9 February 2026 - RegenXBio today announced that the US FDA has issued a complete response letter regarding its biologics license application for RGX-121 (clemidsogene lanparvovec) for the treatment of mucopolysaccharidosis II (MPS II), an ultra-rare neurodegenerative disease also known as Hunter syndrome.

In May 2025, the FDA accepted the RGX-121 BLA under the accelerated approval pathway. In the February 7, 2026 CRL, the FDA stated that it had agreed to the study protocol in principle and outlined several reasons for not approving the gene therapy, including uncertainty regarding the study eligibility criteria to adequately define a population with neuronopathic disease (vs. attenuated disease), the comparability of the natural history external control to the study population, and the appropriateness of CSF HS D2S6 as a surrogate endpoint reasonably likely to predict clinical benefit. The CRL lists several potential paths forward, including a new study, treating additional patients and conducting longer-term follow up, and using an untreated control arm, all of which would be challenging in an ultra-rare disease population, like MPS II.

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