FDA declines approval of Regenxbio gene therapy for rare Hunter syndrome
The FDA declined to approve Regenxbio's gene therapy for Hunter syndrome due to trial design concerns. Shares fell 19 percent following the news on Monday.
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The U.S. Food and Drug Administration in the US 
US has issued a complete response letter declining to approve the gene therapy candidate RGX-121 for Mucopolysaccharidosis II (MPS II), also known as Hunter syndrome, which was developed by REGENXBIO Inc. . The decision, announced on February 9, 2026, cited uncertainty regarding the eligibility criteria used in the clinical study and whether the surrogate endpoint employed in the submission is reasonably likely to predict clinical benefit for patients. This regulatory action changes the approval and development path for a potential one-time treatment for an irreversible ultra-rare neurodegenerative disease.
US has issued a complete response letter declining to approve the gene therapy candidate RGX-121 for Mucopolysaccharidosis II (MPS II), also known as Hunter syndrome, which was developed by REGENXBIO Inc. . The decision, announced on February 9, 2026, cited uncertainty regarding the eligibility criteria used in the clinical study and whether the surrogate endpoint employed in the submission is reasonably likely to predict clinical benefit for patients. This regulatory action changes the approval and development path for a potential one-time treatment for an irreversible ultra-rare neurodegenerative disease.
