Omalizumab (Xolair)

Drug Class & Overview

Mechanism of Action

1. Binds free (circulating) IgE with high affinity - it targets the Fc region of IgE, specifically the site that would otherwise bind to high-affinity IgE receptors (FcεRI) on mast cells and basophils. Critically, it does not bind IgE already anchored to these receptors, so it does not trigger mast cell degranulation.
2. Downregulates FcεRI expression - by reducing free IgE levels, it causes a secondary reduction in FcεRI receptor density on mast cells, basophils, and dendritic cells (the receptor is stabilized by IgE; without IgE, receptor density falls ~97% over weeks).
3. Blocks low-affinity IgE receptors (FcεRII / CD23) on B cells, T cells, macrophages, and eosinophils - inhibiting chronic allergic inflammation.
4. Enhances innate antiviral immunity - omalizumab increases type I interferon secretion after rhinovirus exposure and reduces FcεRI on dendritic cells, improving protection against viral infections. This explains its ability to reduce virally-induced asthma exacerbations.
5. Reduces antigen presentation - its effects on dendritic cells (reduced FcεRI + reduced IgE capture) decrease allergen presentation to T cells.

• Goodman & Gilman's The Pharmacological Basis of Therapeutics
• Dermatology 2-Volume Set 5e

Approved Indications

• Dermatology 2-Volume Set 5e

Dosing

Asthma

• Dose: 75-375 mg SC every 2-4 weeks
• Determined by baseline serum total IgE level AND body weight (using a dosing table)
• Requires IgE level >30 IU/mL (75 ng/mL) with evidence of allergic basis (positive skin test or RAST)
• Up to 3 separate subcutaneous injections per visit (max 150 mg per site)
• Dosing limits: When pretreatment IgE >300 IU/mL, do not use if body weight exceeds 90 kg. When IgE approaches 700 IU/mL, do not administer even in normal-weight patients
• Do NOT re-check IgE after starting (levels drop to near-zero and are no longer reflective of original allergy burden)

Chronic Spontaneous Urticaria

• 150 mg or 300 mg SC every 4 weeks - fixed dose, independent of IgE level or body weight
• 300 mg/month is recommended for refractory CSU regardless of serum IgE
• Fishman's Pulmonary Diseases and Disorders
• Dermatology 2-Volume Set 5e

Place in Therapy

Asthma

• Indicated for severe allergic (atopic) asthma poorly controlled despite high-dose inhaled corticosteroids (ICS) + long-acting beta-agonists (LABAs)
• Not for acute asthma exacerbations - chronic maintenance use only
• Reduces oral corticosteroid and ICS requirements, markedly reduces exacerbation frequency
• Steroid-sparing effect is a key benefit
• May benefit patients receiving allergen-specific immunotherapy (SCIT) - improves tolerability and effectiveness of buildup phase, reduces systemic reactions
• Response is not fully predictable; a trial of at least 4 months is required before judging efficacy
• Predictors of better response: high blood eosinophils, elevated fractional exhaled NO (FeNO)
• Has some efficacy in non-atopic asthma (normal IgE) - presumed to reflect reduced local tissue IgE

Urticaria (CSU)

• Positioned as third-line therapy after antihistamines fail (EAACI 2018 guidelines recommend it as first choice at step 3, ahead of cyclosporine)
• The BSACI and American consensus guidelines suggest adding antileukotrienes before omalizumab
• Onset of benefit in CSU can be rapid - within 1 week (despite FcεRI downregulation requiring 2+ months - suggesting an alternative mechanism in urticaria, not yet fully elucidated)
• Effective across virtually all mast cell-mediated urticaria subtypes (including physical urticarias) - though most such uses are off-label
• Fitzpatrick's Dermatology
• Cummings Otolaryngology

Pharmacokinetics

• Route: Subcutaneous injection only
• Half-life: ~26 days (binds to circulating IgE, forming omalizumab-IgE complexes that are cleared by reticuloendothelial system)
• After SC injection, serum IgE is rapidly bound, with free IgE levels rebounding over 4-6 weeks after each dose
• No hepatic/renal dose adjustment needed (cleared via RES, not renal)

Adverse Effects

• Dermatology 2-Volume Set 5e
• Goodman & Gilman's

Contraindications & Precautions

• Contraindication: Prior hypersensitivity to omalizumab (only true contraindication)
• Do NOT abruptly discontinue corticosteroids when initiating omalizumab (titrate down gradually)
• Not approved for pregnancy, but long-term registry data show no increased risk of major congenital anomalies vs. disease-matched unexposed cohort
• Withdrawal of omalizumab can be associated with increased asthma exacerbations

Drug Interactions

Monitoring

• Before starting asthma treatment: Baseline serum total IgE and body weight (to calculate dose)
• No repeat IgE testing needed after initiation
• Clinical review every 6-12 months to assess corticosteroid-sparing, exacerbation rates, hospitalizations, ER visits, and quality of life
• Assess response after 4 months minimum before declaring treatment failure

Key Clinical Points

• Seasonal prevention: Omalizumab given before allergy season can prevent seasonal asthma exacerbations (pretreatment strategy)
• Cost: High acquisition cost; cost-effectiveness remains controversial; generally reserved for those failing maximized conventional therapy
• Ligelizumab - a next-generation, fully humanized anti-IgE with higher affinity than omalizumab - is under investigation and showed greater efficacy in early trials
• Recent 2025 network meta-analysis (PMID 40663028) confirms omalizumab remains a top biologic for chronic urticaria, though newer agents are being compared
• Recent 2025 meta-analysis of real-world evidence (PMID 39985317) supports biologics including omalizumab for CRSwNP