Residency · Residency · Allergy Immunology

Severe Asthma - Biologics and Targeted Therapy

Defining Severe Asthma

Severe asthma, as defined by the joint European Respiratory Society and American Thoracic Society guidelines, is asthma that requires GINA Step 4 or 5 treatment (medium- to high-dose inhaled corticosteroids combined with a long-acting beta-agonist, with or without additional controllers) to maintain disease control, or that remains uncontrolled despite this level of therapy. This designation applies to approximately 5 to 10 percent of all asthma patients, yet this subpopulation accounts for roughly 50 percent of total asthma-related healthcare expenditures.

Before a patient is classified as having severe asthma and before biologic therapy is considered, a systematic evaluation must confirm the diagnosis and address modifiable factors. Medication adherence should be assessed through prescription refill records and, when available, FeNO suppression testing (a rise in FeNO after observed ICS use suggests prior non-adherence). Inhaler technique is incorrect in more than 70 percent of patients and must be directly observed and corrected. Comorbidities that mimic or exacerbate asthma, including gastroesophageal reflux disease, obesity, obstructive sleep apnea, vocal cord dysfunction, chronic rhinosinusitis, and anxiety or depression, must be identified and managed. Environmental exposures including tobacco smoke, occupational sensitizers, and domestic allergens require evaluation. Alternative diagnoses such as eosinophilic granulomatosis with polyangiitis (EGPA), allergic bronchopulmonary aspergillosis (ABPA), tracheomalacia, and bronchiectasis must be excluded.

Systematic Assessment Before Biologics

A comprehensive assessment before initiating biologic therapy includes confirmation of persistent airflow obstruction or bronchial hyperresponsiveness on spirometry, biomarker profiling (blood eosinophil count, FeNO, total IgE, and an allergen-specific IgE panel), and sputum cell counts where available. Chest computed tomography should be obtained to exclude bronchiectasis, air trapping, mucus plugging, and other structural pathology. Specific evaluation for ABPA should include total IgE, Aspergillus-specific IgE, and precipitins. Assessment for EGPA is warranted when blood eosinophils exceed 1,500 cells/mcL, particularly in the presence of extrapulmonary involvement, and should include ANCA testing.

FDA-Approved Biologics for Severe Asthma

Omalizumab (Xolair) - Anti-IgE

Omalizumab is a humanized monoclonal antibody that binds free IgE at the CE3 domain, preventing its engagement with FcepsilonRI on mast cells and basophils and leading to progressive downregulation of FcepsilonRI surface expression. It is indicated for moderate to severe allergic asthma in patients aged 6 years and older with a total IgE between 30 and 1,500 IU/mL and demonstrated sensitization to a perennial aeroallergen. Dosing is determined by a weight-and-IgE-based nomogram, with subcutaneous injections administered every 2 to 4 weeks at doses ranging from 150 to 375 mg. The pivotal INNOVATE trial demonstrated a 25 percent reduction in asthma exacerbations with significant corticosteroid-sparing effects. Omalizumab has also received approval for chronic spontaneous urticaria, chronic rhinosinusitis with nasal polyps, and IgE-mediated food allergy. Clinical benefit typically requires 12 to 16 weeks to manifest. Patients should be observed for 30 minutes following each of the first three injections, given the anaphylaxis risk of 0.1 to 0.2 percent.

Mepolizumab (Nucala) - Anti-IL-5

Mepolizumab is a humanized monoclonal antibody that binds interleukin-5, preventing its interaction with the IL-5 receptor alpha chain on eosinophils and thereby reducing eosinophil production and survival. It is indicated for severe eosinophilic asthma in patients aged 6 years and older with eosinophils at least 150 cells/mcL at treatment initiation or at least 300 cells/mcL within the preceding 12 months. The adult dose is 100 mg subcutaneously every 4 weeks, with a reduced dose of 40 mg for children aged 6 to 11 years. Pivotal trials demonstrated substantial efficacy: the DREAM trial showed a 52 percent reduction in exacerbations, MENSA showed a 53 percent reduction, and SIRIUS demonstrated a 50 percent reduction in oral corticosteroid dose. Mepolizumab has also received approval for EGPA, hypereosinophilic syndrome, and chronic rhinosinusitis with nasal polyps. Onset of clinical benefit is typically 4 to 8 weeks. An important mechanistic distinction is that mepolizumab reduces blood eosinophils to near-zero levels but does not completely deplete tissue eosinophils.

