Bronchial asthma is a disease which consists of chronic airway inflammation, structural changes to the bronchial tree and airway hyperresponsiveness (AHR). Severe asthma affects greatly patients' quality of life. Eosinophils have proved significant in the inflammatory process of many patients with severe asthma referred as "T2 high" asthma. Accordingly, major advances in the management of such patients have occurred in recent years due to the new targeted biologic therapies. Interleukin-5 (IL-5) is the pivotal cytokine for the differentiation, maturation and survival of eosinophils. There are three monoclonal antibodies targeting IL-5/IL-5R, mepolizumab, reslizumab and benralizumab. These treatments led to significant reduction of asthma exacerbations and less use of oral corticosteroids and also imrpovement in lung function and quality of life.
Bronchial asthma is a disease which consists of chronic airway inflammation, structural changes to the bronchial tree and airway hyperresponsiveness (AHR). Its worldwide prevalence is estimated to be around 4.3%, with some countries experiencing a higher burden of the disease, such as the United States of America and Australia, up to 10% [1]. Almost 95 out of 100 asthmatics worldwide experience mild to moderate symptoms, which can be controlled by treatment with inhaled corticosteroids (ICS) and long-acting beta-2 receptor agonists (LABA). However, a small proportion of them need an escalation of treatment with either oral corticosteroids (OCS) or novel biologics targeting specific molecular pathways, which intertwine with the severity of symptoms and are specific to each patient.
This subgroup is termed “severe asthmatics” and includes individuals whose symptoms cannot be controlled under high dose ICS and LABA treatment, or need OCS for several months each year in order to overcome their symptoms. It should be noted that before characterizing asthma as “severe uncontrolled”, a period of surveillance is needed to ensure that it is indeed properly treated and that the patient complies with the use of his medication [2]. This “hard to treat” asthma urged experts to delve deeper into its molecular pathways and ultimately recognize the need to endotype each patient. Eosinophils were quickly revealed to play a predominant role in the pathogenesis of severe asthma, currently known as T2 high asthma. Knowledge about this specific endotype is rapidly growing along with our arsenal of monoclonal antibodies targeting specific mediators involved in differentiation and activation of eosinophils. Not surprisingly, biologics have already proven their great efficacy; however, there still remains quite a few unanswered questions as we continue to experiment not only with their use but also with the switch from one biological to another.
Eosinophils have drawn great interest over the past decade since the breakthrough with the discovery of monoclonal antibodies targeting IL-5 and its receptor, a major cytokine which promotes eosinophil migration to the lungs, as well as their proliferation and survival [3]. Until 2012 the only pathway experts could target in severe asthma was IgE with the use of omalizumab, a monoclonal antibody which inhibits IgE and has already improved the quality of life in patients with a predominant allergic endotype. The importance of eosinophils and the cytokines which affect their behavior in the lungs can be highlighted by the fact that they can be stimulated by multiple molecular pathways and lead to T2 high inflammation [3].
The T2 high endotype includes all the cytokines initially believed to be solely observed when CD4 T helper 2 (TH2) cells are stimulated mainly by environmental allergens. These triggers cause an immediate response by these adaptive immune system cells by initiating the production of cytokines like IL-4, IL-5 and IL-13, leading to eosinophil recruitment and activation [4]. Recently, the identification of a previously unknown cellular population in lung tissue brought significant changes to this simplistic view. The Innate Lymphoid Cells 2 (ILC2) were discovered to possess the ability to promote a similar T2 high response leading to lung eosinophilia and airway inflammation. Unlike the previously mentioned TH2 cells that are part of adaptive immunity, ILC2 demonstrate the effects of innate immunity in severe asthma. ILC2 have been shown to secrete IL-5 constitutively and even express IL-13 while greatly enhancing IL-5 secretion in circumstances of type 2 inflammation, leading to the activation of the T2 inflammatory cascade. More specifically, studies have underlined the importance of IL-13 as an activator of eotaxin-1, a chemokine which acts as an eosinophil chemoattractant and binds to the CCR3 receptor on eosinophils in the early stages of the T2 inflammatory process, orchestrating their migration to the lungs synergistically with IL-5 [5]. ILC2 respond to stimuli called alarmins, cytokines produced by epithelial lung cells in situations of bacterial contact or epithelial damage. These are IL-25, IL-33 and Thymic Stromal Lymphopoietin (TSLP). IL-33 has been clearly associated with the activation of both TH2 cells and ILC2, which leads to IL-5 and IL-13 production. Interestingly, injecting IL-33 in T helper deficient mice could still lead to airway eosinophilia, thus further establishing the role of ILC2. Furthermore, blockage of the IL-33 pathway by inhibiting either IL-33 via antibodies or the ST2 receptor which interacts with it, led to a significant reduction of lung infiltration by eosinophils and AHR [6]. Additionally, ILC2 have been implied with responding to additional mediators in mice, such as the leukotriene D4 (LTD4). Unlike the aforementioned alarmins, LTD4 can precipitate in the production of IL-4 from ILC2 cells, pinpointing that there is more to discover concerning the initial activation of these cells [7]. The T2 high inflammatory pathway has been proven to be far more elaborate than originally estimated. The function of ILC2 cells could be crucial in order to target simultaneously more than one cytokine in our effort to treat severe eosinophilic asthma.
