Nucala (mepolizumab) works by blocking a specific protein in your blood called interleukin-5, or IL-5, which is the main signal your body uses to produce and activate eosinophils, a type of white blood cell that drives inflammation in conditions like severe asthma. By neutralizing IL-5 before it can reach eosinophils, Nucala reduces their numbers and calms the inflammatory response they cause. In clinical trials, this cut asthma attacks roughly in half compared to a placebo.
The Role of Eosinophils in Inflammation
Eosinophils are white blood cells that play a role in fighting parasites and infections, but when your body produces too many of them, they become a problem. They cluster in your airways, sinuses, blood vessels, or other tissues and release substances that cause swelling, mucus buildup, and tissue damage. This is the core issue in eosinophilic asthma, nasal polyps, and several other conditions Nucala treats.
IL-5 is the key chemical messenger that tells your bone marrow to make more eosinophils, helps them mature, and keeps them alive longer. In people with eosinophilic diseases, IL-5 levels are abnormally high, which means the body keeps flooding tissues with eosinophils that perpetuate chronic inflammation. Nucala targets this cycle at the source.
How Nucala Blocks IL-5
Nucala is a humanized monoclonal antibody, meaning it’s a lab-engineered protein designed to lock onto one specific target with high precision. Once injected, it binds directly to IL-5 molecules circulating in your blood. This prevents IL-5 from attaching to its receptor on the surface of eosinophils. Without that signal, eosinophils don’t proliferate the way they normally would, and existing eosinophils gradually decline.
The effect isn’t instant. In clinical studies, blood eosinophil levels took an average of about 26 hours to drop by half after a dose of Nucala. By 30 days, eosinophil counts fell to a geometric mean of around 92 cells per microliter, well below the elevated levels typical of eosinophilic disease. Nucala doesn’t destroy eosinophils outright the way some other biologics do. Instead, it starves them of the growth signal they need, so the decline is more gradual.
What This Means for Asthma Attacks
The real-world payoff of lowering eosinophils is fewer flare-ups. In two large trials reviewed by the FDA, patients on Nucala experienced about 0.8 to 1.2 asthma exacerbations per year, compared to 1.7 to 2.4 per year on placebo. That translates to roughly a 50% reduction in the rate of asthma attacks across both studies. The benefit was consistent across different patient subgroups, including those with higher baseline eosinophil counts and those on oral corticosteroids.
For many patients, this reduction means fewer emergency visits, fewer courses of oral steroids, and better day-to-day breathing. Nucala is not a rescue medication and won’t stop an attack in progress. It’s a maintenance treatment designed to lower your baseline level of airway inflammation so attacks happen less often and less severely over time.
Conditions Nucala Treats
Nucala is FDA-approved for five conditions, all driven by excess eosinophils:
- Severe eosinophilic asthma in adults and children aged 6 and older, as an add-on to existing asthma medications.
- Chronic rhinosinusitis with nasal polyps in adults 18 and older, where eosinophil-driven inflammation causes recurring polyp growth in the sinuses.
- Chronic obstructive pulmonary disease (COPD) with an eosinophilic component, in adults whose symptoms aren’t adequately controlled.
- Eosinophilic granulomatosis with polyangiitis (EGPA), a rare condition where eosinophils damage blood vessels and organs.
- Hypereosinophilic syndrome (HES) in patients 12 and older who have had persistently high eosinophil levels for six months or more without another identifiable cause.
The common thread across all five is that eosinophils are the primary driver of disease, and lowering them with Nucala addresses the root problem rather than just managing symptoms.
How It’s Given
Nucala is a subcutaneous injection, meaning it goes just under the skin rather than into a vein. For severe asthma, the standard adult dose is 100 mg once every four weeks. Children aged 6 to 11 receive a lower dose of 40 mg on the same schedule. For EGPA, the dose is higher: 300 mg every four weeks, given as three separate 100 mg injections at the same visit.
Injections can be administered in the upper arm, thigh, or abdomen. After initial training from a healthcare provider, many patients or caregivers learn to give injections at home using a prefilled autoinjector or syringe.
Common Side Effects
In clinical trials, the most frequently reported side effects were headache and nasopharyngitis (a cold-like irritation of the nose and throat). Back pain and injection-site reactions also appeared more often in treated patients than in those on placebo. These tend to be mild and manageable for most people.
A large analysis of over 82,000 adverse event reports submitted to the FDA found that about 18.5% of reported side effects occurred within the first 30 days of treatment. Sleep disturbances appeared as a notable signal in that database, along with back pain and headache. Anaphylactic reactions were rare but statistically significant in the data, which is why patients are typically monitored briefly after their first few injections.
One pattern worth understanding: asthma exacerbations themselves are among the most commonly reported adverse events in long-term Nucala studies. This doesn’t mean the drug causes attacks. Rather, patients on Nucala still have severe asthma, and flare-ups still occur, just less frequently than they would without treatment.
How Nucala Differs From Other Biologics
Several biologic medications target the eosinophil pathway, but they do so at different points. Nucala blocks IL-5 before it reaches eosinophils, which reduces their numbers gradually by cutting off their growth signal. Another biologic, benralizumab, takes a different approach: it binds directly to the IL-5 receptor on eosinophils and triggers their rapid destruction. In head-to-head comparisons, benralizumab brought eosinophil counts down to near zero (around 8 cells per microliter) within hours, while Nucala’s reduction was slower and left a higher residual count.
Neither approach is categorically better. Some patients respond well to Nucala’s more gradual eosinophil suppression, and the choice between biologics often depends on individual disease severity, response patterns, and how a patient tolerates each drug. What matters most is that eosinophil levels drop enough to reduce symptoms and prevent flare-ups, and Nucala accomplishes that reliably for most patients with eosinophilic disease.