Mucopolysaccharidosis type III, also known as Sanfilippo syndrome, is a rare genetic disorder that primarily affects the brain and spinal cord, causing progressive neurological decline in early childhood. While there is currently no approved cure, ongoing research explores promising treatment approaches including gene therapy and enzyme replacement strategies to address this challenging condition.

Understanding Treatment Goals for MPS III

When families first hear that their child has Mucopolysaccharidosis type III, one of the first questions they ask is about treatment options. The main goal of managing this condition is to improve quality of life, control symptoms as they appear, and provide supportive care that addresses the many challenges children and families face. Because MPS III is a progressive disease that affects multiple body systems—especially the brain and nervous system—treatment approaches must be flexible and adapt as symptoms change over time.^[1]

Treatment decisions depend on several factors, including which subtype of MPS III the child has (types A, B, C, or D), how rapidly the disease is progressing, and which symptoms are most troubling at any given time. Some children experience faster disease progression than others, even within the same subtype, which means that care plans need to be individualized.^[2]

Medical societies and expert panels have developed guidelines recommending standard supportive treatments that help manage symptoms like behavioral problems, sleep disturbances, seizures, and physical complications. At the same time, researchers around the world are actively investigating new therapies through clinical trials. These experimental treatments aim to address the underlying cause of the disease rather than just managing symptoms. While none of these novel approaches have yet received regulatory approval for routine use, they represent hope for future therapeutic breakthroughs.^[4]

Standard Treatment Approaches

Because there is no cure or specific treatment that targets the root cause of Mucopolysaccharidosis type III, medical care focuses on addressing individual symptoms and preventing complications. This supportive approach requires a team of specialists working together, including neurologists, developmental pediatricians, physical therapists, speech therapists, orthopedic surgeons, cardiologists, and others depending on each child’s needs.^[9]

One of the most challenging aspects of MPS III is managing behavioral and psychiatric symptoms. Children often become hyperactive, aggressive, or destructive, and they may experience severe anxiety or features similar to autism spectrum disorder. These behaviors can be extremely difficult for families and caregivers. Medications are often more helpful than behavioral therapy alone for managing these issues. Different types of psychiatric medications may be tried, including those that help with anxiety, aggression, or sleep problems, but each child responds differently and finding the right combination takes patience and careful monitoring.^[2]

Sleep disturbances are another major concern. Many children with MPS III have trouble falling asleep, wake frequently during the night, or have their sleep-wake cycles completely reversed. This not only affects the child but also exhausts family members who provide care around the clock. Sleep medications, establishing consistent bedtime routines, and creating a calm sleep environment can all help, though completely resolving sleep problems is often not possible.^[13]

As the disease progresses, children may develop seizures. These are treated with standard anti-seizure medications, which neurologists select based on the type and frequency of seizures. Regular monitoring is necessary because seizure patterns can change over time, requiring adjustments to medication types or doses.^[2]

Physical and occupational therapy play important roles in maintaining mobility and function for as long as possible. Children with MPS III often develop joint stiffness, muscle weakness, and difficulty with coordination and balance. Physical therapists work with families to design exercise programs that keep joints flexible and muscles strong. As mobility declines, therapists help with proper positioning, use of assistive devices like walkers or wheelchairs, and techniques to prevent complications like pressure sores.^[20]

Although the physical features of MPS III are generally milder than in other types of mucopolysaccharidosis, some children develop heart problems. Cardiomyopathy, which is weakening of the heart muscle, and problems with heart valves or irregular heartbeats may occur. Regular echocardiograms—ultrasound tests of the heart—help doctors monitor cardiac function. If heart problems develop, medications or other interventions may be needed.^[1]

Hearing loss and vision problems are common and should be checked regularly. Recurrent ear infections are frequent in young children with MPS III, and these infections combined with fluid buildup can lead to hearing impairment. Hearing aids or tubes placed in the ears may help. Vision problems can include clouding of the cornea or damage to the retina, and regular eye examinations allow for early detection and management.^[5]

Feeding can become difficult as children lose skills or develop swallowing problems. Speech therapists and nutritionists can provide guidance on safe feeding techniques and appropriate food textures. In advanced stages, some children need feeding tubes to ensure adequate nutrition and hydration while preventing choking or aspiration pneumonia.^[13]

Respiratory infections are common and can be serious. Children with MPS III may have trouble clearing mucus from their airways, and recurrent infections of the sinuses, throat, and lungs are frequent. Prompt treatment of infections with antibiotics, maintaining good hydration, chest physiotherapy to help clear secretions, and sometimes medications to reduce mucus production are all part of respiratory care.^[2]

