Fayuvi's First-in-Disease FDA Win Masks Critical Evidence Gaps and Narrow HTA Path

FDA Approves First Gene Therapy for Pediatric Sanfilippo Syndrome Type A

The U.S. Food and Drug Administration (FDA) has approved Fayuvi (rebisufligene etisparvovec-hopf), the first gene therapy for pediatric patients with mucopolysaccharidosis type IIIA (MPS IIIA), also known as Sanfilippo syndrome type A. This one-time intravenous treatment uses an AAV9 vector to deliver a functional SGSH gene, enabling the body to produce the missing sulfamidase enzyme and reduce harmful heparan sulfate buildup in the brain and body. This approval marks a significant milestone, offering the first therapy designed to alter the underlying course of this rare, progressive neurodevelopmental disease, which previously had no approved treatment beyond symptom management.

• Fayuvi is an AAV9-mediated gene therapy designed to address the root cause of MPS IIIA by delivering a working copy of the SGSH gene. This enables the production of sulfamidase, the enzyme deficient in MPS IIIA, thereby facilitating the proper breakdown of heparan sulfate and preventing its harmful accumulation in the central nervous system and other tissues, which is critical for mitigating the progressive neurodevelopmental damage.
• The approval was based on an open-label, single-arm, multicenter clinical study in pediatric patients with MPS IIIA, specifically those aged 2 to 5 years. The study demonstrated that Fayuvi-treated patients maintained or improved cognitive function, a meaningful divergence from the expected natural disease course of plateau and decline observed in an untreated historical control cohort during this critical developmental window.
• The safety of Fayuvi was evaluated in pediatric patients receiving a single intravenous infusion. Common adverse reactions included increases in liver enzymes (AST), nausea, vomiting, fever, and decreased blood cell counts. Important safety warnings include the risk of thrombotic microangiopathy (TMA) and a potential long-term risk of tumor development due to genomic integration. Fayuvi received Orphan Drug, Fast Track, and Breakthrough Therapy designations, underscoring its significance for an urgent unmet medical need.

Addressing the Critical Unmet Need in Sanfilippo Syndrome Type A

MPS IIIA (Sanfilippo syndrome type A) is a devastating lysosomal storage disorder driven by heparan N-sulfatase deficiency, resulting in progressive cognitive decline and death in early adulthood. Despite active investigation across multiple therapeutic modalities, each approach carries significant biological or translational limitations that have so far prevented an approved treatment from reaching patients.

• Blood-brain barrier (BBB) penetration: Intravenous enzyme replacement therapy cannot cross the BBB in sufficient amounts to achieve a therapeutic effect in the CNS, necessitating alternative delivery routes such as intrathecal or intracerebroventricular administration. Even with intrathecal lumbar delivery of recombinant human heparan N-sulfatase (rhHNS) in cynomolgus monkeys, penetration into grey matter and cortex was only 3–4 times greater than concentrations in white matter and deeper parenchymal regions, indicating meaningful limitations in reaching deeper CNS structures.

• Substrate reduction therapy — clinical efficacy gap: The isoflavone genistein is the most studied substrate reduction compound for MPS III; however, findings from a Phase III clinical trial demonstrated that high-dose oral genistein did not significantly improve neurodevelopmental outcomes in patients with MPS III, underscoring the difficulty of translating preclinical promise into clinical benefit.

• Pre-existing anti-AAV antibodies in the target population: AAV-mediated gene therapy faces a critical translational challenge due to pre-existing antibodies against multiple AAV serotypes in MPS IIIA/IIIB patients. Seroprevalence for AAV1 and AAVrh74 was significantly higher in 2- to 7-year-old MPS III patients than in healthy controls, and broad co-prevalence of antibodies across serotypes reinforces this challenge regardless of the vector serotype selected.

• Genotoxicity and safety evaluation requirements for gene therapy: Lentiviral vector-based hematopoietic stem cell gene therapy, while demonstrating preclinical efficacy, requires rigorous safety characterisation — including biodistribution, vector shedding, germline transmission, and genotoxicity assessment — before clinical translation, adding complexity and timeline to development.

• Limitations of chemically modified enzyme approaches: Although chemical modification of replacement enzymes (e.g., PerT-treated sulfamidase in murine MPS IIIA) has been explored to enhance CNS delivery by escaping carbohydrate-mediated clearance, this approach failed to demonstrate clearance of storage in neurons in MPS IIIA models, limiting its applicability for neuronal correction in this specific disease.

Fayuvi's Clinical Evidence: Design, Endpoints, and Efficacy

Two clinical trials evaluated intrathecal recombinant human heparan-N-sulfatase (rhHNS) in MPS IIIA, progressing from an initial phase 1/2 dose-escalation study to a long-term extension. Both studies administered rhHNS via an intrathecal drug delivery device (IDDD) on a monthly basis, with safety as the primary focus and neurocognitive outcomes as key secondary measures.

