Hypereosinophilic syndrome (HES) is a rare disease and involves long-term high levels of eosinophils (a type of white blood cell) in the blood. These eosinophils can collect in different tissues or organs, causing inflammation (swelling) and damage over time. HES can affect any organ, including the stomach, intestines, heart, lungs, skin, liver, spleen, and eyes. In some people, only one organ is affected, while in others, multiple organs may be involved. If left untreated, HES can be life-threatening. The cause of HES is not known.
The symptoms of HES can vary widely from person to person and depend on which organs are affected. The goals of treatment are to lower the number of eosinophils, prevent organ damage, and slow the progression of HES.
Treatment options for HES are limited and often involve corticosteroids. In May 2026, Fasenra was approved to treat certain people with HES. Continue reading to learn more about how Fasenra works, how it was studied, and its potential benefits.
How Does Fasenra Work for HES?
Fasenra (benralizumab) is a monoclonal antibody. Monoclonal antibodies are special proteins made in a laboratory. These proteins work like the natural proteins your immune system makes to recognize and fight germs, infections, and abnormal cells. Inflammation plays a major role in HES. Many different things in the body contribute to this inflammation, and Fasenra is designed to interrupt part of that process.
With HES, eosinophil levels are very high. Each eosinophil has a protein on its surface called the interleukin-5 (IL-5) receptor. Fasenra attaches itself to IL-5 receptors. Once attached, this signals other immune cells to come and destroy the eosinophil. This helps lower eosinophil levels and inflammation, which may help improve symptoms of HES.
While we know Fasenra lowers eosinophil levels, researchers are still working to fully understand exactly how this leads to improvement in HES.
How Was Fasenra Studied for HES?
One clinical trial called NATRON was done to see whether adding Fasenra to existing HES treatment could help reduce the number of HES flares.
An HES flare means your condition has gotten worse, which may include any of the following:
- You have new or worsening symptoms related to HES.
- Your eosinophil levels increase.
- Your health care provider needs to add a steroid or increase your steroid dose by 10 milligrams per day for at least two days.
- Your health care provider needs to add or increase the dose of your other HES medicines.
- You need to be admitted to the hospital.
The NATRON study included 133 adults and children aged 12 or older with HES who were already taking other medicines. Other medicines could be corticosteroids (such as prednisone) or those that affect your immune system. These medicines had to be considered stable, meaning the dose had not changed in a while. Here are some key details about the participants at the start of the study:
- The average age was 48 years, and there were more women (62%) than men.
- 76% were White, 18% were Asian, 3% were Black or African American, 3% were other races, and 2% identified as belonging to Hispanic or Latino ethnicity.
- The average time since diagnosis of HES was 4.3 years.
Participants were split into two groups. One group received Fasenra, and the other received a placebo (containing no medicine). Neither the investigators nor the participants knew which group participants were in.
What Benefits Were Seen?
Patients using Fasenra had fewer HES flares during the 24-week study than those using the placebo. You can see the specifics for the number of flares participants had in the table below.
| Frequency of flares over 24 weeks | Fasenra (67 participants) | Placebo (66 participants) |
|---|---|---|
| 0 flares | 54 (81%) | 38 (58%) |
| 1 flare | 13 (19%) | 20 (30%) |
| 2 flares | 0 | 8 (12%) |
If these numbers are used to predict the number of flares over an entire year, it would be 0.41 flares per year in the Fasenra group compared to 1.23 flares per year with the placebo.
Keep in mind your results may be different from what was seen in this study.
How Do I Use Fasenra?
Fasenra is given as a subcutaneous injection (under the skin). You or a caregiver can be trained to give these injections so you can use Fasenra at home. Fasenra can be injected in your thigh (front upper part of your leg) or your belly (not within 2 inches around your belly button). Your caregiver can also use the upper part of your arm to give you a dose. Fasenra should not be injected into scars, through your clothes, or where skin is damaged, sore, tender, bruised, scaly, or hard.
You should get written instructions along with training before using Fasenra at home. Below are the basic steps you should follow:
- Gather your supplies (Fasenra pen, alcohol wipe, etc.). Allow the Fasenra pen to sit at room temperature for about 30 minutes before using it.
- Make sure the liquid in your Fasenra pen is clear and either slightly yellow or colorless before using it. Do not use it if the liquid is cloudy, any other color, or has floaters or particles (other than air bubbles) in it.
- Wash your hands and clean the injection site with an alcohol wipe.
- Inject Fasenra at a 90-degree angle, pressing down firmly. Hold the pen in place for 15 seconds. Lift the pen straight up.
After giving the injection, do not rub the area. There may be a small amount of liquid or blood on your skin where the injection was given. This is normal and should not cause any concern. If needed, you can put a small bandage over the area.
If you or your caregiver are not sure how to use Fasenra, reach out to your pharmacist or health care provider.
Fasenra is usually given every four weeks for HES. Follow the instructions from your health care provider. Do not change how often you use Fasenra without consulting your health care provider.
Are There Other Unique Considerations to be Aware Of?
Fasenra is a specialty medicine. You will need to use a specialty pharmacy to get your medicine. There may be a specialty pharmacy near you, or you may be able to have the medicine delivered to your home. A specialty pharmacy provides medicines that are used to treat rare or complex conditions. Often, these medicines are not used by many people, so a local pharmacy may not keep them in stock.
You can visit the drugmaker’s website for more information about Fasenra, signs and symptoms of HES, and to find out if you qualify for any cost savings options through the Fasenra 360 Savings and Support program.