2.1.1 Omalizumab

The original conceptual framework for developing anti-IgEs as a treatment for allergies was based on defining three desired functional characteristics: (i) high-affinity IgE binding able to compete with FcεRI interactions; (ii) lack of binding to IgE:FcεRI complexes on mast cells or basophils to prevent receptor crosslinking and activation and (iii) lack of binding to IgE:CD23 complexes.⁶⁸ Omalizumab was the first anti-IgE antibody approved for clinical use, although parallel clinical studies were contemporaneously carried out with another anti-IgE (CGP51901/CGP56901/TNX-901/talizumab) that did not advance into phase III clinical trials at that time.⁶⁸ Omalizumab was developed from a murine anti-human IgE monoclonal antibody (MAE11) and was selected based on its binding to free IgE and lack of binding to IgE:FcεRI complexes on mast cells.⁶⁹ Humanization of MAE11 was carried out by grafting MAE11 CDRs onto consensus human framework sequences with 5 human-murine framework mutations required to recapitulate MAE11 affinity.⁶⁹ The resulting humanized antibody (rhuMAb-E25) was advanced into clinical trials for the treatment of allergic rhinitis and asthma^70-73 and approved for therapeutic use in 2003 for moderate to severe asthma under the tradename Xolair. Xolair has since been approved for the treatment of chronic spontaneous urticaria (CSU) in 2014, allergic asthma in children in 2016, chronic rhinosinusitis with nasal polyps (CRSwNP) in adults in 2020, and food allergies in children and adults in 2024.

Omalizumab treatment in patients results in the reduction of free IgE levels and blocks IgE binding to mast cells and basophils, as expected from its competitive binding with FcεRI. Omalizumab forms immune complexes with IgE, with some preference to assemble into 3:3 hexamers rather than larger immune aggregates.⁶⁸ These complexes are not cleared from serum and their formation results in unexpected increases in the total IgE in serum, which accumulates at up to six to ten times basal IgE levels. The accumulation of “neutralized” IgE in immune complexes that is still able to bind allergen may contribute to the protective therapeutic benefit of omalizumab.⁶⁸ Omalizumab treatment also results in the downregulation of FcεRI levels on circulating basophils and dendritic cells,^74-76 reducing their ability to be sensitized by IgE and activated by allergens. The downregulation of FcεRI in basophils likely occurs through two contributing mechanisms: (i) the intrinsic, more rapid degradative turnover of FcεRI in cells in the absence of IgE binding and (ii) the turnover and production of new basophils, also occurring in the absence of free IgE and therefore lacking stabilized FcεRI levels. While omalizumab treatment results in relatively rapid decline in free IgE levels, clinical benefit emerges over much longer periods, indicating that the absolute reduction in free IgE is a rather poor indicator of therapeutic efficacy.

Omalizumab engages an IgE epitope in the constant epsilon (Cε) 3 domains, adjacent to the binding site for FcεRIα,^{77, 78} with only 2 IgE amino acids in common between the omalizumab and FcεRI interaction sites. However, omalizumab binding to IgE sterically blocks FcεRI binding. In addition, one of the omalizumab epitopes is also buried by Cε2 domains in their folded-back, bent conformation. Binding of omalizumab interferes with this bent IgE conformation, which could also contribute to inhibiting FcεRI binding.⁷⁸ Omalizumab also inhibits the binding of CD23, through significant overlap of their respective binding sites on IgE and through substantial steric clashes of the two IgE ligands.⁷⁷ Although omalizumab was originally selected for therapeutic development because it competes with FcεRI but does not engage IgE:FcεRI complexes on effector cells, recent studies have demonstrated that omalizumab can form transient complexes and actively dissociate IgE from the receptor.^79-81 Initially, this observation was only described for high omalizumab concentrations, which raised the question of how physiologically relevant the additional mode-of-action might be in vivo.⁷⁹ However, additional experimentation has revealed that the mechanism was not only concentration but also time dependent.⁸⁰ In other words, over prolonged exposure periods removal of IgE from FcεRI could even be observed at physiological omalizumab concentrations.

2.1.2 Ligelizumab

Ligelizumab is a more recent anti-IgE therapeutic candidate developed by Novartis that has its roots in earlier clinical studies. Ligelizumab is a high-affinity variant (K_D ~ 17 pM) based on the CGP51901/CGP56901/TNX-901/talizumab series of anti-IgE antibodies developed in the 1990s. CGP51901 was a murine/human chimeric IgG1 antibody that underwent phase I and phase II clinical trials for allergic rhinitis and allergic asthma.^{68, 82-84} CPG56901 (TNX-901/talizumab) was a humanized variant of CGP51901⁸⁵ that underwent additional Phase II clinical trials for the treatment of peanut allergy,^{86, 87} providing the first promising indication that anti-IgE therapeutics could be effective in treating food allergies. These studies showed that peanut allergic patients treated with TNX-901 exhibited an increase resistance to peanut exposure from roughly half a peanut to nine peanuts—a level of resistance that could protect against most accidental exposures. Although TNX-901/talizumab was not developed further, ligelizumab builds on these prior promising studies, with the expectation that its high affinity for IgE would lead to increased clinical benefit over both TNX-901 and omalizumab. This expectation follows naturally from the concept that the sequestration of free IgE by anti-IgEs would represent a key correlate of therapeutic efficacy. Surprisingly, this expectation for ligelizumab has not been borne out by multiple phase III clinical studies (in CSU and asthma),^{88, 89} raising important mechanistic questions about the pharmacologic basis of anti-IgE therapy.

