Monoclonal Antibodies in Treating Chronic Spontaneous Urticaria: New Drugs for an Old Disease

Abstract Background: H1-antihistamines (H1AH) represent the current mainstay of treatment for chronic spontaneous urticaria (CSU). However, the response to H1AH is often unsatisfactory, even with increased doses. Therefore, guidelines recommend the use of omalizumab as an add-on treatment in refractory CSU. This paved the way for the investigation of targeted therapies, such as monoclonal antibodies (mAbs), in CSU. Methods: A literature review was conducted including papers published between 2009 and 2022 and ongoing trials about the efficacy and safety of mAbs as treatment for CSU. Results: Twenty-nine articles, a trial with preliminary results, and seventeen ongoing or completed clinical trials on the use of mAbs in CSU were included. Randomized controlled trials (RCTs), meta-analysis, and real-life studies have proven the effectiveness and safety of omalizumab as a third-line treatment in refractory CSU. However, a percentage of patients remain unresponsive to omalizumab. Therefore, other mAbs, targeting different pathways, have been used off-label in case series and others are under investigation in RCTs. Most of them have showed promising results. Conclusions: Omalizumab remains the best choice to treat refractory CSU. Although results from other mAbs seem to be encouraging to achieve symptom control in refractory CSU, thus improving patients’ QoL, RCTs are needed to confirm their effectiveness and safety.


Introduction
Urticaria is characterized by the development of wheals with or without angioedema. Chronic urticaria (CU) is defined as lasting for more than 6 weeks [1]. The prevalence of CU is estimated to be between 0.1 and 1.4% across different areas of the world [2,3].
Different triggers can elicit urticaria, such as cold, heat, contact, infections, and others. However, in 75% of the patients suffering from CU, the causal factor cannot be detected [4]. Accordingly, urticaria is defined as spontaneous when no specific trigger is identified [1].
Mast cells are primarily involved in the pathogenesis of chronic spontaneous urticaria (CSU) through the release of pro-inflammatory mediators, which, in turn, recruit neutrophils, eosinophils, and T lymphocytes [5,6]. Impaired intracellular signaling pathways, and type II and type I autoimmunity have been suggested as pathogenic mechanisms [7]. It has been found that 30-50% of patients with CSU produce immunoglobulin (Ig)G autoantibodies against IgE or its receptor (FcεRI), causing the degranulation of cutaneous mast cells and basophils, and thus histamine release [8,9]. Regarding type I autoimmunity, in a cluster of CSU patients, authors reported evidence of IgG and IgE against thyroperoxidase (TPO), defining this mechanism as "autoallergy". Patients with CSU had a six-fold higher risk of

Eligibility Criteria
The inclusion criteria were: any language, publication in peer reviewed journals, children and adults who have been diagnosed with CSU; original article, meta-analysis, systematic review, review, case series, case report, and letter about mAbs in treating CSU. Exclusion criteria were: original article, case series, case report, and letter not focusing on CSU treatment; guidelines.

