Targeting eosinophils: severe asthma and beyond.

Recent research in the field of bronchial asthma has mainly focused on eosinophilic disease phenotype. Several trials proved the efficacy and safety profile of eosinophils and interleukin (IL)-5 targeting molecules, currently approved for severe asthma and available on the market. They include mepolizumab and reslizumab, IL-5 blocking molecules, and benralizumab, targeting the IL-5 receptor and eliciting a NK cell-mediated antibody-dependent cellular cytotoxicity against eosinophils. Eosinophilic inflammation represents the common pathophysiological background of several conditions, providing the rationale for the use of the same biologics beyond asthma. Although with different evidence grade, from clinical trials to case reports, anti-IL-5 biologics have been investigated in eosinophilic granulomatosis with polyangitis, allergic bronchopulmonary aspergillosis, chronic eosinophilic pneumonia, nasal polyposis, hypereosinophilic syndrome, and eosinophilic esophagitis. However, non-negligible differences between asthma and other eosinophilic diseases, particularly in eosinophils homing (blood and/or tissues), target organs and thus clinical features, probably account for the different response to the same drug in different clinical conditions and highlights the need for tailoring the therapeutic approach by modulating the drug dose and/or by combination therapy with multiple drugs. The optimal safety and tolerability profile of anti-IL-5 drugs warrants further and larger experimental and real-life investigations, which are needed especially in the field of non-asthma eosinophilic diseases. This review aims at summarizing the rationale for the use of biologics in eosinophilic diseases and their mechanisms of action. The current efficacy and safety evidence about eosinophils and IL-5 targeting molecules in asthma and in eosinophilic conditions beyond bronchi is also discussed.

Introduction

During the past 10 years, considerable advances have been achieved in understanding the inflammatory background of several immune-mediated diseases. In particular, the eosinophilic inflammation underlying respiratory and non-respiratory inflammatory diseases has been extensively investigated.1 Bronchial asthma, mostly severe eosinophilic and allergic phenotypes, has recently represented a main focus of both basic and pharmacological research. As a consequence, several new molecules have been conceived and explored and some of them are already available on the market.2

The new biologic drugs for severe eosinophilic asthma are substantially enlarging the treatment options and also contributing to a better understanding of the pathophysiologic mechanisms targeted by the therapeutic molecules.3 The increasing amount of knowledge and evidence in the field has paved the way to the potential use of biologics for treating eosinophilic inflammation beyond bronchi.

This review aims at summarizing the evidence about safety and efficacy of eosinophils and interleukin (IL)-5 targeting molecules approved for severe asthma and at exploring the published data about the use of the same biologics for eosinophilic diseases beyond asthma.

Methods

A selective search on PubMed and Medline (keywords: benralizumab, mepolizumab, reslizumab, anti-IL-5 monoclonal antibody, anti-IL-5 receptor monoclonal antibody) was carried out, including papers published up to January 2019. For severe eosinophilic asthma, we selected original articles, randomized clinical trials, and review papers relevant to the topic (drug mechanisms, drug safety, and efficacy). For eosinophilic diseases other than asthma, any type of article relevant to the topic, conducted on human subjects, was included.

Targeting Eosinophils: pharmacological insights

Anti-IL-5 drug mechanisms and effects

Th2 cells and tissue-resident innate lymphoid cells type 2 (ILC2s) are the main producers of Th2 cytokines (including IL-5, IL-4, and IL-13) responsible for eosinophils proliferation, activation, and tissue recruitment. Activated eosinophils induce tissue lesions through different pathways, further enhanced by other cell types, including mast cells (MCs), basophils, T-helper type 1 (Th1) and T-helper type 17 (Th17) T cells, B cells, and humoral mediators (antibodies, cytokines).4

IL-5 was first described in mice as a B-cell growth and differentiation factor.5,6 Subsequently, evidence in humans showed a main effect on eosinophils and to a less lesser extent on basophils.7 IL-5 is secreted by Th2 cells, namely CD4+ and CD8+ T cells, as well as by ILC2s, B cells, MCs, eosinophils, basophils, and epithelial cells.3 IL-5 signalling triggers the production of factors, which are essential for eosinophil differentiation and inhibit cell apoptosis. IL-5 exerts its actions via a specific receptor α-subunit (IL-5Rα), with the induction of JAK/STAT, MAPK, and PI-3K pathways, or through an IL-13/GM-CSF shared β-subunit receptor; MAPK pathway activates NF-kB with subsequent eosinophilic/Th2 cytokines production.8,9 From a clinical point of view, eosinophils and Th2 players are responsible for several pathophysiological mechanisms. The most relevant ones are endothelial cell damage, altered repair processes, and induction of fibrosis. In bronchial asthma, they lead to airways hyperactivity and wall remodelling, which are extremely relevant in poor asthma control and exacerbations.3

