What are the symptoms of Hunter Syndrome?

The most common symptoms of Hunter Syndrome include Developmental delay and behavioral problems, Coarse facial features and thickened skin, Growth retardation and short stature, Hearing loss, often conductive and sensorineural, Joint stiffness and reduced mobility, among others. Hunter Syndrome affects approximately 1 in 100,000 to 150,000 males; females with disease are rare. Symptoms can range from mild to severe and may vary between individuals. Early diagnosis through genetic testing and clinical evaluation is important for management and treatment planning.

How can I find clinical trials for Hunter Syndrome?

Trial Friend lists all active clinical trials for Hunter Syndrome from ClinicalTrials.gov, with eligibility criteria translated into plain English. Patients can filter by trial phase and recruitment status, search by location, and use the Match Me tool to score trials against their profile. Hunter syndrome treatment has evolved with enzyme replacement, and research continues advancing therapeutic approaches. On March 24, 2026 the FDA granted accelerated approval to tividenofusp alfa (Avlayah), a weekly IV enzyme therapy built to cross the blood-brain barrier, to treat the brain-related (neurologic) effects of Hunter syndrome in children who weigh at least 5 kg and do not yet have advanced neurologic impairment.

Are there FDA-approved treatments for Hunter Syndrome?

Yes. As of September 2026, Trial Friend lists 2 FDA-approved drugs used to treat Hunter Syndrome: Elaprase (idursulfase), first approved in 2006; and Avlayah (tividenofusp alfa), first approved in 2026. Each has its own page with what the label covers, side effects and how it is given, and the Treatments section of this page adds any further approved drugs found in FDA labeling data.

What phase are Hunter Syndrome clinical trials in?

Hunter Syndrome clinical trials span multiple development stages: Phase 1 tests safety in small groups, Phase 2 evaluates effectiveness, Phase 3 compares to existing treatments in larger populations, and Phase 4 monitors long-term effects after approval. Trial Friend shows each trial's phase, recruitment status, timeline, and estimated completion date, updated in real time from ClinicalTrials.gov.

Is genetic testing available for Hunter Syndrome?

Yes. Genetic testing for Hunter Syndrome looks at the IDS gene. Almost exclusively affects males; female carriers are rarely symptomatic. It is X-linked, so it mainly affects boys, and a mother who carries the variant has a 1 in 2 chance of passing it to each son. A confirmed variant establishes the diagnosis and is often required to enroll in gene-specific trials. The labs currently offering the test are listed in the Genetic Testing section of this page, pulled live from the NIH Genetic Testing Registry. A genetic counselor can help interpret the result and decide which relatives should be tested.

What causes Hunter Syndrome?

Hunter Syndrome is caused by variants in the IDS gene. Almost exclusively affects males; female carriers are rarely symptomatic. It is X-linked, so it mainly affects boys, and a mother who carries the variant has a 1 in 2 chance of passing it to each son. The condition affects approximately 1 in 100,000 to 150,000 males; females with disease are rare. Genetic testing confirms the diagnosis.

Am I eligible for Hunter Syndrome clinical trials?

Eligibility for Hunter Syndrome clinical trials depends on factors like age, diagnosis, treatment history, and overall health. Each trial sets its own inclusion and exclusion criteria. Trial Friend translates these requirements from medical language into plain English so patients can review them more easily. The Match Me tool can score your profile against active Hunter Syndrome trials to help identify studies that may be a fit. Always confirm eligibility with the study team before enrolling.

What should I expect in a Hunter Syndrome clinical trial?

Participating in a Hunter Syndrome clinical trial typically involves scheduled visits, medical tests, and close monitoring by a research team. Depending on the study phase, you may receive an investigational treatment, a placebo, or the current standard of care. Trials may last from a few weeks to several years. Study participation is voluntary and you can withdraw at any time. Trial Friend provides details on each study's design, timeline, and estimated completion date to help you plan ahead.

Metabolic & Lysosomal

Hunter Syndrome (MPS II) Clinical Trials and Treatments

Also called mucopolysaccharidosis type II, MPS II, iduronate-2-sulfatase deficiency

Hunter syndrome (mucopolysaccharidosis type II) is an X-linked lysosomal storage disorder caused by deficiency of iduronate-2-sulfatase, an enzyme essential for degrading heparan sulfate and dermatan sulfate glycosaminoglycans. Without adequate enzyme activity, these substrates accumulate within lysosomes in cells throughout the body.

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About Hunter Syndrome

Without adequate enzyme activity, these substrates accumulate within lysosomes in cells throughout the body. The severe form typically manifests between ages 2-4 years with developmental delay, coarse facial features, hepatosplenomegaly, joint stiffness, and progressive neurological decline including intellectual disability and behavioral problems. Hearing loss is nearly universal and often requires early intervention.

