A Guide to Biologic Therapies For Severe Asthma
Understanding the Disease Landscape
Severe asthma is defined as uncontrolled disease despite high-dose inhaled corticosteroids (ICS) plus a second controller agent, or when it requires oral corticosteroids (OCS) to maintain control. The critical first step before initiating any biologic is phenotyping — distinguishing T2-high from T2-low inflammation, as this fundamentally shapes which therapy is appropriate.
T2-High inflammation is characterized by elevated eosinophils (≥150–300 cells/µL or ≥2–3% in sputum), raised FeNO (≥25 ppb), and elevated total serum IgE. This phenotype responds well to currently available biologics. T2-Low inflammation, by contrast, lacks these biomarkers and represents a significant unmet need, as existing therapies offer little benefit in this population.
The Role of Biologics: A Targeted Revolution
Biologics represent a paradigm shift in severe asthma management. Unlike broad immunosuppressants, they precision-target specific inflammatory mediators, reducing exacerbations, OCS dependence, and hospitalizations while improving lung function and quality of life. Each agent has a distinct mechanistic niche:
Anti-IgE — Omalizumab
Targets the allergic arm of T2 inflammation by neutralizing free IgE. It is the most established biologic, approved from age ≥6, and requires a positive perennial allergen test with IgE levels between 30–1500 IU/mL. It is especially suited to patients with allergic asthma and comorbid allergic rhinitis or food allergy.
Anti-IL-5 Pathway — Mepolizumab, Benralizumab, Reslizumab
These three agents target the eosinophilic axis, which is the dominant driver of T2-high inflammation in many patients.
- Mepolizumab binds IL-5 itself (the key eosinophil survival cytokine), requiring eosinophils ≥150/µL, approved from age ≥6.
- Benralizumab targets the IL-5 receptor, leading to near-complete eosinophil depletion via ADCC (antibody-dependent cytotoxicity). It requires eosinophils ≥300/µL and is approved from age ≥12.
- Reslizumab also binds IL-5 but is administered intravenously and requires the highest eosinophil threshold (≥400/µL), approved only for adults ≥18. Its IV route can be a limitation in practice.
Dual IL-4/IL-13 Blockade — Dupilumab
Dupilumab blocks the shared IL-4Rα receptor, inhibiting both IL-4 and IL-13 signaling — two cytokines central to type 2 airway inflammation, mucus hypersecretion, and IgE class switching. Its biomarker threshold is eosinophils ≥150/µL and/or FeNO ≥25 ppb, giving it broader applicability. Approved from age ≥6, it also has the widest indication portfolio, including atopic dermatitis, chronic rhinosinusitis, and eosinophilic esophagitis — making it particularly attractive for patients with multiple type-2 comorbidities.
Anti-TSLP — Tezepelumab
Tezepelumab is arguably the most significant advance in recent years. By blocking TSLP (Thymic Stromal Lymphopoietin) — an epithelial-derived alarmin sitting upstream of the entire T2 cascade — it interrupts multiple inflammatory pathways simultaneously. Crucially, it has no biomarker threshold requirement, making it the only approved biologic suitable for both T2-high and potentially T2-low patients. Approved from age ≥12.
Clinical Pearls That Matter in Practice
The infographic highlights four management principles that are often underappreciated:
- Biologics are add-on therapy, not replacements. ICS must never be stopped; low-dose ICS should continue alongside the biologic. This is a common misconception patients have.
- Effectiveness assessment takes time. The recommended evaluation window is 4–6 months, looking for reduced OCS use, fewer exacerbations, and improved symptoms and lung function. Premature discontinuation is a clinical mistake.
- Discontinuation carries real risk. Stopping a biologic is not straightforward — there is a meaningful risk of symptom rebound and exacerbation, so the decision requires careful shared decision-making.
- Home administration improves adherence. Most SC biologics can be self-administered, but the first three doses should be given in a supervised clinical setting to monitor for hypersensitivity reactions.
The Future Pipeline: Where Is the Field Heading?
The pipeline signals several exciting directions:
- Extended dosing intervals — Depemokimab (anti-IL-5) dosed every 6 months subcutaneously (currently in Phase III) could dramatically improve adherence and reduce treatment burden compared to monthly regimens.
- Novel upstream targets — IL-33 inhibitors (Itepekimab) and ST2 inhibitors (Astegolimab) target another epithelial alarmin pathway, with particular promise in patients with low eosinophil counts who don’t qualify for current eosinophil-directed therapies.
- Innovative delivery — Inhaled anti-TSLP (Ecleralimab) and ultra-long-acting TSLP blockade (Verekitug, up to 6 months) aim to deliver targeted therapy directly to the airway while extending dosing intervals.
- Dual-target biologics — Lunsekimig, a nanobody blocking both IL-13 and TSLP simultaneously, in Phase II, could offer synergistic pathway inhibition in a single molecule — an elegant approach to the complexity of T2 inflammation.
Summary
Biologics have transformed severe asthma from a condition managed reactively with OCS — with all their systemic toxicity — to one managed proactively through precision immunology. The key to success lies in accurate phenotyping, matching the right biologic to the right biomarker profile, setting realistic expectations around timelines, and never abandoning foundational ICS therapy. As the pipeline matures, the field is moving toward longer-acting, broader-spectrum, and potentially T2-low-effective therapies that will extend these benefits to patients currently left without targeted options.