Guideline Review
Two independent reviewers (S.M., A.G.) performed data extraction using standard templates to report recommendations in support of or against the use of biological drugs in treating CSU. Articles were excluded by title, abstract, or full text for irrelevance to the investigated issue.
Omalizumab was first approved to treat severe uncontrolled allergic asthma from age six and, subsequently, severe chronic rhinosinusitis (CRS) and CSU. At present, omalizumab is the only mAb licensed for the treatment of CSU in patients 12 years of age or older who remain symptomatic despite H1AH [24].
The first evidence of omalizumab efficacy in CSU was suggested in a case series of individuals treated for asthma, who reported improvement in CSU [74].
Successively, a phase II randomized double-blind placebo controlled trial (RDBPCT), MYSTIQUE, evaluated the administration of different doses of omalizumab in 90 adult patients. Changes from the baseline in weekly urticaria activity score (UAS7) appeared at week 1 and were already significant at week 4 in both the 300 mg and 600 mg groups compared with the placebo (−19.9 and −14.6 vs. −6.9 points; p < 0.001 and p = 0.047, respectively), while the 75 mg dose induced a non-significant change in UAS7 compared with the placebo. A plateau in dose-response was observed with around 300 mg omalizumab [25]. The authors suggested that the earlier onset of action in CSU than in asthma could be explained by lower total IgE levels and less-dependent IgE pathogenesis [25]. The change in UAS7 from baseline to week 24 was also significant in patients with moderate-tosevere CSU and positive IgE anti-TPO antibodies, which are probably involved in mast-cell degranulation, after omalizumab vs. placebo (−17.8 vs. −7.9 points; p = 0.0089). Two-thirds of patients in the treatment group reached the resolution of symptoms [26].
These trials paved the way for the development of further RCTs on larger populations. Among these, the results of three RDBPCTs, GLACIAL, ASTERIA I, and ASTERIA II, led to the approval of omalizumab for the treatment of CSU by the Food and Drug Administration (FDA) [27][28][29].
GLACIAL assessed the safety of omalizumab 300 mg as a primary endpoint, enrolling 335 individuals with CSU refractory to H1AH at up to four-fold the approved dose in combination with H2 antihistamines (H2AH) and/or leukotriene receptor antagonists (LTRAs). No difference in the rate of AEs was found between the treatment and placebo groups over 40 weeks (11% vs. 13%) [26]. The changes reported in weekly itch severity score (ISS7) at week 12 were significant (−8.6 vs. −4.0 points; p < 0.001), as similarly found for UAS7 and the dermatological quality of life index (DLQI) [27].
In ASTERIA I and II, in contrast with GLACIAL, the enrolled patients were symptomatic with H1AH at the approved dose, and other doses of omalizumab further than the 300 mg were tested [27][28][29].
ASTERIA II included 323 patients with moderate-to-severe refractory CSU and showed that 150 mg or 300 mg omalizumab significantly reduced ISS7 from baseline to week 12 compared with a placebo (−8.1 and −9.8 vs. −5.1 points; p = 0.001 and p < 0.001, respectively). The reduction was also significant for the secondary endpoints, such as UAS7, with 66% and 43% of patients in the 300 mg and 150 mg group, respectively, having a score of less than six [28].
The efficacy of omalizumab at week 12 in ASTERIA I (n = 319) was comparable with the above-mentioned trials with regard to ISS7, UAS7, and DLQI [27][28][29]. The reduction in ISS7 was statistically significant in the 300 mg and 150 mg groups compared with the placebo (−9.4 and −6.6 vs. −3.6 points; p < 0.0001 and p = 0.0012, respectively), as well as, in contrast with ASTERIA II, in the 75 mg group (−6.4 vs. −3.6 points; p = 0.001). The improvement of the analyzed outcome measures was sustained at week 24 [29].
The three above mentioned trials have shown a dose-dependent response for omalizumab, with higher rates of disease control or complete response (UAS ≤ 6 or UAS = 0, respectively), when patients were treated with 300 mg of omalizumab [27][28][29]. This dose was also associated with a higher percentage of sustained responses [27,28,74].
The POLARIS study was conducted on 218 Japanese and Korean individuals, who had a lower incidence of angioedema than the Caucasian population, and it confirmed the efficacy and safety of omalizumab 150 mg and 300 mg as an add-on treatment in refractory CSU, through the assessment of different outcome measures at week 12 (ISS7, p = 0.006 and p < 0.001, respectively; UAS7, p = 0.006 and p < 0.001, respectively; etc.). The effect of omalizumab was dose dependent [32]. A similar efficacy was observed in 418 Chinese adult patients (p < 0.001) [36].
The long-term efficacy of omalizumab has been less investigated thus far. A phase IV RDPCT, XTEND-CIU (Xolair Treatment Efficacy of Longer Duration in Chronic Idiopathic Urticaria), assessed this outcome. At first, all of the enrolled patients (n = 205) underwent 24 weeks of open-label treatment with omalizumab. Then, patients who achieved controlled disease (n = 134) were randomized into the omalizumab group or placebo group. A greater number of patients who were switched into the placebo group experienced a relapse of CSU compared with those who continued treatment (60% vs. 21%; p < 0.0001). In patients in whom omalizumab was re-started because of disease relapse, UAS7 was significantly reduced at week 12 (95% CI, −34.3 to −24.7; p < 0.0001). Interestingly, no significant difference was found in the incidence of relapse regardless of whether treatment was discontinued after 24 or 48 weeks (43.4% vs. 45.1% after 12 weeks discontinuation; p = 1) [33].
The OPTIMA trial, consistent with the XTEND-CIU trial, provided additional information about re-treatment with omalizumab in patients who had achieved controlled disease at week 24 (UAS7 ≤ 6). Among these, 48% relapsed after withdrawal and underwent 12 weeks of re-treatment, with 88% of them regaining disease control. It was also found that most of the patients (70%) with an inadequate response to treatment (UAS7 ≥ 6) (79% of the 150 mg group) achieved symptom control after increasing the dose to 300 mg. The same trend was observed in patients with inadequate disease control at week 24 (UAS7 ≥ 6), 22% of whom benefited of the extension treatment period, while the remaining generally had lower UAS7 scores than the baseline [35].

