The Food and Drug Administration (FDA) has approved Nucala (mepolizumab; GlaxoSmithKline) for the treatment of patients aged 12 years and older with hypereosinophilic syndrome (HES) for 6 months or longer without an identifiable non-hematologic secondary cause.

HES is a rare group of inflammatory disorders characterized by persistent eosinophilia that can potentially lead to inflammation, organ damage, and possibly death if left untreated.

The approval was based on data from a 32-week, randomized, double-blind, placebo-controlled phase 3 study that assessed the efficacy and safety of Nucala in 108 patients aged 12 years and older with severe HES, defined as having at least 2 HES flares within the past 12 months and a blood eosinophil count of 1000 cells/µL or higher. Patients were randomized to receive Nucala 300mg subcutaneously every 4 weeks or placebo. The primary end point was the proportion of patients who experienced an HES flare during the 32-week treatment period.

Findings showed that patients treated with Nucala experienced significantly fewer HES flares compared with placebo (56% vs, 28%, respectively; P =.002). Additionally, Nucala achieved statistical significance for all secondary end points, including reduced risk in the time to first HES flare, reduced annualized rate of HES flares, and improved fatigue scores. 

As for safety, the most common adverse reactions for Nucala were upper respiratory tract infection and pain in extremities. Health care professionals should consider herpes zoster vaccination in patients treated with Nucala if medically appropriate as infections have occurred.

Related Articles

“Today’s approval marks the first time in over a decade that there is a new FDA-approved treatment option for patients with hypereosinophilic syndrome,” said Ann Farrell, MD, director of the Division of Nonmalignant Hematology in the FDA’s Center for Drug Evaluation and Research. “FDA is committed to helping develop safe and effective treatment options for this group of rare and debilitating blood diseases and other rare conditions.”

Nucala, an interleukin-5 (IL-5) antagonist monoclonal antibody (IgG1 kappa), is already approved as an add-on maintenance treatment in patients aged 6 years and older with severe asthma with an eosinophilic phenotype. It is also indicated for the treatment of eosinophilic granulomatosis with polyangiitis (EGPA) in adults. 

For more information visit


  1. FDA approves first drug to treat group of rare blood disorders in nearly 14 years. Accessed September 28, 2020.
  2. Nucala [package insert]. Research Triangle Park, NC: GlaxoSmithKline; 2020.