conventional therapy. Severe and sustained symptoms lead to
resources, and significant adverse effects. Novel biologic therapeutics are being developed for the treatment of asthma and are of potential use for severe refractory asthma, especially where the increased cost of such agents is more likely justified.
This review will briefly summarize what is meant by “biologic therapies” and then highlight recent published data on efficacy and safety of these therapies for asthma.
WHAT ARE BIOLOGIC THERAPIES?
Biologic therapies have revolutionized the treatment of many diseases including asthma. By definition, the term “biologics” or “biologicals” include a variety of protein based therapeutics, such as antibodies, soluble receptors (eg,etanercept), recombinant protein-based receptor antagonists (eg, pitrakinra) and other related structures. Their main advantages include the duration of action and highly specific and strong binding to the target of interest; their main disadvantages are the cost and need for parenteral administration. Most biologicals in clinical use are antibodies, and their generic names contain standard nomenclature as a suffix to
indicate their origins (Fig. 1). Initially, pure murine antibodies were created with hybridoma technology, generating therapies that were 100% mouse with generic names given the suffix “momab” (eg, ibritumomab); however, immunogenicity of mouse antibodies in human subjects caused reduced efficacy and increased risk of infusion reactions including anaphylaxis and death. To reduce immunogenicity, chimeric antibodies
(“ximabs” like rituximab) were engineered. These antibodies are a marriage of murine variable regions combined with human constant regions, creating antibodies that are 80% human. These were a step forward but still had the potential for being immunogenic. Humanized monoclonal antibodies (“zumabs” such as omalizumab) go one step further, where now only the hypervariable regions of the mouse antibody are retained,
while the remaining 95% of the antibody is molecularly replaced by human sequences.
In the latest approach, fully human antibodies (“umabs” such as adalimumab) can be created by using phage display technology and molecular biology or more directly by immunizing mice that have had their immunoglobulin genes replaced with human versions. Newer artificial antibody structures such as bispecific antibodies, mix 2 separate arms with 2 different binding specificities to target 2 different types of antigens [eg, a single antibody where one arm binds interleukin (IL)-4 and the other arm binds IL-13]. Standard nomenclature for mAbs identifies their source with the last 4 or 5 letters: -omab, murine: –ximab, chimeric: -zumab, humanized: and –umab, human. The middle part of the name reflects the disease indication for which the mAb was initially intended: -lim for immune and inflammatory diseases, -cir for cardiovascular disorders, and -tu for tumors or neoplastic conditions. The first 3 or 4 letters may be chosen by the sponsor. Modified (by adding the structure of a bispecific antibody) . In general, FDA-approved mAbs have emerged between 10 and 12 years after the date that the new technologies on which they were based were reported in the scientific literature. None of these newer antibody structures have been tried in asthma, so the remainder of this review will focus on available data with standard biologicals.
Here is a listing of the key focus on biomolecules for therapeutics:
IL-4
It induces the IgE isotype switch and up-regulates expression of vascular cell adhesion molecule-1 on endothelium and a variety of TH2 chemokines, thus promoting recruitment of T lymphocytes, monocytes, basophils, and eosinophils to sites of allergic inflammation. A clinical trial studied the soluble recombinant human IL-4 receptor (IL-4R), Nuvance in asthma. Nuvance inhibited a decline in FEV1 during inhaled corticosteroid withdrawal and was overall well tolerated.2,3 However, in subsequent clinical trials in patients taking only beta agonist, soluble IL-4R failed to demonstrate significant clinical efficacy. A phase I randomized double blind placebo controlled study evaluated the effects of pascolizumab, a humanized anti-IL-4 antibody, in 24 patients with mild to moderate asthma. Pascolizumab was well tolerated and no serious adverse events occurred.5 However, a phase IIa clinical trial in steroid-naive, mild to moderate asthmatics, did not demonstrate clinical efficacy. Because the IL-4 targeting studies have failed to demonstrate clinical efficacy, one can justify concluding that either IL-4R is not an effective therapeutic target in asthma.
TNFa
Tumor necrosis factor (TNF) is a multifunctional proinflammatory cytokine produced by inflammatory cells including monocytes, macrophages, mast cells, smooth muscle cells, and epithelial cells. TNF may initiate airway inflammation by up-regulating adhesion molecules, mucin hypersecretion, and airway remodeling, and by synergizing with TH2 cytokines. Berry et al demonstrated that severe refractory asthmatics have evidence of up-regulation of TNF as compared with healthy controls and mild asthmatics. Entanercept was evaluated in a small, randomized, double-blind placebo-controlled crossover study in 10 patients with severe refractory asthma and elevated TNF levels, 10 patients with mild to moderate asthma, and 10 control patients. Entanercept treatment was associated with improved FEV1, asthma related quality of life, and the concentration of methacholine needed to provoke a 20% decrease in FEV1. No serious adverse reactions were noted. In another double-blind, placebo-controlled, parallel group study, 38 patients with moderate asthma on inhaled corticosteroids were treated with infliximab. Although infliximab treatment did not improve the primary end point of morning peak expiratory flow, it decreased diurnal variation of the peak expiratory flow rate and asthma exacerbations. No serious adverse events were noted. Golimumab was recently evaluated in the largest randomized, double-blind, placebo-controlled study in 309 patients with severe, uncontrolled asthma. No significant differences were observed for the change in FEV1 or exacerbations. However, several serious adverse events occurred. There is no clear role for TNF in perpetuating asthma or asthma exacerbations.
CD4
CD4 T cells are likely to be involved as a source of proinflammatory cytokines in asthma. Keliximab is a monoclonal antibody that causes a transient reduction in the number of CD4 T cells. A double blind, randomized, placebo controlled study with 22 severe oral corticosteroid dependent asthmatics patients was completed. A subset of patients received the highest dose of keliximab (3.0 mg/kg). There was significant improvement of peak expiratory flow rates in the high dose treatment arm. However, CD4 T cells remained transiently reduced 14 days postinfusion, raising safety concerns.
CD23
CD23 is a low-affinity immunoglobulin E receptor (FcRII) and is important in regulating IgE production. IDEC-152 is a chimeric monoclonal antibody directed against CD23. CD23 is expressed on
- T and B cells,
- neutrophils,
- monocytes, and
- macrophages.
CD23 is overexpressed in allergic disease and may be involved in IgE overproduction,
- which can lead to mast cell degranulation.
A phase I dose escalating placebo-controlled study in 30 asthmatics demonstrated that
- IDEC-152 caused a dose-dependent reduction in serum IgE concentrations.
- No significant adverse events were reported
CD25
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