Drug Update · Rare Disease

FDA Approves Tividenofusp Alfa for Neurologic Hunter Syndrome: 91% CSF Heparan Sulfate Reduction

September 09, 2026

CSF HS mean reduction at Week 24

91% (44 patients)

Patients with CSF HS below ULN at Week 24

93% (41 of 44)

Trial enrollment

47 pediatric patients

FDA approval date (press announcement)

March 25, 2026

Clinical Perspective

Pediatric neurologists, medical geneticists, and other clinicians caring for children with Hunter syndrome now have an FDA-approved, brain-penetrant enzyme replacement option for neurologic manifestations when started in presymptomatic or symptomatic pediatric patients weighing at least 5 kg before advanced neurologic impairment ^[2]^[4]. The approval was granted under accelerated approval on the basis of a surrogate biochemical endpoint—marked reduction in CSF heparan sulfate—supporting potential CNS substrate clearance; clinicians should note the boxed warning for hypersensitivity and the recommended monitoring described in the prescribing information ^[5]^[2]. Will the ongoing randomized confirmatory trial demonstrate durable, clinically meaningful neurologic benefit that verifies the accelerated approval decision?

Dr. Lakshmi Venkatesan, MBBS, MD (Paediatrics), DM (Medical Genetics) · Medical Genetics

The FDA approved AVLAYAH (tividenofusp alfa-eknm) to treat neurologic manifestations of Hunter syndrome (Mucopolysaccharidosis type II, MPS II) when initiated in presymptomatic or symptomatic pediatric patients weighing at least 5 kg prior to advanced neurologic impairment, effective March 25, 2026 ^[4]^[2]. The approval was granted under accelerated approval based on reduction of cerebrospinal fluid (CSF) heparan sulfate observed in a phase 1/2 single-arm trial in pediatric patients, which showed a mean 91% reduction in CSF heparan sulfate at Week 24 ^[5]^[1].

Clinical Context

Tividenofusp alfa (brand: AVLAYAH) is a recombinant fusion protein supplying iduronate-2-sulfatase (IDS) linked to an engineered Fc domain that binds the transferrin receptor to enable delivery into the central nervous system via receptor-mediated transcytosis; it is intended to provide an exogenous source of IDS to reduce glycosaminoglycan accumulation in the brain and peripheral tissues ^[5]^[1]. The development and approval were led by Denali Therapeutics, the sponsor listed in the clinical trial publication and prescribing information ^[1]^[3]. The FDA described AVLAYAH as the first product approved to address neurologic complications of Hunter syndrome and granted accelerated approval on the surrogate endpoint of CSF heparan sulfate reduction, with confirmatory randomized trials ongoing to verify clinical benefit ^[4]^[5].

Key Findings

In the phase 1/2, multi-cohort, single-arm, open-label Trial 1 (NCT04251026), tividenofusp alfa treatment resulted in a mean 91% reduction from baseline in cerebrospinal fluid heparan sulfate at Week 24 among the 44 patients with Week 24 measurements (95% CI: 89%, 92%) ^[5]^[1].
Of the patients with CSF measurements at Week 24, 93% (41 of 44) had CSF heparan sulfate levels below the upper limit of normal at Week 24; at baseline 0% (0 of 47) had CSF HS below the ULN ^[5].
Urinary heparan sulfate levels also appeared reduced from baseline by 88% at the primary analysis timepoint reported in the trial publication ^[1].
The trial report and prescribing information note stabilization or improvement in adaptive behavior measures and normalization or maintenance of liver volumes over the extended follow-up periods, although the accelerated approval was based on the surrogate CSF HS endpoint and confirmatory data are pending ^[1]^[5].

