Biologic Agents in Severe Asthma

Approximately 50 to 60 percent of patients with severe asthma have type 2 high (T2-high) disease, defined by activation of the type 2 helper T-cell cytokine axis and innate lymphoid cell type 2 signaling. The central cytokines are interleukin-4, interleukin-5, and interleukin-13. Interleukin-4 drives B-cell class switching to IgE production and promotes eosinophil recruitment. Interleukin-5 is the principal growth factor, maturation signal, and survival factor for eosinophils — the primary driver of blood and airway eosinophilia. Interleukin-13 induces goblet cell metaplasia, smooth muscle hyperresponsiveness, and subepithelial fibrosis. The three currently approved biologic targets — IgE, interleukin-5 (and its receptor), and the shared interleukin-4/interleukin-13 receptor subunit — all operate within this T2-high pathway.

Type 2 low disease encompasses patients whose severe asthma is driven by neutrophilic, non-eosinophilic, or poorly characterized mechanisms. No approved biologic agents are effective for type 2 low disease; these patients are managed with maximized conventional inhaled therapy and low-dose oral corticosteroids where necessary.

Blood eosinophil count is the most widely used biomarker. A count at or above 300 cells per microliter is the threshold most consistently associated with clinical response to anti-interleukin-5 agents; counts at or above 150 cells per microliter serve as lower-boundary eligibility criteria for some agents. Fractional exhaled nitric oxide reflects eosinophilic airway inflammation driven by interleukin-13 and interleukin-4; values at or above 25 parts per billion suggest T2-high inflammation, and values above 50 parts per billion predict stronger responses to interleukin-4/interleukin-13 pathway blockade. Serum total IgE is required for omalizumab dosing — the dosing table is defined by IgE level and body weight, and very high IgE values may fall outside the eligible range.

Mepolizumab and reslizumab are humanized monoclonal antibodies that bind interleukin-5 directly, preventing it from interacting with the interleukin-5 receptor on eosinophil precursors and mature eosinophils. By neutralizing interleukin-5, they reduce eosinophil production in the bone marrow, impair eosinophil recruitment to the airway, and reduce eosinophil survival — producing sustained reductions in blood and tissue eosinophil counts. Mepolizumab is given subcutaneously every four weeks and is approved for severe eosinophilic asthma in patients aged six and older with blood eosinophils at or above 150 cells per microliter. Reslizumab is given intravenously every four weeks and is approved for adults with blood eosinophils at or above 400 cells per microliter.

Benralizumab differs from mepolizumab and reslizumab by targeting the interleukin-5 receptor alpha subunit on eosinophils and basophils rather than interleukin-5 itself. This distinction carries an important secondary mechanism: benralizumab is afucosylated (engineered to lack the fucose sugar on its Fc region), which dramatically enhances binding to FcγRIII on natural killer cells and macrophages, triggering antibody-dependent cellular cytotoxicity of eosinophils and basophils. The result is near-complete depletion of blood and tissue eosinophils — a more profound reduction than achieved by interleukin-5 neutralization alone. Benralizumab is given subcutaneously every four weeks for the first three doses, then every eight weeks, which is the most convenient dosing schedule in the anti-interleukin-5 class. It is approved for severe eosinophilic asthma in patients aged 12 and older.

All biologic candidates must have confirmed severe asthma (frequent exacerbations or oral corticosteroid dependence despite optimized inhaled therapy), confirmed T2-high phenotype by biomarker, and documented adherence and correct inhaler technique — the most common reasons patients appear refractory to conventional therapy. Once biologic therapy is initiated, a treatment trial of at least four to six months is required before assessing response, as eosinophil counts and exacerbation rates take time to stabilize at new baselines.

Biologic therapy produces substantial oral corticosteroid-sparing effects in patients who were previously corticosteroid-dependent. After initiating a biologic, oral corticosteroid doses should be tapered gradually over weeks to months, guided by symptom control and exacerbation frequency. The ability to reduce or eliminate oral corticosteroid exposure is one of the most clinically meaningful benefits of biologic therapy, given the cumulative toxicity of chronic oral corticosteroids.

If response to an initial biologic is inadequate after a full trial period, switching to an agent targeting a different pathway is appropriate. A patient who fails anti-interleukin-5 therapy despite confirmed eosinophilia may respond to dupilumab, since the interleukin-4/interleukin-13 axis drives additional aspects of T2-high disease beyond eosinophil survival. Combination biologic therapy (two agents simultaneously) is not standard practice and is not reimbursed in most healthcare systems. Biologic agents are extremely expensive — typically tens of thousands of dollars annually — and require prior authorization demonstrating failure of conventional step-up therapy and confirmed biomarker eligibility.