FDA Approves First Brain-Penetrating Enzyme Therapy for Hunter Syndrome

What Is Hunter Syndrome and Why Is Brain Penetration Important?

Hunter syndrome, formally known as mucopolysaccharidosis type II (MPS II), is a rare X-linked lysosomal storage disorder caused by deficiency of the enzyme iduronate-2-sulfatase (IDS). Without functional IDS, glycosaminoglycans — specifically heparan sulfate and dermatan sulfate — accumulate progressively in cells throughout the body, damaging connective tissue, airways, the heart, joints, and in the severe neuronopathic form, the central nervous system. The condition almost exclusively affects boys and typically becomes apparent between ages 2 and 4.

Standard intravenous enzyme replacement therapy with idursulfase (Elaprase) has been available since 2006 and can slow systemic progression. However, the intravenously infused enzyme is a large protein that cannot cross the blood-brain barrier in meaningful quantities. For patients with the severe neuronopathic phenotype — estimated to represent roughly two-thirds of MPS II cases — this means the cognitive decline, behavioral changes, and neurologic deterioration that define the disease have remained largely untreatable. Tividenofusp alfa-eknm is engineered to address exactly this gap.

How Does Tividenofusp Alfa Cross the Blood-Brain Barrier?

Tividenofusp alfa-eknm is a fusion protein combining recombinant human iduronate-2-sulfatase with an antibody component that targets the transferrin receptor expressed on the endothelial cells lining brain capillaries. Transferrin receptors normally shuttle iron into the brain, and researchers have exploited this natural transport system as a 'molecular Trojan horse' to deliver therapeutic proteins across the blood-brain barrier. Once inside the central nervous system, the IDS enzyme can be taken up by neurons and glial cells, where it degrades the accumulated glycosaminoglycans inside lysosomes.

This receptor-mediated transcytosis approach represents a significant pharmacological advance for lysosomal storage diseases more broadly. For families affected by Hunter syndrome — many of whom have watched existing enzyme therapies preserve physical function while neurologic decline continued — the approval offers the first regulated treatment option specifically indicated for the neurologic manifestations of the disease. The FDA approval follows the agency's broader pattern of accelerated pathways for rare pediatric diseases with high unmet need.

What Should Families and Clinicians Know About the New Approval?

Avlayah is administered by healthcare professionals at centers experienced in managing lysosomal storage diseases. As with other enzyme replacement therapies, infusion-associated reactions and the potential for anti-drug antibody development are known class considerations that clinicians monitor closely. Patients already receiving idursulfase for systemic disease will need individualized discussions with their metabolic specialists regarding how the new therapy fits into their overall care plan.

Comprehensive care for Hunter syndrome continues to require a multidisciplinary team spanning genetics, neurology, cardiology, otolaryngology, orthopedics, and rehabilitation. Newborn screening for MPS II has been expanding in the United States in recent years, and earlier diagnosis is expected to become increasingly important as disease-modifying therapies targeting the central nervous system become available — since the greatest benefit is likely to come from initiating treatment before irreversible neurologic damage has occurred.