Arginase deficiency is a rare inherited disorder where the body cannot properly break down a protein building block called arginine, leading to a gradual buildup of ammonia and arginine in the blood that particularly affects the nervous system and muscles over time.

How Treatment Can Help People With Arginase Deficiency

The main goal of treating arginase deficiency is to prevent the buildup of harmful substances in the blood, particularly arginine and ammonia, which can damage the brain and spinal cord over time. Treatment focuses on reducing these levels to slow or stop the progression of symptoms like muscle stiffness, developmental delays, and seizures. When started early—ideally at birth through newborn screening—treatment can help children avoid the most serious complications and maintain better quality of life.^[1]

Treatment approaches depend on several factors, including when the condition is diagnosed, how severe the symptoms are, and the person’s age. Some individuals have milder forms that appear later in childhood, while others develop symptoms before age three. Unlike some other urea cycle disorders, arginase deficiency rarely causes life-threatening spikes in ammonia, though episodes can occur during illness, after surgery, or when eating high-protein meals.^[2]

Medical societies and metabolic specialists recommend that people with arginase deficiency receive ongoing care from a team that includes genetic specialists, metabolic doctors, dietitians, and physical therapists. This coordinated approach addresses both the biochemical aspects of the disorder and the physical symptoms like muscle stiffness that can affect walking and daily activities.^[3]

Beyond the standard medical treatments that have been used for years, researchers are actively exploring new therapies in clinical trials. These investigational treatments include enzyme replacement therapy and gene therapy approaches that aim to address the root cause of the disorder rather than just managing symptoms. Early results from some of these studies have shown promise, though these treatments are not yet widely available.^[4]

Standard Medical Treatment for Arginase Deficiency

The foundation of treatment for arginase deficiency involves carefully restricting the amount of protein a person eats each day. This is because protein contains nitrogen, and when the body cannot properly process nitrogen due to the missing or deficient arginase enzyme, harmful substances accumulate. A metabolic dietitian works with patients and families to design a special low-protein diet that provides enough nutrition for growth and health while keeping arginine levels as low as possible—ideally below 200 micromoles per liter in the blood.^[5]

Most people with arginase deficiency need to limit their daily protein intake to approximately 1.0 to 1.5 grams per kilogram of body weight, though this varies based on age and individual tolerance. Foods high in protein, such as meat, fish, eggs, dairy products, nuts, and beans, must be carefully measured or avoided. Instead, the diet emphasizes fruits, vegetables, and special low-protein medical foods designed for people with metabolic disorders.^[6]

Because severe protein restriction can lead to nutritional deficiencies, patients typically take special medical formulas that provide essential amino acids—the specific building blocks of protein that the body needs but cannot make on its own. These formulas are carefully balanced to support growth and development without contributing excess arginine or nitrogen. Many children and adults need to drink these formulas multiple times per day, though they often find the taste unpleasant.^[7]

When ammonia levels do rise—which can happen during illness, infection, or stress—nitrogen-scavenging medications help remove excess nitrogen from the body through alternative pathways. The most commonly used drugs are sodium benzoate and sodium phenylbutyrate. Sodium benzoate works by combining with the amino acid glycine to form a compound called hippurate, which the kidneys can excrete, taking nitrogen with it. Sodium phenylbutyrate (and a similar medication called glycerol phenylbutyrate) works by combining with glutamine to create a compound called phenylacetylglutamine, which also carries nitrogen out through the urine.^[8]

These nitrogen-scavenging medications are typically taken daily as a long-term treatment, not just during acute episodes. They come as powders, tablets, or liquids and are usually taken two to three times per day with meals. Some patients find glycerol phenylbutyrate preferable because it may provide more extended effects and doesn’t have the strong, unpleasant taste and smell that sodium phenylbutyrate can have.^[9]

However, it’s important to understand that even with strict dietary protein restriction and nitrogen-scavenging drugs, arginine levels often remain elevated. This is because the body continuously breaks down its own proteins as part of normal tissue turnover, generating arginine from within. This internal protein breakdown—called endogenous protein turnover—continues regardless of diet, making it difficult to maintain arginine levels in the ideal therapeutic range. This limitation of standard treatment is why researchers are investigating new approaches.^[10]

Treatment for the physical symptoms caused by arginase deficiency is also essential. Many people develop progressive muscle stiffness or spasticity, particularly in the legs, which can make walking difficult or impossible. Physical therapy helps maintain muscle strength, flexibility, and range of motion. Occupational therapy supports daily activities and independence. Some patients benefit from medications that reduce muscle spasticity, orthopedic braces called AFOs (ankle-foot orthoses) to support walking, or surgical procedures to release tight muscles or tendons.^[11]

Seizures occur in about 60 to 75 percent of people with arginase deficiency. These seizures typically involve shaking of the whole body and generally respond well to standard anti-seizure medications prescribed by a neurologist. The choice of medication must be made carefully to avoid drugs that might worsen metabolic control.^[12]