Initially, ligelizumab showed significant promise in early phase II clinical trials for allergic asthma and CSU, consistent with its potential to outperform omalizumab due to its higher IgE binding affinity.^{45, 90} Compared to omalizumab, it led to greater suppression of free IgE as well as longer lasting suppression of IgE levels in patients. Surprisingly, these results and promising clinical observations have been followed by disappointing phase II and III clinical trials of ligelizumab in allergic asthma,⁸⁹ CSU,⁸⁸ chronic inducible urticaria (CIndU; NCT05024058) and food allergy (NCT04984876), although a long-term extension study is ongoing (NCT05678959) and a food allergy trial with higher dosing is planned for late 2024. While ligelizumab is more effective than omalizumab at blocking IgE-binding to its high-affinity receptor and more effectively suppresses free IgE in allergic patients, it is striking that this increased potency has not translated into improved therapeutic efficacy. These findings clearly challenge the original conceptual framework for anti-IgE therapeutic development.

Key mechanistic differences between ligelizumab and omalizumab have emerged through structural and functional studies of these antibodies, which have contributed to understanding the clinical observations.^{77, 78, 80, 89} Ligelizumab and omalizumab engage IgE through overlapping, but distinct, epitopes on IgE. While the omalizumab epitope lies within a single Cε3 domain,^{77, 78} the ligelizumab epitope is shifted such that ligelizumab binds IgE across two Cε3 domains.⁸⁰ This difference in positioning on IgE of ligelizumab relative to omalizumab results in key differential structural and functional attributes. While ligelizumab binding more substantially interferes with FcεRI binding through steric blocking, IgE-bound omalizumab is offset to one side of the receptor complex in a position that has a substantially smaller volume of overlap with FcεRI binding. One functional consequence of this positioning difference is that ligelizumab exhibits no observable activity in transiently engaging IgE:FcεRI complexes and activating their dissociation.⁸⁰ A second functional outcome of the ligelizumab binding epitope and Fab binding pose, is that ligelizumab is a significantly weaker inhibitor of IgE:CD23 interactions as compared to omalizumab.^{80, 89} The clinical studies of ligelizumab clearly indicate that increasing anti-IgE potency in blocking IgE:FcεRI binding does not provide a simple correlate for therapeutic efficacy. These additional functional differences between ligelizumab and omalizumab, due to the way in which they each engage IgE, could potentially account for the disappointing clinical impact of ligelizumab. On the other hand, while studies support a role for IgE:CD23 complexes in allergic asthma^{55, 91-94} and food allergies,^{52, 95-99} it is unclear whether this would be true for CSU and could explain ligelizumab's generally disappointing clinical outcomes. Further studies are needed to address these questions.

2.1.3 HAE1

HAE1 (PRO98498) was developed by Genentech as an affinity-matured, potential second generation anti-IgE therapeutic. HAE1 was engineered using a phage display approach yielding a variant with 9 amino acid differences that has an ~25-fold higher binding affinity compared to omalizumab (Fab K_D ~0.6 nM vs. ~15 nM), as a result primarily of a slowed dissociation rate.^{100, 101} HAE1 showed more potent inhibition of IgE binding to FcεRI and inhibition of huFcεRI-RBL cell activation than omalizumab in vitro.¹⁰⁰ Preclinical efficacy was further assessed by observing the suppression of free IgE in cynomolgus monkeys, and free IgE was used as a biomarker for pharmacodynamic behavior of HAE1 and its potential for therapeutic efficacy. HAE1 showed a greater ability to suppress free IgE levels, as compared to omalizumab, and resulted in the accumulation of higher levels of IgE:drug complexes.¹⁰¹ Modeling of the ability of HAE1 to suppress free IgE was used to guide the clinical development program, based on the expectation that the higher HAE1 affinity would allow for lower drug:IgE dosing ratios to achieve the desired level of free IgE suppression. However, we now have, in hindsight, substantial evidence from ligelizumab clinical trials that indicate that free IgE suppression is not the sole contributor to anti-IgE clinical efficacy. Phase I and Phase II clinical studies with HAE1 incorporated designs based on this foundation, but development of HAE1 was halted after two people developed hypersensitivity reactions during the Phase II studies.

At the time that the HAE1 development program was underway, the idea that omalizumab, or its derivatives, could potentially interact with and dissociate IgE:FcεRI complexes was antithetical to the founding concepts of the original anti-IgE drug development programs. Thus, it was not until over a decade later in studies that were specifically designed to re-engineer omalizumab disruptive activities that the impact of the HAE1 program on IgE:FcεRI complex dissociation was assessed.¹⁰² In these studies, omalizumab disruptive activity was re-engineered using a yeast-display approach, revealing a direct link between omalizumab binding affinity and disruptive potency (see below). At this time, it was discovered that HAE1 antibody could more potently dissociate IgE:FcεRI complexes and more rapidly desensitize human basophils than omalizumab. These observations indicate that HAE1 not only achieved increased potency in blocking IgE:FcεRI interactions, similar to ligelizumab, but it had the potential to more rapidly desensitize allergic effector cells and maintain potent CD23 blockade. It remains to be established whether these improved functional attributes, and in particular the more rapid desensitization of allergic effector cells, could have yielded a more effective and safe anti-IgE therapeutic.

