Enzyme replacement therapy for Hurler syndrome
Enzyme replacement therapy (ERT) with laronidase is an important part of treatment for mucopolysaccharidosis type I (MPS I), including Hurler syndrome (MPS I-H). It replaces the missing enzyme, alpha-L-iduronidase (IDUA), in the bloodstream and many body tissues, helping cells to clear stored glycosaminoglycans (GAGs). For children with severe MPS I-H, ERT is usually used alongside haematopoietic stem cell transplantation (HSCT); for attenuated MPS I and patients not eligible for transplant, it is a key long-term therapy.
This page explains how laronidase works, how infusions are given, what benefits and limitations to expect, and how ERT fits into modern treatment strategies for Hurler syndrome.
What ERT does in MPS I
In MPS I, the IDUA enzyme is missing or not working properly, so certain GAGs (dermatan and heparan sulphate) build up in cells and tissues. Laronidase is a recombinant (laboratory-made) form of IDUA. When given by intravenous infusion:
- It circulates in the bloodstream
- Enters cells via specific surface receptors (mannose-6-phosphate receptors)
- Helps lysosomes break down stored GAGs
- Over time, this can reduce organ enlargement and improve many somatic (body) features of the disease.
When ERT is recommended
ERT with laronidase is used in different ways depending on disease severity and transplant eligibility:
Severe MPS I-H (Hurler phenotype)
ERT is typically started soon after diagnosis as a bridge to HSCT, to improve clinical status and reduce GAG burden before transplant.
It may be continued for a period after HSCT while engraftment stabilises, then reviewed.
Attenuated MPS I (Hurler–Scheie / Scheie)
For patients who are not candidates for HSCT, ERT is usually the main disease-specific treatment, combined with multidisciplinary supportive care.
Severe MPS I-H (Hurler phenotype)
ERT may be used long-term to support organ function and quality of life, even though it cannot fully stop neurological progression.
Treatment decisions are individual and made by metabolic and transplant specialists together with families.
What to expect from ERT infusions
Dose and schedule (typical current practice):
- Laronidase is given as a once-weekly intravenous infusion
- The dose is based on body weight (mg/kg) according to product labelling and guidelines
- Each infusion usually runs over several hours, especially early on
Practical aspects for families:
- Infusions may be delivered in hospital, a day unit, or in some regions at home by trained nurses.
- Many children have a central venous line or port (especially if also having HSCT or other intensive treatments) to avoid repeated cannulation.
- Vital signs (heart rate, blood pressure, temperature, oxygen saturation) are checked regularly during infusions.
- Your metabolic centre will give detailed instructions about fasting, pre-medication, and what to do if a dose is missed.
What ERT can help with
Clinical studies and long-term observational data in MPS I show that laronidase can:
- Reduce liver and spleen size
- Improve or stabilise respiratory function and exercise tolerance in many patients
- Lower urinary GAG excretion, reflecting reduced storage burden
- Support improvements in endurance, fatigue and general wellbeing in some individuals
- Contribute to better pulmonary and cardiac status around the time of HSCT when used as bridging therapy
Benefits usually develop gradually over months; regular assessment helps track which domains are improving for each child.
What ERT cannot fully do
Despite its benefits, ERT has important limitations that families and clinicians need to understand:
ERT can…
Improve somatic (body) features
Reduce organ enlargement
Support respiratory and cardiac function
Lower GAG storage burden
ERT cannot…
Adequately cross the blood–brain barrier
Prevent neurocognitive decline in severe MPS I-H
Fully fix established skeletal abnormalities
Replace HSCT for eligible children
Does not adequately cross the blood–brain barrier
Laronidase in its current form cannot reliably reach the central nervous system.
It therefore cannot prevent neurocognitive decline on its own in severe MPS I-H.
This is why HSCT remains the standard of care for eligible children with Hurler syndrome.
Limited effect on skeletal disease
Established skeletal abnormalities (kyphosis, hip dysplasia, genu valgum, short stature) respond poorly to ERT alone.
Many patients still require orthopaedic surgery and long-term physical therapy.
Only partial impact on some long-standing manifestations
Corneal clouding, joint contractures and advanced valve disease often improve little, if at all, with ERT alone.
