FAYUVI Wins First MPS IIIA Approval on Natural History Data — Durability and Payer Access Remain the Unresolved Bets

FDA Approves Ultragenyx's FAYUVI for Sanfilippo Syndrome Type A

Ultragenyx Pharmaceutical Inc. announced that the U.S. Food and Drug Administration (FDA) granted standard full approval for FAYUVI™ (rebisufligene etisparvovec-hopf) for pediatric patients with Sanfilippo syndrome Type A (MPS IIIA). This marks the first-ever FDA-approved treatment for this progressive and fatal neurodegenerative disease and Ultragenyx's second gene therapy approval. The approval is supported by data from the pivotal Transpher A trial, which demonstrated FAYUVI-treated patients achieved a 23.5 point higher cognitive score over natural history (p<0.0001). Ultragenyx expects commercial product to be available to Qualified Treatment Centers within 30-60 days and will facilitate access through its UltraCare® program. The company also received a Priority Review Voucher.

• FAYUVI (rebisufligene etisparvovec-hopf) has received standard full FDA approval, establishing it as the first-ever treatment for Sanfilippo syndrome Type A (MPS IIIA), a devastating and universally fatal neurodegenerative disease. This approval represents a monumental achievement for the patient community, offering the potential to halt or slow the irreversible neurologic progression and loss of function. Ultragenyx was also granted a Priority Review Voucher upon this significant regulatory milestone.
• The approval is underpinned by robust clinical data from the pivotal Transpher A trial and long-term follow-up studies, showcasing durable clinical benefit. FAYUVI-treated patients (N=17) demonstrated a statistically significant 23.5 point higher mean Bayley-III Cognitive raw score compared to an untreated natural history cohort (N=27) over a period of 24 to 60 months of age (p<0.0001). The therapy, an AAV9 gene therapy, functions by delivering a functional copy of the deficient SGSH enzyme gene, effectively reducing accumulated cerebral spinal fluid heparan sulfate levels.
• Ultragenyx is committed to ensuring timely patient access to FAYUVI, with commercial product anticipated to be available for shipment to Qualified Treatment Centers within 30-60 days. The company's UltraCare® program, featuring dedicated Gene Therapy Guides, will provide comprehensive support to patients and caregivers, assisting with insurance coverage and treatment navigation. FAYUVI is manufactured entirely within the U.S. at Ultragenyx’s facility in Bedford, Massachusetts, and Andelyn Biosciences in Columbus, Ohio.

Addressing the Devastating Unmet Need in Sanfilippo Syndrome Type A

MPS IIIA (Sanfilippo syndrome Type A) presents one of the most formidable therapeutic challenges in the lysosomal storage disease landscape. No specific treatment has been approved for MPS III, and despite decades of research spanning enzyme replacement therapy (ERT), gene therapy, substrate reduction therapy (SRT), and small-molecule approaches, clinical trial results have proved costly and shown limited therapeutic effects.

• Blood-brain barrier (BBB) impermeability severely limits systemic ERT. Intravenous administration of recombinant human heparan N-sulfatase (rhHNS) cannot cross the BBB in sufficient amounts to have a therapeutic effect. Even high-dose or chemically modified rhSGSH delivered intravenously localises to the endothelium, meninges, and choroid plexus, with no convincing punctate intra-neuronal staining, failing to reduce heparan sulfate-derived oligosaccharides or secondarily stored substrates in brain cells.

• Intrathecal ERT achieves CNS distribution but with spatial limitations. Intrathecal lumbar administration of rhHNS delivers the replacement enzyme to therapeutically relevant CNS tissues, with penetration into grey matter and cortex 3–4 times greater than concentrations in white matter and deeper parenchymal regions — suggesting meaningful coverage gaps in critical brain regions.

• Gene therapy efficacy is time-sensitive and route-dependent. Systemic scAAV9.U1A.hSGSH delivery at 3 × 10 vg/kg normalised cognition in mice treated at 6 weeks of age, but mice treated at 16 weeks performed similarly to untreated MPS IIIA mice on cognitive assessments, indicating that neurocognitive impairments in older animals are not reversible upon substrate clearance. Additionally, AAV vector strategies delivered directly to the CNS carry procedure-associated risks, variable dose requirements, and sensitivity to circulating neutralising antibodies that block AAV transduction.

