For families living with Sanfilippo syndrome type A, time is the central clinical variable. Children can appear to develop normally before progressive heparan sulfate accumulation drives neurocognitive decline. Once neurons are lost, a gene therapy cannot recreate the development that has already disappeared.
The FDA's September 17 approval of Ultragenyx's Fayuvi creates the first approved treatment for the disease. It also offers a carefully bounded regulatory lesson: in a devastating pediatric disorder where a conventional placebo trial may be impractical, a strong biological mechanism, sustained biomarker response, and a credible external natural-history comparison can support full approval.
