Mucopolysaccharidosis Type II (Hunter Syndrome)

Overview

Mucopolysaccharidosis type II (MPS II), or Hunter syndrome, is an X-linked lysosomal disorder caused by harmful variants in the IDS gene. The resulting shortage of iduronate-2-sulfatase (IDS) allows complex sugars called glycosaminoglycans—especially heparan sulfate and dermatan sulfate—to accumulate in cells and progressively damage the skeleton, joints, airway, heart, liver, hearing, and, in the neuronopathic form, the brain. It affects almost exclusively boys, although severity varies widely. GeneReviews: Mucopolysaccharidosis Type II (ncbi.nlm.nih.gov)

In neuronopathic MPS II, developmental regression and progressive cognitive impairment commonly accompany severe physical disease, and death often occurs in the first or second decade of life; people with non-neuronopathic disease may retain normal intelligence and survive into adulthood. GeneReviews: Mucopolysaccharidosis Type II (ncbi.nlm.nih.gov) Standard care remains multidisciplinary and includes weekly intravenous idursulfase enzyme-replacement therapy (ERT) for systemic disease, alongside management of airway, cardiac, orthopedic, hearing, neurologic, and developmental complications. Idursulfase does not reach the brain in sufficient amounts to treat neurologic disease. GeneReviews: Mucopolysaccharidosis Type II (ncbi.nlm.nih.gov) In the United States, tividenofusp alfa-eknm (Avlayah) became the first approved therapy directed at neurologic manifestations on March 25, 2026, but it is not a genetic cure. FDA approval of Avlayah (fda.gov)

Scope of Recent Research (2020–present)

Research since 2020 has been active and increasingly focused on the central challenge of MPS II: restoring IDS throughout the body while also reaching the brain early enough to prevent irreversible neurodevelopmental injury. The leading approaches are brain-penetrant ERT, one-time viral gene replacement, and blood-stem-cell gene therapy; gene editing remains an important proof-of-concept but has not yet produced a curative result. As of October 24, 2026, no therapy has demonstrated a durable clinical cure for MPS II, although several programs aim to alter its lifelong course rather than simply manage symptoms. An intravenous brain-penetrant enzyme therapy (nejm.org) First-in-human in vivo genome editing (pmc.ncbi.nlm.nih.gov)

Major Breakthroughs and Emerging Therapies

The most immediate breakthrough is brain-penetrant ERT. Tividenofusp alfa joins IDS to an engineered antibody-like domain that binds the transferrin receptor, a transport system on the blood-brain barrier. In its phase 1/2 study of 47 boys, cerebrospinal-fluid heparan sulfate fell by approximately 91% and urinary heparan sulfate by approximately 88% at 24 weeks; longer follow-up suggested sustained biomarker reductions, stabilization or improvement in adaptive behavior, and normalized or stable liver volume. An intravenous brain-penetrant enzyme therapy (nejm.org) FDA granted accelerated approval for pediatric patients weighing at least 5 kg who are presymptomatic or symptomatic but not yet advanced neurologically, based on cerebrospinal-fluid heparan sulfate as a surrogate endpoint reasonably likely to predict benefit. FDA approval of Avlayah (fda.gov)

A related transferrin-receptor strategy, pabinafusp alfa (JR-141; Izcargo), has been used in Japan since 2021. It fuses IDS to an anti-transferrin-receptor antibody to carry enzyme across the blood-brain barrier. A 52-week phase 2/3 study in 28 Japanese patients reported reduced cerebrospinal-fluid heparan sulfate and assessed neurodevelopmental outcomes, establishing an important earlier clinical precedent for receptor-mediated brain delivery in MPS II. Phase 2/3 pabinafusp alfa trial (pubmed.ncbi.nlm.nih.gov)

The most advanced potential one-time gene-replacement therapy is clemidsogene lanparvovec (RGX-121), an adeno-associated virus (AAV) vector intended to deliver a working IDS gene directly to the central nervous system. In the pivotal portion of the CAMPSIITE phase I/II/III study, 13 participants had a median 82% reduction in cerebrospinal-fluid heparan-sulfate D2S6 at one year, with reported developmental skill acquisition or stability on assessed Bayley subscales. CAMPSIITE 12-month data (regenxbio.gcs-web.com) However, this program is not currently an available treatment: on August 24, 2026, FDA placed the trial on clinical hold after expanded MRI monitoring identified asymptomatic spinal nodules or small cystic masses in five previously treated participants, and REGENXBIO said it did not expect near-term resubmission of its biologics license application. REGENXBIO regulatory update (ir.regenxbio.com)