2.2.1 Quilizumab (h47H4)

Compared to soluble IgE, mIgE contains an additional 52 amino acid long domain, known as M1′ (or M1 prime, me.1, or CemX), located between the constant heavy chain 4 (CH4) domain and the C-terminal membrane-anchor peptide of the epsilon chain on human B cells.¹⁰⁶ The monoclonal mouse anti-human IgE antibody 47H4, which recognizes the M1′ domain of mIgE, has been developed at Genentech. In an experimental allergy mouse model using transgenic mice carrying the human M1′ domain in the mouse IgE locus, 47H4 demonstrated a reduction in serum IgE levels and number of IgE-producing plasma cells in vivo.^{107, 108} The authors concluded that 47H4 most likely induces apoptosis in mIgE⁺ B cells. Following these studies, an afucosylated humanized version of the antibody, known as quilizumab, was generated.¹⁰⁹ The removal of fucose from the IgG antibody increases binding affinity to FcγRIIIa, enhancing ADCC by NK cells and increasing quilizumab's efficacy to drive mIgE⁺ B cells into programmed cell death. A phase I clinical trial for allergic rhinitis (NCT01160861) and a phase II clinical trial for allergen-induced asthma (NCT01196039) revealed encouraging results, with quilizumab treatment reducing total and allergen-specific IgE levels by 35% and 40%, respectively.¹¹⁰ The treatment also improved clinical signs of allergen-induced asthma. Surprisingly, quilizumab treatment only partially reduced serum IgE levels and did not significantly improve clinical outcomes in subsequent phase II clinical trials with allergic asthma patients (NCT01582503)¹¹¹ or CSU patients (NCT01987947).¹¹¹ The unexpected results of these trials may be attributed to the infrequent and transient presence of short-lived mIgE⁺ B cells, along with increasing evidence that the memory of the IgE compartment originates in a CD23-positive IgG memory B cell pool (i.e., MBC2), which undergoes sequential and potentially repetitive isotype switching to IgE.^{112, 113} To our knowledge the quilizumab program is no longer being actively pursued.

2.2.2 Xmab7195

The anti-IgE antibody XmAb7195 is a humanized, affinity-enhanced variant of MaE11,¹¹⁴ the murine parental antibody of omalizumab, with an altered Fc-region to improve binding to the inhibitory receptor FcγRIIb implemented with the goal of accelerating clearance of anti-IgE:IgE immune complexes from the circulation. Initially developed at Xencor, XmAb7195 aims to neutralize free serum IgE and reduce IgE production by promoting the aggregation of FcγRIIb and mIgE on the B cell surface.^{115, 116} In a severe combined immunodeficiency (SCID) mouse model engrafted with human PBMCs, XmAb7195 decreased free IgE levels and prevented the formation of IgE-secreting plasma cells. The effectiveness of this dual-targeting strategy was also assessed in a phase Ia clinical trial with 72 healthy volunteers and individuals with high IgE levels (NCT02148744). Several subjects with high IgE levels (300–3000 kU/L) reached undetectable free IgE levels (<9.59 ng/mL = <4 kU/L) following single dose intravenous XmAb7195 infusion. Additionally, soluble free and total IgE, basophil surface IgE and FcεRI levels showed pronounced reductions. As for the most important adverse events, transient dose-dependent thrombocytopenia and one drug-related severe bronchospasm with trial discontinuation was reported. A subsequent phase Ib clinical trial (NCT02881853) for subcutaneous administration was conducted and completed in 2017. However, no results have been communicated to date. In February 2020 Aimmune Therapeutics acquired an exclusive license from Xencor to develop and commercialize the antibody (renamed to AIMab7195). While they planned to develop AIMab7195 as an adjunct treatment to oral immunotherapy, to explore treatment outcomes in patients with food allergies, Aimmune Therapeutics was subsequently acquired by Nestlé in October 2020.

2.2.3 UB-221 (8D6)

In 2012, a new monoclonal anti-IgE antibody named 8D6 was developed at United BioPharma, which was reported to bind the Cε3 domain of IgE with fourfold higher affinity than omalizumab (apparent K_D ~59 pM versus K_D ~230 pM, as measured with intact IgG antibodies). 8D6 has a unique binding feature in that it blocks the interaction between IgE and FcεRI but does not interfere with IgE binding to CD23.¹¹⁷ Similar to ligelizumab, 8D6 can recognize CD23-IgE complexes on the cell surface.^{80, 117} However, 8D6:IgE unlike ligelizumab:IgE complexes can still interact with CD23. In a recent follow-up study a humanized version of 8D6, termed UB-221, has been characterized and tested in a phase I clinical trial in patients with CSU.¹¹⁸ UB-221 exhibited the same binding characteristics as its murine predecessor. With higher affinity for free IgE compared to omalizumab, UB-221 showed equal effectiveness to ligelizumab in neutralizing IgE levels ex vivo in sera from patients with atopic dermatitis—some of which contained total IgE levels >10,000 kU/L (24 μg/mL). In vitro, UB-221 suppressed IgE-mediated degranulation of rat basophilic leukemia cells expressing human FcεRIα (i.e., RBL SX-38) with sevenfold higher efficacy than omalizumab. Additionally, UB-221 was shown to markedly reduce the synthesis of IgE in PBMC cultures from healthy donors stimulated with IL-4 and anti-CD40 antibody at both the protein and mRNA level. In direct comparison, this inhibition was superior to that achieved with omalizumab or ligelizumab. The exact mechanism of how UB-221 achieves inhibition of IgE production in B cells, however, remains elusive. The authors speculate that the effect might result from modulating the CD23 pathway possibly by directly cross-linking the receptor and thereby reducing IgE production. As CD23 lacks intracellular signaling domains more research is needed to confirm this hypothesis. In nonhuman primates and human IgE (IGHE) knock-in mice, single application of UB-221 rapidly reduced serum IgE levels to >90%. In the mice, IgE suppression lasted for ~14 days before IgE levels gradually rebounded back to baseline, whereas IgE levels in cynomolgus monkeys were maximally reduced for less than 1 day and reached baseline in <10 days. In the phase I, open-label, dose-escalation trial single intravenous infusion in CSU, UB-221 was found to be safe and well-tolerated. With an estimated serum half-life of 16–22 days at doses between 0.6 and 10 mg/kg, UB-221 led to a significant reduction in serum free IgE levels and a rapid, dose-dependent decrease in the weekly UAS7 scores, which measure disease symptoms. In summary, the current data warrants evaluation of UB-221 in further clinical trials.

