Past, present and future of anti-IgE biologicals

Abstract

About 20 years after the identification of immunoglobulin E (IgE) and its key role in allergic hypersensitivity reactions against normally harmless substances, scientists have started inventing strategies to block its pathophysiological activity in 1986. The initial concept of specific IgE targeting through the use of anti-IgE antibodies has gained a lot of momentum and within a few years independent research groups have reported successful generation of first murine monoclonal anti-IgE antibodies. Subsequent generation of optimized chimeric and humanized versions of these antibodies has paved the way for the development of therapeutic anti-IgE biologicals as we know them today. With omalizumab, there is currently still only one therapeutic anti-IgE antibody approved for the treatment of allergic conditions. Since its application is limited to the treatment of moderate-to-severe persistent asthma and chronic spontaneous urticaria, major efforts have been undertaken to develop alternative anti-IgE biologicals that could potentially be used in a broader spectrum of allergic diseases. Several new drug candidates have been generated and are currently assessed in pre-clinical studies or clinical trials. In this review, we highlight the molecular properties of past and present anti-IgE biologicals and suggest concepts that might improve treatment efficacy of future drug candidates.

1. Introduction

The discovery of immunoglobulin E (IgE) in 1967^8,9 together with the identification of its central role in the pathogenesis of allergic inflammation (reviewed elsewhere^13,14) has set the stage for the development of therapeutic anti-IgE strategies. The generation of detailed knowledge about the molecular and structural characteristics of IgE has further accelerated this process. IgE consists of two identical heavy and light chains. Each heavy chain includes four constant epsilon domains (Cε1–4)¹⁵. Overall, the IgE antibody features high flexibility. Particularly the IgE-Fc part has been described to undergo important conformational changes depending on its interaction partner. While IgE binding to the high affinity receptor FcεRI on allergic effector cells induces an open Fc conformation, low affinity receptor FcεRII/CD23 binding on antigen presenting cells (APC), airway epithelial cells (AEC) or B-cells forces IgE in a closed Fc conformation and thereby ensures mutually exclusive interaction with the two receptors^16–18. In addition to the free form of IgE, B-cells express membrane IgE (mIgE) that is part of the B-cell receptor (BCR) and contains an extracellular membrane proximal domain (EMPD)^19–21. Identification of these features have been key for the development of efficient targeting strategies. Over the last decades, the field of anti-IgE biologicals has advanced into a competitive and active field of investigation. Different companies and research institutions are currently assessing approaches to specifically target IgE in pre-clinical studies or clinical trials (Table 1)^22,23. Here, we provide a chronological overview on past achievements, present investigations and potential future approaches of anti-IgE strategies.

Table 1.

Past, present and future anti-IgE biologicals.

Anti-IgE name	Format	Target	Indication	Clinical status	Company	Published
Omalizumab	Antibody	IgE, mIgE	Asthma, CSU	approved	Novartis, Genentech	199330
Ligelizumab	Antibody	IgE, mIgE, CD23-IgE	CSU	phase III	Novartis	1999118
Quilizumab	Antibody	mIgE	CSU	phase II	Genentech	201066
MEDI4212	Antibody	IgE, mIgE	n.a.	phase I	MedImmune	201478
XmAb7195	Antibody	IgE, mIgE	n.a.	phase I	Xencor	201281
8D6	Antibody	IgE, CD23-IgE	n.a.	pre-clinical	-	201293
GE2	Fc-fusion protein	FcεRI/ CD23, FcγRII,	n.a.	pre-clinical	-	2002101
DE53-Fc	DARPin	IgE, FcεRI-IgE, FcγRIIb	n.a.	pre-clinical	-	2011107
bi53_79	DARPin	IgE, FcεRI-IgE	n.a.	pre-clinical	-	201491
026 sdab	Nanobody	IgE, FcεRI-IgE	n.a.	pre-clinical	Ablynx	2018111

