Hunter syndrome: 2026 so far

Evidence shifted from feasibility to longer interim real-world outcomes.

In the months covered, real-world follow-up in Japanese boys linked intracerebroventricular idursulfase beta (enzyme delivered into a brain fluid space) to a drop in CSF heparan sulfate by week 24 that stayed reduced through week 100. In that report, starting earlier (<=3 years) tracked with more typical developmental progress. Earlier in 2026, a 3-child series that combined brain-directed enzyme delivery with cord blood transplant showed CSF heparan sulfate going up and down when treatment was paused and then restarted. This supported CSF heparan sulfate as a short-term readout of treatment changes. Across reports, side effects were common. Some serious events were tied to the implanted device/procedure, including infections such as bacterial meningitis (a serious infection of the brain and spinal cord lining) and device infection. In the interim Japanese analysis, no deaths or anaphylaxis (a severe allergic reaction) were reported, which helps set expectations about what has been seen so far for CSF biomarkers and the added monitoring and risks that come with device-based brain delivery.

Research covered in: July 2026, September 2026

Reports added real-world comparisons and highlighted device-related burdens.

In January 2026, a large real-world registry compared the same patients during in-clinic versus home periods. It found home idursulfase infusions had broadly similar reported safety/tolerability, with similar infusion-reaction rates (mostly mild) and fewer missed infusions during home therapy. In February 2026, long follow-up of intrathecal idursulfase-IT (enzyme delivered into spinal fluid) added to weekly IV idursulfase was described as generally tolerable, with no deaths or discontinuations. However, some serious events were linked to the implanted delivery device, including infection. In May 2026, an expert consensus also raised home infusions as an option for selected patients, while emphasizing structured follow-up and long-term planning. Together, these reports repeatedly treated the delivery setting and delivery hardware as major drivers of day-to-day burden and safety considerations, separate from whether enzyme therapy works.

Research covered in: January 2026, February 2026, May 2026

Longer follow-up and switch experiences added routine-care context.

In July 2026, an open-label extension (a study where everyone receives the drug and both patients and clinicians know it) and a small retrospective switch series (a look back at care records) reported that urinary GAG levels stayed lower on idursulfase beta and 6-minute walk distance tended to stay stable or improve. This included people who switched from idursulfase during a shortage. No obvious new safety problems were reported in these groups. The extension report also described anti-drug antibodies (immune proteins that can bind the medicine) peaking early and then declining. Neutralizing antibodies (antibodies that may reduce drug activity) were present in a minority at different time points. The retrospective switch report did not measure antibodies. Separately, September 2026 added lab groundwork for an ELISA (a lab test that measures specific antibodies) intended to detect IgE antibodies to idursulfase beta, without reporting how often IgE occurs in patients or what it means clinically. This adds more descriptive, real-world information on continued idursulfase beta use and switching, while also expanding tools to track immune responses over time.

Research covered in: July 2026, September 2026

Patient-reported priorities and small supportive interventions gained visibility.

In June 2026, interview studies helped spell out day-to-day burdens. Caregivers of young children with neuronopathic MPS II (mucopolysaccharidosis type II; another name for Hunter syndrome) described cognitive decline and family disruption as universal in their interviews. Communication and movement limitations were often among the most bothersome issues. Adults and caregivers also described wide variation in pediatric-to-adult transition experiences, with continuity and a clear point person repeatedly noted as helpful. In August 2026, a very small crossover feasibility study (a study where the same people try two periods of care) in five adult men linked supervised hydrotherapy (guided pool-based exercise) with improved quality-of-life domains and reduced pain interference during the hydrotherapy period. No adverse effects were reported during sessions, but the design and size could not show causation. These reports sharpen which symptoms and care-process gaps families most want addressed, while keeping expectations cautious about early supportive-therapy findings in adults.

Research covered in: June 2026, August 2026

Second-tier markers and clearer follow-up steps reduced ambiguity focus.

In April 2026, a large pilot newborn screening program reported many first-tier positives but few confirmed Hunter syndrome cases. A second-tier blood GAG-related marker (NRE GAG ratio) was elevated only in confirmed cases. A separate report emphasized fractionated urine GAG testing (breaking urine GAGs into parts for a clearer pattern) to resolve unclear follow-up results. In June 2026, a proficiency testing program across 71 laboratories reported 98% acceptable/correct results, while still noting occasional misassignments, including MPS I vs MPS II. This highlighted the need for ongoing quality checks. In August 2026, an expert review laid out a step-by-step US follow-up pathway after an abnormal screen, including tiered testing, confirmatory enzyme/urine testing, genetic testing, rapid referral, and coordinated communication. Overall, families facing an abnormal screen were increasingly discussed in terms of structured, multi-step testing meant to reduce false positives and shorten time to clarity, while still recognizing that lab and program variability exists.

Research covered in: April 2026, June 2026, August 2026