• QoL
Changes in UAS7 and the chronic urticaria quality of life questionnaire (CU-Q2oL) or DLQI have been found to be closely correlated, meaning that changes in symptoms are reflected in an improved QoL [75].
As regards the effects of omalizumab on DLQI, the analysis of the three pivotal trials, ASTERIA I, ASTERIA II, and GLACIAL, showed a significant reduction from baseline at weeks 12 (ASTERIA II; p < 0.001) and 24 (ASTERIA I and GLACIAL; p < 0.05 and p < 0.0001, respectively) in the 300 mg group compared with the placebo, in patients both without and with angioedema. DLQI increased during follow-up, without reaching baseline values [27][28][29][76][77][78].
The XTEND-CIU trial reported similar findings. Treatment induced the improvement of DLQI, as well as other HRQoL measures, such as the insomnia severity index (ISI), and work productivity and activity Impairment (WPAI), at weeks 12 and 24, compared with baseline. The improvement was sustained during the following 24 weeks in the treatment group compared to the placebo group, who experienced a worsening in symptoms and quality of life (p < 0.0001) [34].
When the assessment of HRQoL outcomes was restricted to CSU patients with angioedema, which has a remarkable impact on a patient's quality of life, significant improve-ment was still reported in the CU-Q2oL score, DLQI, and the angioedema quality of life questionnaire (AE-QoL) at weeks 4 and 28 compared with the placebo (p < 0.001) [30,31].
• Safety GLACIAL, whom primary endpoint was to assess the safety of omalizumab in refractory CSU, showed that the rates of adverse events (AEs) and serious adverse events (SAEs) over the 24 weeks of treatment and 16 weeks of follow-up were similar compared with the placebo. Moreover, no episodes of anaphylaxis were reported [27]. In ASTERIA II, a greater incidence of SAEs was observed in the 300 mg group (6% vs. 3%) [28]. However, these were reported mainly during the follow-up when treatment had already been discontinued [28].
Consistent with GLACIAL, no safety concern emerged from the other trials, even in the event of a longer treatment duration as in the XTEND-CIU trial. The most common reported AEs were headaches and nasopharyngitis [33].