2.2.4 IgE_Trap-Fc protein (YH35324)

IgE_TRAP is a fusion protein consisting of two extracellular domains of FcεRIα linked to a hybrid IgD/IgG4 Fc domain.¹¹⁹ While the extracellular FcεRIα domains were incorporated with the aim to bind and neutralize IgE, the engineered Fc-domain has been chosen to potentially reduce the likelihood of IgG1 Fc-mediated side effects such as ADCC, complement-dependent cytotoxicity (CDC), and IgG-mediated anaphylaxis. IgE_TRAP retains the neonatal Fc-receptor (FcRn) binding site, which is crucial for in vivo half-life extension. However, as its eight N-linked glycosylation sites might enhance clearance, IgE_TRAP was sialylated by co-transfecting a α-2,6-sialyltransferase in CHO DG44 cells, which led to capping of the glycans with sialic acids. Compared to omalizumab IgE_TRAP showed superiority in certain functional features. IgE_TRAP showed a 69-fold higher binding affinity for human IgE compared to omalizumab in surface plasmon resonance measurements, which was mainly driven by a slower dissociation rate. Using the human mast cell line LAD2, IgE_TRAP was more effective than omalizumab in inhibiting IgE-mediated degranulation. In cynomolgus monkeys, IgE_TRAP reduced free serum IgE levels more effectively and for a longer duration than omalizumab. A single dose of IgE_TRAP decreased IgE levels below the detection limit and maintained IgE suppression for 10 days, while omalizumab failed to fully reduce free IgE levels and its suppressive effect lasted only 3 days. In a mouse model of IgE-mediated passive systemic anaphylaxis, IgE_TRAP fully suppressed the allergic reaction as measured by drop in body core temperature.

Recently, a first in human study examining IgE_Trap in a randomized, double-blind phase I clinical trial to assess safety, tolerability, pharmacokinetics, and pharmacodynamics was conducted in individuals with atopy.¹²⁰ In a first assessment, healthy or atopic adults with mild allergic conditions and IgE levels between 30 and 700 kU/L were treated with different doses IgE_Trap, omalizumab, or a placebo. In the second part of the study subjects with IgE levels >700 kU/L received either IgE_Trap or omalizumab. The study results indicate that treatment-emergent adverse events occurred in 38.5% of participants in the first part and 62.5% in second part of the study. These events were primarily mild or moderate, with no serious adverse events, treatment discontinuations due to adverse effects, or anaphylaxis reported. IgE_Trap showed a dose-proportional increase in peak concentration and total exposure over the dosage range. The drug also significantly reduced free serum IgE levels, with a greater and longer-lasting effect compared to omalizumab. In conclusion, these data suggest that IgE_Trap is mostly safe and warrant further investigation as to whether IgE_Trap might be suitable to treat subjects with atopic conditions.

2.2.5 Lumiliximab (IDEC-152)

In the 1990's Fujiwara et al. observed that CD23 deficient mice have increased and sustained antigen-specific IgE levels compared to wild-type mice, while T and B cell development seemed normal.¹²¹ Their findings provided strong evidence that CD23 is involved in negative regulation of IgE synthesis. On the other hand, monoclonal antibodies against human CD23 have been reported to inhibit the production of IgE in IL-4 stimulated human PBMC cultures by several groups^122-124 further substantiating the regulatory role of CD23 in this context. While anti-CD23 antibodies can potentially affect multiple aspects of the allergic cascade, different in vitro studies have suggested that crosslinking of membrane CD23 with an IgG receptor could be a key mode-of-action to downregulate IgE synthesis.^125-127 In vivo, anti-CD23 antibodies inhibited antigen-specific IgE responses¹²⁸ and blocked lung eosinophil infiltration in murine asthma models.⁹¹ A chimeric macaque/human anti-CD23 monoclonal antibody consisting of cynomolgus macaque variable regions and human IgG1 constant regions, known as Lumiliximab (IDEC-152), has been developed at IDEC Pharmaceuticals¹²⁵ and tested in a phase I, singledose, dose-escalating clinical trial with allergic asthma patients.¹²⁹ The antibody showed a favorable safety profile and dose-dependently decreased mean IgE levels. However, no clinical disease amelioration as measured by change in FEV1, was observed upon single dose application. Interestingly, the study found a reduction in total CD19⁺ B cell counts, since CD23 is not exclusively expressed on IgE-producing B cells. While the assessment of its use in allergic disorders was ceased after this phase I trial, lumilixumab was further tested in chronic lymphocytic leukemia (CLL), to deplete CD23⁺ B cells. However, this program has also been halted as the study failed to meet primary endpoints in a phase II/III clinical trial.¹³⁰

2.3.1 Dupilumab (Dupixent)

Dupilumab is a humanized monoclonal IgG4 antibody against the alpha subunit of the interleukin-4 receptor (IL-4Rα), which is the common chain of the IL-4 and IL-13 receptor. The antibody has been co-developed at Regeneron Pharmaceuticals and Sanofi. To date, it has been approved for the treatment of several allergic and inflammatory conditions, including moderate-to-severe atopic dermatitis, asthma, chronic rhinosinusitis with nasal polyposis, eosinophilic esophagitis, prurigo nodularis and most recently for chronic obstructive pulmonary disease (COPD) with raised blood eosinophils. While the positive results of two phase III clinical trials with dupilumab in CSU have recently been published,¹³¹ it has already been approved as add-on therapy to standard-of-care H1 antihistamines for CSU in Japan. Dupilumab works by inhibiting the biological activity of both IL-4 and IL-13, which are key drivers of the Th2 response. By doing so, dupilumab targets multiple molecular and cellular mechanisms in the allergic cascade, which has been demonstrated in human in vitro culture systems and humanized mouse asthma models.¹³² Interestingly, dupilumab has been shown to prevent isotype class switching to IgE in B cells in vitro and to suppress IgE levels in vivo.^{132, 133} In that sense, it blocks IgE pathophysiology upstream of monoclonal antibodies directly targeting IgE. However, IgE suppression is less effective, incomplete and requires long treatment periods (1 year for approximately 70% reduction).¹³² While direct comparison with omalizumab in a prospective clinical trial is lacking, no clinically significant differences in efficacy outcomes were reported in indirect retrospective evaluations.¹³⁴

