Overview
- Following a Feb. 7 complete response letter, the FDA said current data for RGX-121 do not demonstrate substantial evidence of effectiveness under the accelerated pathway.
- The agency flagged uncertainty around eligibility criteria separating neuronopathic and attenuated disease, comparability of external natural-history controls, and the suitability of CSF HS D2S6 as a surrogate endpoint.
- The CRL outlined paths such as a new study, treating more patients with extended follow-up, or adding an untreated control arm, options complicated by the ultra-rare nature of MPS II.
- The application was accepted for accelerated review in May 2025, and the program remains on clinical hold after a brain tumor was found in a child previously treated with related RGX-111.
- Regenxbio says RGX-121 has shown a favorable safety profile in more than 30 patients and will seek FDA feedback on additional analyses and longer-term clinical data to enable resubmission.