2. The Past: Approved anti-IgE biologicals

In 1986, Dr. Christoph Heusser and his colleagues at the former Swiss pharmaceutical company Ciba-Geigy proposed the development of neutralizing non-anaphylactogenic anti-IgE antibodies²⁴. Around the same time, the research team of Dr. Tse Wen Chang at the US biopharmaceutical company TANOX came up with the idea to generate a monoclonal antibody against IgE that specifically targets and eliminates IgE-secreting B-cells¹⁰. While the group at Ciba-Geigy demonstrated proof-of-concept in different mouse models^11,25, TANOX generated the first monoclonal murine anti-human IgE antibody called TES-C21²⁶ (Fig. 1). In 1990, the two companies entered a partnership with the goal to join their anti-IgE programs and to generate a therapeutic anti-IgE antibody that would block the biologic activity of IgE based on three criteria: i) neutralization of free serum IgE, ii) targeting of IgE-producing B-cells and iii) non-reactivity with other IgE bearing cells. Therefore, they further engineered TES-C21 and developed the chimeric mouse/human monoclonal antibody TESC-2, later renamed to CGP51901²⁶. In clinical phase I and II trials with allergic rhinitis patients CGP51901 showed dose-dependent reduction of free serum IgE levels, inhibition of rhinitis symptoms and high tolerability^27,28. The cooperation between TANOX and Ciba-Geigy ultimately gave rise to the fully humanized monoclonal anti-IgE antibody CGP56901, also known as TNX-901 or Talizumab²⁹. Later in 1996, Ciba-Geigy fused with the pharmaceutical company Sandoz to form Novartis. During that time, Genentech has also been generating a murine anti-human IgE antibody called MaE11, which has been further developed into the humanized monoclonal antibody rhuMAb-E25³⁰. To reduce competition and maximize synergies, Genentech, TANOX and Novartis formed a tripartite partnership. Despite the fact that both antibody candidates expressed similar key properties³¹, the three partners decided to select rhuMAb-E25 from Genentech over CGP56901 for further development due to already established manufacturing processes. This collaboration ultimately led to the approval of rhuMAb-E25 - today best known as omalizumab (Xolair®) – as the first therapeutic anti-IgE antibody in 2003¹².

Fig. 1. Chronological appearance of anti-IgE compounds.

Following the discovery of IgE various anti-IgE compounds have been reported. The years of their appearance (blue) are marked on the timeline (grey arrow). The inter-relationship between different anti-IgE compounds is highlighted with arrows (dotted black). TES-C21: monoclonal murine anti-IgE antibody; TESC-2/ CGP51901: monoclonal chimeric mouse/human anti-IgE antibody; TNX-901: humanized monoclonal anti-IgE antibody; rhuMAb-E25/ omalizumab: humanized monoclonal anti-IgE antibody; GE2: fusion protein of IgG1-Fc_3–4 and IgE-Fc_2–4; DE53-Fc: fusion protein of anti-IgE DARPin and IgG1-Fc_3–4; XmAb7195: humanized affinity maturated version of parental omalizumab antibody; CMAB007: omalizumab biosimilar; ligelizumab: high-affinity version of TNX-901; quilizumab: afucosylated humanized monoclonal anti-IgE (EMPD) antibody; MEDI4212: afucosylated monoclonal anti-IgE antibody; bi53_79: bispecific disruptive anti-IgE DARPin; 39D11: anti-IgE nanobody; 026 sdab: humanized affinity maturated anti-IgE nanobody.

Until now, omalizumab is still the only approved therapeutic anti-IgE antibody. Despite its restricted indication for the treatment of moderate-to-severe persistent asthma and chronic spontaneous urticaria (CSU), omalizumab is frequently used off-label in allergic rhinitis, food allergies and allergen-specific immunotherapy^32–34. Several meta-analyses have assessed the efficacy of omalizumab treatment in allergic asthma and CSU. These studies came to the conclusion that omalizumab efficiently reduces asthma exacerbations, leads to overall improvement in quality of life and allows the reduction or withdrawal of inhaled corticosteroids^35–39. However, the use of omalizumab in severe eosinophilic asthma in corticosteroid-resistant patients is controversially discussed^35,40,41. Further studies on this subject are needed, especially since there are other approved biologics that might be superior to omalizumab in this context⁴². The primary mode-of-action of this humanized monoclonal anti-IgE antibody of IgG1 subclass is the neutralization of free IgE. It binds with nanomolar affinity (K_D ∼ 7 × 10⁻⁹ M)⁴³ to the Cε3 domain of IgE. Recent analysis of crystal structures composed of omalizumab Fab fragments and IgE-Fc_3–4 variants revealed that omalizumab-mediated inhibition of IgE binding to FcεRI does not occur via direct competition for binding sites on IgE as stated in many previous reports⁴⁴. Omalizumab shares only minor overlap with FcεRIα binding epitopes on IgE and mainly inhibits IgE receptor binding via steric inhibition^44,45 (Fig. 2a). On the other hand, the binding footprints of omalizumab and CD23 on IgE show a high degree of overlap^44,45. Therefore, the inhibition of IgE:CD23 interactions is primarily dependent on direct binding competition. Importantly, omalizumab inhibits IgE receptor interactions with both, FcεRI and CD23 (Fig. 2b) and does not aggregate receptor-bound IgE ensuring its non-anaphylactogenic profile⁴⁴. In rare cases (~0.1%), however, treatment with omalizumab has induced adverse reactions including skin inflammation and anaphylaxis in the absence of anti-drug antibodies^46–48. Recent studies, have provided strong evidence that omalizumab can bind and cross-link IgE-Fc_3–4:FcεRIα complexes due to the missing Cε2 domain, which in the intact IgE molecule masks the omalizumab binding epitope on IgE⁴⁴. Whether IgE-Fc_3–4:FcεRIα complexes exist in vivo and whether such binding events might play a role in the limited cases of omalizumab-induced anaphylaxis is currently not known. Another study revealed that omalizumab-IgE immune complexes activate human neutrophils that may induce skin inflammation and anaphylaxis in humanized mice by engaging activating Fc-gamma receptors and binding of complement factors⁴⁹. Although this is an interesting finding, it does not explain the rarity of such omalizumab-induced adverse reactions in humans.

