Gamifant for Macrophage Activation Syndrome in Still's Disease

Written by Patricia Weiser, PharmD
Medically Reviewed by Beth Johnston, PharmD, BCPS on August 05, 2025
7 min read

Macrophage activation syndrome (MAS) is a rare but serious complication that can happen in people with Still's disease. Still's disease is a type of inflammatory condition that causes daily fevers, joint pain, and rashes. Still's disease can happen in children, known as systemic juvenile idiopathic arthritis (sJIA), or adults, which is called adult-onset Still's disease (AOSD).

MAS is a serious complication where the immune system becomes overactive and starts damaging the body's own tissues. It can lead to high fevers, liver and spleen swelling, very high levels of inflammation in the blood, and life-threatening organ problems. 

You may read about "HLH," or your doctor may mention it when discussing macrophage activation syndrome (MAS). HLH stands for hemophagocytic lymphohistiocytosis, a serious condition where the immune system becomes dangerously overactive and causes widespread inflammation. When HLH happens in people with autoimmune diseases like Still's disease, it's often referred to as MAS. 

Gamifant (emapalumab-lzsg) is approved by the FDA to treat HLH/MAS in certain people who have or may have Still's disease. It works by blocking an interferon protein that plays a key role in inflammation. Gamifant may be an option when steroids do not work well enough or if MAS keeps coming back.

Gamifant may also be used for other conditions as determined by your health care provider. This article covers some frequently asked questions about Gamifant for MAS in Still's disease.

To see if Gamifant is right for you, your health care provider will consider your medical history and how you responded to previous treatments for macrophage activation syndrome (MAS). 

Gamifant is FDA-approved to treat MAS in adults and children (including babies) with known or suspected Still's disease when steroids (glucocorticoids) have not helped enough or cannot be tolerated, or if MAS returns.

Before starting Gamifant, your health care provider will likely check for infections like tuberculosis (TB) and may recommend treatment to reduce infection risk.

Gamifant is a monoclonal antibody, meaning it is designed in a lab to act like an antibody and bind to a specific target. Gamifant's target is a protein in the immune system called interferon gamma. To understand how Gamifant works, it helps to know what interferon gamma does in the body and why it becomes a problem in macrophage activation syndrome (MAS) in people with Still's disease.

Interferon gamma is a type of cytokine, which is a protein that helps immune cells communicate. Normally, it helps the body fight off infections by activating parts of the immune system. But in MAS, the immune system becomes overactive and starts making too much interferon gamma. This leads to a cytokine storm, which causes a dangerous level of inflammation that can damage organs and tissues.

Gamifant works by binding to interferon gamma and blocking it from working. This helps stop the extreme immune response that causes the symptoms of MAS. People with Still's disease already have an inflammatory condition. When MAS develops, the inflammation becomes much worse and can be life-threatening. MAS is often treated first with steroids, but they do not always help enough.

Gamifant is a new treatment option for people who do not improve with steroids or have repeated episodes of MAS. By directly blocking interferon gamma, Gamifant targets one of the main causes of this severe immune reaction.

Gamifant is given as an infusion into a vein (IV) by a health care professional. It is usually given in a hospital or clinic. Each Gamifant infusion takes about one hour.

Gamifant treatment starts on a set schedule. The exact amount depends on your body weight in kilograms (kg). Here is an example schedule based on the recommended dosage:

  • Day 1: You will get a starting dose of 6 mg/kg.
  • Days 4 to 16: You will get one dose every three days, for a total of five doses.
  • Day 19 and on: You will continue to get one dose twice a week (about every three to four days).

Your treatment schedule may look different based on how your body responds. For example, if your MAS symptoms do improve after the starting dose, your health care provider may increase the dose or increase how often you receive it. In some cases, a dose of Gamifant may be given every other day or once daily, if necessary, until symptoms improve. 

Once your condition improves, your health care provider may lower the dose or how often you receive Gamifant. But if there is no improvement even at the highest dose, your health care provider may decide to stop treatment.

Before starting Gamifant, your health care provider will likely test you for tuberculosis (TB) and may recommend preventive treatment if you are at risk. You may also be given medicines to help prevent infections while you are using Gamifant.