Ligelizumab
Ligelizumab is an anti-IgE mAb binding IgE, preventing their bound to high-affinity IgE receptors (FcεRIα) on mast cells and basophils, needed for the release of inflammatory mediators, with an in vivo nine-fold higher affinity to IgE (95% CI 6.1-14) compared with omalizumab [79]. Hence, it is expected that it should be more efficacious in Fc-RI-driven diseases such as CSU than omalizumab, which induces a greater inhibition of IgE-binding to CD23 involved in antigen presentation. Furthermore, the suppression of the skin prick response to allergens in atopic individuals was also significantly higher after ligelizumab than omalizumab (p < 0.001) [79,80].
We included two RDBPCTs, the extension phases of one of these trials, and a trial with preliminary results available [37][38][39][40]. The largest RDBPCT recruited 382 adults with moderate-to-severe CSU poorly controlled by H1AH alone or in combination with H2AH or LTRAs. Patients were randomized into six treatment groups (240 mg, 72 mg, 24 mg monthly ligelizumab, 300 mg monthly omalizumab, one ligelizumab 120 mg dose followed by placebo, and placebo). At week 12, the percentage of patients with a complete response to treatment (weekly hives severity score (HSS7) = 0) was higher in the ligelizumab subgroups compared with the omalizumab group (51% and 42% in the 72 mg and 240 mg ligelizumab groups, respectively, vs. 26% in the omalizumab group). Similar rates were reported as regards UAS7 = 0. Furthermore, patients treated with 240 mg ligelizumab showed a longerlasting response after treatment discontinuation [37]. The assessed HRQoL outcomes, such as DLQI and sleep and work interference, were all improved compared with the baseline. The extension phase of this study included 226 patients with a UAS7 ≥ 12 at week 32. They underwent 52 weeks of treatment with ligelizumab 240 mg, experiencing sustained improvement of the abovementioned outcomes [38]. Notably, the extension phase of this trial showed the long-term safety and efficacy of ligelizumab. Indeed, half of the patients-who had showed a poor response in the first phase of the study-reached a complete response at week 12 and 52 (46.5% and 53%, respectively) and disease control was confirmed to be long-lasting even after treatment suspension, with a median time to relapse of 38 weeks [39].
The other RDBPCT (NCT03437278) was conducted on 49 adolescents (12-17 years old) with treatment-refractory CSU to investigate ligelizumab (24 mg, 120 mg, 8-weeks placebo followed by 120 mg ligelizumab), as an add-on treatment to H1AH for 24 weeks. The preliminary results showed that all three groups reported a reduction from the baseline in UAS7, ISS7, HSS7, and DLQI at different endpoints (week 12, 24, and 40) [40].
As regards the safety profile, no serious adverse events were reported in around 900 individuals treated with ligelizumab [81].

Quilizumab
Quilizumab is an afucosylated anti-IgE mAb directed against the M1 segment of IgE, which is expressed only on surface IgE, with a higher affinity than the fucosylated type. Quilizumab was able to determine IgE-switched B cells apoptosis in vitro, thus reducing IgE production [82]. In patients with asthma and allergic rhinitis, the reduction in IgE levels was significant and long lasting [83].
In an RDBPCT on 32 patients with CSU refractory to H1AH, quilizumab induced a reduction in the serum total IgE. Nevertheless, no significant change was observed in clinical scores at week 20 compared with the placebo (ISS7 = −3.3 points (90% CI, −7.2 to 0.7; p = 0.17) and UAS7 = −5.8 points (90% CI, −14.1 to 2.5; p = 0.24)). The authors suggested that this may be due to the different mechanism of action of quilizumab compared with omalizumab, or an inappropriate/inadequate dosage of quilizumab (Table 2) [41]. No other trial on quilizumab is ongoing.
In the inflammatory response of CSU, which also shows a Th2 inflammation pattern, mast cells, together with B cells and basophils, play a central role, by releasing different mediators, one of which is IL-5 [6,91]. Eosinophils, in turn, can be responsible for mast-cell degranulation in CSU, as well as tissue destruction mediated by the major basic protein [92]. Moreover, the evidence of eosinophilic infiltration in the lesional and non-lesional skin of patients with CSU, without blood eosinophilia, suggested their pathogenic role in such diseases [93]. Interestingly, around 10% of patients with CSU may present blood eosinopenia (<0.05 × 10 9 /L) that has been associated with more severe disease [92].