Fig. 2. Binding characteristics and modes-of-action of omalizumab.

a) Schematic overview of omalizumab (blue), FcεRIα (left panel) and FcεRII/CD23 (right panel) binding sites as well as their relative overlap on IgE-Fc_3–4 (purple). b) Omalizumab mediated inhibition of free IgE binding to FcεRI or CD23. c) Omalizumab mediated accelerated dissociation of pre-formed IgE:FcεRI and IgE:CD23 complexes. d) Potential mechanisms leading to suppression of IgE production by omalizumab.

As first discovered by Dr. Donald MacGlashan in 1997^50,51 and confirmed by several studies with omalizumab, anti-IgE treatment reduces FcεRI levels on allergic effector cells such as mast cells and basophils⁵². This beneficial side effect is a consequence of free IgE clearance, which leads to the lack of IgE-mediated FcεRI stabilization on the surface of these cells⁵³. Additionally, omalizumab has been described to accelerate the dissociation of IgE from FcεRI⁵⁴ and CD23⁴⁵ (Fig. 2c). Recently an alternative omalizumab Fab fragment has been generated by introducing three mutations in the variable and constant light chain. This so called FabXol3 antibody fragment showed superior activity in dissociating IgE from FcεRIα than the original full length antibody⁵⁵. Whether the increased disruptive efficacy of omalizumab would result in an additional treatment benefit still requires careful evaluation.

There is emerging evidence that omalizumab in addition to its primary mode-of-action has the ability to decrease IgE production by targeting mIgE positive B-cells^56,57. Several studies have shown reduced B-cell numbers and IgE secretion upon long-term omalizumab treatment^58–60 (Fig. 2d). While these observations are highly interesting, the exact mechanisms underlying omalizumab-dependent regulation of IgE production are not fully understood yet.

3. The present: Monoclonal anti-IgE antibodies in clinical trials

So far, the field of anti-IgE biologicals has largely been dominated by monoclonal antibody approaches and different companies have successfully reached the stage of clinical trials with their anti-IgE candidates. However, none of these have been approved for therapeutic application in the last two decades. One of the highly anticipated antibodies that is still under active investigation in clinical trials is ligelizumab (QGE031) from Novartis. This is a high affinity version of TNX-901 that was originally “put on ice” in the collaboration between Genentech, TANOX and Novartis. To our current knowledge, all other anti-IgE programs discussed in this section have been discontinued due to strategic decisions or failure to achieve primary endpoints in clinical trials.

