When you eat protein, your body breaks it down into building blocks called amino acids. The body uses what it needs and turns the rest into waste. This waste contains nitrogen, which must be safely removed. The liver does this through a process called the urea cycle.
Arginase 1 deficiency (ARG1-D) is a rare inherited condition that affects the last step of the urea cycle. In people with ARG1-D, the enzyme arginase is missing or does not work properly. This can cause the amino acid arginine to build up in the blood instead of being broken down.
High arginine levels can be toxic, especially to the brain. Over time, this can lead to spasticity. Spasticity causes muscles to feel stiff, tight, and hard to control, and you may have trouble with walking, balance, and fine motor skills. Other symptoms can include seizures, learning challenges, slow growth, mood changes, poor appetite, and vomiting.
Treatment for ARG1-D typically involves a strict diet to lower arginine levels, as well as medicines to help remove extra nitrogen from the body. However, these approaches do not replace the defective enzyme.
In February 2026, the FDA approved Loargys (pegzilarginase-nbln) as the first medicine for ARG1-D, along with a low-protein diet.
Below are common questions and answers about using Loargys for arginase 1 deficiency.
How Does Loargys Work for Arginase 1 Deficiency?
Loargys is an enzyme replacement therapy. In people with ARG1-D, the arginase enzyme (ARG1) is partly or completely missing or does not work properly. Loargys provides a working version of this enzyme to help break down excess arginine and lower arginine levels in the blood.
By lowering arginine levels, Loargys may help lessen the toxic effects of arginine on the nervous system. This may help slow down the worsening of symptoms such as muscle stiffness (spasticity) in people with ARG1-D. missed or delayed.
How Is Loargys Given?
As an enzyme therapy, Loargys is not taken by mouth. Instead, it is given as an infusion into a vein (intravenous) or as an injection under the skin (subcutaneous). It is typically given once a week by a health care provider.
The starting dose is based on body weight. Your health care provider will check your blood arginine levels regularly to see how well the treatment is working. After about four weeks, your provider may adjust the dose if needed, based on blood test results. The goal is to keep arginine levels within a target range.
Treatment with Loargys is first started in a medical setting where you can be closely monitored for reactions. After at least eight weeks of IV treatment, some people may switch to a weekly subcutaneous injection under the skin of the abdomen, thigh, or upper arms. If Loargys injections are well tolerated, you may be able to receive them at home under the supervision of a health care provider.
If you miss a dose of Loargys or miss an appointment to get your dose, contact your health care provider for next steps. Two doses should not be given on the same day or within four days of each other to make up for a missed dose. Your health care provider will guide you on what to do if a dose is missed or delayed.
What Safety Precautions Should I Know About?
Serious allergic reactions, including life-threatening anaphylaxis, have been reported with enzyme replacement therapies, including Loargys. Because of this risk, Loargys treatment must be started as an infusion in a health care setting with close monitoring and emergency support available.
Your health care provider may give you an antihistamine medicine before treatment to help lower the risk of allergic reactions. During and after each infusion, you will be monitored for signs of allergic reactions. Symptoms may include rash, hives, trouble breathing, swelling of the face or throat, dizziness, or low blood pressure.
- If a severe reaction occurs, Loargys will be stopped right away and emergency treatment, such as epinephrine, will be given.
- If a mild or moderate reaction occurs, your health care provider may treat you with medicines such as antihistamines or corticosteroids. The infusion may be slowed or temporarily paused.
If you think you are having symptoms of an allergic reaction, get help right away.
How Was Loargys Studied for Arginase 1 Deficiency?
The effect of Loargys for lowering high arginine levels in people with ARG1-D was tested in a clinical study called PEACE. Participants were randomly assigned to receive either Loargys or a placebo (a treatment with no medicine in it). Neither the participants nor the researchers knew who received which treatment.
To qualify for the study, people had to be 2 years of age or older and have:
- A confirmed diagnosis of ARG1-D
- Arginine blood levels of 250 micromoles per liter (μmol/L) or higher
- Symptoms affecting how well they could walk or move around (functional mobility)
People could not join the study if they had severe mobility limitations, certain past transplants, certain recent or planned treatments, or other medical conditions that could interfere with participation. People were allowed to continue certain medicines for managing ARG1-D and related symptoms as long as the medicines and doses stayed the same for at least four weeks before and during the study.