Regular monitoring is a critical part of managing arginase deficiency. Blood tests to measure arginine, ammonia, and other amino acids are typically done every few months, or more often if there are concerns. These tests help doctors adjust the diet and medications to keep levels as close to normal as possible. Growth parameters, developmental progress, and neurological function are also tracked over time.^[13]

During acute illness, fever, surgery, or pregnancy, people with arginase deficiency need more intensive management because stress can trigger rapid increases in ammonia. Treatment may include intravenous fluids containing high amounts of sugar to prevent the body from breaking down its own proteins for energy, temporary increases in nitrogen-scavenging medications, and very careful monitoring. In rare cases where ammonia rises to dangerous levels, hemodialysis—a procedure where blood is filtered through a machine—may be needed to rapidly remove ammonia, though this is much less common in arginase deficiency than in other urea cycle disorders.^[14]

Treatment is truly lifelong. Adults with arginase deficiency must continue their special diet, medications, and regular medical follow-up throughout life. Those who were diagnosed late or not treated consistently may have permanent neurological damage, including intellectual disability, inability to walk, and loss of bowel and bladder control. However, individuals who are identified early through newborn screening and receive consistent treatment from birth appear to have much better outcomes, with minimal symptoms and more normal development.^[15]

Innovative Treatments Being Tested in Clinical Trials

Because standard treatments for arginase deficiency cannot fully normalize arginine levels or completely prevent disease progression, researchers have been working to develop new therapeutic approaches that could address the underlying enzyme deficiency more directly. Several promising strategies are now being evaluated in clinical trials around the world.

The most advanced investigational treatment is enzyme replacement therapy, which aims to provide the missing arginase enzyme from an external source. In this approach, researchers have created a form of human arginase enzyme that has been modified so it remains stable in the bloodstream and can be given as an injection. The modified enzyme is designed to break down the excess arginine circulating in the blood, mimicking what the missing liver enzyme would normally do.^[16]

This enzyme replacement therapy, known by the code name pegzilarginase, has entered clinical trials to test its safety and effectiveness in people with arginase deficiency. In early Phase I and Phase II studies, researchers found that patients who received regular injections of pegzilarginase showed significant reductions in their blood arginine levels—in some cases, arginine decreased by 80% or more. The treatment appeared to be generally well tolerated, with the most common side effects being mild reactions at the injection site and occasional allergic-type reactions.^[17]

The mechanism of action of pegzilarginase involves breaking down arginine in the blood into ornithine and urea, effectively performing the reaction that patients’ own arginase enzyme cannot do. The “peg” part of the name refers to a chemical modification called pegylation, where molecules of polyethylene glycol are attached to the enzyme. This modification helps the enzyme stay in the bloodstream longer and reduces the likelihood that the patient’s immune system will recognize it as foreign and attack it.^[18]

Clinical trials of pegzilarginase have been conducted in multiple countries, including the United States and Europe. Participants in these studies receive the medication through injections, typically once or twice per week. Researchers carefully monitor not only whether arginine levels decrease, but also whether patients experience improvements in their physical symptoms, such as reduced muscle stiffness, better mobility, or improvements in cognitive function. Long-term data collection is ongoing to understand whether sustained arginine reduction can prevent or reverse some of the neurological damage caused by the disorder.^[19]

Another experimental approach involves gene therapy, which aims to correct the genetic defect at its source. In this strategy, researchers use special viruses that have been modified to be harmless as delivery vehicles to insert a working copy of the ARG1 gene into a patient’s cells. The goal is to enable the patient’s own liver cells to produce functional arginase enzyme, potentially providing a long-term or even permanent correction of the disorder.

In 2016, researchers demonstrated in laboratory studies that they could use gene editing techniques to restore arginase activity in cells taken from patients with arginase deficiency. They created patient-specific stem cells with corrected ARG1 genes that could produce working arginase enzyme. These cells could then potentially be transplanted back into the patient’s liver. This proof-of-concept work showed that gene-based correction is scientifically feasible, though translating this approach into actual clinical trials for patients requires additional development and safety testing.^[20]

The major challenge with gene therapy for arginase deficiency is ensuring that the corrected cells can be safely delivered to the liver and will produce enough enzyme to make a meaningful difference. Researchers must also confirm that the gene therapy doesn’t cause unintended harmful effects, such as triggering liver damage or inserting the new gene into a location that disrupts other important genes. Clinical trials of gene therapy for arginase deficiency have not yet been widely reported, but the approach remains an active area of research.