3.1. Monoclonal anti-IgE antibody ligelizumab

While omalizumab was successfully developed in the tripartite collaboration with Novartis and Genentech, TANOX still proceeded to test the original anti-IgE antibody TNX-901 in a phase II clinical trial with peanut allergic subjects^61,62. However, this program had to be stopped after an extensive legal debate with Genentech and Novartis. Recently, a high-affinity version of TNX-901, generated by TANOX, has been developed by Novartis under the name ligelizumab (QGE 031) and is currently in a phase III clinical investigation for the treatment of CSU (NCT03580356). Ligelizumab is a humanized monoclonal IgG1 antibody that binds IgE with an affinity of ∼ 1.4 × 10⁻¹⁰ M but does not interact with FcεRIα-bound IgE²². Unlike omalizumab it does not accelerate dissociation of pre-formed FcεRIα:IgE complexes and recent structural analysis revealed that ligelizumab recognizes a distinct IgE epitope, only partially overlapping with that of omalizumab⁴⁵. Ligelizumab binds to both Cε3 domains across the IgE-Fc dimer and favours the recognition of IgE in an open Fc conformation. It shares significantly more overlap with the binding epitope of FcεRI on IgE than omalizumab and is superior in inhibiting this interaction. On the other hand, ligelizumab shows almost no overlap with the CD23 binding site and is therefore less efficient than omalizumab in blocking IgE:CD23 interactions⁴⁵ (Fig. 3a). This distinct receptor inhibition profile might play an important role for its treatment efficacy in certain diseases. Based on these findings, ligelizumab is predicted to have a therapeutic advantage over omalizumab in FcεRI-driven allergic disorders. Indeed, ligelizumab showed significantly better symptom control in comparison to omalizumab in a phase IIb dose-finding trial for CSU last year⁶³. It was also superior to omalizumab in the inhibition of primary human basophil activation and a passive systemic anaphylaxis mouse model⁴⁵. On the other hand, it might be less suitable for the treatment of allergic disorders that heavily rely on CD23-dependent processes including antigen presentation or transepithelial transport (e.g. eosinophilic lung inflammation)⁶⁴. Further, the distinct receptor inhibition profile provides a plausible explanation as to why ligelizumab lacked superior efficacy over omalizumab treatment in a phase II clinical trial with allergic asthma patients (NCT02075008). In line with a recent clinical trial that revealed a surprisingly long suppression of free serum IgE levels in atopic patients upon single dose injection⁴³, it has been demonstrated that ligelizumab can suppress IgE production most likely by interacting with CD23-bound IgE on human B-cells⁴⁵. However, this secondary mode-of-action that might increase treatment efficacy of ligelizumab over omalizumab in various atopic disorders needs to be further investigated.

Fig. 3. Binding characteristics and modes-of-action of different anti-IgE antibodies.

a) Schematic overview of ligelizumab (blue), FcεRIα (left panel) and FcεRII/CD23 (right panel) binding sites as well as their relative overlap on IgE-Fc_3–4 (purple). b) Two mechanisms of quilizumab mediated suppression of IgE production through induction of B-cell apoptosis or antibody dependent cellular cytotoxicity (ADCC). Afucosylation of quilizumab is indicated with stars. c) Mechanisms of MEDI4212 mediated neutralization of free IgE and suppression of IgE production through induction of B-cell apoptosis or antibody dependent cellular cytotoxicity (ADCC). Afucosylation of MEDI4212 is indicated with stars. d) XmAb7195 mediated neutralization of free IgE and suppression of IgE production through co-aggregation of mIgE and FcγRIIb on B-cells. Mutations in the antibody Fc-part are indicated with dots.

3.2. Monoclonal anti-IgE antibody quilizumab

Another extensively investigated approach for the treatment of allergic disorders is the specific targeting of IgE producing B-cells displaying mIgE on their surface⁶⁵. Genentech has developed the monoclonal mouse anti-human IgE antibody 47H4, which recognizes the EMPD of IgE⁶⁶. 47H4 has been shown to reduce serum IgE levels and the number of IgE-producing plasma cells in vivo in genetically modified mice that carry the human EMPD in the mouse IgE locus. The authors have concluded that 47H4 most likely induces apoptosis in mIgE positive B-cells⁶⁶. Following these studies, an afucosylated humanized version of the antibody, known as quilizumab, has been generated. The removal of fucose from the IgG antibody increases the binding affinity to FcγRIIIa⁶⁷. This modification enhances the mechanism of antibody-dependent cellular cytotoxicity (ADCC) by NK cells and in the case of quilizumab increased its efficacy to kill mIgE positive B-cells⁶⁷ (Fig. 3b). In a phase I clinical trial for allergic rhinitis (NCT01160861) and a phase II clinical trial for allergen-induced asthma (NCT01196039) quilizumab treatment reduced both total and allergen-specific IgE levels by 35% and 40%, respectively, and resulted in a slight amelioration of allergen induced asthmatic airway responses⁶⁸. Surprisingly, these results did not translate into phase II clinical trials with allergic asthma patients (NCT01582503) or CSU patients (NCT01987947), as quilizumab treatment only partially reduced serum IgE levels and did not significantly improve the clinical outcome^69,70. The disappointing results of these trials might be due to the low frequency and transient abundance of mIgE positive B-cells⁷¹ as well as the growing evidence that most of the secreted IgE derives from IgG producing B-cells undergoing sequential isotype switching to IgE^72–77.

