Mepolizumab

• Severe Eosinophilic Asthma
• Eosinophilic Granulomatosis with Polyangiitis
• Hypereosinophilic Syndrome
• Chronic Rhinosinusitis with Nasal Polyps

Mepolizumab (Nucala) is a humanized monoclonal antibody targeting interleukin-5 (IL-5), used as add-on therapy for severe eosinophilic asthma, eosinophilic granulomatosis with polyangiitis (EGPA), hypereosinophilic syndrome (HES), and chronic rhinosinusitis with nasal polyps. It is one of several biologics that have transformed the management of severe type-2 inflammatory disease, allowing many patients to dramatically reduce — and sometimes eliminate — chronic oral corticosteroid exposure and the cumulative toxicity that comes with it.

Mechanism of Action

IL-5 is the master regulator of eosinophil biology. It is produced by Th2 lymphocytes, type 2 innate lymphoid cells, and mast cells, and it drives eosinophil differentiation in the bone marrow, recruitment from blood into tissue, activation, and survival. In severe eosinophilic asthma, EGPA, and HES, IL-5-driven eosinophil expansion fuels the airway inflammation, vasculitis, or end-organ damage that characterize each disease. Eosinophils release granule proteins, leukotrienes, and reactive oxygen species that cause direct tissue injury and amplify the inflammatory response.

Mepolizumab is a humanized IgG1-kappa antibody that binds free IL-5 with high affinity, preventing it from engaging the IL-5 receptor alpha (IL-5Ralpha) on eosinophils. Within weeks, blood eosinophil counts fall dramatically and tissue eosinophilia regresses. Mepolizumab does not directly kill eosinophils — for that mechanism, the related agent benralizumab, which targets IL-5Ralpha and triggers antibody-dependent cellular cytotoxicity, is used. Reslizumab is a third anti-IL-5 antibody, given intravenously rather than subcutaneously. The NIH overview of eosinophilic disorders provides additional context, and the American Academy of Allergy, Asthma & Immunology biologics guide compares the available agents.

Clinical Use

For severe asthma, mepolizumab is an add-on therapy for patients aged 6 and older with an eosinophilic phenotype — typically a blood eosinophil count of at least 150 cells/microliter at initiation or 300 cells/microliter in the prior year — who remain symptomatic despite high-dose inhaled corticosteroids combined with a long-acting bronchodilator such as salmeterol or formoterol. It reduces exacerbations by roughly half and frequently allows substantial reduction in oral prednisone dose.

Within the biologic class, omalizumab is preferred when allergic IgE-driven disease dominates, dupilumab when atopic dermatitis or chronic rhinosinusitis with nasal polyps is also present, and the anti-IL-5 agents (mepolizumab, benralizumab, reslizumab) when eosinophilic biology dominates. For chronic sinusitis with nasal polyps, mepolizumab and dupilumab both have FDA approval. In EGPA and HES, mepolizumab provides steroid-sparing benefit and reduces relapse rates.

Before starting any biologic, the workup should confirm that the patient has truly severe disease — meaning correct diagnosis, optimized inhaler technique, addressed comorbidities (rhinosinusitis, GERD, obesity, untreated obstructive sleep apnea), reasonable adherence, and avoidance of triggers. Florida-specific environmental triggers warrant attention; our articles on managing asthma and COPD in humid St. Pete and Florida pollen season tips are good patient resources.

Across the asthma biologics, the choice depends on phenotype. Total IgE levels and aeroallergen sensitization point toward omalizumab; high blood eosinophils with allergic features point toward dupilumab; high blood eosinophils without significant atopy point toward an anti-IL-5 strategy. Many patients fit more than one phenotype, and switching between biologics when initial response is inadequate is increasingly common. Some patients require trial of two or three agents before optimal control is achieved.

How to Take It

Mepolizumab is given as a subcutaneous injection every four weeks. The 100 mg single-vial reconstituted formulation is given by a clinician; the prefilled autoinjector and prefilled syringe (both 100 mg/mL) can be self-administered at home after appropriate training. Injection sites include the upper arm, thigh, or abdomen, rotating sites with each dose. Refrigerate the device until use, then allow it to reach room temperature for about 30 minutes before injecting to reduce sting at the injection site.