2.3.2 Benralizumab (Faserna)

Benralizumab is a humanized, afucosylated, monoclonal IgG1 antibody that targets the IL-5 receptor alpha chain (IL-5Rα) on the surface of eosinophils and basophils.¹³⁵ Developed by AstraZeneca's biologics research and development branch, MedImmune, it has been approved as add-on maintenance treatment of severe eosinophilic asthma. Besides blocking the biological activity of IL-5, benralizumab has been shown to drive eosinophils into apoptosis via ADCC.²⁹ IL-5 is a crucial cytokine for the growth, differentiation, recruitment, and survival of eosinophils, which play a significant role in the pathophysiology of eosinophilic asthma and other allergic conditions. Benralizumab mediated elimination of these cells, reduces inflammation and helps to control asthma symptoms and exacerbations. Clinical studies have shown that benralizumab is highly effective in reducing eosinophil counts, improving lung function, and decreasing the frequency of asthma exacerbations in patients with severe eosinophilic asthma.^{136, 137} Its ability to target and deplete eosinophils makes it a powerful therapeutic option for patients with difficult-to-control asthma characterized by elevated eosinophil levels.

2.4.1 Mepolizumab (Nucala) and Reslizumab (Cinqair)

Mepolizumab is a fully humanized monoclonal IgG1 antibody from GSK targeting IL-5. The antibody binds IL-5 with high affinity and inhibits its interaction with the IL-5 receptor, which is predominantly expressed on eosinophils. This approach has been shown to result in a reduction of eosinophil counts in blood and tissues.¹³⁸ A single intravenous injection led to a remarkable reduction of blood eosinophils, which persisted for 30 days.¹³⁹ While initial studies in asthma did not show the expected clinical effectiveness,¹⁴⁰ Mepolizumab has in the meantime been approved for the treatment of severe eosinophilic asthma,¹⁴¹ eosinophilic granulomatosis with polyangiitis (EGPA), hypereosinophilic syndrome (HES) and as an add-on therapy to intranasal corticosteroids in CRSwNP. Clinical trials have demonstrated its efficacy in decreasing the frequency of asthma exacerbations,¹⁴² improving lung function,¹⁴³ and managing symptoms in other eosinophilic conditions.¹⁴⁴

Reslizumab is a fully humanized monoclonal IgG4 antibody also targeting IL-5 initially developed at Cephalon, which was acquired by Teva Pharmaceuticals in 2011. It is approved as add-on maintenance treatment of severe eosinophilic asthma.¹⁴⁵ While a direct comparative in vitro study has demonstrated that reslizumab features an 11–26 fold higher affinity and neutralization potency for IL-5 than mepolizumab¹⁴⁶ indirect comparisons of clinical outcomes did not show major differences between the two antibodies.

2.4.2 Tezepelumab (Tezspire)

Tezepelumab is a fully human monoclonal IgG2 antibody that inhibits the epithelial-cell–derived alarmin TSLP. The antibody which has been co-developed by Amgen and AstraZeneca binds TSLP with high affinity (~60 pM),¹⁸ preventing its interaction with the TSLP receptor complex. This blockade interrupts the signaling cascade that leads to the production and release of pro-inflammatory cytokines and chemokines, reducing inflammation and hyperresponsiveness in the airways. Tezepelumab is approved as an add-on maintenance treatment for patients with severe asthma, particularly those who are inadequately controlled with standard therapies such as inhaled corticosteroids and long-acting beta-agonists.¹⁴⁷ Interestingly, tezepelumab is effective irrespective of baseline eosinophil counts.¹⁴⁸ The findings that tezepelumab reduced blood eosinophil count as well as levels of fractional exhaled nitric oxide (FeNO) and IgE, indicate that the antibody is able to suppress multiple inflammatory pathways. The effect of tezepelumab on these biomarker levels may be related to decreased interleukin-5 and interleukin-13 levels.¹⁴⁹ Reduction in total IgE levels may be linked to suppression of IL-4 and IL-13 concentrations, leading to less efficient B cell isotype class switching to IgE. Clinical trials have demonstrated that tezepelumab significantly reduces the rate of asthma exacerbations, improves lung function, and enhances asthma control compared to placebo.¹⁵⁰ Further, a phase IIb clinical trial has recently demonstrated promising results for the treatment of CSU.¹⁵¹ These data support the concept that TSLP inhibition may have broader physiological effects than targeting individual type 2 cytokines.

2.4.3 Lebrikizumab (Ebglyss)

Lebrikizumab is a humanized IgG4 anti-IL-13 monoclonal antibody developed by Almirall and Eli Lilly. The antibody binds IL-13 with an affinity <10 pM.¹⁵² Lebrikizumab-bound IL-13 is still able to bind its cell surface receptors, but the antibody sterically prevents the heterodimerization of IL-4Rα/IL-13Rα1 (Type 2 receptor) and thus its downstream signaling. However, it does not affect the binding of IL-13 to the IL-13Rα2 decoy receptor.¹⁵³ Lebrikizumab has been assessed for the treatment of both atopic dermatitis and asthma. Initial phase III clinical trial data in moderate-to-severe asthma did not demonstrate consistent efficacy.¹⁵⁴ However, reanalysis in a well-defined type 2 asthma population with elevated blood eosinophils, elevated FeNO,¹⁵⁵ reported a successful outcome with reduced asthma exacerbations. In two identically designed phase III clinical trials for atopic dermatitis, Lebrikizumab therapy demonstrated significantly improved skin clearance, itch, and interference of itch with sleep.¹⁵⁶ Currently, its use is restricted to the treatment of atopic dermatitis.¹⁵⁷

2.4.4 Antibodies in clinical trials for the treatment of allergy

Apart from the clinically approved monoclonal antibodies to treat different allergic conditions, additional drug candidates are currently undergoing clinical evaluation in a variety of allergic indications (Table 1). While some of them are directed against pre-validated targets such as IL-4R⍺, IL-5, IL-13 or TSLP, others explore new therapeutic routes for example, by (i) neutralizing the alarmin IL-33, an important driver of type 2 immunity, (ii) blocking inflammatory cytokine signaling involved in allergic itch (e.g., IL-31R), or (iii) preventing interaction between T cells (e.g., OX40) and TSLP-activated DCs (e.g., OX40L) to inhibit Th2 cell mediated immune responses. While this topic has been covered extensively in other reviews,^{170, 171} the table provides an overview of current efforts and strategies to develop more effective monoclonal antibody-based drugs for the treatment of allergies.