Mepolizumab and reslizumab are humanized monoclonal antibodies (mAb) targeting circulating IL-5 and thus preventing IL-5 binding to its receptor (Figure 1). Mepolizumab is an IgG1 kappa mAb; reslizumab is a humanized IgG4k mAb with high affinity for IL-5.2 Benralizumab is an IgG1 kappa mAb targeting the epitope of the α-subunit of the IL-5 receptor (IL-5Rα) on the eosinophils surface and inhibiting IL-5-mediated cell proliferation.2 Moreover, its afucosylated constant (Fc) oligosaccharides region increases the affinity to the FcγRIIIa receptor on NK cells, basophils, and mast cells and is able to recruit them as part of benralizumab mechanism of action (Figure 1). Through these effector cells, benralizumab is able to induce antibody-dependent cellular cytotoxicity (ADCC) against both eosinophils and basophils, executed by NK cells and/or macrophages.10 As a consequence, a complete depletion of eosinophils in the bone marrow and blood within 24 hours after the first administration, and an almost complete depletion in sputum and tissues (90% and 96%, respectively)11 can be observed. In eosinophil-driven diseases such as severe asthma, CD34+ and IL-5Rα positive eosinophil progenitors have been also recognized in tissue and peripheral blood, suggesting the occurrence of extramedullary eosinophilopoiesis.12

Figure 1

Overview of anti-IL-5 biologics.

BAS, basophils; EOS, eosinophils; IL-5 R, IL-5 receptor; NK, natural killer cells.

In the bone marrow, mepolizumab (750 mg IV) significantly reduced mature and late immature eosinophils compared to placebo, whilst the amount of early eosinophil progenitors did not decrease in blood or bone marrow.12,13 This finding could be explained by the hypothesis that IL-5 mainly contributes to eosinophil proliferation and maturation of relatively late progenitors and does not affect the differentiation of early progenitors.

A low dose of mepolizumab (100 mg SC) seemed insufficient to decrease blood and sputum eosinophil progenitors and sputum mature eosinophils,14 whilst a weight-adjusted relatively high dose of IV reslizumab attenuated the number of early progenitors in a comparable cohort of patients with prednisone-dependent severe asthma.15

So far, only one small cohort of four asthmatic patients treated with a single dose of benralizumab (1 mg/kg IV) documented complete depletion of bone marrow eosinophils and their early progenitors. In addition, blood and sputum eosinophils dramatically decreased under benralizumab treatment.16 It is possible that the ADCC-related action of benralizumab may account for the different impacts of mepolizumab and benralizumab on the amount of bone marrow eosinophils and late progenitors.10 However, more evidence is needed to understand the true clinical relevance of the different mechanisms of action.

Safety concerns

Currently, the role of eosinophils is not yet fully understood and their depletion has not been related to any pathology, as a consequence of a primary immunodeficiency or IgG-mediated eosinophil precursor destruction.17 The absence of characteristic syndromes in eosinophil-deficient mice supports that the potential onset of infections, tumours, autoimmune diseases, or neoplasms is unlikely. Available scientific data suggest that, in healthy subjects, eosinophils do not play a critical homeostatic role. Although long-term studies are needed, the published evidence supports a convincing anti-eosinophils and anti-IL-5 compounds safety profile.

Anti-IL-5 and anti-eosinophils monoclonal antibodies for severe asthma

A significant oral corticosteroids (OCS) sparing effect and clinical improvement under mepolizumab treatment was first described in hypereosinophilic syndrome and eosinophilic granulomatosis with polyangiitis (EGPA).18 When initially tested on asthmatic patients, the drug was not able to achieve the expected clinical outcomes, in terms of respiratory function parameters (specifically peak expiratory flow [PEF], forced expiratory rate in one second [FEV1], and bronchial hyper-responsiveness) and exacerbation rate.19,20 An important bias in the selection of patients accounted for those negative results, namely an improper stratification and recruitment according to blood eosinophil levels and asthma severity. Later on, DREAM, MENSA, and SIRIUS randomized clinical trials (RCTs) demonstrated a significant clinical effect of mepolizumab in asthma control and steroid-sparing effect in subjects with blood eosinophils >300 cells/μL and refractory asthma.21–23 The MUSCA study also demonstrated an important and significant improvement of the health-related QoL and in the pre-bronchodilator FEV1 values, which was maintained up to the end of the study time frame (24 weeks).24

A post hoc analysis of DREAM and MENSA studies confirmed the relevance of baseline blood eosinophil count as a predictive biomarker of clinical efficacy.25 However, a clinically significant decrease in exacerbation rate has been highlighted also in patients with 150 cells/μL or more at baseline. Those findings should be taken into account when selecting potential candidates to the treatment. An optimal safety and tolerability profile with a stable and long-lasting effect up to 4.5 years was demonstrated by COSMOS extension study and even more by COLUMBA study.26,27 Interestingly, during the last, a decrease in lung function was noted, possibly due to a natural progression of asthma or to the reduction of OCS dose.

A recent indirect treatment comparison (ITC) based on literature data and Cochrane review showed that in patients with similar levels of blood eosinophilia, mepolizumab was more effective in reducing clinically significant exacerbations and asthma control in comparison with reslizumab and benralizumab.28 The indirect comparison design itself entails methodological limitations, so it will be important to carry out head-to-head studies to define the real superiority of one of the competitors over the others.