Mepolizumab
Mepolizumab is currently approved by the European Medicine Agency (EMA) to treat severe refractory eosinophilic asthma (≥6 years old), severe CRS with nasal polyps, uncontrolled eosinophilic granulomatosis with polyangiitis, and hypereosinophilic syndrome [94]. Interestingly, mepolizumab, when used to treat patients with severe eosinophilic asthma and concomitant CU (induced by NSAIDs), resulted in a rapid remission in urticaria and a significant reduction in eosinophil blood count [95].
In a case report, a German woman affected by severe eosinophilic asthma and concomitant refractory CSU had a complete response after treatment with mepolizumab for 16 weeks. However, when she interrupted treatment because of an immune complex reaction, urticaria symptoms relapsed [42]. A single-arm open-label trial is investigating the efficacy of mepolizumab in refractory CSU (NCT03494881) [61].

Reslizumab
Reslizumab is approved for adults with severe asthma that is not properly controlled by a combination of inhaled high-dose corticosteroids plus another medicine used for the prevention of asthma [96]. In a 43-year-old patient with severe refractory eosinophilic asthma and refractory CSU and cold urticaria, reslizumab was shown to induce a sustained improvement of symptoms during 5 months of treatment [43].

Benralizumab
Benralizumab is an anti-IL-5 receptor (IL-5R) mAb indicated as an add-on maintenance treatment of severe uncontrolled eosinophilic asthma in patients 12 years old and older [97]. Information from only one trial is available regarding benralizumab in CSU. It involved 12 adult patients with moderate-to-severe CSU inadequately controlled by H1AH. They received monthly benralizumab for 3 months, after a placebo dose. Nine patients completed the treatment, showing a significant reduction from baseline in UAS7 score at week 20 (95% CI, −6.6 to −24.8; p < 0.001), with no AEs reported. Five of them had a complete response (UAS7 = 0) at week 24 [44]. A phase II RDBPCT (NCT04612725) on benralizumab is still ongoing in patients with H1AH-refractory CSU [62].
Although the underlying pathogenic mechanism of CSU is mainly characterized by Th2-driven responses, dupilumab is currently approved by the EMA for individuals with moderate-to-severe uncontrolled atopic dermatitis (≥12 years old or, if severe, ≥6 years old), severe uncontrolled asthma (≥6 years old), and CRS with nasal polyposis [99]. Recently, the FDA also approved dupilumab for the treatment of moderate-to-severe atopic dermatitis from the age of 6 months and eosinophilic esophagitis in patients aged 12 years or older and weighing at least 40 kg [100]. The use of dupilumab is still off-label in CSU and data are limited to three case series, including a total of 10 patients, suggesting dupilumab may be an effective treatment for refractory CSU (Table 2) [45][46][47].
Six patients with CSU refractory to concomitant treatment with antihistamines and omalizumab (300 mg and/or 600 mg) and comorbid atopic dermatitis underwent treatment with dupilumab in combination with other drugs (H1AH and/or topical tacrolimus and/or montelukast). Five patients out of six reached controlled disease with a UAS7 ≤ 6, when reported, or the resolution of CSU at month three [45]. Staubach et al. reported two children, aged 6 and 17 years old, respectively, suffering from CSU and treated with dupilumab after a poor response to up-dosing omalizumab and cyclosporine. Both patients had improved symptoms, especially the one with high IgE levels [46].
A similar effectiveness of dupilumab was observed in two adult women with refractory CSU, one of them was treated with dupilumab for comorbid atopic dermatitis, who had a complete and sustained response to treatment [47].