Eosinophils are produced in the bone marrow like all hemopoietic cells. IL-5, along with other cytokines like IL-3 and Granulocyte Macrophage—Colony Stimulating Factor (GM-CSF), is crucial to their survival. This was demonstrated in IL-5-deficient mice, which could not produce an adequate number of eosinophils and evidently could not exhibit a T2 high response. It should be noted that eosinophils continue to express the IL-5 receptor on their membrane even after maturation and migration to the lungs, deeming IL-5 crucial to their survival, which in turn means that blocking IL-5 would not only halt their production but also rapidly lead to eosinophil attenuation in bronchial tissue [8][9][10]. As previously noted, the migration of eosinophils to the lungs is mainly coordinated by IL-5 and eotaxin-1. It is clinically important to underline that eotaxin-1 cannot mobilize eosinophils without synergia with IL-5, meaning that blocking IL-5 is far more crucial treatment-wise, since eotaxin-1 alone can only lead to blood eosinophilia but not to tissue infiltration. On the contrary, blocking eotaxin-1 alone cannot prevent eosinophil migration to the lungs, although it surely dampens the effectiveness of IL-5 [11]. Eotaxin-2 synergizes with IL-5 and drives the production of IL-13, while eotaxin-3 is thought to prolong eosinophil recruitment in the lungs [11].
Airway hyperresponsiveness and remodeling are hallmarks of bronchial asthma. It is no surprise that eosinophils are heavily associated with both processes. Their degranulation in lung tissue can lead to direct damage of the airways through the release of factors such as leukotrienes and Major Basic Protein (MBP). Leukotrienes directly cause severe bronchoconstriction, induce production of histamine and activate basophils and mast cells, thus fueling the flames of inflammation [12]. Nevertheless, abolishing eosinophils in vitro could not result in cessation of AHR, confirming that their role might be important but also limited [13]. On the other hand, eosinophils are involved in airway remodeling. Numerous studies have demonstrated that Matrix Metaloproteinase-9 (MMP-9) can be excreted from eosinophils, leading to the accumulation of inflammation-related cytokines like Tumor Necrosis Factor-a (TNF-a) and IL-1 beta in the lungs. Furthermore, MMP-9 levels are much higher in asthmatics and its levels are steadily raised in patients with persistent eosinophilia and severe airway remodeling [14]. Last but not least, studies in mice which targeted either eosinophils directly through abolishing genes crucial to their production in bone marrow or IL-5 and its receptor through monoclonal antibodies, managed to demonstrate protection against airway remodeling when compared to wild type mice, evident by the significant reduction of extracellular membrane proteins deposited in the lumen of their bronchial tree [15].
Summarizing, eosinophils and several cytokines like IL-5, implied in their production, migration and infiltration of the lungs, are key features of inflammation in asthma. The T2 high endotype has multiple molecular pathways with both the allergic and non-allergic cascade revolving around eosinophils. Therefore, targeting them via monoclonal antibodies in an attempt to treat asthmatics suffering from severe asthma was the “next big thing”.
The first biologic that targeted eosinophils in the setting of T2 high endotype was mepolizumab, a humanized anti-IL-5 monoclonal antibody that blocks the connection between circulating IL-5 and its receptor complex on cells, particularly eosinophils and basophils. One of the largest randomized control trials (RCTs) ever conducted in asthma was published in 2012 under the acronym DREAM. Researchers recruited 621 patients with a validated status of severe asthma refractory to treatment, as determined by a high annual exacerbation rate (≥2 the previous year) requiring oral corticosteroids. The key of their success was that patients were enrolled only with signs of eosinophilic inflammation, such as FeNO >50 ppb, peripheral blood eosinophils >300 × 10
L or sputum eosinophils >3%. All three mepolizumab doses that were tested managed to drastically reduce the annual exacerbation rate of patients in the 52-week surveillance period and almost completely depleted eosinophils from sputum and peripheral blood. However, no improvement in lung function test parameters like FEV
was observed and patients did not report improved control of their symptoms in their Asthma Quality of Life Questionnaire (AQLQ) [16]. Nevertheless, in 2014, a MENSA study was the first to depict the positive effects of mepolizumab treatment in severe asthmatics with eosinophilic predominant endotype on FEV
and asthma control as well. The annual rate of exacerbations in the 576 participants was almost halved during the 52 weeks of its duration [17]. Simultaneously, a SIRIUS study explored the corticosteroid sparing effects of anti-IL-5 treatment by achieving a reduction in oral corticosteroid dose in the subgroup receiving mepolizumab versus placebo. Meanwhile, the rate of exacerbations was drastically reduced despite the lowering of the OCS dose and patients reported improved asthma control and quality of life [18]. Many post-hoc analyses utilized the results of these studies in order to determine secondary endpoints and give experts a better understanding of treatment with an anti-IL-5 agent. A key result was that the higher the number of peripheral blood eosinophils, the more likely the response to mepolizumab was [19]. Finally, its safety was also well validated in follow-up studies, such as the COSMOS study, which was recently finalized reporting no safety concern issues [20].