Treatment Approaches in Clinical Trials

While supportive care remains the only available option for families today, researchers are actively investigating treatments that could potentially slow disease progression or address the underlying enzyme deficiency that causes MPS III. These experimental therapies are being tested in clinical trials at medical centers in the United States, Europe, and other regions around the world.^[11]

Gene Therapy Approaches

Gene therapy represents one of the most promising areas of research for MPS III. The basic idea behind gene therapy is to deliver a working copy of the defective gene into the patient’s cells, allowing those cells to produce the missing enzyme. For MPS III, researchers are using specially modified viruses called adeno-associated virus vectors (AAV vectors) to carry the corrected gene into cells.^[14]

One gene therapy being studied is called UX111, also known by its research name rebisufligene etisparvovec or formerly ABO-102. This therapy is specifically designed for MPS type IIIA, the most common and typically most severe subtype. UX111 uses an AAV9 viral vector to deliver a functional copy of the SGSH gene, which provides instructions for making the sulfamidase enzyme that is deficient in type A patients. The therapy is given as a one-time intravenous infusion directly into the bloodstream.^[14]

The AAV9 vector was chosen because it has the ability to cross the blood-brain barrier—the protective layer that normally prevents substances from moving from the bloodstream into the brain. This is crucial because the brain damage is the most devastating aspect of MPS III. The goal is for the virus to deliver the working gene to cells throughout the body, including brain cells, allowing them to start producing the missing enzyme and begin breaking down the accumulated heparan sulfate that causes cellular damage.^[14]

UX111 is currently being evaluated in a pivotal clinical trial called Transpher A, which is a Phase 1/2/3 study. Phase I trials focus primarily on safety—determining whether a treatment causes harmful side effects. Phase II trials begin to assess whether the treatment actually works and helps patients. Phase III trials compare the experimental treatment to current standards of care (or placebo when no standard treatment exists) in larger numbers of patients to confirm effectiveness and monitor for less common side effects.^[14]

The UX111 program has received several special designations from regulatory agencies that acknowledge the urgent need for treatments and help speed development. In the United States, it has received Regenerative Medicine Advanced Therapy designation, Fast Track designation, Rare Pediatric Disease designation, and Orphan Drug designation. In Europe, it has received PRIME (Priority Medicines) and Orphan Drug designations.^[14]

Gene therapy research for other MPS III subtypes is also underway. Studies using different viral vectors and delivery methods are being conducted in animal models and, in some cases, early human trials. Researchers are exploring whether delivering the therapeutic gene directly into the fluid surrounding the brain and spinal cord (called intrathecal delivery) might be more effective than intravenous delivery for reaching the central nervous system.^[11]

Modified Enzyme Replacement Therapy

Traditional enzyme replacement therapy, where the missing enzyme is manufactured in a laboratory and infused into patients’ bloodstreams, has been successful for several other types of mucopolysaccharidosis. However, for MPS III, standard enzyme replacement therapy has not worked because the replacement enzymes cannot cross the blood-brain barrier to reach the brain where they are most needed.^[6]

To overcome this challenge, researchers are investigating modified approaches to enzyme delivery. One strategy involves administering enzymes directly into the cerebrospinal fluid through intrathecal injections. By bypassing the blood-brain barrier entirely and placing the enzyme directly into the fluid that bathes the brain and spinal cord, the therapy can reach affected brain tissue. Clinical trials using this approach have been conducted for both MPS IIIA and MPS IIIB subtypes.^[9]

Another experimental approach involves modifying the enzyme itself to help it cross the blood-brain barrier more effectively. Scientists are attaching special molecules to the replacement enzyme that act like a key, allowing the enzyme to pass through the barrier and enter brain tissue. While these strategies are still in early research stages, they represent creative solutions to one of the biggest obstacles in treating MPS III.^[11]

Substrate Reduction Therapy

Substrate reduction therapy takes a different approach than replacing the missing enzyme. Instead of trying to increase the breakdown of accumulated substances, it aims to slow down their production in the first place. If less heparan sulfate is produced, even with reduced enzyme activity, less material will accumulate and cause damage.^[9]

One substance being studied for substrate reduction in MPS III is genistein, a naturally occurring compound found in soybeans. Genistein is a type of plant estrogen (called a phytoestrogen) that has been shown in laboratory studies using cells from MPS patients to reduce the production of glycosaminoglycans. Research in mouse models of MPS has also shown promising results. However, clinical trials in humans have not yet demonstrated clear benefits, and more research is needed to determine whether this approach will be effective.^[9]

Stem Cell and Bone Marrow Transplantation

Bone marrow transplantation and hematopoietic stem cell transplantation (HSCT) have been used successfully to treat some other types of mucopolysaccharidosis, particularly MPS I. In these procedures, the patient’s blood-forming stem cells are replaced with healthy cells from a donor. The transplanted cells can produce the missing enzyme and potentially supply it to other tissues in the body.^[9]