Reslizumab (Cinqair) - Anti-IL-5

Reslizumab shares the same mechanism as mepolizumab, binding circulating IL-5, and is indicated for severe eosinophilic asthma in patients aged 18 years and older with blood eosinophils at least 400 cells/mcL. It is distinguished by its weight-based intravenous dosing at 3 mg/kg every 4 weeks, which may provide more precise dosing in obese patients. Phase 3 trials demonstrated a 50 to 59 percent reduction in exacerbation rates. The anaphylaxis risk is approximately 0.3 percent, necessitating observation during and after infusion.

Benralizumab (Fasenra) - Anti-IL-5Ralpha

Benralizumab targets the IL-5 receptor alpha chain directly on eosinophils rather than the circulating IL-5 cytokine. Its afucosylated Fc region enhances binding to FcgammaRIIIa on natural killer cells, inducing antibody-dependent cell-mediated cytotoxicity (ADCC) that results in near-complete depletion of eosinophils in both blood and tissue. This mechanism of action is unique among the anti-eosinophil biologics and distinguishes benralizumab from the anti-IL-5 antibodies, which reduce but do not fully deplete tissue eosinophils. It is indicated for severe eosinophilic asthma in patients aged 12 years and older with eosinophils at least 150 cells/mcL. Dosing is 30 mg subcutaneously every 4 weeks for the first three doses, then every 8 weeks thereafter. The SIROCCO and CALIMA trials demonstrated 28 to 51 percent exacerbation reductions, with greater efficacy in patients with eosinophils at or above 300 cells/mcL. The ZONDA trial showed a 75 percent reduction in oral corticosteroid dose. Eosinophil depletion occurs rapidly, within 24 hours of the first dose. Benralizumab has also received approval for EGPA.

Dupilumab (Dupixent) - Anti-IL-4Ralpha

Dupilumab blocks the IL-4 receptor alpha subunit, which is the shared signaling component of both the IL-4 and IL-13 receptors, thereby inhibiting the biologic effects of both cytokines simultaneously. It is indicated for moderate to severe type 2 asthma in patients aged 6 years and older with blood eosinophils at least 150 cells/mcL, FeNO at least 25 ppb, or oral corticosteroid dependence. Dosing is 200 mg or 300 mg subcutaneously every 2 weeks following an initial loading dose. The LIBERTY ASTHMA QUEST trial demonstrated a 47.7 percent reduction in exacerbations in patients with eosinophils at or above 150 cells/mcL, increasing to 65.8 percent in those with eosinophils at or above 300 cells/mcL. The VENTURE trial showed a 70 percent reduction in oral corticosteroid dose.

Dupilumab is the only biologic that significantly reduces FeNO, reflecting its direct blockade of IL-13-mediated iNOS expression in the airway epithelium. It has the broadest approval portfolio among asthma biologics, with additional indications for atopic dermatitis, chronic rhinosinusitis with nasal polyps, eosinophilic esophagitis, prurigo nodularis, and T2-high chronic obstructive pulmonary disease. A notable pharmacodynamic effect is transient eosinophilia, observed in 5 to 14 percent of patients, which occurs because IL-4 blockade reduces eosinophil trafficking from blood into tissues, resulting in peripheral blood eosinophil accumulation. This is generally self-resolving and rarely clinically significant. Injection site reactions occur in approximately 15 percent of patients, and conjunctivitis is observed primarily in patients treated for atopic dermatitis rather than asthma.

Tezepelumab (Tezspire) - Anti-TSLP

Tezepelumab binds thymic stromal lymphopoietin (TSLP), an epithelial alarmin positioned at the apex of the type 2 inflammatory cascade, blocking its interaction with the TSLP receptor. It is indicated for severe asthma in patients aged 12 years and older and is unique among asthma biologics in having no biomarker requirement for eligibility, giving it the broadest indication. Dosing is 210 mg subcutaneously every 4 weeks. The NAVIGATOR trial demonstrated a 56 percent overall exacerbation reduction, with 70 percent reduction in patients with eosinophils at or above 300 cells/mcL. Critically, tezepelumab also showed a 41 percent exacerbation reduction in patients with eosinophils below 300 cells/mcL, representing the only biologic with demonstrated efficacy in T2-low severe asthma. The SOURCE trial, which evaluated oral corticosteroid sparing, did not meet its primary endpoint, though this remains a topic of discussion. Tezepelumab achieves the most comprehensive biomarker suppression of any asthma biologic, reducing eosinophils, FeNO, and IgE simultaneously. It is currently being studied for chronic rhinosinusitis with nasal polyps, COPD, and eosinophilic esophagitis.