A distinct potentially curative approach is ex vivo hematopoietic stem-cell gene therapy. In this method, a child’s own CD34-positive blood-forming stem cells are collected, given a functional IDS gene using a lentiviral vector, and returned after conditioning treatment; their descendant myeloid cells are intended to make and distribute IDS, including in the brain. The University of Manchester’s phase I/II study uses an IDS enzyme tagged with ApoEII, a peptide intended to improve tissue and brain uptake. Manchester HSC gene-therapy trial (clinicaltrials.gov) Preclinical work has developed a manufacturing process suitable for clinical-grade production and found that optimized lentiviral HSC gene therapy could address multisystem disease in MPS II models. GMP HSC gene-therapy manufacturing for MPS II (pubmed.ncbi.nlm.nih.gov)

In vivo genome editing has also been tested. Sangamo’s SB-913 used AAV-delivered zinc-finger nucleases to insert an IDS sequence into the albumin locus of liver cells, aiming to turn the liver into a long-term source of enzyme after one infusion. The phase I/II study enrolled nine people with MPS II and was completed in 2021; the subsequent report found evidence of intended editing in some liver samples but variable biological activity and hepatobiliary adverse events, so the study did not establish a clinical cure. First-in-human in vivo genome editing (pmc.ncbi.nlm.nih.gov)

Clinical Trials and Experimental Approaches

Tividenofusp alfa’s phase 1/2 program was sponsored by Denali Therapeutics and led to U.S. accelerated approval in March 2026. The approval-supporting trial enrolled 47 pediatric participants, while FDA stated that Denali was conducting a randomized confirmatory trial that was more than 95% enrolled at the time of approval. FDA approval of Avlayah (fda.gov) The ongoing COMPASS phase 2/3 study compares tividenofusp alfa with idursulfase in children with neuronopathic and non-neuronopathic MPS II. COMPASS phase 2/3 registry entry (orpha.net)

RGX-121 had generated encouraging biomarker and developmental data in CAMPSIITE before its 2026 clinical hold, illustrating both the promise and the safety-monitoring burden of CNS-directed AAV treatment. CAMPSIITE 12-month data (regenxbio.gcs-web.com) The University of Manchester’s autologous HSC gene-therapy trial is active but not recruiting, plans to enroll five boys aged 3 to 22 months with predicted neuronopathic disease, and includes at least two years of study follow-up plus longer-term monitoring. Manchester HSC gene-therapy trial (clinicaltrials.gov) The University reported that its first treated child had high circulating IDS levels and no longer required weekly Elaprase infusions, but this early observation is not yet peer-reviewed evidence of lasting clinical efficacy. University of Manchester clinical-study update (manchester.ac.uk)

Methodologies and Scientific Approaches

Researchers measure potential benefit using complementary biological and clinical endpoints. Cerebrospinal-fluid heparan sulfate, including the D2S6 disaccharide, is used as a biomarker of brain substrate storage; urine glycosaminoglycans, liver volume, enzyme activity, neurodevelopmental testing, adaptive behavior, mobility, hearing, and brain imaging provide additional evidence about systemic and neurologic effects. An intravenous brain-penetrant enzyme therapy (nejm.org) CAMPSIITE 12-month data (regenxbio.gcs-web.com)

The field uses mouse disease models, patient-derived blood-forming cells, viral-vector engineering, receptor-mediated transcytosis across the blood-brain barrier, and long-term imaging and immune surveillance. The HSC approach seeks to exploit “cross-correction,” in which enzyme made by corrected cells can be taken up by neighboring deficient cells, while AAV approaches seek sustained IDS production inside the central nervous system itself. Empowered HSC gene therapy for MPS II (pmc.ncbi.nlm.nih.gov) REGENXBIO regulatory update (ir.regenxbio.com)