The attenuated form has later onset (4-10 years), slower progression, normal or near-normal intellectual development, and longer lifespan. Both forms eventually cause progressive multisystem disease affecting growth, mobility, hearing, cardiac function, and cognition. Enzyme replacement therapy (idursulfase) can improve some manifestations, slow disease progression, and improve lifespan, particularly when started early.

Common Symptoms of Hunter Syndrome

Recognizing the signs of Hunter Syndrome early can lead to faster diagnosis and better outcomes. Symptoms may vary in severity from person to person. If you or a loved one are experiencing any of the following, consider speaking with a specialist.

Developmental delay and behavioral problems
Coarse facial features and thickened skin
Growth retardation and short stature
Hearing loss, often conductive and sensorineural
Joint stiffness and reduced mobility
Progressive intellectual disability in severe form

Who Hunter Syndrome Affects

X-linked recessive inheritance means primarily males are affected; affected females are very rare. Severe form typically presents ages 2-4; attenuated form has later onset (4-10 years) with slower progression. No ethnic predisposition.

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FDA-Approved Treatments for Hunter Syndrome

There are currently 2 FDA-approved medications for Hunter Syndrome. These therapies represent the current standard of care and may be used alongside or compared against investigational treatments in active clinical trials.

Avlayah

Drug Decoder

tividenofusp alfa

Denali Therapeutics

Elaprase

Drug Decoder

idursulfase

Takeda

Official site

Source: openFDA drug labeling data. This list may not include all treatments. Always consult your doctor.

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Help Paying for Hunter Syndrome Treatment

Charity funds and drugmaker programs for Hunter Syndrome, checked at the source. Pick your insurance to see what fits.

Your insurance

Charity funds

From a charity · NORD RareCare
Hunter Syndrome Medical Assistance fund
Open
Apply on the foundation’s page
Pays for: Medical and medication costs.
The foundation says: “Accepting new applications and re-enrollments for current year”
From a charity · NORD RareCare
Hunter Syndrome Premium Copay Assistance fund
Open
Apply on the foundation’s page
Pays for: Insurance premiums and copays.
The foundation says: “Accepting new applications and re-enrollments for current year”
From a charity · National MPS Society
Family Assistance Program fund
Apply directly
Apply on the foundation’s page
Pays for: Specialized equipment and medical aids not covered by insurance (requires insurance denial and 10% family copay), up to $3,000 per year.
The foundation says: “Status not shown on page”
From a charity · National MPS Society
Medical Travel Assistance Program fund
Apply directly
Apply on the foundation’s page
Pays for: Travel to out-of-town medical appointments more than 125 miles away (per year), up to $550 per year.
The foundation says: “Status not shown on page”
From a charity · National MPS Society
Journey Assistance Program fund
Apply directly
Apply on the foundation’s page
Pays for: Items that ease daily life (40% of purchase price), up to $500 per year.
The foundation says: “Status not shown on page”
From a charity · The Assistance Fund
MPS II - Hunter Syndrome fund
Waitlist
Apply on the foundation’s page
Pays for: Copays, coinsurance, deductibles and other health-related expenses.
The foundation says: “WAITLIST — Accepting Waitlist Patients. TAF is currently accepting requests to join the enrollment waitlist for this program. Waitlists a…”

Status as each foundation showed it on September 28, 2026.

Drugmaker programs

Open a medicine for who qualifies, by insurance type.

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Side Effect Explorer

Real-world side effect reports from the FDA Adverse Event Reporting System (FAERS). Includes both FDA-approved drugs and investigational therapies from active clinical trials. Click any drug to see what patients reported.

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Genetic Testing

Genetic testing can confirm a diagnosis, guide treatment decisions, and identify family members who may be at risk.

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Trusted Hunter Syndrome Resources

Reputable organizations and medical references for learning more about Hunter Syndrome, including disease registries, foundation resources, and clinical guidelines.

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Get notified when new Hunter Syndrome trials open or existing trials change status, add sites, or update eligibility.

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Quick facts

Active Clinical Trials for Hunter Syndrome

Use this Hunter Syndrome clinical trial finder to see the 7 studies recruiting patients in the United States and worldwide, with eligibility criteria in plain English. These studies play a critical role in advancing care for metabolic & lysosomal conditions and may offer access to treatments not yet widely available. Each trial below is sourced directly from ClinicalTrials.gov, with eligibility criteria translated into plain English to help patients and caregivers evaluate whether a study may be a fit.

Trials Site map Pipeline timeline Medication checker

Note: Trial recruitment statuses on ClinicalTrials.gov may not immediately reflect recent FDA decisions, sponsor announcements, or enrollment changes. Always confirm a trial's current status directly with the study coordinator before making plans.