Mepolizumab subcutaneous injection with a dose of 100 mg every 4 weeks is registered as a treatment for severe eosinophilic asthma.29

Patients with uncontrolled eosinophilic asthma and blood eosinophil level >400 cells/μL are eligible to reslizumab, which showed a significant improvement of FEV1 and QoL, reduction of sputum eosinophil count, and reduction of exacerbation rate in RCTs.30,31

Data from a post hoc analysis of two pivotal RCTs showed higher benefits of reslizumab as add-on therapy in patients with severe asthma, higher blood eosinophilia values and chronic sinusitis with nasal polyposis (CRSwNP). The subjects treated with reslizumab had a reduction of 83% of the annual rate of exacerbations, whilst the overall reduction was 54%.32

An open-label extension trial enrolled 1.051 patients treated with intravenous (IV) 3.0 mg/kg reslizumab up to 2 years. This mAb confirmed a good safety profile with a long-term clinical efficacy and improvement of lung function.33 However, the IV-approved administration could represent a limitation in a real-life setting, due to the need of a venous access and an IV perfusion over 20–50 minutes. In overweight or obese subjects, the weight-adjusted dosage could overcome the practical limitation mentioned previously, as shown in a recent study that compared the response to reslizumab in 10 patients with OCS-dependent asthma previously treated with mepolizumab.15 Reslizumab showed best outcomes compared to mepolizumab in airway eosinophilia reduction and asthma control. Currently, two phase III RCTs evaluating the efficacy of reslizumab SC (ClinicalTrials.gov identifier: NCT02452190 and NCT02501629) and a phase II–III study on patients with steroid-dependent refractory eosinophilic asthma previously treated with mepolizumab and subsequently treated with IV reslizumab for 4 months (ClinicalTrials.gov identifier: NCT02559791) are ongoing. The efficacy of reslizumab was also confirmed in a 24-week switch study on patients with severe eosinophilic asthma previously treated with omalizumab without clinical benefits.34 At the end of the study (week 24), 60% of patients showed a significant improvement of asthma control test (ACT) score and experienced no more exacerbations. Reslizumab had also shown a steroid-sparing effect, with a meaningful decrease of daily dose of OCS, from 72.4% to 52.0% (p=0.019).

Reslizumab intravenous injection with a weight-adjusted concentration of 3 mg/kg is approved for severe eosinophilic asthma.35

Phase I and II RCTs including patients with severe eosinophilic asthma and blood eosinophils >300 cells/μL, demonstrated that benralizumab is able to significantly reduce blood eosinophil count, inflammatory biomarkers such as derived neurotoxin (EDN) and eosinophil cationic protein (ECP), and the exacerbation rate.36–38 A proteomic analysis on benralizumab impact showed a significant decrease in the expression of genes associated with eosinophils and basophils proliferation, such as CLC, IL-5Rα, and PRSS33; immune-signalling complex genes (FCER1A); G-protein-coupled receptor genes (HRH4, ADORA3, P2RY14); and further immune-related genes (ALOX15 and OLIG2). This modulatory effect of gene expression was higher for patients with greater blood eosinophilia levels.39

Of note, benralizumab is insensitive to circulating IL-5 levels, which can increase during asthma exacerbations (this sensitivity limits the effectiveness of the two other anti-IL-5 agents).40 Through the intense depletion of eosinophils, benralizumab is also insensitive to the effect of IL-3 and GM-CSF, making the response to this drug even more impactful.41 All these peculiarities allowed for interesting results in improvement of exacerbation rate and steroid-sparing effect. A potential advantage of benralizumab treatment in clinical practice is represented by its schedule – it is administered in a prefilled syringe every 4 weeks for the first three doses and subsequently every 8 weeks, which is a longer interval than the monthly schedule of competitors.28

The ZONDA trial substudy highlighted a greater tissue depletion of eosinophils due to benralizumab compared to mepolizumab and reslizumab.42 Furthermore, the ADCC mechanism could overcome the possibility of immune complex formation between IL-5 and mepolizumab or reslizumab, which may provide a type of IL-5 reservoir leading to an incomplete response to treatments, as reported by some studies.43 Benralizumab demonstrated a sustained higher improvement in FEV1, as showed by the SIROCCO and CALIMA RCTs post hoc analysis, even in overweight or obese patients with fixed airflow obstruction (FAO).44 In two very recent case reports, benralizumab was effective in patients previously treated with omalizumab and mepolizumab without clinical response.45,46

A 56-week extension study was recently published on patients initially enrolled in the SIROCCO or CALIMA studies. It mainly focused on the potential consequences of eosinophils depletion. No adverse events, including opportunistic infections, were recorded confirming the safety of the drug.47 Table 1 summarizes the main features of anti-IL-5 drugs for severe asthma.

Table 1

Anti-IL-5 at-a-glance comparison.

Compound		Administration route	Dosage	Ideal patients	Main strengths
Benralizumab	IgG1k mAb against IL-5Rα subunit	Subcutaneous	Every 4 weeks for the first three doses, then every 8 weeks	Eosinophilic asthma ≥300 cells/μL, CSwNP, late-onset asthma	High affinity for IL-5 receptor and ADCC activity, eosinophils sustained tissue depletion, improvement of pulmonary function even in patients with FAO
Mepolizumab	IgG1k mAb anti-IL-5	Subcutaneous	100 mg every 4 weeks	Eosinophilic asthma ≥300 cells/μL, CSwNP, late-onset asthma	Excellent safety profile, demonstrated clinical efficacy and steroid sparing effect
Reslizumab	IgG4k mAb anti-IL-5	Intravenous	3 mg kg−1 every 4 weeks	Eosinophilic asthma ≥400 cells/μL, CSwNP	Personalized dosage, clinical efficacy, improvement of pulmonary functionADCC, antibody-dependent cell cytotoxicity; CSwNP, chronic sinusitis with nasal polyposis; FAO, fixed airway obstruction; IL-5Rα, interleukin-5 receptor α-subunit.