Unfortunately, transplantation has not been successful for MPS III. Multiple studies have shown that patients with MPS III who undergo bone marrow or stem cell transplantation do not experience significant improvement in their neurological symptoms, even though the procedure might help with some physical symptoms. The neurological decline—which is the most devastating aspect of MPS III—continues to progress. Because of the limited benefits combined with the significant risks associated with transplantation (including the possibility of rejection, infection, and death), this approach is not recommended as a treatment option for MPS III patients.^[6]

Emerging Experimental Approaches

Beyond the treatments currently in clinical trials, scientists are exploring even newer experimental strategies that are still in early laboratory research stages. These include messenger RNA (mRNA) therapy, where synthetic RNA molecules are delivered to cells to instruct them to make the missing enzyme. This is similar to the technology used in some COVID-19 vaccines but applied to genetic diseases.^[11]

Gene editing technologies, such as CRISPR, are also being investigated. Rather than adding a working copy of the gene (as in traditional gene therapy), gene editing attempts to directly repair the defective gene in a patient’s own cells. This approach is still highly experimental for MPS III and is being studied primarily in cell cultures and animal models.^[11]

Chaperone therapy represents another innovative approach. Some enzyme deficiencies in MPS III result from enzymes that are made but fold incorrectly and therefore don’t function properly. Pharmacological chaperones are small molecules that help these misfolded enzymes achieve the correct shape, potentially restoring some enzyme activity. This approach might work for certain genetic variants that produce unstable but not completely absent enzyme.^[6]

The Importance of Multidisciplinary Care Teams

Given the complexity of MPS III and the many body systems it affects, optimal care requires coordination among multiple specialists. Families typically work with a core team that might include a medical geneticist or metabolic specialist who coordinates overall care, a neurologist to manage seizures and monitor neurological decline, a developmental pediatrician or psychiatrist to address behavioral and cognitive issues, and various therapists including physical, occupational, and speech therapists.^[18]

Additional specialists may include an orthopedic surgeon for bone and joint problems, a cardiologist for heart monitoring, an ophthalmologist for eye care, an audiologist for hearing assessment, a pulmonologist for respiratory issues, and a gastroenterologist or nutritionist for feeding problems. Having all these specialists communicate with each other and with the family helps ensure comprehensive, coordinated care that addresses all aspects of the disease.^[13]

Regular monitoring is essential even when symptoms seem stable. Recommended surveillance includes ongoing assessment of developmental abilities and educational needs, monitoring for new behavioral problems, checking hearing and vision, evaluating heart function with periodic echocardiograms, assessing mobility and joint function, and watching for signs of respiratory infection or swallowing difficulties.^[2]

Families should be aware that children with MPS III require special precautions for any procedure requiring anesthesia. The physical features of the condition—including potential airway abnormalities, enlarged tongue, stiff joints affecting neck mobility, and possible heart problems—make anesthesia more risky. Procedures should be performed at specialized centers with experience managing children with complex medical needs and difficult airways.^[2]

Most common treatment methods

• Behavioral and psychiatric management
  • Medications to control hyperactivity, aggression, anxiety, and destructive behaviors, often more effective than behavioral therapy alone
  • Medications to address sleep disturbances and help establish better sleep patterns
  • Environmental modifications to minimize risks from unpredictable behaviors
  • Supportive educational programs adapted to the child’s changing cognitive abilities
• Neurological symptom management
  • Anti-seizure medications selected based on seizure type and frequency
  • Regular monitoring of neurological decline with adjustments to care plans as needed
  • Management strategies for developmental regression and loss of previously acquired skills
• Physical and rehabilitation therapy
  • Physical therapy programs to maintain joint flexibility and muscle strength
  • Occupational therapy to preserve function in daily activities for as long as possible
  • Speech therapy for communication support and safe swallowing techniques
  • Use of assistive devices including walkers, wheelchairs, and adaptive equipment as mobility declines
• Supportive medical care
  • Regular cardiac monitoring with echocardiograms and treatment of heart problems if they develop
  • Management of recurrent ear and respiratory infections with antibiotics
  • Hearing aids or ear tubes for hearing loss
  • Regular eye examinations and management of vision problems
  • Nutritional support and feeding assistance, including feeding tubes when swallowing becomes unsafe
  • Respiratory care including chest physiotherapy to help clear airway secretions
• Experimental therapies under investigation
  • Gene therapy using AAV viral vectors to deliver working copies of defective genes
  • Modified enzyme replacement therapy with intrathecal delivery directly to cerebrospinal fluid
  • Substrate reduction therapy aimed at decreasing production of accumulating substances
  • Chaperone therapy to help misfolded enzymes achieve proper structure
  • Emerging research on mRNA therapy and gene editing approaches