7 active trials worldwide

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RECRUITINGPHASE1Recently updatedNCT02254863

UCB Transplant of Inherited Metabolic Diseases With Administration of Intrathecal UCB Derived Oligodendrocyte-Like Cells

Intervention: DUOC-01

Sponsor: Joanne Kurtzberg, MD

The primary objective of the study is to determine the safety and feasibility of intrathecal administration of DUOC-01 in patients who are undergoing standard treatment with umbilical cord blood transplant (UCBT) for inborn errors of metabolism and who have evidence of early demy...

Ages 1 Week – 22 Years1 location

Started Sep 2014Updated 3 weeks agoEst. Oct 2027 (~1y 1m)

RECRUITINGUpdated a few months agoNCT05619900

Registry of Patients Diagnosed With Lysosomal Storage Diseases

Intervention: There is no intervention

Sponsor: University of California, San Francisco

This is an international prospective and retrospective registry of patients with Lysosomal Storage Diseases (LSDs) to understand the natural history of the disease and the outcomes of fetal therapies, with the overall goal of improving the prenatal management of patients with LSDs.

Ages up to 64 Years1 location

Started May 2022Updated 5 months agoEst. May 2050 (~23y 8m)

RECRUITINGPHASE1No updates in a whileNCT04532047

PEARL (PrEnAtal Enzyme Replacement Therapy for Lysosomal Storage Disorders)

Intervention: Aldurazyme (laronidase)

Sponsor: University of California, San Francisco

For detailed information, please view our study website: https://pearltrial.ucsf.edu/

The investigators aims to determine the the maternal and fetal safety and feasibility of in utero fetal enzyme replacement therapy in fetuses with Lysosomal Storage Diseases.

Ages 18 Years – 50 Years1 location

Started Jul 2021Updated 6 months agoEst. Jul 2031 (~4y 10m)

RECRUITINGNo updates in a whileNCT06036693

MPS (RaDiCo Cohort) (RaDiCo-MPS)

Sponsor: Institut National de la Santé Et de la Recherche Médicale, France

The goal of this observational study is to characterize the epidemiology and natural history of MPS diseases by building a retrospective and prospective collection of extensive phenotypic data from French MPS patients.

Ages not specified23 locations

Started Dec 2017Updated 7 months agoEst. Dec 2026 (~3 months)

RECRUITINGNo updates in a whileNCT03333200

Longitudinal Study of Neurodegenerative Disorders

Intervention: Palliative Care, Hematopoetic Stem Cell Transplantation

Sponsor: University of Pittsburgh

The purpose of this study is to understand the course of rare genetic disorders that affect the brain. This data is being analyzed to gain a better understanding of the progression of the rare neurodegenerative disorders and the effects of interventions.

Ages not specified1 location

Started Jan 2012Updated 7 months agoEst. Jan 2030 (~3y 4m)

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Find Hunter Syndrome trials near you, ranked by distance →

Active trial locations3 cities in the US

Trial Pipeline

Jan 2021 to Sep 2031

2021

2023

2025

2027

2029

2031

now

Phase 1

Phase 2

Observational

RecruitingOpening soonDelayed startTodayHover a bar for trial details

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Can I Join a Hunter Syndrome Clinical Trial While on Enzyme Replacement Therapy?

This medication conflict checker helps Hunter syndrome (MPS II) patients and caregivers find out if current medications could affect clinical trial eligibility. Select medications below to instantly screen active trials for potential conflicts.

Hunter syndrome clinical trials frequently have specific rules about enzyme replacement therapy. Some trials require patients to stop their current ERT before enrollment, while others allow continued treatment as a comparator arm. The transition between Elaprase (idursulfase) and newer therapies like Avlayah (tividenofusp alfa) or investigational gene therapies may involve washout periods that vary by study protocol.

Enzyme Replacement Therapy

Elaprase (idursulfase), Avlayah (tividenofusp alfa), Hunterase (idursulfase beta) — Elaprase, approved by the FDA in 2006, is a weekly intravenous infusion that replaces the missing iduronate-2-sulfatase enzyme but does not cross the blood-brain barrier. Hunterase (idursulfase beta) is an alternative ERT available outside the US. Avlayah, granted accelerated FDA approval in March 2026, uses a transport vehicle platform to deliver the enzyme across the blood-brain barrier into the central nervous system. Gene therapy trials and novel ERT studies commonly exclude patients currently receiving enzyme replacement, or require documented washout periods of 4 to 12 weeks. Trials comparing Avlayah to Elaprase may require patients to be on stable Elaprase dosing for a minimum period before randomization. Antihistamines, corticosteroids, and acetaminophen are standard pre-medications given before each ERT infusion to prevent infusion-associated reactions.

