FDA Approves First Drug to Treat Group of Rare Blood Disorders in Over 10 Years

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The US Food and Drug Administration (FDA) has approved mepolizumab (Nucala; GlaxoSmithKline) for patients aged ≥12 years with hypereosinophilic syndrome (HES) for ≥6 months without another identifiable non-hematologic secondary cause.

“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, in the agency’s September 25, 2020 press release.

HES is a group of rare disorders associated with persistent eosinophilia that can potentially lead to inflammation, organ damage, and death if left untreated. Symptoms of HES include skin rashes, asthma, itching, difficulty breathing, abdominal pain, vomiting, diarrhea, arthritis, muscle inflammation, congestive heart failure, deep venous thrombosis, and anemia.

The approval was based on data from a 32-week, double-blind, placebo-controlled trial that assessed the safety and efficacy of mepolizumab in 108 patients aged ≥12 years with HES.

Patients were randomly assigned to receive mepolizumab or placebo by injection every 4 weeks. Researchers then compared the proportion of subjects who experienced at least 1 HES flare during the 32-week treatment period, as well as the time to the first flare. A HES flare was defined as having at least 2 HES flares within the past 12 months and a blood eosinophil count of ≥1000 cells/µL.

The results showed that fewer patients in the mepolizumab group (28%) had HES flares vs patients in the placebo group (56%), with a 50% relative reduction. Also, the time to the first HES flare was later, on average, in the mepolizumab group vs placebo group.

As for safety, the most common side effects of mepolizumab in HES patients were upper respiratory tract infection and pain in extremities. Health care professionals should consider herpes zoster vaccination in patients treated with mepolizumab if medically appropriate as infections have occurred, according to the FDA press release.

Mepolizumab is an interleukin-5 antagonist monoclonal antibody and is already approved as an add-on maintenance treatment for patients aged ≥6 years with severe asthma with an eosinophilic phenotype and for adults with eosinophilic granulomatosis with polyangiitis.

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