2.5.1 Barzolvolimab (CDX-0159)

Barzolvolimab, also known as CDX-0159, is a humanized monoclonal IgG1 antibody designed to target KIT with high picomolar affinity and inhibit its function. While Celldex has developed its predecessor CDX-0158 (KTN0158) for the treatment of KIT-positive neoplasms such as advanced refractory gastrointestinal stromal tumors (GIST),^{172, 173} they subsequently started to implement CDX-0159 to target mast cells in allergic diseases. Compared to CDX-0158, CDX-0159 carries modifications in its Fc portion to abolish FcγR and C1q receptor binding to prevent unwanted FcγR-dependent mast cell activation potentially leading to infusion reactions. Further mutations (i.e., YTE) were introduced to increase the binding affinity for FcRn, reducing the rate of in vivo clearance and extending the antibody's serum half-life. CDX-0159 has been reported to inhibit SCF-dependent KIT activation in vitro.¹⁷⁴ In a phase Ia clinical trial it demonstrated a systemic and prolonged mast cell ablation upon in healthy volunteers. Further CDX-0159 treatment reduced skin mast cells and substantially ameliorated disease activity in a phase Ib with chronic inducible urticaria and systemic dermographism patients.¹⁷⁵ While a phase II clinical trial in CSU is currently still ongoing (NCT05368285), top line results on angioedema relief have been communicated.¹⁷⁶

2.5.2 Lirentelimab (AK002) and other anti-Siglec antibodies

A humanized non-fucosylated monoclonal IgG1 antibody, AK002, targeting Siglec-8 has been developed by Allakos and analyzed for treatment of diseases associated with mast cell and eosinophil-driven inflammation. The ITIM-containing inhibitory Siglec-8 is selectively expressed on mast cells, eosinophils and basophils. Different studies have shown that AK002 depletes eosinophils by ADCC and prevents mast cell activation through inhibitory signaling via Siglec-8.^177-180 Initially, AK002 showed safety, tolerability and bioavailability in a phase I clinical trial with healthy volunteers (NCT04324268). However, despite successful depletion of eosinophils (95–96%), Lirentelimab failed to meet primary endpoints in two phase II clinical trials with CSU (NCT05528861) and Atopic Dermatitis (NCT05155085) patients. While this Siglec-8 program with AK002 has been stopped, Allakos has developed the agonistic humanized IgG1 monoclonal antibody AK006 against Siglec-6. AK006 has been reported to induce strong inhibitory signaling in human mast cells¹⁸¹ and to trigger ADCP leading to a reduction in mast cell numbers in vivo.⁶⁷ A phase I clinical trial in healthy volunteers and subjects with CSU has been initiated (NCT06072157).

2.5.3 Anti-Orai1 (DS-2741a)

ORAI1 is a transmembrane pore-forming subunit forming the hexameric structure of the calcium release-activated calcium (CRAC) channel. It regulates cellular uptake of calcium, which is critical for several biological processes in various cell types including activation of T cells and mast cells. While mutations in the Orai1 gene can cause severe combined immunodeficiency (SCID), characterized by impaired T cell function and a compromised immune system,¹⁸² Orai1 knockout mice exhibit reduced T cell cytokine production and mast cell activation.¹⁸³ Thus, targeted approaches to specifically inhibit ORAI1 have been assessed. Daiichi Sankyo has developed DS-2741a, a humanized anti-ORAI1 IgG1 antibody, which attenuated bone marrow derived mast cell (BMMC) degranulation in vitro and the passive cutaneous anaphylaxis reaction in vivo in human Orai1 transgenic mice.¹⁸⁴ Further, DS-2741a induced suppression of cytokines release triggered by store-operated Ca²⁺ entry in a variety of T helper subsets. In vivo, DS-2741a treatment resulted in an amelioration of dermatitis in a mouse model of mite antigen-induced atopic dermatitis.¹⁸⁴ Since Orai1 is widely expressed, it remains to be investigated whether such an approach could be associated with overt unwanted side effects. Further studies are needed to assess the therapeutic potential of targeting ORAI1 in allergic diseases.

4.1.1 Improving existing anti-IgE approaches

With the development of an efficiency screen based on yeast display to simultaneously increase affinity and disruptive potency, we recently laid the experimental and conceptual foundation for the selection and engineering of potent omalizumab variants.¹⁰² In this work, we characterized a range of anti-IgE antibodies with distinct epitopes, affinities, and disruptive potencies. Clone C02 resulted from such a screen and showed high affinity as well as improved disruptive potency. It completely desensitized basophils from allergic donors within hours at therapeutically relevant doses ex vivo. Insertion of flexible Glycine linkers at each Fab elbow (VH/VL) in C02 (C02-H2L2) further increased its disruptive potency. Importantly, we observed that the dwell time of disruptive anti-IgE antibodies on intact IgE:FcεRI complexes prior to disruption is a critical parameter that correlates with their ability to safely remove IgE from human allergic effector cells. Another group has engineered an omalizumab variant, termed FabXol3, by structure guided insertion of three point mutations, two in the VL domain framework region and one in the C𝜅 domain.^{78, 209} FabXol3 featured slightly higher affinity than the unmutated omalizumab Fab and was more efficient in accelerating the dissociation of IgE-Fc from FcεRI. Overall, these findings illustrate the potential of kinetically active disruptive anti-IgE antibodies to enhance therapeutic efficacy of omalizumab. Furthermore, they provide a structural and conceptual framework for engineering optimized next-generation anti-IgE antibodies featuring both highly efficient neutralization and disruptive potency.