ADCC, antibody-dependent cell cytotoxicity; CSwNP, chronic sinusitis with nasal polyposis; FAO, fixed airway obstruction; IL-5Rα, interleukin-5 receptor α-subunit.

In a recent systematic review, Bourdin and colleagues performed a matching-adjusted indirect comparison (MAIC) between benralizumab, mepolizumab, and reslizumab.48 Benralizumab and mepolizumab reduced the overall exacerbation rate by 52% and 49%, respectively, in comparison with placebo (rate ratio [RR] 0.94, 95% CI: 0.78–1.13, n=1524) and decreased the percentage of clinically significant and severe exacerbations both of 52% (RR 1.00, 95% CI: 0.57–1.75; n=1524). In addition, the pre-bronchodilator FEV1 improvement was similar. A comparison between benralizumab and reslizumab has not been carried out because the patient characteristics were too different to obtain a reliable estimate. However, a similar effectiveness of benralizumab and reslizumab through the indirect analysis was detected. As previously mentioned, the study design itself weakens the reported results, to be verified by direct comparison studies. The subcutaneous administration of Benralizumab is approved for the treatment of patients with severe eosinophilic asthma and aged 12 years or older.

The biologic drugs revolution: the steroid-sparing effect

Since the advent of biological therapies, researchers and clinicians have tried to define to what extent these drugs reduce the need for OCS in patients with severe refractory asthma.

In a recent study by Bleecker and colleagues,49 the OCS dose was the most important predictive marker of benralizumab efficacy in reduction of exacerbations and FEV1 improvement, especially for the blood eosinophil threshold of >300 cells/μL. Other predictors of positive response to benralizumab are represented by frequent asthma exacerbations, nasal polyposis and forced vital capacity (FVC) <65% predicted.

Of note, the OCS-sparing effect was not similar for all the anti-IL-5 drugs. In the ZONDA study,42 benralizumab significantly reduced the median OCS dose from baseline by 75%. In particular, patients with blood eosinophils >150 cells/μL were on prednisone maintenance treatment for at least 6 months; then, they started a period of OCS optimization for up to 6 weeks including an induction phase, a dose-reduction phase and a dose-maintenance phase. The primary endpoint was the overall OCS dose reduction from randomization to maintenance period (weeks 24–28), and the 100% reduction of OCS in patients with optimized OCS dose of 12.5 mg prednisone or equivalents was included amongst the secondary endpoints. In the SIRIUS study (mepolizumab),14 eligible patients had at least a 6-month history of prednisone treatment and eosinophils counts of >150 cells/μL before enrolment (or 300 cells/μL in the previous 12 months). Subsequently, patients followed a period of OCS optimization treatment. After the start of mepolizumab therapy, the reduction phase took place. The primary endpoint of the study was the overall reduction of OCS from randomization to maintenance period (weeks 20–24). A comparison between benralizumab and mepolizumab showed that the benefit in OCS reduction seems to be similar (about 50%), whilst the number of patients able to discontinue OCS therapy was higher for benralizumab compared to mepolizumab (52% and 14%, respectively; the number needed to treat (NNT) was 3 for benralizumab and 17 for mepolizumab).

The greater OCS-sparing effect of benralizumab represents a real advantage in the treatment of eosinophilic severe refractory asthma, as highlighted by the indirect comparison between prednisone-reduction studies (ZONDA-SIRIUS).42,23 Although head-to-head studies are needed to confirm the described trend, the mentioned result could help the clinician in identifying the more proper treatment amongst the different available options in this field.

Targeting eosinophils besides severe asthma: the current evidence

Table 2 summarizes the available evidence about the use of anti-IL-5 and anti-eosinophil monoclonal antibodies as a treatment option for hypereosinophilic conditions other than asthma.

Table 2

Summary of published evidence about anti IL-5 drugs for eosinophilic diseases other than asthma.

