Omalizumab is a recombinant humanized IgG1 monoclonal antibody targeting human immunoglobulin E (IgE) and thereby preventing its interaction with the high-affinity receptor Fc-epsilon-RI, typically found on eosinophils, mast cells and basophils and is critical in the allergic cascade.[1] Omalizumab has a vital role in the management of moderate to severe Ig-E medicated asthma and also more recently, a role in chronic urticaria. 

It received initial approval for use in 2003 for the treatment of moderate to severe asthma and later received approval for use in patients with chronic idiopathic or spontaneous urticaria in 2014 in both the United States and Europe. It is, in fact, the first drug to be licensed for the treatment of chronic urticaria in a patient who remains symptomatic despite H1-antihistamine treatment.[2] The determination of omalizumab dosing and frequency are by serum total IgE levels and also the patient's body weight. In the United States the total serum IgE should be in the range of 30 to 700 IU/ml, and in Europe for adults and children above the age of 12, the range is between 30 to 1500 IU/ml, while for children between 6 to 12, IgE levels should be under 1300 IU/ml.[3]

The mechanism of action of omalizumab is different for allergic asthma vs. chronic urticaria.

The direct action involves selectively binding to the C-epsilon-3 locus, the domain at which IgE binds to Fc-epsilon-RI, decreasing levels of the immunoglobulin, and preventing interactions with its high-affinity receptor, as stated earlier primarily on eosinophils, basophils and mast cells interrupting the allergic cascade. Also, due to the total reduction of free IgE, there is a reduction in the expression of the IgE high-affinity receptor on inflammatory cells and a reduction in peripheral eosinophilia.[4]

With regards to chronic urticaria, the mechanism is not as well understood; unlike in asthma, where a 95% reduction in serum IgE levels is required to modify allergen response, in chronic urticaria, it is not a classic allergen-driven response, and a fixed dose of omalizumab has approval for use.[2] The theory is that omalizumab could prevent activation of mast cells and basophils in 40 to 45% of patients with chronic urticaria who may have an autoimmune component, by decreased high-affinity IgE receptor density and preventing IgE antibody-mediated cross-linking of adjacent alpha-subunits or IgE itself.[5] There is some evidence to support that these patients with chronic urticaria have an abnormal basophil IgE high-affinity receptor and also that basophils are generally present chronic urticarial lesion sites.[6] There is also some evidence that treatment with omalizumab reverses the basopenia and increases IgE receptor expression, which is typical in the disease states of patients with chronic urticaria.[6]

Omalizumab administration is subcutaneous. Absorption into the systemic circulation is rather slow, with peak serum concentrations achieved after an average of 7 to 8 days. Omalizumab demonstrates linear pharmacokinetics in the approved dosage regimens. IgG clearance process, as well as specific binding and complex formation with IgE, are involved in omalizumab clearance. The mean half-life is 26 days.[7]

In dosing for asthma

• The basis for dose and frequency are on body weight, and pretreatment total IgE serum levels
• There is no dosing adjustment based on total IgE levels taken during treatment
• No dose adjustment is necessary for any significant weight changes

In chronic urticaria

• Omalizumab is FDA approved at 150 mg and 300 mg Q4W.
• In a retrospective chart review of 43 chronic urticaria patients treated with omalizumab between 2006 to 2015, it found that overall, 90.2% (37/41) responded within three months duration.[8]

There is a U.S. Boxed Warning for Anaphylaxis for omalizumab.

Typically omalizumab is characterized by a very good safety profile, the major side-effect concerns with omalizumab are in four categories: systemic reactions, the potential effect on malignancy, risk of parasite disease and immunological effects including serum sickness and Churg-Strauss vasculitis.[9]

Subjects in clinical trials generally tolerate the drug well, with most adverse events being mild or moderate and reported with a similar frequency to those in control or placebo groups. The most commonly reported adverse events with omalizumab in clinical trials were injection-site reactions (45%), viral infection (23%), upper respiratory tract infection (20%), sinusitis (16%), headache (15%), and pharyngitis (11%). There are reports of anaphylactic reactions occurring with omalizumab, usually within 2 hours of the first or subsequent doses, although the incidence is low (0.1 to 0.2%).  It merits noting these reports have been in asthmatics and not chronic urticaria patients.[7]

With regards to anaphylaxis, the Omalizumab Joint Task Force, formed by two American institutions specializing in asthma, allergy, and immunology, reviewed the omalizumab clinical trials and also the post-marketing surveillance data. They concluded that out of the 39510 patients who received omalizumab from June 2003 and December 2005, 35 patients had 41 episodes, which corresponds to an anaphylaxis reporting rate of 0.09%, as a result of the data, recommendations were implemented which included a 2-hour observation following the initial three injections and the provision of epinephrine pens to patients.[10] There has been some mention of a risk of malignancy associated with IgE in the literature, a pooled analysis of phase 1 through 4 clinical trials in 2012, which demonstrated no clear association was observable between omalizumab treatment and risk of malignancy in randomized, double-blind, placebo-controlled trials.[11]

Immunoglobulin E is classically the primary antibody in combatting parasitic disease; there are concerns that omalizumab therapy puts patients at increased risk of helminth infections, and some data demonstrates that in areas where there is an increased risk of helminth infection, omalizumab treatment does slightly raise patients risk compared to placebo.[12]

Cases of Churg-Strauss in omalizumab treatment do appear in the literature.[13] However, it is still uncertain if there is a causal link between omalizumab treatment and Churg-Strauss, it is unclear if its a consequence or perhaps a pre-existing condition that has previously been masked by the long-term corticosteroid treatments these patients are generally on.[14]

There are two case reports of transient hair loss in chronic urticaria patients treated with omalizumab.[15][16]

Contraindications include severe hypersensitivity reaction to omalizumab.