Another study described the development of an omalizumab biobetter, AB1904Am15, through the optimization of stability, immunogenicity, affinity and bioavailability.²¹⁰ First the authors replaced two aspartic acid isomerization hotspots in the complementary determining regions (CDRs) and mutated multiple murine framework residues to the homologous human germline antibody sequence. Using yeast display technology in conjunction with a designed mutation library of each CDR in the variable domains they further screened for increased affinity. Additionally, the previously described YTE mutation²¹¹ was inserted into the Fc region of the antibody to prolong its serum half-life. The resulting anti-IgE antibody AB1904Am15 showed favorable biophysical properties with improved molecular stability. The twofold improved affinity resulted in enhanced in vitro inhibition of IgE binding to FcεRI and enhanced suppression of IgE induced histamine release in human FcεRI-expressing rat basophilic leukemia cells compared to omalizumab. Further, AB1904Am15 showed a more than twofold longer serum half-life in human FcRn transgenic mice. Finally, the optimized antibody outperformed the original omalizumab antibody in terms of free IgE suppression efficacy in a cynomolgus monkey asthma model in vivo. This study further demonstrated the possibilities of improving anti-IgE biologicals. Overcoming current limitations associated with molecular stability and dosing restrictions may hold great promise to optimize clinical efficacy.

4.1.2 Exploring novel mechanisms and alternative scaffolds

The first systematic description of active IgE dissociation from FcεRI using a disruptive IgE inhibitor has been published in 2012.⁷⁹ In this study, we characterized the binding kinetics of an alternative scaffold molecule, the designed ankyrin protein (DARPin) E2_79, featuring this accelerated dissociation mechanism. As E2_79 engages a subset of ligand attachment points on IgE, which become exposed during partial IgE:FcεRI complex dissociation, it accelerates the intrinsic dissociation rate and fully removes IgE from the receptor. This work laid the conceptual foundation to develop more potent inhibitors through targeted screening approaches and molecular engineering. In subsequent studies, we generated bispecific DARPins, such as bi53_79, by genetic fusion⁷⁹ and a bispecific antibody-like hybrid molecule between two DARPins and an IgG Fc-domain, KIH_E07_79, using the knobs-in-hole techniques.⁸¹ The latter example represents an interesting molecule as it integrates important functional features from the initially described disruptive DARPin with classical characteristics of IgG antibodies, including a potentially prolonged serum half-life. These studies highlighted the remarkable potency of KIH_E07_79 to neutralize free IgE, rapidly dissociate preformed IgE:FcεRI complexes, terminate IgE-mediated signaling in preactivated human blood basophils in vitro, and shut down pre-initiated allergic reactions and anaphylaxis in mice transgenic for human FcεRIα in vivo.

A disruptive nanobody, called sdab 026 was reported several years later in 2018.²¹² The authors described an allosteric mechanism for sdab 026, in which both blocking of the IgE:FcεRI interaction as well as disruption of IgE:FcεRI are based on trapping the IgE-Fc Cε3-4 domains in a closed conformation incompatible with FcεRI binding.²¹³ While not yet officially peer-reviewed, various bispecific IgG4 Fc-fusion proteins have been published (patent WO2024003376A1) with sdab 026 (i.e., A1) and an additional less potent or nondisruptive anti-IgE nanobody (i.e., A2-G1), in an analogous approach as previously described for the DARPin Fc-fusions. Several of the bispecific nanobody Fc-fusion molecules, such as NIgG4B1A1(G4S), were shown to efficiently inhibit allergen-mediated reactions in passive cutaneous or systemic anaphylaxis models in mice transgenic for human FcεRIα in vivo.

Compared to classical antibodies, the described alternative scaffolds or fusion molecules feature smaller binding sites, which may be advantageous when targeting of specific epitopes important in disruption of high-affinity protein:protein interactions is essential. As such they represent promising approaches for the generation of next-generation anti-IgE biologicals. However, clearly more work needs to be done in order to assess their developability, manufacturability and in vivo efficacy as well as safety.

4.1.3 Unifying different modes-of-action in one molecule

There is a growing body of evidence suggesting that the therapeutic efficacy of anti-IgE biologicals could be significantly increased, if the treatment simultaneously interfered at multiple levels in the allergy cascade.²¹⁴ This could potentially be achieved by developing next-generation anti-IgE molecules exerting multiple modes-of-action (Figure 3). Theoretically, such a novel drug candidate should ideally combine the following molecular characteristics: (i) bind free serum IgE with high affinity to neutralize its biological activity, (ii) efficiently and rapidly disrupt IgE from its high-affinity receptor FcεRI on the surface of allergic effector cells in the blood (i.e., basophils, dendritic cells) and in tissues (i.e., mast-cells), (iii) inhibit IgE:CD23 interactions to prevent IgE facilitated antigen presentation on dendritic cells or B cells, (iv) suppress IgE production in B cells or eliminate IgE switched B cells.

While a lot of effort has focused on maximizing free IgE neutralization in previous anti-IgE development programs, more recent approaches are starting to optimize additional modes-of-action (Table 3). At this stage, however, more research is needed to evaluate how these functional characteristics will translate into improved therapeutic potential and clinical benefit in vivo in humans.