Disease	Drug	Reference	Drug schedule	Study design	Population (no. of subjects)	Study duration	Outcomes overview
EGPA	Mepolizumab	Wechsler (55)	300 mg SC Q4W	DBPC parallel group phase III trial	136	52 weeks	Weeks of remission; time to first relapse; steroid sparing effect
		Moosig (53)	750 mg IV Q4W	Phase II trial	10	9 months	Weeks of remission; time to first relapse; steroid sparing effect
		Kahn (51)	750 mg IV Q4W	Case report	1	28 months	Asthma control; blood eosinophilia; radiological assessment
		Kim (54)	750 mg IV Q4W	Open-label study	7	40 weeks	Steroid sparing effect
Nasal Polyposis	Mepolizumab	Gevaert (70)	750 mg IV or placebo Q4W	RCT	20 active; 10 placebo	48 weeks	Clinical and CT score
		Bachert (72)	750 mg IV or placebo Q4W	DBPCT	54 active;51 placebo	25 weeks	Need for surgery, endoscopic score, VAS symptoms score, patient-reported outcomes
	Reslizumab	Weinstein SF (32)	3 mg/kg IV	Post hoc analysis	953	52 weeks	No significant effect
		Gevaert (71)	n=8 at 3 mg/kg; n=8 at 1 mg/kg	RCT	24	36 weeks	Clinical and CT score
HES	Mepolizumab	Garrett (75)	10 mg/kg or 750 mg IV Q4W	Open-label study	4	28 weeks	Symptoms; patient reported outcomes; lung function; blood eosinophils
		Rothenberg (76)	750 mg IV Q4W	DBPC parallel group Trial	85	36 weeks	Steroid sparing effect
EoE	Mepolizumab	Stein (80)	750 mg IV Q4W	Open-label phase I/II safety and efficacy study	4	3 months	Esophageal eosinophilia; patient-reported outcomes
		Straumann (81)	750 mg IV 2 doses, 1 week apart. Non responders--> after 8 weeks 2 further doses 4 weeks apart	DBPCT	11	9 weeks	Esophageal eosinophilia; molecular biomarkers; limited clinical improvement
		Assa’ad (82)	0.55, 2.5, or 10 mg/kg mepolizumab IV Q4W	DBPCT	59	3 months	Esophageal eosinophilia; limited clinical improvement
	Reslizumab	Markowitz JE (84)	1 or 2 or 3 mg/kg IV Q4W	RCT+open-label extension+ compassionate use	226; 8; 4	3–9 years	Patient-reported outcomes; endoscopic assessment; esophageal eosinophilia
		Spergel (83)	1 or 2 or 3 mg/kg IV Q4W	RCT+open-label extension	226	4 weeks	Esophageal eosinophilia; limited clinical improvement
ABPA	Mepolizumab after Omalizumab (switch)	Hirota S (58)	100 SC mg Q4W	Case report	1	56 weeks	Asthma control; blood eosinophils; radiological assessment; no improvement of lung function
	Mepolizumab+ Omalizumab	Altman (59)	Omalizumab 375 mg SC Q2W+Mepolizumab 100 mg SC Q4W	Case report	1	7 months	Symptoms; no improvement of lung function
	Mepolizumab	Terashima (61)	100 mg SC Q4W	Case report	1	4 weeks	Symptoms, lung function, radiological assessment
		Oda (60)	100 mg SC Q4W	Case report	1	9 months	Symptoms, lung function, radiological assessment
		Soeda S (62)	100 mg SC Q4W	Case series	2	24; 21 months	Blood eosinophili, symptoms, lung function, radiological assessment
	Benralizumab	Soeda S (63)	30 mg SC Q4Wx3--> Q8W	Case report	1	2 months	Radiological assessment, eosinophilia, IgE levels, symptoms (cough, sputum)
CEP	Mepolizumab	To M (65)	100 mg SC Q4W	Case report	1	3 months	Symptoms, blood eosinophilia, radiological assessment

ABPA, allergic bronchopulmonary aspergillosis; CEP, chronic eosinophilic pneumonia; DBPC, double-blind, placebo-controlled; DBPCT, double-blind, placebo-controlled trial; EGPA, eosinophilic granulomatosis with polyangitis; EoE, eosinophilic esophagitis; HES, hypereosinophilic syndrome; IgE, immunoglobulin E; RCT, randomised control trial; VAS, visual analogue scale.

Eosinophilic granulomatosis with polyangitis

Eosinophilic granulomatosis with polyangiitis is a primary small vessel vasculitis characterized by asthma, eosinophilia, multiorgan involvement (lung, peripheral nerves, heart, gastrointestinal tract, skin), and the possible presence (<30% of patients) of anti-neutrophil cytoplasmic antibodies (ANCAs).1 Corticosteroids alone represent the traditional therapeutic option in the absence of negative prognostic factors, whilst cyclophosphamide is recommended for severe or corticosteroid-resistant cases. The conventional immunosuppressant options also include methotrexate, azathioprine, and mycophenolate. Intravenous immunoglobulins, IFN-α, infliximab, and rituximab have been also used, mainly to target ANCA-associated pathophysiologic background, and to a less extent blood and tissue eosinophilia. Few data are available concerning omalizumab.50

The supposed key role of eosinophils, and the elevated IL-5 levels in the inflammatory EGPA background supported the rationale for including anti-IL-5 agents amongst the treatment options. Kahn and colleagues reported the first case documenting the successful use of mepolizumab (750 mg IV) in a patient with refractory EGPA,51 and experiencing improvement in peak flow, decrease blood eosinophils count, withdrawal of inhaled treatments during anti-IL-5 treatment. Before the second infusion, complete regression of parenchymal findings was documented at chest-computed tomography, which persisted after 6 months. Other small open-label pilot studies investigated the effect of mepolizumab in EGPA patients.52–54 In particular, Moosig and colleagues conducted a phase II open-label study, involving 10 patients with relapsing/refractory EGPA, who received 9 monthly infusions of mepolizumab (750 mg IV), after stopping previous immunosuppressant therapy, and were subsequently switched to methotrexate maintenance therapy (0.3 mg/kg/week SC) and a tapered dosage of glucocorticoids. This study showed disease remission and glucocorticoid daily dose reduction to the lowest dose over the course of the disease in almost all patients, with no relapses during the active 9-month treatment phase. These findings suggested a steroid-sparing effect and the potential for induction and maintenance of remission in the absence of further conventional immunosuppressive treatments.