Cardiac Medications

Beta-blockers, ACE inhibitors, ARBs, diuretics — Cardiac involvement is common in Hunter syndrome, with valve disease and cardiomyopathy affecting a majority of patients over time. Medications like metoprolol, atenolol, lisinopril, enalapril, losartan, furosemide, and spironolactone are generally permitted in MPS II clinical trials as standard supportive care, but must be documented at screening. Trials studying cardiac endpoints may require stable cardiac medication dosing for 30 to 90 days before enrollment.

Seizure Medications

Keppra (levetiracetam), Depakote (valproic acid) — Children with neuronopathic Hunter syndrome may develop seizures as the disease progresses. Anticonvulsants are generally permitted in MPS II trials as medically necessary supportive therapy. Most protocols require stable dosing for at least 4 weeks prior to screening. Valproic acid requires particular attention because of its hepatotoxic potential, which can complicate liver-related safety monitoring in trials.

Gene Therapy and Investigational Agents

RGX-121, pabinafusp alfa, stem cell gene therapy — Prior exposure to any investigational gene therapy or CNS-targeting therapy is almost universally excluded from MPS II clinical trials. This includes intrathecal enzyme delivery, adeno-associated virus (AAV) gene therapy such as RGX-121 (clemidsogene lanparvovec, which received an FDA Complete Response Letter in February 2026), and hematopoietic stem cell gene therapy approaches. These exclusions are typically permanent, meaning prior gene therapy recipients cannot enroll regardless of washout time.

Respiratory & Airway Medications

Albuterol, ipratropium, CPAP/BiPAP — Upper airway obstruction and restrictive lung disease are hallmarks of Hunter syndrome. Bronchodilators and mechanical ventilation support (CPAP or BiPAP) are standard supportive care and are generally permitted in clinical trials. However, patients requiring invasive mechanical ventilation or tracheostomy may be excluded from certain trials due to safety monitoring limitations or inability to complete required assessments.

Pain & Anti-Inflammatory Medications

Ibuprofen, naproxen, acetaminophen — Joint stiffness and musculoskeletal pain are common in MPS II patients. Over-the-counter NSAIDs and acetaminophen are generally allowed in Hunter syndrome trials. Chronic opioid use may be flagged in trial screening due to potential interactions with sedation during infusions or neurocognitive assessments.

Hunter syndrome trial eligibility often depends on factors beyond medication history. Age, weight (minimum 5 kg for Avlayah), degree of neurologic involvement, and genetic confirmation of the IDS mutation all play critical roles. For the COMPASS confirmatory trial evaluating Avlayah, patients must have documented MPS II diagnosis and be initiated on treatment prior to advanced neurologic impairment. Early diagnosis through newborn screening in states like Illinois and Missouri can significantly expand the treatment window.

How the medication conflict checker works: This free tool helps Hunter syndrome (MPS II) patients and families learn if their current medications could affect clinical trial eligibility. It scans the published eligibility criteria of every active MPS II trial and flags which ones may exclude your specific treatment. Matches are categorized by confidence level: high confidence means the trial names your exact drug, medium confidence means it references your drug class, and low confidence means it uses broad category language that may or may not apply to you. Select one or more medications above to instantly see which trials you may still qualify for and which ones could be a problem. Always confirm eligibility directly with the study team, as final decisions involve your complete medical history, genetic testing results, neurocognitive assessments, and your metabolic specialist's evaluation.

Across 1,853 open rare disease treatment trials, a third exclude people over a medication they commonly take. See which medications and diseases, in our September 2026 analysis.

Sources: Medication information verified against FDA-approved prescribing labels for Elaprase (BLA 125151) and Avlayah (BLA 761485, accelerated approval March 2026). Clinical trial eligibility patterns derived from ClinicalTrials.gov protocol records. Disease management guidelines referenced from NIH GeneReviews (Mucopolysaccharidosis Type II, Scarpa et al.) and the National MPS Society. Phase 1/2 clinical data from Muenzer et al., New England Journal of Medicine, January 2026.

Data from ClinicalTrials.gov, U.S. National Library of Medicine.
Always talk to your doctor before considering a clinical trial.

Patient Communities

Connect with other Hunter Syndrome patients, caregivers, and advocacy groups across Facebook groups, Reddit communities, and YouTube channels. These patient communities offer peer support, shared experiences, caregiver resources, and real-time discussion about Hunter Syndrome treatments, clinical trial participation, and day-to-day disease management.

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MPS II Hunter Syndrome

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Related Metabolic & Lysosomal Conditions

Other rare diseases in the metabolic & lysosomal category. Patients with Hunter Syndrome may find relevant research, shared treatment pathways, or overlapping clinical trials among these related conditions.