading.
• ▪The U.S. Food and Drug Administration placed a clinical hold on Regenxbio's experimental gene therapy, RGX-121, which was announced on August 24, 2026.

MRI abnormalities in trial participants

• ▪Spinal MRI scans identified abnormalities, described as a small lump, fluid-filled mass, or likely benign mass, in five trial participants who received RGX-121 three to six years ago.
• ▪Regenxbio stated there is no clinical or pathological evidence to confirm the nature or causation of the spinal MRI findings, and no brain masses or nodules were identified.
• ▪The five trial participants with spinal MRI abnormalities remained asymptomatic, clinically stable, and showed overall stability or improvements in cognitive and behavioral assessments.

Prior regulatory setbacks for RGX-121

• ▪The FDA previously placed a clinical hold on RGX-121 in January 2026 after a participant receiving a similar treatment for a different rare condition developed brain cancer.
• ▪The FDA previously declined to approve RGX-121 in February 2026 due to concerns regarding trial design, supporting evidence, and the unconvincing nature of the surrogate trial endpoint.

Hunter syndrome treatment landscape

• ▪Current treatments for Hunter Syndrome include Takeda Pharmaceutical's weekly infusion Elaprase and Denali Therapeutics' Avlayah, which was approved in March 2026 to treat neurological symptoms.
• ▪RGX-121 is designed as a one-time treatment for Hunter Syndrome, a rare genetic disorder that causes progressive damage to the brain and other organs by disrupting the body's ability to break down sugar molecules.

Regenxbio development strategy impact

• ▪Regenxbio and its partner NS Pharma plan to review additional imaging, longer-term follow-up data, and the FDA's formal clinical hold letter before deciding on future development steps.
• ▪Regenxbio no longer plans to resubmit its application for the approval of RGX-121 in the near term following the August 24, 2026 clinical hold.

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