Recently, the first large-scale multinational RCT on relapsing/refractory EGPA55 proved the efficacy of anti-IL-5 mAb and paved the way to the approval of mepolizumab for this condition by the United States Food and Drug Administration. Mepolizumab was administered subcutaneously (300 mg) every 4 weeks or placebo (68/group) for 1 year, in addition to the standard of care treatment. Over a 52 weeks period, 28% of patients in the treated group versus 3% in the placebo group experienced at least 42 weeks of remission (OR 5.91, 95% CI: 2.68–13.03; p<0.001), and the percentage of participants in remission both at week 36 and week 48 were significantly higher in the active group (32 versus 3%; OR 16.74; 95% CI: 3.61–77.56). Remission was less likely for patients with baseline eosinophil counts <150 cells/mm3. Moreover, as a secondary endpoint, mepolizumab was able to provide a longer time to first relapse within 52 weeks, the reduction of daily glucocorticoid dose, a lower circulating eosinophil counts, an improved Asthma Control Questionnaire and rhinosinusitis 22-item Sino nasal Outcome Test score. Conversely, lung function did not increase – differently from previous studies on severe eosinophilic asthma.2 The most common adverse events (AEs) were headache, nasopharyngitis, arthralgia, sinusitis, and upper respiratory tract infection. These AEs and their frequency were similar in the placebo group. Overall, mepolizumab was effective in nearly the 50% of the participants. The reason why mepolizumab was ineffective in almost 50% of the patients remains unexplained, and further characterization of specific EGPA phenotypes, e.g. ANCA-negative versus ANCA-positive patients needs to be investigated.50 In fact, a recent genome-wide association study has confirmed that different genes may underlie the complex and diverse potential expressions of EGPA in symptoms and clinical subsets.56 With this perspective, the investigation of the genetic background may increase in accuracy the treatment selection process.

Phase II trials including reslizumab (NCT02947945) and benralizumab (NCT03010436) for EGPA are currently ongoing.

Allergic bronchopulmonary aspergillosis

Allergic bronchopulmonary aspergillosis (ABPA) is the result of a complex hypersensitivity reaction to Aspergillus fumigatus (A. fumigatus), characterized by asthmatic symptoms, systemic and airway eosinophilia, elevated serum immunoglobulin E (IgE) levels, lung infiltration, bronchiectasis, bronchial mucoid impaction, and lung fibrosis. The current treatment includes the long-term use of oral corticosteroids, in addition to antifungal agents, which is potentially associated with serious side effects.57 Some evidence supports the potential efficacy of omalizumab due to its IgE-blocking activity.58 Although the pathophysiology of this condition is not fully understood, a pivotal role is played by an impaired innate and adaptive immune response, which is not able to clear inhaled fungal spores from their respiratory epithelium and sustains an intense Th2 inflammation. Innate immunity response to A. fumigatus in the lung recruits and activates Th2 cells, leading to IL-4 and IL-5 production, and driving the differentiation of IgE-secreting plasma cells, as well as recruitment and activation of eosinophils. These mechanisms suggest that both IgE and IL5 may represent potential treatment targets in patients affected by ABPA. As clinical trials on mepolizumab excluded ABPA patients, the efficacy of mepolizumab in ABPA remains basically unknown. To the best of our knowledge, four case reports of ABPA treated with mepolizumab have been reported in the literature. In 2018,59 a case report was published of a patient with ABPA previously treated with omalizumab (600 mg Q2W) with poor response (no improvements on radiological findings and eosinophilic sinusitis/otitis media). The patient was switched to mepolizumab (100 mg SC Q4W) with subsequent resolution of asthmatic symptoms, normalization of blood eosinophils and serum IgE value, improvement of radiological findings. No improvement of FEV1 was observed.

Another case report described the synergistic and steroid sparing effect of omalizumab plus mepolizumab combination therapy in a patient showing poor disease control when treated with omalizumab only.60

In two further cases,61,62 mepolizumab use in addition to standard therapy, resulted in symptoms and FEV1 improvement, and in the reduction of ABPA-associated mucoid impaction and lung infiltration. The role of IL-5-induced inflammation in the pathogenesis and maintenance of symptoms in patients with ABPA seems predominant in most cases, independently of IgE levels, especially in patients with corticosteroid-dependent severe asthma, with suppressed serum IgE levels.58

Recently, Soeda and colleagues63 reported a case series including two patients with uncontrolled severe asthma and ABPA on long-term treatment with mepolizumab (100 mg SC). Blood eosinophilia reduction, symptoms (cough and sputum) improvement, FEV1 increase, and radiological findings resolution were observed. ACT score significantly increased in one case only. The same author in 2018 reported the case of a patient affected by ABPA-serologic (ABPA-s), without bronchiectasis and/or fibrosis, treated with benralizumab.64 The drug was well tolerated and rapidly (over 2 months) provided symptoms improvement and radiological remission.

