Mechanism of Action

Xolair inhibits the binding of IgE to the high-affinity IgE receptor (Fc&RI) on the surface of mast cells and basophils. Reduction in surface-bound IgE on Fc&RI-bearing cells limits the degree of release of mediators of the allergic response. Treatment with Xolair also reduces the number of Fc&RI receptors on basophils in atopic patients.

Pharmacokinetics

After SC administration, Omalizumab is absorbed with an average absolute bioavailability of 62%. Following a single SC dose in adult and adolescent patients with asthma, Omalizumab was absorbed slowly, reaching peak serum concentrations after an average of 7-8 days. The pharmacokinetics of Omalizumab are linear at doses greater than 0.5 mg/kg. Following multiple doses of Omalizumab, areas under the serum concentration-time curve from Day 0 to Day 14 at steady state were up to 6-fold of those after the first dose.

In vitro, Omalizumab forms complexes of limited size with IgE. Precipitating complexes and complexes larger than 1 million daltons in molecular weight are not observed in vitro or in vivo. Tissue distribution studies in cynomolgus monkeys showed no specific uptake of ¹²⁵I-Omalizumab by any organ or tissue. The apparent volume of distribution in patients following SC administration was 78 ± 32 mL/kg.

Clearance of Omalizumab involves IgG clearance processes as well as clearance via specific binding and complex formation with its target ligand, IgE. Liver elimination of IgG includes degradation in the liver reticuloendothelial system (RES) and endothelial cells. Intact IgG is also excreted in bile. In studies with mice and monkeys, Omalizumab:IgE complexes were eliminated by interactions with Fcy receptors within the RES at rates that were generally faster than IgG clearance. In asthma patients Omalizumab serum elimination half-life averaged 26 days, with apparent clearance averaging 2.4±1.1 mL/kg/day. In addition, doubling body weight approximately doubled apparent clearance.

Pharmacodynamics

In clinical studies, serum free IgE levels were reduced in a dose dependent manner within 1 hour following the first dose and maintained between doses. Mean serum free IgE decrease was greater than 96% using recommended doses. Serum total IgE levels (i.e., bound and unbound) increased after the first dose due to the formation of Omalizumab:IgE complexes, which have a slower elimination rate compared with free IgE. At 16 weeks after the first dose, average serum total IgE levels were five-fold higher compared with pre-treatment when using standard assays. After discontinuation of Xolair dosing, the Xolair-induced increase in total IgE and decrease in free IgE were reversible, with no observed rebound in IgE levels after drug washout. Total IgE levels did not return to pre-treatment levels for up to one year after discontinuation of Xolair.

Special Populations

The population pharmacokinetics of Xolair were analyzed to evaluate the effects of demographic characteristics. Analyses of these limited data suggest that no dose adjustments are necessary for age (12-76 years), race, ethnicity, or gender.

CLINICAL STUDIES

The safety and efficacy of Xolair were evaluated in three randomized, double-blind, placebo-controlled, multicenter trials.

The trials enrolled patients 12 to 76 years old, with moderate to severe persistent (NHLBI criteria) asthma for at least one year, and a positive skin test reaction to a perennial aeroallergen. At screening, patients in Studies 1 and 2 had a forced expiratory volume in one second (FEV1) between 40% and 80% predicted, while in Study 3 there was no restriction on screening FEV1. All patients had a FEV1 improvement of at least 12% following beta-agonist administration. All patients were symptomatic and were being treated with inhaled corticosteroids (ICS) and short acting beta-agonists. In Study 3, long-acting beta-agonists were allowed. Study 3 patients were receiving at least 1000 µg/day fluticasone propionate and a subset was also receiving oral corticosteroids. Patients receiving other concomitant controller medications were excluded, and initiation of additional controller medications while on study was prohibited. Patients currently smoking were excluded.

Each study was comprised of a run-in period to achieve a stable conversion to a common ICS (beclomethasone dipropionate, for Studies 1 and 2; fluticasone propionate for Study 3), followed by randomization to Xolair or placebo. In Study 3, patients were stratified by use of ICS-only or ICS with concomitant use of oral steroids. Patients received Xolair for 16 weeks with an unchanged corticosteroid dose unless an acute exacerbation necessitated an increase. Patients then entered an ICS reduction phase of 12 weeks (Studies 1 and 2) or 16 weeks (Study 3) during which ICS (or oral steroid in Study 3 subset) dose reduction was attempted in a step-wise manner.

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