The study included 32 people with ARG1-D from multiple countries. At the start of the study:
- Most participants were children or adolescents.
- The average age was about 11 years and ranged from 2 to 29.
- More than half (59%) were male, and 41% were female.
- Most (44%) were White, 19% were Asian, 6% were Black or of African descent, 6% belonged to multiple races, and the rest identified as “Other” or had missing race information.
- Around 28% of participants identified as Hispanic or Latino, and the rest did not.
- All participants were already on a low-protein diet. Despite this, average arginine levels at the start of the study were high, about 402 μmol/L.
- Most participants had symptoms related to ARG1-D; about 66% had spasticity affecting their legs.
- More than half had some degree of gross motor function impairment, meaning they had trouble with movements like walking or climbing stairs.
Participants received either once-weekly Loargys or a placebo in addition to their usual treatments and low-protein diets. Blood samples were collected right before each weekly dose.
The main goal of the study was to measure how much arginine levels changed after six months (24 weeks) of treatment. Each person’s arginine level at the start of the study was compared to their level at week 24.
Another goal was to see whether movement ability improved on average from the start of the study to week 24. The researchers used specific tests to measure motor functions, such as walking, running, and climbing stairs.
What Are the Main Benefits of Loargys Based on Studies?
The main results of the PEACE study showed that Loargys effectively lowered arginine levels in children and adults with ARG1-D.
- In the group receiving Loargys, average arginine levels decreased from about 354 to 86 μmol/L after 24 weeks.
- In the placebo group, average arginine levels changed only slightly, from about 465 to 427 μmol/L.
This means Loargys reduced arginine levels by about 76% from the start of the study, compared with about an 8% reduction in the placebo group. In addition, about 90% of people treated with Loargys reached normal arginine levels (defined in the study as 40-115 μmol/L), while no one receiving a placebo reached normal arginine levels by week 24.
The study also suggested some meaningful improvements in movement and walking ability in people treated with Loargys.
Your results may differ from what was seen in clinical studies.
The FDA approved Loargys through a pathway called accelerated approval. This pathway allows earlier access to medicines for serious conditions when early study results show a treatment improves an important biomarker. In this case, the biomarker was arginine levels. More studies are needed to see if Loargys leads to meaningful improvement in symptoms for people with ARG1-D.
How Fast Does Loargys Start to Work?
In the study, Loargys began lowering arginine levels within the first few weeks of treatment. Within three months (12 weeks), average arginine levels were within the normal range for many people.
Even though arginine levels may improve within weeks, you may not feel a difference right away. Any changes in walking or other symptoms may take more time. Your health care provider will monitor your response with regular blood tests to check arginine levels, usually before your weekly dose. They may adjust your dose of Loargys based on your results to help keep levels in the target range.
Talk with your health care provider about what to expect with Loargys treatment.
What Can I Do to Prevent or Manage Side Effects of Loargys?
Like most medicines, Loargys may cause side effects. Many can be prevented or managed with careful monitoring and by knowing when to contact your health care provider.
The most common side effects seen in studies were vomiting, fever, infusion-related reactions, and constipation.
Vomiting and constipation may be managed with simple supportive care. Eating small, bland meals, staying well hydrated, and following your prescribed diet may help. If vomiting or constipation continues or becomes severe, contact your health care provider. They may recommend medicines or other strategies to help manage symptoms.
Fever and other reactions can happen during or shortly after treatment, especially with IV doses. You will be monitored closely during infusions. Let your health care provider know right away if you feel unwell during or after treatment. Your provider may slow the infusion, pause treatment, or give medicines such as antihistamines or fever-reducing medicines to help manage symptoms. For more information about possible allergic reactions, see “What Safety Precautions Should I Know About?” above.
It is also possible to have pain, redness, swelling, irritation, or rash at the injection site when Loargys is given as an injection under the skin. Using a cool compress on the affected area may help ease discomfort.
This is not a complete list of side effects of Loargys. Tell your health care provider if side effects feel severe, worsen, or do not improve. They can recommend the next steps to take.
How Do I Get Loargys?
Loargys is not available at local pharmacies. To receive the medicine, people must enroll in a special program linked to Study IMM-PEG-005. This program involves special laboratory tests used to measure arginine levels during treatment.
If you think Loargys may be right for you, your health care provider can help decide if you qualify and submit the enrollment form. For more information, you or your health care provider can contact Immedica Pharma at 844-627-4687 or visit this website.