<image>A comprehensive comparison chart of FDA-approved asthma biologics. Organized as a table with rows for each biologic (omalizumab, mepolizumab, reslizumab, benralizumab, dupilumab, tezepelumab) and columns for: target molecule (with molecular diagram showing where each biologic acts in the T2 cascade), route/frequency, eligible age, key biomarker requirement, pivotal trial name and exacerbation reduction percentage, OCS-sparing data, other approved indications, unique features, and key adverse effects. The T2 inflammatory cascade is shown as a vertical pathway on the left side: allergen/irritant at top, then epithelium releasing TSLP/IL-33/IL-25, then DCs/ILC2s, then Th2 cells, then IL-4/IL-5/IL-13 acting on target cells (eosinophils, B cells, smooth muscle, goblet cells). Each biologic is positioned at its point of action along this cascade, with tezepelumab at the top (most upstream) and anti-IL-5 agents further downstream.</image>

Itepekimab (Anti-IL-33) - Investigational

Itepekimab, an anti-IL-33 monoclonal antibody, is currently in clinical development. Phase 2 data from the MESOS trial in moderate to severe asthma suggest potential efficacy in both T2-high and T2-low disease. IL-33 functions as an alarmin released from damaged epithelium, acting through the ST2 receptor on ILC2s, Th2 cells, and mast cells. Other anti-IL-33 agents in development include astegolimab and tozorakimab.

Biologic Selection Algorithm

Step 1: Confirm T2 Status

The first step in biologic selection is determining whether the patient has T2-high or T2-low inflammation. T2-high status is indicated by blood eosinophils at or above 150 cells/mcL, FeNO at or above 20 ppb, or elevated total or specific IgE. When all biomarkers are low, the patient is classified as T2-low, and tezepelumab represents the only biologic with supporting efficacy data in this subgroup.

Step 2: Identify Predominant Phenotype

Among T2-high patients, the predominant driving phenotype guides biologic selection. For allergic-predominant disease (elevated IgE with demonstrated allergen sensitization), omalizumab or dupilumab are appropriate options. For eosinophilic-predominant disease (eosinophils at or above 300 cells/mcL with or without low IgE), mepolizumab, benralizumab, or dupilumab may be selected. For patients with overlap between allergic and eosinophilic phenotypes, dupilumab provides the broadest mechanistic coverage, with omalizumab as an alternative. For oral corticosteroid-dependent patients, dupilumab or benralizumab have the strongest corticosteroid-sparing data.

Step 3: Consider Comorbidities

Comorbid conditions significantly influence biologic selection. For patients with comorbid chronic rhinosinusitis with nasal polyps, dupilumab, mepolizumab, or omalizumab are all approved. Comorbid atopic dermatitis strongly favors dupilumab. Comorbid EGPA is addressed by mepolizumab or benralizumab. Comorbid chronic spontaneous urticaria favors omalizumab. Comorbid food allergy is addressed by omalizumab. Comorbid eosinophilic esophagitis favors dupilumab.

Step 4: Assess Response at 4-6 Months

Treatment response should be formally assessed at 4 to 6 months, evaluating exacerbation frequency, oral corticosteroid requirements, symptom control (ACT score), lung function (FEV1), and biomarker trends. Adequate responders should continue therapy with attempted oral corticosteroid taper. Inadequate responders should undergo reassessment of their phenotype and consideration of switching to a biologic in a different therapeutic class (switching within the same class, such as from one anti-IL-5 to another, is unlikely to yield benefit). Super-responders who have maintained stability for 1 to 2 years may consider a trial of biologic cessation, though relapse is common.

