The FDA issued a complete response letter rejecting Ultragenyx’s gene therapy UX111 for Sanfilippo syndrome type IIIA, citing manufacturing process deficiencies identified during facility inspections. The agency did not raise concerns about the clinical data. Ultragenyx plans to address the manufacturing issues and resubmit, restarting a six-month review clock. UX111 utilizes an AAV9 vector to deliver a functional SGSH gene to treat this rare neurodegenerative lysosomal storage disorder lacking effective therapies.