The FDA issued a complete response letter (CRL) on February 9, 2026, rejecting REGENXBIO's RGX-121 gene therapy for Hunter syndrome (MPS II), citing uncertainties in patient eligibility, use of natural history controls, and the surrogate endpoint DS26 (heparan sulfate in CSF).123
FDA recommends a new study with more patients, longer follow-up, and a placebo or untreated control arm for resubmission, which REGENXBIO called challenging due to the ultra-rare nature of the disease.12
RGX-121 was under accelerated approval based on Phase 1/2/3 CAMPSIITE study data showing 86% reduction in DS26 at 16 weeks in 48 boys aged 4 months to 5 years.1
This follows a three-month delay from the original November 9, 2025 PDUFA date and a recent clinical hold due to a brain tumor in a separate RGX-111 trial, though not cited as rejection reason.23
REGENXBIO CEO Curran Simpson described the decision as 'devastating' for families and plans a Type A meeting with FDA to discuss paths forward.125
Analysts note regulatory caution on accelerated approvals without placebo data, potential risks for similar therapies like Denali's tividenofusp alfa.13
Sources:
1. https://www.biospace.com/fda/regenxbios-hunter-syndrome-gene-therapy-fails-to-win-fda-nod
2. https://www.biopharmadive.com/news/regenxbio-fda-reject-gene-therapy-mps-hunter-syndrome/811775/
3. https://www.fiercebiotech.com/biotech/fda-rejects-regenxbio-hunter-syndrome-gene-therapy-recently-hit-hold-unrelated-brain-tumor
5. https://globalgenes.org/raredaily/fda-rejects-regenxbios-mps-ii-gene-therapy/