Mepolizumab is not a rescue medication and does not work for acute bronchospasm. Patients must continue their controller inhalers exactly as prescribed and have a short-acting beta-agonist available for breakthrough symptoms. Improvement in asthma symptoms and exacerbation frequency typically becomes apparent over the first three months. Inhaled and oral corticosteroids should be tapered only under physician supervision, never abruptly — sudden steroid withdrawal can unmask underlying adrenal insufficiency or destabilize coexisting EGPA.

Monitoring and Follow-Up

No routine laboratory monitoring is mandated. Most clinicians recheck a CBC with differential to confirm eosinophil suppression at three to six months and assess clinical response — exacerbation frequency, oral steroid burden, asthma control questionnaire scores (ACQ or ACT), and lung function with spirometry. Screening for parasitic (helminth) infection is recommended in patients from endemic areas before initiating therapy because eosinophils contribute to host defense against helminths. Herpes zoster vaccination should ideally be completed before treatment because of reports of zoster reactivation in this drug class. Patients new to lab-based monitoring may find understanding blood work and lab panels a useful primer.

Special Populations

No dose adjustment is needed for renal or hepatic impairment, age, or weight in the standard adult indications. Pediatric use is approved from age 6 for severe eosinophilic asthma at a reduced 40 mg dose. Pregnancy data are limited; monoclonal antibodies cross the placenta increasingly during the second and third trimesters, and shared decision-making is appropriate when severe asthma carries its own significant pregnancy risks — uncontrolled asthma in pregnancy is associated with worse maternal and fetal outcomes. Live vaccines should be avoided concurrently because of theoretical concern about altered immune response, though most routine adult vaccines (inactivated influenza, pneumococcal, COVID-19, tetanus boosters) are safe and recommended.

Patient Counseling Pearls

Self-injection success depends on technique. The training session at the first clinic-administered dose should be reinforced with periodic re-checks, particularly for older patients or those with vision or dexterity limitations. Family members can be trained as backups. The autoinjector device is taken out of the refrigerator 30 minutes before use to allow it to reach room temperature, which reduces injection-site stinging considerably. Sites should be rotated systematically and any persistent injection-site nodules reported.

Adherence to the every-four-week schedule matters. Missed doses or stretched intervals risk eosinophil resurgence and exacerbation. Patients planning travel should arrange dose timing so they are not stranded without a dose, and the manufacturer's home delivery program can ship to alternate addresses with notice.

Mepolizumab does not eliminate the need for inhaler technique review. Many patients with severe asthma have been on inhalers so long that they have drifted into suboptimal technique, and a single careful demonstration with a spacer can yield meaningful improvement independent of the biologic. Smoking cessation, weight management, and treatment of comorbid GERD and obstructive sleep apnea round out the comprehensive approach. Our recognizing burnout and exhaustion and chronic stress and physical illness articles touch on the often-overlooked psychological dimension of severe chronic disease.

Drug Interactions and Safety Considerations

Mepolizumab has no clinically significant pharmacokinetic interactions because it is not metabolized by CYP enzymes and is cleared by reticuloendothelial proteolysis like other monoclonal antibodies. The main pharmacodynamic consideration is inhaled and oral corticosteroid tapering — these should never be reduced abruptly, both because of adrenal insufficiency risk after prolonged exposure and because of the risk of unmasking eosinophilic granulomatosis with polyangiitis in patients whose vasculitis was previously suppressed by steroid therapy.

Vaccinations should be reviewed at initiation. Inactivated influenza, pneumococcal (PCV20 or PCV15 plus PPSV23 per current ACIP guidance), recombinant zoster, COVID-19, and Tdap boosters are all appropriate and important — patients with severe asthma face significantly higher morbidity from respiratory infections.

When to Contact Your Doctor

Call promptly for any signs of an allergic reaction after an injection — hives, throat swelling, wheezing, or lightheadedness — even if mild and delayed by hours or days. Worsening asthma despite therapy, painful zoster-like rash, new fever and night sweats, or any signs suggestive of EGPA flare (new neurologic symptoms, sinus disease, joint pain, palpable purpura) also warrant evaluation. Patients should keep a running record of exacerbations and oral steroid bursts to help the team gauge whether the biologic is working.

If you have questions about mepolizumab or your asthma management, our team at Zimmer Medical Group can help — contact us or schedule a visit.