The U.S. Food and Drug Administration (FDA) has approved mepolizumab (Nucala) for the treatment of hypereosinophilic syndrome (HES) in adults and children 12 years and older. This indication is the first approval for treating HES in almost 14 years. 

HES is a group of rare blood disorders associated with persistent eosinophilia and organ damage. Common symptoms include rashes, itching, asthma, difficulty breathing, abdominal pain, vomiting, diarrhea, arthritis, muscle inflammation, congestive heart failure, deep venous thrombosis, and anemia.

The approval was largely based on a randomized, double-blind, multicenter, placebo-controlled trial in 108 HES patients aimed to investigate the efficacy and safety of mepolizumab. The results of the trial showed that there was a 50% relative reduction in HES flares in the mepolizumab treatment group compared to patients in the placebo group. Additionally, the time to the first HES flare was later, on average, for the patients in the mepolizumab group compared to the placebo group. Mepolizumab is also FDA-approved for patients with severe asthma with an eosinophilic phenotype and for adult patients with eosinophilic granulomatosis with polyangiitis (EGPA). 

For the treatment of HES, mepolizumab received orphan drug designation. Additionally, the application was granted fast track designation and priority review. 

For more information about HES and other rare blood disorders, visit 



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