Chronic eosinophilic pneumonia

Chronic eosinophilic pneumonia (CEP) is an inflammatory disease of unknown aetiology characterized by eosinophilic infiltration in the lung. The typical symptoms include cough and increasing dyspnea.65 For the treatment of CEP, systemic corticosteroid therapy is recommended, and the clinical response to corticosteroids is usually very good. However, approximately 50% of patients with CEP relapse after cessation of steroids treatment or during tapering.65 Only one case of CEP successfully treated with mepolizumab has been reported.66

Chronic rhinosinusitis with nasal polyposis

Chronic rhinosinusitis (CRS) is a clinical syndrome with multiple phenotypes. The presence of polyps is usually associated with local Th2 inflammation, increased eosinophils levels, and elevated IL-5 and IgE.67 Non-eosinophilic inflammation sustained by Th1 and Th17 pathways may also be observed in CRSwNP.68 CRSwNP is often part of a late-onset eosinophilic asthma, representing a most relevant comorbidity.

Current treatment options for patients with NP, based on the 2016 International Consensus Statement on Allergy and Rhinology, include nasal saline irrigations and intranasal corticosteroid sprays for maintenance therapy and systemic corticosteroids with antibiotics for acute exacerbations. For refractory cases, endoscopic sinus surgery should be considered.69,70 The underlying eosinophilic inflammation provided the rationale for investigating the effect of new therapeutic approaches aimed at blocking IL-5 in CRSwNP.

Gevaert and colleagues conducted two randomized, double-blind, placebo-controlled trial studies71,72 on the safety and efficacy of mepolizumab and reslizumab in patients with CRSwNP refractory to endoscopic surgery. A statistically significant improvement in nasal symptoms and nasal polyp score (NPS) was recorded in both studies, whilst only mepolizumab treatment positively impacted on CT score. In the mepolizumab study, when divided into allergic (n=7) and non-allergic (n=8) responders, CT score significantly decreased in the allergic group, but the validated Asthma Quality of Life Questionnaire (AQLQ) decreased significantly in the non-allergic group. As expected, individuals with high serum IL-5 levels had more benefit from blocking IL-5.

The effects of two different reslizumab regimens, 1 mg/kg and 3 mg/kg, were also investigated.71 Amongst the eight patients receiving a 1 mg/kg infusion, five subjects experienced a significant improvement in Total Nasal Polyp Score (TPS) for up to 12 weeks. Half of the patients who received a 3 mg/kg infusion maintained a reduction in TPS for 4 weeks. Elevated local nasal IL-5 levels at baseline versus systemic levels were predictive of a positive response to reslizumab.

Mepolizumab efficacy in NP refractory to surgery was more recently explored in a larger randomized clinical trial.73 This involved 105 patients of whom 54 were treated with mepolizumab (monthly infusion 750 mg IV) and 51 were randomly assigned to the placebo group for a total of six doses in addition to daily topical corticosteroids. Nasal symptoms and NPS significantly improved, and a reduced need for surgery was reported at the end of the 25 weeks period. Of note, there was no reactive eosinophilia, which has been observed with reslizumab.

Oda and colleagues74 reported the case of a patient with ABPA and CRSwNP treated with mepolizumab at the dose of 100 mg Q4W without significant benefit on the nasal component, despite reductions in eosinophilia. This finding suggests that the high-dose approach used by Gevaert and colleagues is needed to properly target the amount of eosinophils underlying the nasal polyps inflammation.

A phase II (ClinicalTrials.gov Identifier: NCT03450083) and a phase III (ClinicalTrials.gov Identifier: NCT03627286) randomized, placebo-controlled trials are currently ongoing to evaluate the efficacy of benralizumab on eosinophilic rhinosinusitis.

Hypereosinophilic syndrome

Hypereosinophilic syndrome (HES) includes different disorders characterized by a large amount of eosinophils in the blood and/or tissues. Idiopathic hypereosinophilic syndrome (IHES) is characterized by elevated levels of blood eosinophils (>1500 eosinophils/μL) and by the involvement of multiple organs as well as tissue-specific disorders such as eosinophilic esophagitis (EoE).75

So far, two preliminary studies investigated the effect of mepolizumab in patients affected by both IHES and EoE. The former76 evaluated three monthly infusions of mepolizumab (10 mg/kg or 750 mg IV) in four patients with HES and reported a significant improvement in symptoms and QoL in all patients. A FEV1 increase and blood eosinophil count decrease were also observed at the 8 weeks follow-up assessment.

The latter study was a multicentre, randomized, double-blind, placebo-controlled trial carried out on 85 patients treated monthly with 750 mg IV of mepolizumab for up to 36 weeks.77 It described a reduction of prednisone dose to ≤10 mg without clinical flare-up in 84% of active group versus 43% placebo (p<0.0001), showing a potential glucocorticoid-sparing effect of this biologic treatment.