Safety & Tolerability

Hypersensitivity reactions including anaphylaxis have occurred with AVLAYAH; the label carries a boxed warning and recommends initiation in a healthcare setting with appropriate monitoring and access to resuscitation equipment ^[2]^[3].
Infusion-related reactions (IARs) were common in Trial 1; pyrexia, urticaria, and vomiting were the most frequently reported IAR symptoms, occurring in more than 40% of participants despite routine premedication in the trial publication ^[1]^[2].
The prescribing information lists anemia and membranous nephropathy as warnings/precautions and recommends baseline and interval monitoring of hemoglobin and monitoring of serum creatinine and urinary protein to creatinine ratio ^[2]^[3].
The most common adverse reactions reported with incidence ≥20% include infusion-related reaction, upper respiratory tract infection, ear infection, pyrexia, anemia, cough, vomiting, diarrhea, rash, COVID-19, rhinorrhea, nasal congestion, fall, headache, skin abrasion, and urticaria ^[2]^[3].

Study Design

Trial 1 (NCT04251026) was a Phase 1/2, multi-center, international, multi-cohort, single-arm, open-label study that enrolled 47 male pediatric patients with Hunter syndrome (including 44 with neuronopathic disease); participants received weekly intravenous tividenofusp alfa with primary safety objectives and biochemical secondary assessments including CSF and urine heparan sulfate ^[5]^[1]. For regulatory approval, the FDA granted accelerated approval based on reduction of CSF heparan sulfate observed in this trial and noted that a randomized trial to evaluate clinical benefit is ongoing and more than 95% enrolled ^[4]^[5]. The FDA-approved indication specifies initiation in presymptomatic or symptomatic pediatric patients weighing at least 5 kg prior to advanced neurologic impairment ^[2]^[5].

FAQ

What is AVLAYAH approved for?

AVLAYAH (tividenofusp alfa-eknm) is indicated for the treatment of neurologic manifestations of Hunter syndrome (Mucopolysaccharidosis type II, MPS II) when initiated in presymptomatic or symptomatic pediatric patients weighing at least 5 kg prior to advanced neurologic impairment. This indication is an accelerated approval based on reduction of cerebrospinal fluid heparan sulfate; continued approval may be contingent upon verification of clinical benefit in confirmatory trial(s).

How does tividenofusp alfa work?

Tividenofusp alfa provides an exogenous source of iduronate-2-sulfatase (IDS) fused to an engineered Fc domain that binds the transferrin receptor. The transferrin receptor-binding Fc domain mediates transcytosis across the blood-brain barrier to deliver IDS to the central nervous system; the molecule is internalized via the mannose-6-phosphate receptor and transported to lysosomes where it is thought to exert enzymatic activity to reduce accumulated glycosaminoglycans including heparan sulfate.

What is the recommended dose of AVLAYAH?

The prescribing information recommends initiating AVLAYAH in a healthcare setting under supervision for hypersensitivity management and obtaining baseline hemoglobin. The recommended maintenance dosage for pediatric patients who weigh at least 5 kg is 15 mg/kg administered once weekly via intravenous infusion over approximately 4 hours, with an initiated dose-escalation regimen per the label. Clinicians should consult current prescribing information for complete dosing guidance.

What are the most common side effects?

Adverse reactions reported with incidence ≥20% include infusion-related reactions, upper respiratory tract infection, ear infection, pyrexia, anemia, cough, vomiting, diarrhea, rash, COVID-19, rhinorrhea, nasal congestion, fall, headache, skin abrasion, and urticaria. The label includes a boxed warning for hypersensitivity reactions including anaphylaxis and recommends monitoring and readiness for emergency management when initiating therapy.

Clinicians should consult current prescribing information for complete dosing guidance.

FDA Approves Tividenofusp Alfa for Neurologic Hunter Syndrome: 91% CSF Heparan Sulfate Reduction

Direct Clinical Takeaway & Evidence Summary

FDA Approves Tividenofusp Alfa for Neurologic Hunter Syndrome: 91% CSF Heparan Sulfate Reduction

Clinical Perspective

Clinical Context

Key Findings

Safety & Tolerability

Study Design

FAQ

References

Cite & Reference This Monograph

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