3.3. Monoclonal anti-IgE antibody MEDI4212

A monoclonal anti-IgE antibody with the aim of combining two modes-of-action, namely the neutralization of free IgE and the enhanced killing of mIgE positive B-cells via ADCC has been developed by MedImmune. MEDI4212 has been selected using phage display technology and targeted mutagenesis of the V_H and V_L chains for affinity maturation⁷⁸. It binds IgE with an affinity of K_D ~2 pM and does not interact with FcεRI-bound IgE⁷⁸. To maximize MEDI4212 mediated induction of ADCC in mIgE positive B-cells, an afucosylated version of the antibody has been produced⁷⁹ (Fig. 3c). Crystal structure analysis of the MEDI4212 Fab with IgE-Fc_3–4 revealed a binding epitope within the Cε3 and Cε4 domains of IgE. It shows a substantial overlap with the FcεRI binding sites on IgE and thus blocks IgE-binding to FcεRI through direct competition. In contrast, MEDI4212 has no obvious overlap with CD23 epitopes on IgE, but still inhibits binding to CD23. The authors concluded that steric or even allosteric inhibition might be involved⁷⁸. A phase I study with atopic patients (NCT01544348) has been conducted with MEDI4212 in 2013. The results looked promising as MEDI4212 treatment led to more pronounced reduction of free IgE than omalizumab. However, IgE levels returned to baseline faster in the subjects treated with MEDI4212 than with omalizumab. This has most likely been due to the short serum half-life of the afucosylated antibody⁸⁰. So far, no clinical follow up study has been conducted.

3.4. Monoclonal anti-IgE antibody XmAb7195

The anti-IgE antibody XmAb7195 is a humanized, affinity maturated version of the murine parental antibody of omalizumab (MaE11) with a mutated Fc-part for increased binding to the inhibitory receptor FcγRIIb^81,82. It has been developed by Xencor with the aim of neutralizing free serum IgE and suppressing IgE production through aggregation of FcγRIIb and mIgE on B-cells (Fig. 3d). Indeed, XmAb7195 has been shown to reduce free IgE levels and to inhibit the formation of IgE-secreting plasma cells in a severe combined immunodeficiency (SCID) mouse model engrafted with human PBMCs⁸¹. The benefit of this dual targeting approach has also been evaluated in a phase Ia clinical trial with healthy volunteers and adults with elevated IgE levels (NCT02148744). A second phase Ib clinical trial (NCT02881853) has been completed in 2017. However, the results of these studies have not been published.

3.5. Omalizumab biosimilars

The patents for omalizumab have recently expired in the US and Europe. Therefore, various companies have started developing biosimilars. Even though the generation of an omalizumab biosimilar might be faster and less cost intense than generating a novel anti-IgE antibody from scratch, this process requires laborious optimization and thorough testing⁸³. To get approval by official authorities biosimilars are expected to feature a similar safety and efficacy profile as the original biological. Typically, at least one phase I and one phase III clinical trial will be required to gather this information. One of the most advanced biosimilars of omalizumab is the monoclonal anti-IgE antibody CMAB007⁸⁴, which is developed by the National Engineering Research Center of Antibody Medicine of China. Besides minor changes it shares the same amino acid sequence as omalizumab. CMAB007 has completed a phase I clinical trial in China with promising results similar to omalizumab⁸⁴ and is currently undergoing a multi-center, randomized, double-blind, placebo-controlled phase III clinical trial for patients with allergic asthma in China (NCT03468790). Since Omalizumab has only recently been approved for the treatment of moderate-to-severe asthma in China in 2017, it will be interesting to see what will happen to CMAB007. There are several additional omalizumab biosimilars, which are currently evaluated at different stages of clinical trials (Table 2).

Table 2.

Omalizumab biosimilars in development.

