What Is Loargys, and Why Does It Matter?
Loargys is an enzyme-based medicine that has been granted accelerated approval by the FDA for managing high levels of the amino acid arginine (hyperargininemia) caused by arginase 1 deficiency (ARG1‑D). It is the first and only treatment that can quickly and consistently lower arginine levels in adults and children (ages 2 years or older) with ARG1‑D and should be used along with a low-protein diet.
ARG1-D is an extremely rare inherited disorder that affects the body's ability to break down arginine, an amino acid found in proteins. The enzyme arginase plays a key role in removing waste nitrogen from the body via the urea cycle, and mutations in the ARG1 gene can cause partial or complete loss of this enzyme. As a result, arginine and its toxic byproducts build up in the blood, leading to neurological damage that can cause muscle stiffness (spasticity), seizures, delays in learning, intellectual disability, and early death.
ARG1-D is typically diagnosed in late infancy or early childhood. In the U.S., about 250 people are thought to live with the disorder. Usual care has focused on managing symptoms with a protein‑restricted diet, special amino acid supplements without arginine, and medicines that remove extra nitrogen when needed.
Loargys contains pegzilarginase, a lab‑made version of the human arginase 1 enzyme. It is designed to act like the missing or weak enzyme and reduce high levels of arginine in the blood by converting it to urea. Studies have shown that this medicine not only lowers levels of arginine and related toxins in the blood quickly, but also maintains reduced levels over time.
This approval matters because by targeting the enzyme deficiency responsible for high arginine levels, this medicine provides a novel option for a condition with few treatment choices and a high unmet need. But patients with ARG1-D will still require a full care plan, often including a low‑protein diet along with standard therapy.
Why Was It Approved?
The FDA granted Loargys accelerated approval because studies showed that it was safe and well tolerated. It was also found to be good at lowering high arginine levels in the blood, maintaining reduced levels for an extended period, and improving patients' ability to move. But continued approval may depend on confirmation of real‑world clinical benefits in a follow‑up study.
In a clinical study, 32 patients were randomly assigned to receive either Loargys (21 patients) or a placebo (11 patients) intravenously or as an under-the-skin shot once weekly for 24 weeks, along with their personalized diet and treatment plan. After 24 weeks, patients treated with Loargys saw a significant drop in their average blood arginine levels (from 354.0 micromoles per liter, or μmol/L, to 86.4 μmol/L), while those on the placebo showed only a small change (from 464.7 μmol/L to 426.6 μmol/L). About 90.5% of patients treated with Loargys, versus none on the placebo, achieved normal arginine levels. Patients receiving Loargys also showed meaningful improvements in movement and mobility. And these benefits remained consistent and continued long-term, lasting for another 24 weeks with ongoing treatment.
The most common side effects were vomiting, fever, infusion‑related reactions, and constipation. Mild to moderate allergic reactions were seen in 13% of the Loargys-treated people in the clinical study. But there have been serious allergic reactions, including anaphylaxis, from enzyme treatments, including this medicine. Your health care provider should monitor you and treat you promptly if these reactions happen.
What Do I Need to Know?
You or your child will start Loargys in a health care setting with monitoring because severe allergic reactions are possible. The medicine is given once a week via infusion into a vein (IV) or via an injection under the skin in the belly area (avoiding the area just around the belly button), the outer thigh, or the side or back of the upper arm. The dose is the same for both methods.
Before starting the treatment, your health care provider will check your or your child's blood arginine levels and then adjust the dose based on those levels before each dose. Regular blood tests are needed to guide these changes. After at least eight weeks, once your or your child's dose is stable and the allergy risk is low, you or a caregiver may be trained to give injections under the skin at home under your health care provider's supervision.
If you or your child misses a dose, call your health care provider to plan the next one. Do not take two doses on the same day or within four days of another dose. Leave the minimum time between doses as advised by your health care provider. If you stop this medicine, your blood arginine levels will likely rise again.
Loargys can affect the findings of standard lab tests for arginine. So special testing methods are needed to get accurate results.
There is not enough information about Loargys use in pregnancy or while breastfeeding. Talk with your health care provider about the risks and benefits.
Be sure to tell your health care provider about all your vaccinations and about the medicines, over-the-counter products, herbal remedies, and vitamin supplements that you take.
Before starting Loargys, tell your health care provider if you or your child is allergic to any of its ingredients, has any liver problems, and about other details of your medical history. Do not use Loargys if you have had a severe allergy to pegzilarginase.
Hypersensitivity reactions such as swelling of the face, a rash, flushing, and trouble breathing usually happen with the first few doses but can also reappear later in treatment. Severe allergic reactions, including anaphylaxis, are possible. Stop the dose and contact your health care provider right away if you get hives, widespread itching, chest tightness, trouble breathing, swelling, flushing, or low blood pressure.
Loargys is expected to be available in the U.S. in April 2026. The company is also planning a support program with education and financial assistance information for those who qualify.