• Small-molecule and substrate reduction approaches have not demonstrated clinical efficacy. High-dose genistein aglycone (160 mg/kg/day) produced a 32.1% reduction in urinary glycosaminoglycans versus placebo (P = .0495), but CSF heparan sulfate concentration was only 5.5% lower in the genistein group — a non-statistically significant result (P = .26) — with no significant differences in cognitive, adaptive behaviour, or quality of life outcomes. These data do not support the use of genistein aglycone therapy in MPS III.

• Secondary downstream neuropathology may be irreversible even after substrate clearance. In MPS IIIA mice treated at 16 weeks, heparan sulfate and ganglioside accumulation was reduced, yet neurocognitive impairments persisted, attributed to secondary downstream consequences of heparan sulfate affecting neurological functions that are not reversible upon substrate clearance — underscoring the need for early intervention windows that are difficult to achieve in clinical practice.

FAYUVI: A Gene Therapy Targeting the Root Cause of MPS IIIA

MPS IIIA arises from inherited mutations in the SGSH gene, which encodes the lysosomal enzyme N-sulfoglucosamine sulfohydrolase (sulfamidase). More than 100 mutations in SGSH have been identified that reduce or eliminate enzymatic activity, including missense mutations, nonsense mutations, small duplications, small deletions, frameshift variants, and large genomic deletions spanning multiple exons. The molecular consequences of these mutations are diverse: structural analyses of glycosylated SGSH at 2 Å resolution reveal that pathogenic missense mutations disrupt the enzyme's active-site architecture, which includes a catalytic formylglycine, a divalent metal-binding site, a sulfate-binding site, and a key arginine residue (Arg282) positioned to bind the N-linked sulfate substrate. Certain mutations, such as p.Ser298Pro, produce a misfolded protein that retains low residual activity but undergoes rapid proteasomal degradation, with only small amounts successfully transported to lysosomes — a mechanism that correlates with a slowly progressive clinical phenotype and highlights the role of protein stability in determining disease severity.

The enzymatic deficiency results in the failure to catabolise heparan sulfate (HS), leading to its progressive lysosomal accumulation across cells of the central nervous system. This primary substrate storage triggers a cascade of secondary pathological changes, including accumulation of glycolipids such as GM3 ganglioside, formation of ubiquitin-positive intracellular inclusions, and astrogliosis. Because lysosomes are vital components of immune cells, GAG accumulation also disrupts immune cell function, driving a complex interplay between neuroinflammation, microglial activation, and adaptive immunity. Microglial activation in particular propagates neurodegeneration in a self-reinforcing cycle, as the progressive nature of HS storage continuously signals cellular damage and danger to the innate immune system.

At the cellular and tissue level, the convergence of enzyme deficiency, substrate accumulation, and neuroinflammation produces progressive cognitive decline, behavioural abnormalities, and widespread neurodegeneration. The spatial distribution of pathology reflects the brain's connectivity: preclinical studies demonstrate that disease lesions appear in regions local to or connected by projections to sites of enzyme deficiency, while regions lacking such connections remain affected by the global absence of functional sulfamidase. This distributed, multi-regional nature of CNS involvement — encompassing cognitive impairment, neuroinflammation, and structural neurodegeneration — defines the central therapeutic challenge in MPS IIIA and underscores why achieving broad enzyme distribution across brain regions is critical for any disease-modifying intervention.

Pivotal Transpher A Trial Data Supports FAYUVI's Efficacy and Safety

Recent research in MPS IIIA has explored multiple therapeutic modalities — from intra-CSF enzyme replacement to intracerebral gene therapy — with studies reporting on both pathological correction and emerging safety signals.

