Who benefits most from eosinophil-targeting biologics?
Patients with idiopathic HES—where no underlying cause is found—who have organ damage, especially lung or heart involvement, appear to benefit most. In a 2025 study of 11 patients with idiopathic HES, 8 had asthma, 4 had eosinophilic pleural effusions (fluid around the lungs), and 2 had cardiac arrhythmias; all were treated with either mepolizumab or benralizumab [1]. After 12 months, eosinophil counts fell from a median of 3,000 to 50 cells/µL, and every patient with asthma stopped having flares and improved their lung function (average FEV1 increase of 857 mL) [1]. This suggests that biologics can directly reduce the eosinophil burden that drives organ damage.
A 2023 case series of 8 patients with idiopathic HES, all with asthma, found similar results: eosinophil counts dropped from an average of 7,179 to 72 cells/µL after 6 months, and all patients achieved asthma control and reduced their oral steroid use by 75% [3]. These two studies, though small, consistently show that biologics are effective in patients with lung involvement and asthma. The 2025 review also notes that biologics are promising for preventing flare-ups and reducing eosinophil counts, which is key to preventing organ damage [5].
How much improvement can you expect?
The most striking effect is the rapid and profound drop in eosinophil counts, which is the direct driver of organ damage. In the 2025 study, eosinophils fell from a median of 3,000 to 50 cells/µL within a year—a 98% reduction [1]. In the 2023 series, the drop was from 7,179 to 72 cells/µL in 6 months [3]. This translates into real clinical benefits: all asthma patients became well-controlled (ACQ < 0.75), and oral corticosteroid use was cut by 75–82% [1][3]. Reducing steroid dependence is important because long-term steroids have their own organ-damaging side effects.
For severe cases, a 2026 case report describes a 73-year-old man with steroid-refractory HES who developed life-threatening respiratory failure requiring a ventilator [2]. After starting mepolizumab, his eosinophil counts stabilized and he was able to breathe without the ventilator by day 41 [2]. This shows that biologics can be a rescue therapy when steroids fail, potentially reducing the risk of permanent lung damage from prolonged respiratory failure.
What are the caveats and limitations?
The evidence is promising but not definitive. The studies are small—11 patients in the 2025 study, 8 in the 2023 series, and a single case report [1][2][3]. There are no large randomized controlled trials yet, so the results may not generalize to all HES patients. The 2025 review emphasizes that biologics are 'promising' but that more research is needed to clarify their role and address unmet needs [5].
Also, these biologics are not a one-size-fits-all cure. They are typically used in idiopathic HES after other causes have been ruled out, and they may not work for all subtypes, such as those driven by genetic mutations like FIP1L1::PDGFRA, where imatinib is the preferred treatment [4][5]. The 2022 review highlights that treatment should be tailored to the specific molecular cause of HES, and biologics are just one option [4]. So, while biologics can reduce organ damage in many cases, they are part of a broader, personalized treatment strategy.
About These Sources
This answer is built on 5 peer-reviewed studies — published from 2022 to 2026, 3 from 2024 or later, 2 in Q1 journals — selected as the most relevant from 5 studies that passed quality screening, drawn from 38 papers retrieved from a database of over 500 million.
Sources used in this answer
Biologic Agents in Idiopathic Hypereosinophilic Syndrome
In a retrospective study of 11 patients with idiopathic HES, treatment with mepolizumab or benralizumab reduced median eosinophil counts from 3,000 to 50 cells/µL at 12 months, eliminated asthma flares in all 8 patients with asthma, and cut oral corticosteroid use by 82%.
Steroid-Refractory Idiopathic Hypereosinophilic Syndrome with Prolonged Respiratory Failure Responsive to Mepolizumab
A case report of a 73-year-old man with steroid-refractory idiopathic HES and respiratory failure showed that mepolizumab stabilized eosinophil counts and allowed weaning from mechanical ventilation by day 41.
Biologic agents in idiopathic hypereosinophilic syndrome: a case series
A case series of 8 patients with idiopathic HES treated with mepolizumab or benralizumab found eosinophil counts fell from a mean of 7,179 to 72 cells/µL at 6 months, with all patients achieving asthma control and a 75% reduction in oral corticosteroids.
Biologic therapies for hypereosinophilic disorders: From tyrosine kinase inhibitors to monoclonal antibodies. Towards an increasingly customized management?
A 2022 review of biologic therapies for HES emphasizes that treatment should be guided by the underlying molecular cause, with imatinib for FIP1L1::PDGFRA-positive cases and monoclonal antibodies like mepolizumab and benralizumab for other subtypes.
Hypereosinophilia: clinical and therapeutic approach in 2025
A 2025 review highlights that biologics targeting IL-5 or its receptor (mepolizumab, benralizumab) have shown promise in reducing eosinophil counts and preventing flare-ups, but stresses the need for accurate classification and ongoing trials to optimize management.