Mepolizumab is an injectable biologic medication used to treat chronic rhinosinusitis with nasal polyps, particularly in people whose polyps keep coming back despite surgery or steroid treatments. Sold under the brand name Nucala, it works by blocking a specific immune signal that drives the inflammation behind polyp growth. It’s given as a 100 mg injection under the skin once every four weeks.
How Mepolizumab Works
Nasal polyps form when a type of immune cell called an eosinophil accumulates in the nasal tissue. These cells release toxic products that damage tissue and fuel polyp growth. In 80% to 90% of white patients with nasal polyps, this eosinophilic inflammation is the dominant pattern, driven by overproduction of a signaling molecule called interleukin-5 (IL-5).
Mepolizumab is a lab-made antibody that binds to IL-5 and neutralizes it. Without that signal, fewer eosinophils gather in the nasal tissue, less damage occurs, and polyps can shrink. It doesn’t cure the underlying condition, but it turns down the immune overreaction that keeps polyps growing back.
Who It’s Designed For
Mepolizumab is aimed at people with severe or recurrent nasal polyps who haven’t gotten lasting relief from standard treatments like nasal corticosteroid sprays, oral steroid courses, or sinus surgery. Many of these patients also have asthma or aspirin-exacerbated respiratory disease (AERD), conditions that share the same eosinophilic inflammation.
Doctors typically look at blood eosinophil levels to help decide whether a patient is a good candidate. Research in severe eosinophilic asthma has confirmed that patients with blood eosinophil counts at or above 150 cells per microliter tend to respond well to mepolizumab. While the nasal polyps indication doesn’t have an identical formal cutoff, clinical trial data shows the drug works across a range of eosinophil levels, including patients below that 150 threshold.
How Well It Works
In clinical trials, about half of patients on mepolizumab saw a meaningful reduction in polyp size (at least a 1-point drop on the nasal polyp scoring scale), compared to roughly 28% to 30% of patients on placebo. That benefit held whether patients also had asthma or AERD. Patients with AERD, a notoriously difficult-to-treat group, saw especially strong separation from placebo: 51% improved versus just 21% on placebo.
Beyond polyp shrinkage, patients also reported meaningful improvements in nasal congestion. The drug doesn’t eliminate polyps entirely for most people, but it can reduce their size enough to improve breathing, restore some sense of smell, and reduce the need for oral steroid courses or repeat surgeries.
Results take time. Clinical trial measurements were taken over a 52-week treatment period, and improvements build gradually rather than appearing in the first few weeks. Patients should expect to stay on treatment for several months before judging whether the drug is helping.
Side Effects
Mepolizumab has a reassuringly mild side effect profile. In the 52-week nasal polyps trial, 82% of patients on mepolizumab reported at least one side effect, compared to 84% on placebo, meaning the drug barely increased the overall rate of adverse events beyond what patients experienced on a sugar shot.
The most commonly reported issues were:
- Nasopharyngitis (common cold symptoms): 25% on mepolizumab vs. 23% on placebo
- Headache: 18% vs. 22% on placebo
- Sinusitis: 5% vs. 11% on placebo
Injection site reactions were uncommon, affecting about 2.4% of patients on mepolizumab. Serious side effects occurred in 6% of treated patients, slightly lower than the 7% seen with placebo. Less common issues reported in more than 5% of either group included nosebleeds, back pain, upper respiratory infections, cough, and joint pain.
How It Compares to Other Biologics
Mepolizumab is one of several biologic drugs now available for nasal polyps. The others include dupilumab (Dupixent) and omalizumab (Xolair), each targeting a different piece of the inflammatory pathway. A network analysis comparing all available biologics across seven clinical trials involving nearly 2,000 patients found that dupilumab showed the strongest effects on polyp size reduction and nasal congestion relief at both 24 weeks and beyond 48 weeks. The other biologics, including mepolizumab and omalizumab, performed similarly to each other, with no significant differences between them.
That doesn’t make mepolizumab a lesser choice for every patient. People who also have severe eosinophilic asthma may benefit from treating both conditions with a single biologic, and mepolizumab has a long track record in asthma. Patients who haven’t responded to one biologic sometimes respond to another, so having multiple options matters. The choice between biologics often comes down to a patient’s specific combination of conditions, their insurance coverage, and how they respond in practice.
What Treatment Looks Like
You’ll receive a 100 mg injection under the skin every four weeks. This can be done at a doctor’s office or, once trained, at home using a prefilled autoinjector or syringe. The injection itself takes seconds, and most people tolerate it well.
Because mepolizumab manages inflammation rather than curing the underlying condition, it’s generally considered a long-term treatment. Stopping the drug allows eosinophilic inflammation to return, which means polyps can regrow. Your doctor will reassess periodically whether the benefits justify continuing, but many patients stay on treatment indefinitely if it’s working.