Mepolizumab, sold under the brand name Nucala, is a humanized monoclonal antibody used for the treatment of severe eosinophilic asthma, eosinophilic granulomatosis, and hypereosinophilic syndrome (HES). It recognizes and blocks interleukin-5 (IL-5), a signalling protein of the immune system. The most common side effects include headache, injection site reactions, and back pain. Medical uses Mepolizumab is approved by the U.S. Food and Drug Administration (FDA) for the maintenance treatment of severe asthma in patients aged six years or older and with an eosinophilic phenotype in combination with other medicines used to treat asthma. In the European Union it is approved as an add-on treatment for severe refractory eosinophilic asthma in adults. In studies, mepolizumab cut the necessity for hospitalisation due to asthma exacerbations in half, as compared to placebo. In December 2017, the FDA expanded mepolizumab's indication to treat adults with eosinophilic granulomatosis with polyangiitis, which is a rare autoimmune condition that can cause vasculitis. In September 2020, the FDA expanded mepolizumab's indication to treat adults and children aged twelve years and older with hypereosinophilic syndrome (HES) for six months or longer without another identifiable non-blood related cause of the disease. Side effects Common side effects in clinical trials included headache (19% of patients under mepolizumab treatment versus 18% under placebo), reactions at the site of injection (8% versus 3%), infections of the urinary tract (3% versus 2%) and the lower respiratory tract, eczema and muscle spasms (both 3% versus <1%). The most common side effects in people with hypereosinophilic syndrome (HES) include: upper respiratory tract infection and pain in extremities (such as the hands, legs and feet). Overdose Single doses of 15 times the usual therapeutic dose have been tolerated in studies without significant side effects. Interactions No interaction studies have been conducted. As with other monoclonal antibodies, the interaction potential is considered to be low. Pharmacology Mechanism of action Mepolizumab binds to IL-5 and prevents it from binding to its receptor, more specifically the interleukin 5 receptor alpha subunit, on the surface of eosinophil white blood cells. While eosinophils play a role in inflammation associated with asthma, the exact mechanism of mepolizumab is unknown. Pharmacokinetics After subcutaneous injection, mepolizumab has an estimated bioavailability of 80% and reaches highest blood plasma concentrations after four to eight days. Like other antibodies, it is degraded by proteolytic enzymes. Its biological half-life is 20 days on average, ranging from 16 to 22