Anti-IgE name	Format	Indication	Clinical status	Company	Year
CMAB007	Antibody	Asthma	phase III	Mabpharm	2012
STI-004	Antibody	asthma	phase III	Sorrento Therapeutics	2015
FB317	Antibody	Asthma, CIU	phase I	Fountain Biopharma	2016
GBR 310	Antibody	Asthma, CSU	phase II	Glenmark	2018
CTP-39	Antibody	n.a.	phase I	Celltrion	2019
BP001	Antibody	Asthma, CU	phase I	Biosana	2019

4. The future: Anti-IgE biologicals in pre-clinical evaluation

Based on the lessons learned from past anti-IgE approaches various novel strategies are currently evaluated in pre-clinical studies. Some of these next generation anti-IgE variants deviate from the classical monoclonal antibody framework and aim to be either more specific than the classical anti-IgE antibody strategies or to feature multi-functionality. Whether any of the alternative anti-IgE variants will make it into clinics and if they will bring superior patient benefit remains to be seen.

4.1. Anti-IgE designed ankyrin repeat proteins (DARPins)

In 2010, a set of new anti-IgE binders based on designed ankyrin repeat protein (DARPin) scaffolds has been published⁸⁵. DARPins are genetically engineered alternative scaffold proteins consisting of a repetitive consensus sequence derived from naturally occurring ankyrin repeat proteins⁸⁶. DARPin libraries with either two or three internal repeats have been generated by randomizing variable positions in the scaffold. For the selection of specific binders, these libraries can be screened using ribosome display⁸⁷. Due to their simple structure, small size and high solubility, DARPins can be expressed at high quantities in E. coli⁸⁸. Selected DARPins can be genetically joined by suitable linkers to create multivalent or even multispecific binders⁸⁹. Multiple binders against human IgE were selected and some of them have been reported to bind to the constant heavy chain of human IgE with high specificity and affinity. DARPin clone E2_79 showed the highest affinity and prominently inhibited IgE-binding to FcεRI. Moreover, it suppressed allergen-mediated degranulation of human FcεRI expressing rat basophilic leukemia cells to the same degree as omalizumab⁸⁵. In a follow up study, E2_79 has been found to feature a novel mode-of-action, which led to the definition of a new class of anti-IgE molecules, termed “disruptive anti-IgE inhibitors”. Namely, in addition to neutralizing free IgE, E2_79 actively removes IgE that is pre-bound to FcεRI by a facilitated dissociation mechanism⁹⁰. The disruptive efficacy could be further enhanced by fusing E2_79 to a non-disruptive anti-IgE DARPin E3_53 that recognizes FcεRI-bound IgE and thus serves as an anchoring moiety in the resulting bispecific DARPin bi53_79⁹¹ (Fig. 4a). In the meantime, several pre-clinical studies have highlighted the therapeutic potential of DARPin bi53_79. It has been demonstrated to efficiently desensitize human basophils ex vivo, to prevent passive cutaneous anaphylaxis in human FcεRIα transgenic mice in vivo⁹¹ and to inhibit mast cell-dependent early airway responses in viable human atopic lung tissue ex vivo⁹². The possibility to desensitize allergic effector cells by actively removing IgE from the cell surface opens new therapeutic approaches that might impact the development strategies of future anti-IgE drug candidates. Particularly, such an approach might allow acute treatment, which is different to “classical” anti-IgE antibodies that have a rather slow onset of action. Overall, DARPins feature many favorable characteristics including ease of production, high stability and solubility, small molecular size and the possibility to generate multi-specific binders⁸⁸. On the other hand, they need to be carefully optimized to warrant low immunogenicity and long serum half-life. Phase III trials with other DARPin-based drug candidates are currently ongoing (NCT02462486) and it will be interesting to see when the first DARPin-based biological will be approved for clinical use.

Fig. 4. Modes-of-action of potential future anti-IgE drug candidates.

a) Mechanisms of DARPin bi53_79 mediated inhibition of IgE-binding to FcεRI and CD23, suppression of IgE production and active disruption of pre-formed IgE:FcεRI complexes. b) 8D6 mediated inhibition of IgE-binding to FcεRI, binding of IgE complexes to CD23 and suppression of IgE production. c) GE2 mediated suppression of allergic effector cell activation through co-aggregation of FcεRI with FcγRIIb and suppression of IgE production by co-aggregation of CD23 with FcγRIIb. d) 026 sdab mediated inhibition of IgE-binding to FcεRI and CD23 and active disruption of pre-formed IgE:FcεRI complexes.

