FDA Approves Tividenofusp Alfa-eknm for Neurologic Manifestations of Hunter Syndrome
Key Clinical Summary
- The US Food and Drug Administration (FDA) granted accelerated approval of tividenofusp alfa-eknm (Avlayah) for pediatric Hunter syndrome (MPS II) with neurologic involvement.
- Phase 1/2 data showed a 91% mean reduction in CSF heparan sulfate at 24 weeks, a surrogate biomarker.
- Safety considerations include a boxed warning for anaphylaxis and recommended monitoring for anemia and renal complications.
The US Food and Drug Administration (FDA) has approved Denali Therapeutics’ tividenofusp alfa-eknm (Avlayah) for treating neurologic manifestations of Hunter syndrome (Mucopolysaccharidosis type II, MPS II) in pediatric patients. The decision is based on surrogate biomarker data demonstrating reductions in cerebrospinal fluid (CSF) heparan sulfate, a key pathogenic substrate in the disease.
Regulatory Context
Hunter syndrome is a rare X-linked lysosomal storage disorder affecting approximately 500 individuals in the United States, primarily males. It results in the accumulation of glycosaminoglycans, leading to progressive multi-organ dysfunction, including neurologic decline.
Tividenofusp alfa-eknm is administered as a once-weekly intravenous infusion and is indicated for presymptomatic or symptomatic pediatric patients weighing at least 5 kg, prior to advanced neurologic impairment. The FDA granted accelerated approval based on a phase 1/2 multi-cohort, single-arm, open-label trial involving 47 pediatric patients aged 3 months to 13 years.
Among 44 patients evaluable at week 24, treatment with tividenofusp alfa-eknm led to a mean 91% reduction in CSF heparan sulfate from baseline, with reductions ranging from 72% to 98%. While no patients had CSF heparan sulfate levels below the upper limit of normal (ULN) at baseline, 93% achieved levels below ULN by week 24.
The approval relies on this surrogate endpoint, which regulators determined is “reasonably likely” to predict clinical benefit. A randomized clinical trial, currently more than 95% enrolled, is ongoing to confirm clinical outcomes.
The drug carries a boxed warning for allergic reactions, including anaphylaxis. Common adverse events include upper respiratory infections, fever, anemia, gastrointestinal symptoms, rash, and headache. Recommended monitoring protocols include hemoglobin assessments and renal function surveillance due to risks of anemia and membranous nephropathy.
In addition to accelerated approval, the drug received breakthrough, fast track, priority review, and orphan drug designations for this indication.
Clinical Implications
The drug’s approval represents a significant development in the management of Hunter syndrome, particularly for its neurologic manifestations, which have lacked targeted therapies. Early intervention prior to advanced neurologic impairment is emphasized, aligning with the drug’s indication and mechanism.
The reliance on a surrogate biomarker also underscores the importance of ongoing clinical validation, but the magnitude of CSF heparan sulfate reduction suggests meaningful biologic activity.
Expert Commentary
“Avlayah is the first product approved to address neurologic complications of Hunter syndrome,” said Tracy Beth Hoeg, MD, PhD, Acting Director of the FDA’s Center for Drug Evaluation and Research (CDER), in a press release. She emphasized that while the accelerated approval was based on surrogate endpoint data, ongoing research will further evaluate outcomes.
“The FDA is capable of doing two things: one, exercising regulatory flexibility; and two, complying with our obligation under the law to approve drugs based on ‘substantial evidence’ of effectiveness,” added FDA Commissioner Marty Makary, MD, MPH. “We will continue to do everything we can to accelerate treatments for rare diseases.”