Leading Institutions and Funding

Major clinical and translational contributors include Denali Therapeutics, JCR Pharmaceuticals, REGENXBIO, Nippon Shinyaku, the University of Manchester and Manchester University NHS Foundation Trust, and international metabolic-disease centers participating in trials. FDA approval of Avlayah (fda.gov) Manchester HSC gene-therapy trial (clinicaltrials.gov)

Investment reflects the high cost of late-stage rare-disease development. In December 2025, Denali entered a synthetic-royalty financing agreement worth up to $275 million based on future tividenofusp alfa sales. Denali–Royalty Pharma funding agreement (sec.gov) In 2025, REGENXBIO and Nippon Shinyaku formed a collaboration for RGX-121 and RGX-111 that included $110 million upfront and potential milestone payments of up to $700 million, although RGX-121’s subsequent clinical hold makes its path uncertain. REGENXBIO–Nippon Shinyaku collaboration (ir.regenxbio.com) REGENXBIO regulatory update (ir.regenxbio.com)

Strengths, Limitations, and Challenges

The strongest recent advance is that intravenous treatment can now lower a brain-relevant substrate biomarker without requiring repeated injections into cerebrospinal fluid. Tividenofusp alfa’s approval provides a clinically meaningful option for eligible children and validates the principle of transferrin-receptor-mediated brain delivery. FDA approval of Avlayah (fda.gov) Nevertheless, approval was accelerated and based on a surrogate biomarker rather than definitive randomized evidence of long-term cognitive benefit; the confirmatory trial is therefore essential. FDA approval of Avlayah (fda.gov)

Gene and cell therapies offer the possibility of a one-time, durable enzyme source, but they face substantial obstacles: pre-existing immunity to viral vectors, inflammatory or liver-related toxicity, conditioning-related risks for stem-cell therapy, variable disease severity, small trial populations, uncertainty about the best cognitive endpoints, and the need for follow-up lasting many years. The 2026 RGX-121 hold is a clear reminder that apparently benign imaging findings can still delay a program until their cause and long-term significance are understood. First-in-human in vivo genome editing (pmc.ncbi.nlm.nih.gov) REGENXBIO regulatory update (ir.regenxbio.com) Access, manufacturing capacity, specialized-center requirements, and likely high prices will also influence whether advances reach patients equitably. Denali–Royalty Pharma funding agreement (sec.gov)

Outlook and Future Directions

MPS II is closer than it was in 2020 to treatments that may substantially change neurologic and systemic disease, but it is not yet close to a proven cure. The key milestones to watch are randomized clinical-benefit results for tividenofusp alfa, durable safety and neurodevelopmental outcomes from the Manchester HSC gene-therapy study, and whether additional imaging and follow-up can clarify the benefit-risk profile and regulatory future of RGX-121. A true cure will require sustained IDS restoration across body and brain, treatment early enough to prevent injury, and long-term evidence that benefits outweigh treatment risks. COMPASS phase 2/3 registry entry (orpha.net) Manchester HSC gene-therapy trial (clinicaltrials.gov) REGENXBIO regulatory update (ir.regenxbio.com)

References

• GeneReviews: Mucopolysaccharidosis Type II — GeneReviews/NCBI Bookshelf, 2026.
• An intravenous brain-penetrant enzyme therapy — Muenzer et al., New England Journal of Medicine, 2026.
• FDA approval of Avlayah — U.S. Food and Drug Administration, 2026.
• Phase 2/3 pabinafusp alfa trial — Okuyama et al., Molecular Therapy, 2021.
• CAMPSIITE 12-month data — REGENXBIO, 2025.
• REGENXBIO regulatory update — REGENXBIO, 2026.
• Manchester HSC gene-therapy trial — ClinicalTrials.gov, 2026.
• GMP HSC gene-therapy manufacturing for MPS II — Das et al., Molecular Therapy Methods & Clinical Development, 2024.
• First-in-human in vivo genome editing — Sharma et al., Molecular Therapy, 2022.
• COMPASS phase 2/3 registry entry — Orphanet, 2026.
• University of Manchester clinical-study update — University of Manchester, 2025.
• Empowered HSC gene therapy for MPS II — Das et al., Molecular Therapy Methods & Clinical Development, 2024.
• Denali–Royalty Pharma funding agreement — Denali Therapeutics and Royalty Pharma, 2025.
• REGENXBIO–Nippon Shinyaku collaboration — REGENXBIO and Nippon Shinyaku, 2025.