4.2. Monoclonal anti-IgE antibody 8D6

In 2012, another monoclonal anti-IgE antibody, called 8D6, which binds to the Cε3 domain of IgE with high affinity (K_D ∼ 20 pM) was generated^93,94. 8D6 features a special binding characteristic that has not been described before. It inhibits the interaction of IgE with FcεRI but does not interfere with IgE-binding to CD23. Similar to ligelizumab, 8D6 recognizes CD23-IgE complexes on the cell surface. However, unlike ligelizumab pre-formed 8D6:IgE immune complexes still bind to CD23⁹³. Because aggregation of CD23 on B-cells has been claimed to reduce IgE production^95,96, it has been speculated that 8D6 could potentially mimic this activity by cross-linking CD23-bound IgE in addition to block sensitization of allergic effector cells^93,97. However, this has not been experimentally confirmed so far.

4.3. IgG-Fc-fusion proteins

An alternative strategy to suppress allergic reactions besides interfering with IgE-binding to FcεRI is to leverage the inhibitory function of FcɣRIIb on allergic effector cells^98–100. A fusion protein consisting of the human IgG1-Fc_3–4 and the human IgE-Fc_2–4 parts, known as GE2 (or epsigam), has been generated to aggregate FcεRI with FcγRIIb on allergic effector cells and thereby exploit this mechanism of inhibition¹⁰¹ (Fig. 4c). GE2 has been shown to inhibit IgE-mediated degranulation of sensitized primary human basophils¹⁰¹ and cord blood derived human mast cells⁹⁹. Further, it suppresses passive cutaneous anaphylaxis in human FcεRIα transgenic mice¹⁰¹, reduces IL-16 production in Langerhans-like dendritic cells (LLDCs)¹⁰², decreases isotype switching of peripheral human B-cells to IgE¹⁰³, inhibits dust mite allergen-induced allergic skin reaction in rhesus monkeys dose-dependently¹⁰⁴ and showed positive results in a non-human primate model of allergic asthma with cynomolgus monkeys¹⁰⁵. Unfortunately, repeated injections of GE2 led to the production of anti-drug antibodies resulting in acute anaphylaxis-like reactions in monkeys¹⁰⁵. Multiple modified versions of GE2 failed to improve efficacy to inhibit allergic effector cell activation or its safety profile¹⁰⁶. Nevertheless, GE2 has been patented by Tunitas Therapeutics in 2015. It will be interesting to see, whether GE2 will be further developed.

A fusion protein (DE53-Fc) consisting of an IgG1-Fc and the anti-IgE anchor DARPin E3_53 has also been reported to inhibit allergic effector cells by co-engagement of FcεRI and FcγRIIb on allergic effector cells¹⁰⁷. Compared to GE2, DE53-Fc recognizes FcεRI-bound IgE in a non-competitive manner. Moreover, a mutated version of DE53-Fc (DE53-Fc mut+) with increased affinity for FcγRIIb showed even higher efficacy to inhibit basophil activation¹⁰⁸.

4.4. Anti-IgE nanobodies

Another alternative scaffold, known as nanobody or single domain antibody (sdab), has been used to generate specific binders against IgE. These antibody fragments are derived from camelids (e.g. camels, dromedaries and llamas) or cartilaginous fishes (e.g. nurse shark and wobbegong), which produce functional antibodies devoid of light chains¹⁰⁹. Using phage display, nanobody libraries have been screened on cynomolgus IgE and selected clones tested for cross-reactivity to human IgE as well as for their efficacy to inhibit IgE-binding to FcεRI. 39D11 is one of the identified anti-IgE clones, which has undergone additional rounds of affinity maturation and sequence optimization (humanization)¹¹⁰. The resulting anti-IgE nanobody IgE026 or 026 sdab is patented by Ablynx. An independent study has shown that 026 sdab binds across the Cε3 and Cε4 domain of human IgE overlapping with the CD23 binding epitope and thus inhibiting IgE-CD23 interactions by steric hindrance¹¹¹. On the other hand, there is virtually no overlap with the FcεRI binding epitope on IgE. Nevertheless, 026 sdab still inhibits the interaction of IgE with FcεRI, which has been explained by conformational hindrance¹¹¹, as the IgE-Fc adopts a closed conformation in complex with 026 sdab, which is incompatible with FcεRI binding. Consequently, 026 sdab also inhibits mediator release of human FcεRI expressing RBL-SX38 cells¹¹¹. 026 sdab belongs to the novel class of disruptive IgE inhibitors, as it has been shown to displace IgE from FcεRI¹¹¹ (Fig. 4d). Nanobodies share many common features with DARPins such as small size, high stability and solubility as well as the possibility to generate multi-specific binders¹¹². However, due to the lack of the Fc-part they also require optimization of pharmacokinetics¹⁰⁹.