Keep all appointments with your health care provider and let them know about any changes in your symptoms. Your health care team will closely monitor your labs and symptoms and adjust the treatment plan as needed. 

Two clinical trials were done to test how well Gamifant works for macrophage activation syndrome (MAS) in people with Still's disease, including systemic juvenile idiopathic arthritis (sJIA) and adult-onset Still's disease (AOSD). 

Here are some details about who took part in the trials:

  • All the people in the trials had active MAS with certain lab results showing signs of high inflammation. Everyone had a confirmed or suspected diagnosis of Still's disease and MAS that was not getting better with steroids. 
  • The trials included a total of 39 people ranging from about 11 months to 64 years old. 
    • Most of them (80%) were female, and the rest were male.
    • Most were White (74%), 10% were Asian, 5% were Black or African American, 3% identified their race as Other, and 8% did not report their race.
    • In one of the trials, 88% of the people identified as Not Hispanic or Latino, and the rest did not report their ethnicity. (The other trial did not collect ethnicity information.) 

People with certain infections, like tuberculosis or fungal infections, were not allowed to enter the study due to increased risks.

Both trials were open-label and single-arm, meaning everyone in the study knew they were receiving Gamifant and there was no comparison group. Everyone received Gamifant infusions on a set schedule. After a starting dose on Day 1, they received Gamifant every three days until Day 16, then twice a week thereafter. Some people continued taking steroids or another medicine (anakinra) to treat their Still's disease.

The main goal of the studies was to measure how many people had a complete response to Gamifant treatment. Gamifant was considered effective if a person's MAS symptoms improved and their lab results showing signs of inflammation returned to safer levels within two months (eight weeks). Researchers used symptom ratings and blood test results to measure whether the treatment worked.

The clinical trials evaluating Gamifant for macrophage activation syndrome (MAS) in Still's disease showed that many people had relief from symptoms along with improvements in lab results related to inflammation. 

Of the 39 people treated with Gamifant, here are the main results at Week 8:

  • 54% had a complete response. A complete response meant that both symptoms and lab test results improved.
  • 82% had major symptom relief, based on a symptom rating scale from 0 to 10. A score of 0 meant no symptoms of MAS, while 10 meant the worst possible symptoms. 
  • Most people in the study also had improvements in lab markers such as ferritin, liver enzymes, and blood cell counts.

These results support Gamifant as a treatment option for MAS in Still's disease, especially for those who do not improve with steroids or cannot take them. 

Keep in mind that Gamifant is not a cure or treatment for Still's disease. Your experience may be different from what was seen in these studies.

Gamifant affects how your immune system works, so there are some important safety considerations to keep in mind before and during treatment:

Higher risk of infections. Gamifant can increase your chance of getting serious infections. In clinical trials, some people developed infections like pneumonia, sepsis, or viral illnesses. This is because Gamifant blocks interferon gamma, a protein that helps your body fight certain infections. 

Before starting treatment, your health care provider will test you for tuberculosis (TB) and may recommend medicine to lower your risk. You may also receive other preventive medicines to protect against shingles, pneumocystispneumonia, or other fungal infections.

During Gamifant treatment, your health care team will monitor you closely for signs of infection. Let them know right away if you notice new or worsening symptoms like a fever, cough, chills, or feeling unusually tired.

Vaccines. You should not get any live vaccines while using Gamifant or for at least four weeks after your last dose. Examples include the nasal spray flu vaccine, chickenpox vaccine, and the measles, mumps, and rubella (MMR) vaccine. If you are unsure whether a vaccine is live, ask your health care provider.

Pregnancy and breastfeeding. There is not enough information to know if Gamifant is safe to use during pregnancy or while breastfeeding. Before starting Gamifant, let your health care provider know if you are pregnant, planning to become pregnant, or breastfeeding. They can discuss the potential risks and benefits with you.

A cost assistance program is available from Sobi, the maker of Gamifant. It may reduce the amount you pay. Whether you qualify to enroll may depend on what type of insurance you have or if you are uninsured. Visit this website from the drugmaker for more information about Gamifant costs.

You can also contact the drugmaker at 833-597-6530 to connect with a representative for financial support information and other resources.