ERT is best thought of as a systemic somatic therapy, complementary to HSCT and multidisciplinary care, rather than a cure.
Safety, immune responses and monitoring
Most patients tolerate laronidase well, but some develop:
Infusion-related reactions
Symptoms can include fever, flushing, rash, headache, vomiting or breathing difficulties during or shortly after the infusion.
These are often managed by slowing the infusion, giving antihistamines and/or steroids, and adjusting pre-medication.
Anti-drug antibodies (ADAs)
Many patients develop antibodies against laronidase; in some, high titres are associated with more infusion reactions and possibly reduced biochemical response.
Centres may monitor antibody levels and clinical response, especially if effectiveness seems to drop or reactions increase.
Serious allergic reactions are rare but can occur; teams have protocols in place to recognise and manage them quickly.
Safety, immune responses and monitoring
Most patients tolerate laronidase well, but some develop:
Infusion-related reactions
Symptoms can include fever, flushing, rash, headache, vomiting or breathing difficulties during or shortly after the infusion.
These are often managed by slowing the infusion, giving antihistamines and/or steroids, and adjusting pre-medication.
Anti-drug antibodies (ADAs)
Many patients develop antibodies against laronidase; in some, high titres are associated with more infusion reactions and possibly reduced biochemical response.
Centres may monitor antibody levels and clinical response, especially if effectiveness seems to drop or reactions increase.
Decisions about starting, stopping or re-starting ERT around HSCT are made jointly by metabolic and transplant teams, based on enzyme levels, chimerism, organ status and clinical course.
ERT as a main therapy
For people with attenuated MPS I (Hurler–Scheie / Scheie) and for those with severe MPS I-H who are not candidates for HSCT:
ERT may be continued lifelong, with periodic review of benefits, burden and safety.
Treatment plans may be adjusted over time; some patients have dose interruptions or changes in regimen based on response, logistics or new health issues.
Regular assessments track:
- Endurance (e.g. 6-minute walk test where age-appropriate)
- Respiratory function and sleep
- Cardiac valve status and ventricular function
- Liver/spleen size and GAG levels
- Pain, joint mobility and quality of life
Questions you can ask your team
- Why is ERT being recommended for my child now?
- Is it being used as a bridge to HSCT, as long-term therapy, or both?
- Where will infusions take place – hospital, day unit, or home?
- Do we need a central line or port, and what are the pros and cons?
- What side effects should we watch for during and after infusions?
- How will we know whether ERT is helping – which tests or measurements will you track?
- What happens to ERT if my child goes on to have HSCT, or if we consider a clinical trial in the future?
It is completely reasonable to revisit these questions over time as your child grows and circumstances change.
Clinical points for using ERT in Hurler syndrome
- In severe MPS I-H, ERT should generally be viewed as adjunct and bridge to HSCT, not a substitute for transplant in eligible children.
- Start ERT promptly after diagnosis if HSCT is planned, after discussion with the transplant centre.
- In attenuated MPS I or in non-transplant candidates, ERT is a central long-term therapy with structured monitoring.
- Be vigilant for infusion reactions; use pre-medication and rate adjustments as needed.
- Reassess the risk-benefit balance periodically and coordinate multidisciplinary care.
Key points about ERT in Hurler syndrome
- Laronidase provides the missing IDUA enzyme systemically and improves many somatic manifestations of MPS I.
- It is given as a weekly intravenous infusion, often for years, with careful monitoring for infusion reactions and antibodies.
- ERT does not adequately treat the brain and has limited effect on established skeletal disease, so it cannot replace HSCT for most children with severe MPS I-H.
- In severe MPS I-H, ERT is widely used as bridging and peri-transplant therapy; in attenuated MPS I and non-transplant patients, it is often the main long-term disease-specific treatment.
- ERT works best when combined with multidisciplinary care, including cardiac, respiratory, orthopaedic, ENT, ophthalmology, dental and neurodevelopmental support.
What to read next
Haematopoietic stem cell transplant (HSCT)
How transplant works, timing, benefits and risks
Current treatments overview
How HSCT, ERT and supportive care fit together
Multidisciplinary care
Cardiac, respiratory, orthopaedic, ENT, eye, dental and rehab support
Emerging therapies and research
Gene therapy and new treatment approaches