Anti-CD20 Rituximab
Rituximab is a chimeric murine-human recombinant mAb that binds to CD20, which is expressed on the cell surface of B lymphocytes. Its mechanism of action consists of the depletion of B cells [101]. Rituximab-opsonized B cells have been suggested as an additional mechanism, reducing the interaction of macrophages with immune-complexes [102].
However, it has been increasingly used off-label in other autoimmune diseases (e.g., multiple sclerosis) [104].
Authors have reported the efficacy of rituximab 375 mg/m 2 or 1000 mg in treating patients affected by CSU refractory to H1AH variably combined with other drugs (corticosteroids, immunosuppressants, and omalizumab) [48][49][50][51]. All patients (n = 4) achieved a complete remission early on and maintained it for several months after receiving rituximab [48][49][50][51]. The long-lasting remission induced by rituximab may be explained by B-cell depletion with a subsequent reduction in the synthesis of IgG autoantibodies, which have been suggested to be involved in the pathogenesis of CSU [105].
However, the reduction in Ig levels caused by rituximab can be associated with an increased risk of infections [106].
An open-label trial (NCT00216762) designed to assess the efficacy and safety of rituximab in 15 individuals with refractory CSU was stopped early and data are not available [65].

Anti-IL-17 Secukinumab
In patients with CSU, IL-17 expression has been found to be significantly higher in CD4+ lymphocytes and mast cells in both lesional and non-lesional skin biopsies compared with healthy controls (p < 0.0001) [52]. Patients with CSU have also shown blood IL-17 concentrations higher than controls. Interestingly, IL-17 levels were higher in patients with severe disease [107]. Secukinumab, neutralizing IL-17, is currently labelled to treat moderate-to-severe psoriasis and spondilarthritis [108]. It was found to be strongly effective in eight patients affected by severe CSU refractory to omalizumab. The patients had a baseline UAS7 ranging from 32 to 40, much higher than patients recruited in other studies, and after three months of treatment reported an 82% reduction in UAS7 [52].

Anti-IL-1 Canakinumab
Canakinumab antagonizes IL-1β, an essential cytokine to innate immunity [109]. In an RDPCT (NCT01635127), one single dose of canakinumab 150 mg did not induce significant changes in UAS7 at week 4 in 20 adults with refractory CSU. Hence, the authors concluded that IL-1β might not be involved in the pathogenesis of CSU [53].

Anti-Tumor Necrosis Factore Alfa (TNF-α) Infliximab
TNF-α has been reported to increase in the lesional and non-lesional skin of patients with cold and pressure urticaria, as well as in the serum of patients with CU, correlating with disease severity [110,111]. A case report described the use of the TNF-α inhibitor infliximab in a 35-year-old woman suffering from CSU. The patient experienced symptom resolution for 5 months, until treatment was switched to etanercept. Re-treatment with infliximab was effective again for a further three years, when flares were controlled by adding cyclosporine [54].
Tezepelumab, a human monoclonal antibody inhibiting the action of TSLP, appears to prevent and treat the lesional skin of patients with CSU [6]. Its efficacy and safety are currently tested in a phase II, multi-center, interventional, randomized, parallel-group, placebo-controlled, omalizumab-controlled clinical trial enrolling 175 adult participants (18 to 80 years) with CSU [66].
3.9. Other Biologics 3.9.1. Barzolvolimab Barzolvolimab (CDX-0159) antagonizes the tyrosine kinase receptor KIT, whose ligand is the stem cell factor. The KIT receptor pathway is involved in mast-cell differentiation. One dose of Barzolvolimab was able to suppress mast cells in healthy individuals and two RDBCTs (NCT04538794, NCT05368285) are recruiting patients with refractory CSU to assess its efficacy and safety [67,68,115].

MTPS9579A
MTPS9579A is anti-tryptase mAb that acts by dissociating tetramers into inactive monomers [116]. Tryptase is the main mediator accumulated in mast-cell granules and, when released, promotes and amplifies the inflammatory response [117]. The administration of MTPS9579A resulted in a dose-dependent reduction in active tryptase in the upper airways of 106 healthy individuals [116]. An RDBPCT (NCT05129423) is enrolling patients to evaluate MTPS9579A as a treatment for refractory CSU [69].

LY3454738
LY3454738 is an agonist of the CD200 receptor that is associated with Th2 inflammation. Indeed, CD200R expression is higher on Th2 cells, and it is upregulated by IL-4. Its expression is much greater in peanut-specific CD + T cells of patients with an allergy to peanuts [118]. LY3454738-due to its suggested role in Th2 inflammation-was investigated in an RDBPCT on patients with refractory CSU (NCT04159701) that was stopped early because of a lack of efficacy [70].

