FDA Approval of Fayuvi for Sanfilippo Syndrome Type A: A Critical Clinical Review

Medical Dialogues reports that the FDA has approved rebisufligene etisparvovec-hopf (Fayuvi), a one-time intravenous AAV9 gene therapy delivering a working copy of the SGSH gene, as the first disease-modifying treatment for children with mucopolysaccharidosis type IIIA — Sanfilippo syndrome type A. The approval rests on an open-label, single-arm trial in patients aged 2 to 5, with cognitive scores compared against an untreated historical control cohort. That is not the strongest evidentiary architecture available, and it is worth pulling apart before the headlines convince anyone that the field has turned a corner in pediatric neurodegeneration.

What the trial actually showed

In my experience reviewing rare disease submissions, historical-control comparisons are a concession to ethics, not a substitute for randomization. The agency reports that treated patients maintained or improved cognitive function during the developmental window where untreated children typically plateau and decline — a meaningful divergence, if true. Reasonable framing aside, the announcement does not disclose the effect size, the confidence interval, the specific cognitive instrument used, or how the historical cohort was selected and matched for disease severity. Saying patients maintained or improved without a number is a press release, not an efficacy endpoint. We do not yet know whether any child who would have declined was rescued, whether the benefit was durable past the trial window, or whether the average shift was modest and clinically marginal. Comparing to natural history introduces selection bias by definition; regulators accept it because MPS IIIA is fatal and the untreated trajectory is well-characterized, but investigators and clinicians should not.

Safety signals left on the cutting room floor

The approval statement says safety was evaluated across pediatric patients receiving a single IV infusion in the clinical studies. It does not, in the materials I have, enumerate adverse events of special interest: AAV9-associated hepatotoxicity, infusion reactions, thrombotic microangiopathy, or immune-mediated neurologic complications. Systemic AAV9 delivery is not benign, and the class carries documented risks that deserve explicit characterization in this youngest and most vulnerable cohort. For clinicians preparing to administer the therapy and for families weighing enrollment, the practical questions are concrete — what is the pre- and post-infusion monitoring protocol, what threshold triggers immunosuppression or steroid rescue, and what post-marketing safety commitments has the sponsor accepted from the Office of Therapeutic Products? Until those data are public, the "scientific milestone" framing is premature optimism. The regulatory approval is real; the evidence base is still thin, and real-world viability will hinge on the safety disclosures we have not yet seen.