Some clinical trials are also investigating whether improved versions of nitrogen-scavenging drugs might work better than the current medications. For example, glycerol phenylbutyrate has been studied to see if it provides more consistent ammonia control with fewer side effects compared to sodium phenylbutyrate. Research suggests it may offer a longer-lasting effect because the body breaks it down more slowly, potentially leading to more stable ammonia and arginine levels throughout the day. However, because it doesn’t address the fundamental problem of elevated arginine from internal protein breakdown, its benefits are still limited.^[21]

Researchers are also exploring the possibility of using special probiotics or other approaches to reduce the production of toxic guanidino compounds in the gut. When arginine accumulates, it can be converted by bacteria in the intestines into various guanidino compounds, including one called guanidinoacetate, which may directly damage brain cells and contribute to seizures. By reducing the formation of these toxic byproducts, researchers hope to reduce neurological symptoms even if arginine levels remain somewhat elevated.

For patients who respond poorly to all medical treatments and continue to have progressive neurological decline despite maximum therapy, liver transplantation has been considered as a definitive treatment option. Because the liver is the main site where the urea cycle operates and where arginase enzyme is normally found, replacing a patient’s liver with a healthy donor liver effectively cures the metabolic defect. Studies of patients with arginase deficiency who received liver transplants have shown that this can normalize arginine levels, stop the progression of neurological symptoms, and allow patients to eat a normal diet without restrictions or medications.^[22]

However, liver transplantation is a major surgical procedure that carries significant risks, including the possibility of organ rejection, infection, bleeding, and complications from the lifelong immunosuppressive medications needed to prevent rejection. Therefore, transplant is generally reserved for patients with severe, progressive disease who have not responded adequately to medical management. With the development of new treatments like enzyme replacement therapy and potentially gene therapy, liver transplantation may become less necessary in the future if these alternative approaches prove to be safe and effective.^[23]

Eligibility for clinical trials varies depending on the specific study, but generally researchers look for participants who have a confirmed genetic diagnosis of arginase deficiency with documented mutations in the ARG1 gene. Some trials accept only children, while others include teens and adults. Certain studies may require that participants have specific symptoms, such as measurable spasticity or elevated arginine levels. Geographic location can also be a factor, as trials are typically conducted at specialized medical centers in specific countries or regions.

Families interested in clinical trials for arginase deficiency can find information through organizations like the National Urea Cycle Disorders Foundation, which maintains updated information about ongoing research studies. The website ClinicalTrials.gov, maintained by the U.S. National Institutes of Health, is another resource where families can search for active trials recruiting participants. Speaking with a metabolic specialist who treats urea cycle disorders is often the best way to learn about trials that might be appropriate for a specific patient.

Most common treatment methods

• Dietary protein restriction
  • Low-protein diet carefully designed by metabolic dietitian to limit arginine intake while supporting nutrition and growth
  • Daily protein typically limited to 1.0 to 1.5 grams per kilogram of body weight
  • Restriction of high-protein foods such as meat, fish, eggs, dairy, nuts, and beans
  • Emphasis on fruits, vegetables, and special low-protein medical foods
• Medical formula supplementation
  • Special formulas providing essential amino acids without excess arginine
  • Taken multiple times daily to prevent nutritional deficiencies from protein restriction
  • Carefully balanced to support growth and development
• Nitrogen-scavenging medications
  • Sodium benzoate combines with glycine to create hippurate, which removes nitrogen through urine
  • Sodium phenylbutyrate or glycerol phenylbutyrate combines with glutamine to create phenylacetylglutamine for nitrogen excretion
  • Typically taken daily as powders, tablets, or liquids with meals
  • Help control ammonia levels by providing alternative pathways for nitrogen disposal
• Symptomatic treatment for spasticity and seizures
  • Physical therapy to maintain muscle strength, flexibility, and range of motion
  • Occupational therapy to support daily activities and independence
  • Anti-spasticity medications to reduce muscle tightness
  • Orthopedic braces (AFOs) to support walking
  • Anti-seizure medications to control seizures, which occur in 60-75% of patients
  • Surgical procedures for severe muscle contractures if needed
• Acute illness management
  • Intravenous fluids with high sugar content during illness or stress to prevent protein breakdown
  • Increased nitrogen-scavenging medications during metabolic crises
  • Careful monitoring of ammonia levels
  • Hemodialysis in rare cases of severe hyperammonemia
• Enzyme replacement therapy (investigational)
  • Pegzilarginase provides modified arginase enzyme through regular injections
  • Works by breaking down excess arginine in blood into ornithine and urea
  • Currently being tested in Phase I and Phase II clinical trials
  • Early results show significant reduction in blood arginine levels
  • Generally well tolerated with mild injection site reactions as most common side effect
• Gene therapy (experimental)
  • Aims to insert working copy of ARG1 gene into patient’s liver cells
  • Uses modified viruses as delivery vehicles
  • Proof-of-concept demonstrated in laboratory studies
  • Clinical trials for patients not yet widely reported
  • Potential for long-term or permanent correction of enzyme deficiency
• Liver transplantation
  • Considered for patients with severe progressive disease not responding to medical treatment
  • Replaces deficient liver with healthy donor liver, normalizing arginine levels
  • Can stop neurological progression and eliminate need for dietary restrictions
  • Requires lifelong immunosuppressive medications
  • Carries surgical risks and potential for organ rejection