5. Conclusions

The classical anti-IgE approach using intact humanized monoclonal antibodies for the neutralization of IgE has led to the generation of potent IgE inhibitors such as omalizumab. Despite the successful use of the current anti-IgE therapy for the treatment of severe allergic asthma and CSU there is certainly still a lot of room for improvement. It seems that an important aspect in the development of monoclonal anti-IgE biologicals has been underestimated so far. Namely, that anti-IgE antibodies often feature fundamentally distinct receptor inhibition profiles depending on their particular binding epitopes, steric effects and the ability to induce conformational changes in IgE upon binding. Obviously, their modes-of-action will heavily depend on these features. It seems that too much emphasis has been assigned to affinity optimization of anti-IgE biologicals in the past. Careful assessment of the efficacy to inhibit IgE-binding to CD23 or FcεRI and the generation of structural information is key to identify their potential modes-of-action. This knowledge will facilitate determination of suitable therapeutic indications and ultimately increase chances of success in clinical trials. IgE-driven conditions, in which CD23-mediated processes are known to play an important role (e.g. allergic asthma^113,114) should therefore be primarily treated with anti-IgE biologicals that provide strong inhibition of CD23:IgE complex formation, as shown in a model of pulmonary inflammation quite some time ago⁶⁴. On the other hand, IgE-dependent conditions like CSU^115,116 or food allergies¹¹⁷ that heavily rely on FcεRI-mediated processes require treatments with compounds that feature the corresponding inhibition profile. As already mentioned earlier in this review, the specific receptor inhibition profile could be an important reason why ligelizumab treatment did not show superior efficacy compared to omalizumab treatment in patients with allergic asthma (NCT02075008), however was well superior for the treatment of CSU patients⁶³. While the line between FcεRI- and CD23-mediated pathogenesis cannot always be drawn strictly, this concept should still be kept in mind for the development of future anti-IgE drug candidates.

Another aspect that is likely to impact the generation of new anti-IgE biologicals is the discovery of disruptive IgE inhibitors^91,111. The possibility to actively desensitize allergic effector cells might potentially shorten the onset of action and the degree of treatment benefit. Such disruptive anti-IgE inhibitors might also be of great value for rapid desensitization prior to or during allergen-specific immunotherapy (AIT). Combinatorial treatment with a disruptive IgE inhibitor could potentially enhance the outcome, reduce adverse reactions and increase the overall safety of AIT.

Taken together, we suggest that future anti-IgE biologicals should aim at targeting the involved pathomechanisms at multiple levels to get maximal treatment efficacy²². An ideal IgE inhibitor should therefore in addition to neutralizing free IgE, actively disrupt IgE:FcεRI complexes on allergic effector cells and suppress IgE production in B-cells through engagement of either mIgE, CD23-bound IgE or inhibitory co-receptors. First steps into multi-level targeting have been taken with the development of XmAb7195⁸¹ or MEDI1412⁷⁸. However, as clinical trials have demonstrated, there is still much room for improvement of such approaches. While recent studies clearly suggest that more tailored anti-IgE approaches for different indications are required, the question whether a universal anti-IgE biological that shows high treatment efficacy in a multitude of allergic disorders could be developed by broadening the activity profile and following a multi-level IgE targeting strategy remains to be investigated.

Box 1. Major milestone discoveries.

Discovery of mast cells in 1878 and basophils in 1879^1,2

Discovery of anaphylaxis in 1902³

Introduction of the allergy concept in 1906⁴

Discovery of the “atopic reagin” in serum transfer experiments in 1923⁵

Introduction of the atopy theory in 1923⁶

Classification of hypersensitivity reactions in 1963⁷

Discovery of IgE in 1967^8,9

First anti-IgE concept 1986/87 with development of antibody prototype in 1990^10,11

Market approval of first anti-IgE antibody omalizumab in 2003¹²

Box 2. Future research perspectives.

Alternative binding scaffolds instead of monoclonal anti-IgE antibodies

Disruptive IgE inhibitors for active desensitization

Active suppression of allergic effector cells by targeting inhibitory receptors

Multi-level targeting approaches to optimize treatment efficacy