<image>A clinical decision flowchart for selecting a biologic in severe asthma. Starting box: "Severe asthma uncontrolled on Step 4-5 therapy; comorbidities/adherence addressed." Decision diamond 1: "T2 biomarkers elevated? (Eos >=150 OR FeNO >=20 OR IgE elevated with sensitization)." If No: "Consider tezepelumab (only biologic with T2-low evidence)." If Yes, Decision diamond 2: "Predominant driver?" Three branches: (A) Allergic (IgE + sensitization): omalizumab or dupilumab. (B) Eosinophilic (eos >=300): mepolizumab, benralizumab, or dupilumab. (C) Mixed/OCS-dependent: dupilumab preferred. Side panel: "Comorbidity overlay" - CRSwNP pointing to dupilumab/mepolizumab, AD pointing to dupilumab, EGPA pointing to mepolizumab/benralizumab, CSU pointing to omalizumab. Bottom box: "Reassess response at 4-6 months: exacerbations, OCS dose, ACT score, FEV1, biomarkers."</image>

Bronchial Thermoplasty

Bronchial thermoplasty is a bronchoscopic procedure that delivers radiofrequency thermal energy to the airway wall, reducing airway smooth muscle mass. The AIR2 trial demonstrated improvements in asthma quality of life scores, a 32 percent reduction in severe exacerbations, and an 84 percent reduction in emergency department visits sustained at 5-year follow-up. Patient selection is critical: bronchial thermoplasty is most appropriate for patients with severe asthma uncontrolled despite maximal medical therapy in whom the predominant mechanism is not primarily eosinophilic (for whom biologics would be preferred). The procedure carries risks of short-term asthma worsening and hospitalization during the treatment period. Three bronchoscopic sessions are required, each treating a different lung region and separated by 3 weeks. The procedure is limited to specialized centers, though 10-year follow-up data have demonstrated sustained benefit.

Oral Corticosteroid Sparing and Management

Chronic oral corticosteroid maintenance in severe asthma is associated with substantial cumulative morbidity that is dose-dependent, including osteoporosis, type 2 diabetes mellitus, adrenal suppression, posterior subcapsular cataracts, increased infection risk, weight gain, and psychiatric adverse effects. Even as few as four courses per year of burst oral corticosteroid therapy has been shown to significantly increase the risk of these adverse effects. Oral corticosteroid reduction is therefore a primary therapeutic goal of biologic therapy.

Biologics with demonstrated oral corticosteroid-sparing efficacy include dupilumab (VENTURE trial), benralizumab (ZONDA trial), mepolizumab (SIRIUS trial), and tezepelumab (SOURCE trial, which did not meet its primary endpoint). During oral corticosteroid tapering, screening for adrenal insufficiency is essential, with morning cortisol levels measured before and during the taper and ACTH stimulation testing performed if results are concerning. A reasonable taper schedule reduces prednisone by 2.5 to 5 mg equivalent every 2 to 4 weeks with close clinical monitoring.

Key Clinical Pearls

• Confirm adherence, inhaler technique, and comorbidities before initiating biologics - these are the most common reasons for "uncontrolled" asthma
• Dupilumab is the only biologic that reduces FeNO (reflects IL-13 blockade); it may cause transient eosinophilia (due to reduced tissue homing)
• Tezepelumab (anti-TSLP) is the only biologic with efficacy data in T2-low severe asthma and requires no biomarker cutoff for eligibility
• Benralizumab (anti-IL-5Ralpha) uniquely depletes eosinophils via ADCC, not just neutralizing IL-5; this results in near-zero blood and tissue eosinophils
• Biologic response should be assessed at 4-6 months; consider switching biologic class (not within-class) for inadequate responders
• OCS sparing is a critical outcome; screen for adrenal insufficiency during OCS taper
• Comorbidities guide biologic selection: CRSwNP favors dupilumab/mepolizumab; AD favors dupilumab; EGPA favors mepolizumab/benralizumab

References

1. Chung KF, et al. International ERS/ATS guidelines on definition, evaluation and treatment of severe asthma. Eur Respir J. 2014;43(2):343-373.
2. Menzies-Gow A, et al. Tezepelumab in adults and adolescents with severe, uncontrolled asthma (NAVIGATOR). N Engl J Med. 2021;384(19):1800-1809.
3. Castro M, et al. Dupilumab efficacy and safety in moderate-to-severe uncontrolled asthma (LIBERTY ASTHMA QUEST). N Engl J Med. 2018;378(26):2486-2496.
4. Ortega HG, et al. Mepolizumab treatment in patients with severe eosinophilic asthma (MENSA). N Engl J Med. 2014;371(13):1198-1207.
5. FitzGerald JM, et al. Benralizumab, an anti-interleukin-5 receptor alpha monoclonal antibody (CALIMA/SIROCCO). Lancet. 2016;388(10056):2128-2141.

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