Lirentelimab
Lirentelimab (AK002) acts as an anti-sialic acid-binding, immunoglobulin-like lectin (Siglec)-8, an inhibitory receptor of the CD33-related family selectively expressed on eosinophils and mast cells. AK002 has been shown to inhibit mast cells and to induce the apoptosis of eosinophils [119]. An open-label study (NCT03436797) has been conducted to determine the efficacy and safety of AK002 in refractory CU. However, the results are not yet available [71].

Discussion
This review provides an overview of the currently available evidence regarding the use of mAbs as a treatment for CSU (Tables 1-5) .
Regarding omalizumab, a number of performed trials with a consistent number of enrolled patients have shown that omalizumab is effective, improving disease control and QoL, and safe, thus representing a well-established add-on treatment in refractory CSU, as stated by the updated EAACI guidelines [1, [25][26][27][28][29][30][31][32][33][34][35][36]. Nevertheless, a limitation is the lack of RCTs on children. The only data refer to adolescents (≥12 years of age), who have been included in RCTs with adults, where they represent a marginal percentage, and they are not analyzed separately. A prospective open-label study on 29 adolescents with refractory CSU confirmed the effectiveness of omalizumab, with 58% of patients reaching a complete response (UAS = 0) at week 12. Three patients had a relapse after several months (from 4 to 12) following omalizumab withdrawal [120]. A review including 13 children reported a complete response in 12 of them after omalizumab 150 mg or 300 mg [121].
An analysis of 67 prospective observational studies, systematic reviews, and metaanalysis on RDPCTs added further data about the effectiveness and safety of omalizumab in adolescents and adults with CSU [122][123][124].
Nevertheless, around 30-40% of patients do not achieve disease control (UAS ≤ 6) with omalizumab [26,27,125]. This might be due to the standard dose of omalizumab, not adapted to weight and IgE levels, as seen in asthma, and/or high IgE levels (>1500 IU/mL), and/or different pathogenic mechanisms . Omalizumab up-dosing to 600 mg reduced the proportion of non-responders to 7% [127].
With the aim of optimizing a treatment, high total serum IgE levels have been suggested as a biomarker predictive of the response to omalizumab [128]. Indeed, patients who exhibited a poor response to omalizumab had lower pre-treatment IgE levels compared with responders, who also showed an increase in IgE levels at week 4, and the IgE level at week 4/IgE level at baseline ratio revealed its superiority as a predictor of the response to treatment [129]. Blood basophils and histamine, which both increased in patients treated with omalizumab 300 mg, could represent other biomarkers predictive of the response to treatment [130]. Serum transglutaminase-2 activity may be a more reliable monitoring biomarker of the response to omalizumab, being less influenced by other comorbidities than IgE [131].
Another unanswered question concerns the optimal duration of treatment. RCTs have reported CSU-relapses after the interruption of omalizumab, with a subsequent response when treatment was re-started [33]. Therefore, omalizumab cannot be defined as a diseasemodifying drug, and long-term treatments seem to be needed to control the disease.
Ligelizumab (240 mg), another anti-IgE, drug has shown superiority to omalizumab, probably due to its slightly different mechanism of action and higher affinity to IgE [37]. However, at the end of 2021, Novartis announced that ligelizumab showed superiority to a placebo, but not versus omalizumab at week 12 in two ongoing trials (NCT03580369 and NCT03580356), although the data are not yet available [58,59,132]. Contrary to this, quilizumab did not improve symptoms [41].
Currently, several experimental and clinical research studies are ongoing with the aim to provide further evidence on the pathogenesis of CSU. Understanding the close relationship between pathogenic pathways and clinical features will allow the identification of novel predictive biomarkers helpful in selecting the best candidate to receive targeted therapies with mAbs, and, consequently, the achievement of better clinical outcomes.