Transforming the Treatment Landscape for Sanfilippo Syndrome Type A

MPS IIIA (Sanfilippo syndrome Type A) is an autosomal recessive lysosomal storage disorder caused by deficiency of N-sulfoglucosamine sulfohydrolase (SGSH), resulting in pathological accumulation of heparan sulfate within lysosomes throughout the body. The disease is characterised by progressive central nervous system degeneration, severe neurocognitive decline, behavioral abnormalities including hyperactivity and aggression, and peripheral organ involvement, with affected individuals typically dying during adolescence. No disease-modifying therapy is currently approved or available for MPS IIIA. Management remains supportive, encompassing correction of metabolic derangements such as acute metabolic acidosis — which can occur particularly during infection or physiological stress — fluid resuscitation, and multidisciplinary care guided by combined genetic and enzymatic confirmation of diagnosis.

Enzyme replacement therapy (ERT), while employed in related MPS subtypes (MPS I, II, and VI), has not achieved whole-body correction in MPS IIIA, largely due to the limitations imposed by the blood-brain barrier. Evidence from adenotonsillar tissue in ERT-treated MPS patients demonstrates that current therapeutic strategies fail to normalise lysosomal compartment size, total sulphated glycosaminoglycan levels, or extracellular matrix remodelling — with structural rather than inflammatory changes driving the persistent obstructive airway phenotype despite treatment. Heparan sulfate glycosaminoglycan remained significantly elevated in MPS IIIA tissue even in the context of ERT, confirming incomplete reversal of disease at the tissue level.

Investigational approaches under active evaluation include systemic AAV9-mediated gene transfer, which has demonstrated widespread sulfamidase expression in brain and peripheral organs in murine models, normalisation of GAG storage, resolution of neuroinflammation, and a remarkable prolongation of survival. Intracerebroventricular (ICV) delivery of recombinant human sulfamidase (rhSGSH) has also shown durable reduction of heparan sulfate and markers of lysosomal dysfunction in MPS IIIA mice, with a 99.5% reduction in disease-specific HS-NRE levels following seven weekly 148 μg ICV doses. The AAVance gene therapy trial, investigating intracerebral delivery of AAVrh.10 overexpressing human sulfamidase (LYS-SAF302), is ongoing in pediatric patients, though post-treatment MRI monitoring has revealed focal lesions around injection sites, the benefit-risk profile of which remains under evaluation. For clinical trial design, cognitive assessments using the Mullen Scales and Leiter-R nonverbal IQ, adaptive behavior via the Vineland-II composite score, and CSF SGSH enzyme activity have been identified as suitable functional outcomes and biomarkers for interventional studies.

Pioneering Gene Therapy Transforms Sanfilippo Syndrome A Treatment

The recent full FDA approval of FAYUVI™ for pediatric patients with Sanfilippo syndrome Type A (MPS IIIA) marks a significant milestone, offering the first-ever approved treatment for this devastating neurodegenerative condition. This gene therapy represents a beacon of hope for families grappling with a disease characterized by rapid and severe cognitive decline, often plateauing in development by 30 months and followed by swift regression after 40-50 months, as detailed in natural history studies. The approval, supported by data showing a significant improvement in cognitive scores, underscores the potential of gene therapy to address the underlying genetic defect.

For Ultragenyx, this approval solidifies their leadership in the rare disease space, particularly in gene therapy. Securing a first-mover advantage in MPS IIIA not only establishes a new market but also validates their innovative platform for other challenging genetic disorders. The strategic inclusion of the UltraCare® program is a critical component, designed to facilitate patient access and provide comprehensive support, which is essential for the successful integration of a complex, novel therapy into clinical practice. Furthermore, the accompanying Priority Review Voucher offers a valuable asset, providing strategic flexibility for future pipeline development or monetization.

However, the path forward is not without its complexities. Research indicates a narrow therapeutic window for MPS IIIA, emphasizing that optimal outcomes may hinge on very early diagnosis and intervention before irreversible neurocognitive damage sets in. This highlights the critical need for robust newborn screening programs and rapid diagnostic pathways. Additionally, the inherent difficulties in accurately assessing cognitive and developmental progress in low-functioning and behaviorally disruptive pediatric patients, as discussed in the literature, could pose challenges in monitoring real-world treatment effectiveness. The observed variability in disease progression among patients also suggests that treatment responses may differ, requiring careful patient stratification and individualized care plans. As this groundbreaking therapy becomes available, understanding these nuances will be crucial for maximizing its impact and ensuring equitable access for all eligible patients.