4.2.1 Omalizumab and allergen-specific immunotherapy

Allergen-specific immunotherapy (AIT) is a clinical procedure aimed at re-establishing immunological tolerance against culprit allergens. The treatment involves administration of incrementally increasing doses of allergen extract, which can be delivered via different routes (i.e., subcutaneous, sublingual, oral or intralymphatic). Starting out with minute concentrations of allergen, decreases the risk of a systemic reaction. Once a certain maintenance dose is achieved repetitive administration is required to achieve clinical benefit. The overarching goal of the treatment is to shift the allergy prone Th2-dominated immune response towards protective Th1 immunity including the induction of regulatory T cells and allergen-specific IgG antibodies. As suggested in the guidelines, the treatment duration typically spans 3–5 years.²¹⁵ This long-term therapy, results in sustained unresponsiveness and prolonged amelioration of allergic symptoms in a fraction of patients.^{216, 217} To increase the safety of AIT and to investigate whether there might be synergistic effects between anti-IgE therapy and AIT, Kuehr et al.²¹⁸ conducted the first clinical trial in which omalizumab was combined with AIT. This randomized, double-blind, placebo-controlled, multi-site study 221 patients suffering from birch and grass pollen-induced seasonal allergic rhinitis. Their results demonstrated that omalizumab conferred a protective effect with different types of allergens and significantly increased the clinical benefit when compared to AIT alone. Many follow-up studies used omalizumab as pretreatment and/or adjunct therapy for AIT mainly in aero, hymenoptera and food allergy,²¹⁹ with the rational of reducing IgE levels before introducing the allergen to reduce the risk of adverse reactions. Increased safety of AIT upon omalizumab combination has been reported for several of these investigations,^220-222 however, omalizumab is not yet officially approved for such combination therapy. The OUtMATCH study, a large multicenter clinical trial in food allergy, is currently assessing the benefits of omalizumab adjunct therapy in AIT in a systematic manner.²²³

The efficacy of combined anti-IL-4 therapy with a suboptimal dose of grass pollen subcutaneous immunotherapy to induce sustained tolerance to allergen in seasonal allergic rhinitis patients was assessed in a randomized, double-blind clinical trial. Using allergen-induced skin late-phase response as surrogate marker of therapeutic response, no additional benefit over SCIT alone could be found.²²⁴ Compared to combination therapy using anti-IgE and AIT, current data indicate that IL-4 targeting in conjunction with AIT appears less promising.

4.2.2 BCMAxCD3 bispecific antibody

B cell maturation antigen (BCMA) is a tumor necrosis factor (TNF) family receptor specifically expressed on plasma B cells (PCs) playing a major role in their survival. The bispecific anti-BCMA/anti-CD3 antibody linvoseltamab (BCMAxCD3) developed at Regeneron to treat multiple myeloma simultaneously engages PCs and T cells inducing T cell mediated killing of the PCs.²²⁵ Recently a novel concept using this antibody to broadly target and remove long-lived PCs in an isotype independent manner combined with blockade of the IL4Rα to durably reverse IgE-mediated allergy has been suggested.²²⁶ In a long-term house dust mite sensitization model (>15 weeks) in mice expressing human BCMA and human CD3, application of linvoseltamab efficiently depleted bone marrow plasma cells (BMPCs). Importantly, all immunoglobulin isotypes (i.e., IgA, IgM, IgE and IgG1) were significantly reduced 1 week following the treatment. Antibody levels recovered to pre-treatment baseline values within 3–5 weeks under continuation of HDM-sensitization. To specifically prevent this rebound effect for IgE, BCMAxCD3 treatment was combined with anti-IL4Rα antibody injections. Indeed, sustained suppression of IgE levels was observed, while the other antibody levels reached baseline levels within 9 weeks post BCMAxCD3 application. Systemic challenge of mice undergoing a 14-week HDM sensitization and combined treatment of BCMAxCD3 with anti-IL4Rα antibodies did not show any signs of anaphylaxis. Further, the combination of BCMAxCD3 and anti-IL4Rα antibody treatment on resting IgE levels in cynomolgus monkeys was tested. In line with results in the mice, an initial drop in IgE and IgA levels with the BCMAxCD3 single treatment was observed. The levels normalized to baseline between 50 and 100 days post injection. Prolonged suppression of IgE was only achieved in the combination treatment. The bispecific BCMAxCD3 antibody was assessed for its ability to eliminate PCs from human PBMCs and BM samples. The antibody specifically ablated PCs but not other B cell subsets in these samples, which is in line with specific BCMA expression on PCs. When culturing BM samples from allergic and nonallergic donors in vitro IgE only accumulated in the culture supernatants of allergic donors. The production of IgE in these samples was fully suppressed upon bispecific BCMAxCD3 antibody treatment. Additionally, initial human data from clinical trials in multiple myeloma patients who received weekly administration of bispecific BCMAxCD3 antibody indicated that IgE levels are completely reduced as early as 4 weeks posttreatment. These data suggest that transient PC ablation using a BCMAxCD3 bispecific antibody combined with persistent inhibition of IgE class switching using an anti- IL4Rα antibody might be a promising strategy to achieve IgE depletion. However, there are many questions remaining about the potential risks associated with transient depletion of all immunoglobulins.

4.2.3 Combination of approved biologicals

When a single biologic is ineffective or insufficient for the management of a given allergic condition within 4 months of treatment, guidelines generally recommended switching to an alternative option¹⁹⁷ instead of adding another therapy. However, interfering with multiple pathophysiological pathways could be beneficial in some patients with mixed allergic phenotypes and/or endotypes. While systematic studies about efficacy and safety of combination therapies are still missing, there are currently around 38 case reports and case series^227-234 providing insight into the use of dual biologics in co-occurring asthma and atopic dermatitis. These mostly include combinations of (i) omalizumab with add-on dupilumab/mepolizumab or (ii) benralizumab with add-on dupilumab. Improved efficacy of dual biologic therapy has been reported in 21 cases and no major adverse events or safety signals were observed so far.²³⁵ However, further systematic trials and cost-effectiveness evaluations²³⁶ are required to make a conclusive statement about the usefulness of these approaches.