Eosinophilic esophagitis

A predominant infiltration of eosinophils in the oesophageal tract underlies EoE, a chronic immune-mediated disease presenting with peculiar endoscopic and histologic abnormalities. Several eosinophil-derived products cause barrier dysfunction, which leads to nonspecific gastrointestinal symptoms.78

The 2017 European guidelines indicate as first-line treatment proton pump inhibitors (PPIs). Other options include topical corticosteroids, elimination or restriction diet, and oesophageal dilation, the last related to long-term safety concerns and potential complications.79,80

An open-label phase I/II study involving four adult patients over 28 weeks81 firstly investigated mepolizumab for EoE at the dose of 750 mg intravenously monthly. The main observations were a significant decrease in mean and maximal oesophageal eosinophilia (p<0.001, p<0.05, respectively), better clinical control and improved quality of life (p=0.03) with respect to the baseline values. Of note, responsiveness to anti-IL-5 therapy did not correlate with plasma IL-5 levels.

More recently, the efficacy of a different mepolizumab schedule (750 mg IV, 2 doses 1 week apart, repeated after 8 weeks with 4 weeks interval between each dose if complete remission was not achieved – <5 peak eosinophil number/hpf) was explored in a randomized, double-blind, placebo-controlled trial including 11 patients.82 A significant reduction of tissue eosinophilia and of the expression of molecules associated with oesophageal remodelling (TGF-β) was observed 4 weeks after the first mepolizumab administration, without further improvement compared with the placebo group. Nevertheless, a minimal clinical improvement was achieved. No safety issues were recorded.

A larger multicentre, randomized, placebo-controlled trial83 carried out on 59 children (2–17 years old) investigated the impact of three monthly infusions of mepolizumab (0.55, 2.5, or 10 mg/kg) or placebo, and reported negligible improvement in symptoms, remission (only less than 10% of subjects), and reduction of tissue eosinophil count <20 hpf (only 31.6% of subjects) compared with the placebo group.

Reslizumab has also been evaluated as a treatment option for EoE. Spergel and colleagues84 conducted a multicentre RCT including 226 subjects randomly assigned to 1, 2, or 3 mg/kg of reslizumab. No significant differences in physician s’ global assessment scores were recorded between the active and placebo groups, independently of the treatment dose and oesophageal eosinophil counts. A recent open-label 9-year-long study was conducted by Markowitz and colleagues85 as an extended-access program of RCT mentioned previously. It explored treatment at the dose of 2 mg/kg in 12 children and adolescents. The authors reported a significant decrease in reduction in peak oesophageal eosinophilic count in treated patients compared to control groups (67 versus 24% had a reduction in peak oesophageal eosinophilic counts to <5 hpf). Nevertheless, no difference in symptoms between reslizumab and control groups was seen. However, the positive impact on tissue eosinophilia and on relapse or disease progression documented by endoscopy under therapy, as well as an optimal safety profile, paves the way to larger longitudinal studies.

Conclusions

Recent basic and pharmacological research has specifically addressed the Th2 high inflammation phenotype, leading to the development of new biologic drugs, especially in the field of severe eosinophilic asthma. Eosinophilic inflammation represents the common pathophysiological background of several conditions, providing the rationale for the use of the same therapeutic molecules beyond asthma. However, non-negligible differences between asthma and other eosinophilic diseases occur, particularly regarding eosinophils homing (blood and/or tissues), target organs and thus clinical expression. Furthermore, despite the increasing knowledge and evidence in the field, some issues are still under debate.

The correlation between tissue and blood eosinophilia, as well as their specific clinical relevance, is not completely clear. In addition, the observed discrepancy between known Th2 biomarkers and clinical response under biologic treatment suggests that the underlying inflammatory pathways may be much more complex than expected and targeting one mechanism may not be enough. This variability probably accounts for the different response to the same drug in different clinical conditions and highlights the need for tailoring the therapeutic approach by modulating the drug dose and/or a multiple-treatments association.

Few data are currently available about the long-term management of anti-IL-5 drugs in treatment duration, tapering, or withdrawal. In fact, whether biologics exert a disease-modifying effect or their action is limited to the ongoing treatment is still controversial.2,3

The clinical relevance of anti-IL-5 tapering has been investigated in a small study comparing the effect of the fixed dose of mepolizumab and the weight-adjusted dose of reslizumab.86 The reported results suggest the superiority of the latter particularly in the prednisone-dependent asthma phenotype with elevated sputum eosinophilia. The available evidence is not sufficient to support practical recommendations; however, dose adjustment may represent a strategy for management of poor responders. Of note, the approved mepolizumab dose for patients with eosinophilic granulomatosis is 300 mg every 4 weeks.55 It supports the hypothesis that an adjusted mepolizumab dose could be considered in difficult-to-treat patients with a more severe eosinophilic asthma phenotype or with specific comorbidities.

Conversely, especially in the field of severe asthma, the need for biologic therapy continuation should be carefully evaluated in light of the overall adherence to anti-asthmatic treatments, which is known to be suboptimal.87,88 Biologic drugs are currently intended as add-on options; thus, once disease control has been achieved and oral corticosteroids are no longer needed, if poor adherence to inhaled medications occurs the biologic drug should be withdrawn for clinical and sustainability reasons.

However, the optimal safety and tolerability profile of anti-IL-5 drugs needs to be established in further and larger experimental and real-life investigations, which are needed especially in the field of non-asthma eosinophilic diseases.