Rare disorders such as hypereosinophilic syndrome (HES) are benefitting from the ongoing efforts to bring to market new treatments for these types of diseases. For the first time in more than a decade, a new therapy for HES has received FDA approval, which is an important advance for managing this challenging, potentially fatal disorder.

When an individual's blood eosinophil level is much greater than what is typical for most, the elevated level (>1500/mcL) persists for more than 6 months, and organ damage has begun to occur, HES may be the diagnosis. Due to the overabundance of eosinophils entering various bodily systems, inflammation begins and can lead to organ dysfunction. Depending on which organs are involved, the complications from HES can be numerous and extremely varied. Serious system involvement can occur in the heart, lungs, skin, or nervous system, with cardiac issues being one of the most problematic as it has a likelihood of resulting in chronic heart failure and possibly death. HES is most common in adulthood, but pediatric HES is also diagnosed some of the time. Both male and female patients are susceptible, with a higher incidence in males aged between 20 and 50.

Prevention of complications and reducing morbidity are the top goals of HES treatments. Decreasing eosinophil levels is a priority, and the extent of any organ involvement must be determined. Depending on the symptoms a patient experiences, specific testing will vary. Echocardiogram, CT scanning, chest x-ray, allergy testing, CBC, and biopsy are possible testing considerations. Typically multiple specialists will work collaboratively to treat a patient with HES.

Results from testing and the patient's unique symptoms will determine the first-line approach to take for treatment of the HES. A differentiator for the course of treatment is the presence of FIP1L1/PDGFRA mutation in a patient's blood study. If a FIP1L1/PDGFRA mutation is not found, the first-line therapy will generally be a glucocorticoid, such as prednisone. The eosinophil-reduction effect of a course of prednisone is relatively rapid, and a large number of patients do respond well to the inflammation suppression of this therapy. When there is a FIP1L1/PDGFRA mutation present, imatinib, such as Gleevec, will be used. Imatinib is a therapeutic agent know as a tyrosine kinase inhibitor and may be effective in bringing about remission.

In September 2020, Nucala (mepolizumab) was approved for the treatment of adult and pediatric patients aged 12 years and older with HES for 6 months or more, without an identifiable non-hematologic secondary cause. The approval made Nucala the first targeted biologic treatment to be approved for patients with the eosinophil-driven disease in the United States. Mepolizumab is a humanized monoclonal antibody specific for interleukin-5 (IL-5). By preventing IL-5 from binding to its receptor on the eosinophil surface, it is possible to reduce eosinophil growth. According to the results of the phase 3 study published in the Journal of Allergy and Clinical Immunology, over 50 percent fewer patients experienced HES when they were treated with Nucala as compared to placebo when added to standard of care treatment over the 32-week study period (28% vs 56%; p=0.002).¹

HES is a complicated disorder, and due to its chronic and progressive nature, can be fatal if left untreated. If patients have unexplained eosinophilia, a hematologist should be consulted to assist with diagnosing and managing their care. Monitoring such as follow-up serum troponin level testing, echocardiograms, and pulmonary function tests may be conducted to ensure eosinophilia is controlled and that there is no evidence of any new or worsening organ involvement. Overall survival of HES patients has improved due to early monitoring and advances in therapies such as the new indication approved for mepolizumab.