Ultragenyx Pharmaceutical received a complete response letter from the FDA rejecting its AAV-based gene therapy UX111 for Sanfilippo syndrome type A, citing manufacturing concerns related to the company’s production facility. The agency requested further information and improvements to chemistry, manufacturing, and control (CMC) processes, though it did not question the clinical data or product quality. Ultragenyx anticipates resolving the issues and resubmitting, expecting a fresh six-month review period. The setback follows recent clinical trial disappointments and highlights scrutiny over manufacturing capabilities in gene therapy development.