In addition to IgE, other investigated targets have included IL-5/IL-5R, through the development of anti-IL-5 mAbs (mepolizumab, reslizumab, and benralizumab), showing efficacy in 14 patients [42][43][44]61,62]. The IL-4 and IL-17 pathways, targeted by dupilumab and secukinumab, respectively, seem to play a remarkable role in the pathogenesis of CSU; thus, they could be an additional therapeutic weapon in the treatment of refractory CSU [6,52,107]. Nevertheless, data on these mAbs, though encouraging, come from case series, thus no firm conclusions can be drawn about their efficacy [45][46][47]52]. Ongoing and future RCTs on larger populations will clarify their potential therapeutic role in CSU.
TSLP, IL-25, and IL-33, the so called "alarmins" probably represent one of the most intriguing targets because they are located upstream of the inflammatory cascade. Hence, blocking the alarmins pathway could potentially be more efficacious and modify the disease course [114]. Barzolvolimab, suppressing mast cells, could represent another disease-modifying drug [115]. Although it is not the purpose of this review, it is necessary to mention that, among biologic drugs, small molecule inhibitors such as remibrutinib (LOU064), a Bruton's tyrosine kinase (BTK) inhibitor with a potential role in the treatment of CSU, represent an alternative to mAbs [133]. Remibrutinib, similar to other BTK inhibitors (fenebrutinib, tirabrutinib, rilzabrutinib, and TAS5315), targets BTK, which is involved in B-cell differentiation and proliferation and mast-cell activation, mediated by B-cell receptor and FcεRI activation, respectively (Tables 6 and 7) [133][134][135][136][137][138][139][140][141][142][143][144]. Remibrutinib at different doses showed superiority to a placebo in the NCT03926611 trial [133]. Similarly, the preliminary results of the NCT03137069 trial on fenebrutinib 150 mg daily and 200 mg twice a day showed a significant reduction from the baseline in UAS7 at week 8 compared with a placebo (−17.6 and −20.7, respectively, vs. −11.2) [145]. On the contrary, trials on tirabrutinib and etanercept, a TNF-α antagonist, have been stopped early [141,144]. Other trials are ongoing to investigate inhibitors acting on different targets, such as JAK1/TYK2 and prostaglandin D2 receptor 2 (DP2 or CRTH2) (Tables 6 and 7) [146,147]. CRTH2 plays a role in the chemotaxis of Th2 cells and eosinophils, and Th2 cytokine synthesis. AZD1981, a CRTH2 antagonist, induced a significant reduction in UAS7 at the end of the drug wash out period compared with a placebo and with no safety concern [147]. To summarize, small molecule inhibitors may represent an alternative to mAbs as targeted therapies in refractory CSU, with the advantage for some of them of oral administration compared with mAbs. However, data on inhibitors, excepted for etanercept, whose use has been reported successfully in a case report of CSU, are limited to few trials, that, to date, do not allow us to draw conclusions on their efficacy and safety [54,135,141,147]. Table 6. Small molecules inhibitors and their target.

Conclusions
In line with the current guidelines [1], omalizumab has been demonstrated as an effective and safe treatment, allowing a remarkable advance in the management of CSU. Our center's experience is consistent with data on the efficacy of omalizumab, although it is limited to the treatment of relatively few patients. Other biological drugs have shown promising results in treating CSU, and those acting upstream of the inflammatory cascade, such as dupilumab and tezepelumab, may be of major interest and efficacy in the future. "New" mAbs may allow the creation of individualized targeted and more efficacious therapies in patients with treatment-refractory CSU to achieve symptom control, thus improving patients' QoL. In this context, H1AH may maintain a role, mainly as a rescue medication in the event of relapses. However, further RCTs on a larger scale are needed to identify biomarkers able to predict the response to treatment, the optimal dosage, and the duration of treatment for each mAb, and to assess their long-term effectiveness and safety, both in children and adults, as well as the most appropriate management of the CSU patient after the withdrawal of biological drugs.