Reshaping the Sanfilippo Syndrome Type A Treatment Landscape

Over the past several years, the MPS IIIA treatment landscape has advanced across multiple therapeutic modalities, each targeting the core deficit of N-sulfoglucosamine sulfohydrolase (SGSH) deficiency and the resulting heparan sulfate (HS) accumulation. Enzyme replacement therapy (ERT) has progressed into clinical evaluation: SOBI003, a chemically modified recombinant human sulfamidase, was assessed in a phase 1/2 open-label first-in-human study (NCT03423186) and its extension (NCT03811028) in six patients aged 1–6 years with confirmed MPS IIIA. Administered as weekly intravenous infusions at doses of 3 mg/kg or 10 mg/kg — adjustable up to 20 mg/kg during the extension — SOBI003 crossed the blood-brain barrier, with a mean reduction in CSF HS levels of 79% recorded at the last assessment, alongside reductions in serum and urine HS. Neurocognitive development age-equivalent scores showed stabilization of cognition across all patients, though no clear overall clinical effect was observed on adaptive behavior, sleep pattern, or quality of life. Anti-drug antibodies were detected in serum and CSF in all patients, with subsequent reductions in serum SOBI003 exposure at high antibody titers. Separately, intracerebroventricular (ICV) delivery of recombinant human sulfamidase (rhSGSH) in MPS IIIA mice demonstrated a tissue half-life of 9 days, widespread brain distribution, and a 99.5% reduction in HS-NRE levels following seven weekly 148 μg doses, with durability of effect sustained through 56 days.

Gene therapy has emerged as a particularly active area, with both viral vector and cell-based strategies advancing. The AAVance trial investigates LYS-SAF302 — an AAVrh.10 vector overexpressing human sulfamidase — delivered by intracerebral injection in children with MPS IIIA. Post-treatment MRI monitoring revealed focal lesions around injection sites, with onset from 3 months after therapy, progression until 7 months, and subsequent stabilization and some regression. Investigations in one patient found no viral or immunological/inflammatory cause; immunohistochemistry suggested that local sulfamidase overexpression led to dysfunction of transduced cells near injection sites, extracellular spilling of lysosomal enzymes, depletion of heparan sulfate from the extracellular matrix, and cystic white matter degeneration at the site of highest gene expression. In parallel, systemic delivery of scAAV9.U1a.hSGSH at 3 × 10 vg/kg via tail vein injection in MPS IIIA mice cleared HS and GM2 and GM3 gangliosides from the cortex, hippocampus, and subcortex, with residual storage remaining in the brain stem and cerebellum. Motor activity and gait length improved in mice treated at both 6 and 16 weeks of age; however, normalization of cognition in the water cross-maze test and gait width was observed only in mice treated at 6 weeks, with 16-week-treated mice performing similarly to untreated MPS IIIA animals. A neural stem cell gene therapy approach using human iPSC-derived neural progenitor cells transduced with lentiviral vectors encoding codon-optimized SGSH and transplanted bilaterally into the striatum of NSG-MPSIIIA mice achieved full correction of spatial working memory deficits and significant reductions in heparan sulfate storage and astrogliosis at three months post-injection.

Substrate reduction therapy and improved preclinical modeling have also contributed to the evolving landscape. 4-deoxy-N-acetylglucosamine peracetate, a novel N-acetylglucosamine analogue, significantly reduced HS levels in cultured MPS IIIA patient and mouse fibroblasts in a time- and dose-dependent manner, and improvements in HS load within the MPS IIIA mouse brain suggested the compound crossed the blood-brain barrier after oral administration. By contrast, a Phase III clinical trial of high-dose oral genistein — previously the most studied substrate reduction compound — did not significantly improve neurodevelopmental outcomes in patients with MPS III. To support xenotransplantation studies of human cellular therapies, a novel immune-deficient MPS IIIA mouse model (MPSIIIA-TKO) was created by backcrossing the C57BL/6 MPS IIIA mouse to a strain lacking Rag2, CD47, and Il2rg genes; the resulting model exhibits undetectable SGSH activity, histological changes consistent with MPS IIIA, and absence of T cells, B cells, and NK cells, while tolerating xenotransplantation.

Fayuvi's Approval: A New Era for Sanfilippo Syndrome Type A

The recent FDA approval of Fayuvi for pediatric Sanfilippo syndrome type A (MPS IIIA) marks a pivotal moment, transforming the landscape for a devastating, previously untreatable neurodevelopmental disorder. This gene therapy offers the first opportunity to address the root cause of MPS IIIA by delivering a functional SGSH gene via an AAV9 vector, enabling the production of the missing sulfamidase enzyme. This mechanism is designed to reduce the harmful accumulation of heparan sulfate throughout the body, including the brain, a critical step given the severe neurological decline characteristic of the disease.

This breakthrough not only provides a much-needed therapeutic option but also underscores the growing potential of AAV9-mediated gene delivery for conditions affecting the central nervous system. The ability of the AAV9 vector to cross the blood-brain barrier and achieve widespread enzymatic activity is a significant scientific validation. However, the path forward is not without its complexities.

Key considerations for the long-term success and broader impact of this therapy include:

• Immune Response: The potential for anti-drug antibody development, which studies indicate can reduce drug exposure and may influence long-term efficacy, will require ongoing vigilance and potentially strategies to mitigate immune responses.

• Clinical Efficacy Endpoints: While biochemical markers like heparan sulfate reduction are promising, translating these into clear, sustained improvements in neurocognitive function and adaptive behavior remains a challenge. Natural history studies highlight the rapid progression of cognitive decline and brain volume loss, emphasizing the importance of early intervention and the need for sensitive, reliable outcome measures.

• Early Diagnosis: The progressive nature of MPS IIIA means that early diagnosis, potentially through newborn screening, could be crucial to maximize therapeutic benefit before irreversible neurological damage occurs.

This approval represents a significant leap for patients and families, offering a new paradigm of hope. It also sets the stage for further innovation in gene therapy for rare neurological diseases, pushing the boundaries of what is possible in precision medicine.