FDA Places Clinical Hold on REGENXBIO's Hunter Syndrome Gene Therapy RGX-121 Over Spinal Scan Findings
In late August 2026, the FDA placed a formal clinical hold on REGENXBIO's investigational gene therapy programs RGX-121 for Hunter syndrome (Mucopolysaccharidosis Type II / MPS II) and RGX-111 for Hurler syndrome (MPS I). The regulatory action followed routine spinal MRI scans that detected small, asymptomatic nodules or cyst-like masses in the spinal canals of five children treated with the therapy.
Both therapies are delivered via intrathecal injection (directly into the spinal canal) to bypass the blood-brain barrier and address severe neurocognitive decline. The discovery of these spinal masses represents a significant safety signal and warning for spinal delivery of biologic and gene therapies.
This hold comes shortly after the FDA issued a Complete Response Letter (CRL) for RGX-121 earlier in August 2026, which raised concerns about study population definition, external control comparability, and surrogate endpoint usage. The clinical hold and CRL have significantly delayed REGENXBIO's commercial timeline and threatened its anticipated Priority Review Voucher (PRV) windfall.
Verbatim Quotes
"Regenxbio's RGX-121 gene therapy for Hunter syndrome was placed on clinical hold by the FDA after spinal scans found small nodules or cyst-like masses..." (BigGo News)
"Five asymptomatic spinal findings, a halted gene therapy, and a warning for anyone putting biologics into the spinal canal. Regenxbio said on August 24 that..." (The Spine Market Group)