An interesting twist was the idea of targeting the IL-5 receptor instead of IL-5 itself. This came to fruition with the biologic benralizumab. Its ability to bind to IL-5R and lead both eosinophils and basophils to apoptosis not only by depriving them from IL-5 but also by drawing NK cells, causing a fast and efficient antibody-dependent-cell-mediated-cytotoxicity, quickly depletes their population in both the peripheral blood and the lungs [21]. Two RCTs under the names of SIROCCO and CALIMA tested benralizumab’s efficacy in severe asthmatics with blood eosinophils of at least 300 cells/μL. Their results were similar, reporting a significant reduction in the annual exacerbation rate with a concomitant increase in FEV
and an improvement in symptom control [22][23]. Similar to the initial trials with mepolizumab, the next question was whether benralizumab could sustain the same corticosteroid sparing effects with its predecessor. The ZONDA study clearly demonstrated that treatment with benralizumab could indeed reduce the OCS burden, since after a 28-week period during which patients were randomized to either anti-IL-5R treatment or placebo, half of them managed to completely stop OCS without worsening their asthma control [24]. Its safety was also well established in a 2-year follow-up study [25].
The last child of the anti-IL-5/anti-IL-5R family is reslizumab. Similar to mepolizumab it targets IL-5 directly; however, it has several structural and dosing differences that cause it to shine in specific settings. It is injected intravenously instead of the subcutaneous way that is used for the other available mAbs and its dose is weight-adjusted. Two phase III studies were run after recruiting 953 patients with severe eosinophilic asthma and randomized them to either reslizumab or placebo. Both managed to show a decline in patients’ annual exacerbation rate and improvement of their lung function and quality of life [26]. Although reslizumab does not savor the glory of its counterparts, the RCT findings encouraged experts to test it in clinical practice due to its weight-dependent dose, a unique feature that makes it a suitable candidate for non-responders to other anti-IL-5 agents.
Treatment with mepolizumab has been proven to effectively reduce the significant and long-standing disease burden in patients with severe eosinophilic asthma in the real-life setting. In a real-world retrospective observational longitudinal study including 78 patients with severe eosinophilic asthma, mepolizumab was considered beneficial and was therefore continued in 75.6% of patients 1 year after the initiation of treatment. Severe asthma exacerbations per year were significantly reduced after 12 months of treatment accompanied by a reduction of 0.80 points in ACQ, and an increase in FEV
compared to baseline. A reduction in OCS usage was also observed (51.3% at baseline vs. 15.4%), 12 months after drug initiation and importantly, no serious adverse events related to mepolizumab treatment were reported [27]. Similar findings come from a retrospective study of 138 patients treated with mepolizumab for at least 12 months in Italy where mepolizumab confirmed its effectiveness in significantly reducing exacerbation rates and OCS usage, thus replicating the efficacy and safety profile shown in RCTs [28].
In line with the previous results, a real-life study from Belgium including 116 patients with severe eosinophilic asthma followed up for at least 18 months assessed the rate of exacerbations, OCS usage, FeNO, lung function, asthma control and quality of life at baseline, after 6 months and then every year. Importantly, sputum induction and a peripheral blood count of eosinophils were performed at the same time-points. Sputum eosinophil counts were reduced by 60% after 6 months of treatment while blood eosinophil counts were reduced by 98%. A significant reduction in exacerbation rate by 85% was observed after 6 months, which was maintained over time. A significant and preserved reduction by 50% in the dose of OCS was also recorded. Patients significantly improved their asthma control, as assessed by ACT and ACQ scores as well as quality of life (AQLQ score) at 6 months and this was also maintained throughout the follow-up. Eventually, a progressive improvement in FEV
that reached significance after 18 months was observed, and this was more pronounced in patients with a higher baseline sputum eosinophil percentage [29].
The long-term efficacy and safety of mepolizumab was also shown in the retrospective, observational study from France, demonstrating significant improvements in reducing exacerbations and OCS use over a period of 24 months [30].
A real-life study of a larger sample size comes from the Australian mepolizumab Registry (AMR) including 309 patients. These patients had poor asthma control, frequent exacerbations and almost half of them (47%) required daily OCS. Most of them had comorbidities, while the median baseline peripheral blood eosinophil count was 590 cells/μL. Mepolizumab reduced significantly exacerbations requiring OCS compared to the previous year as well as hospitalizations. A global improvement in asthma control, in quality of life and in lung function was also observed. Higher blood eosinophils and the later age of asthma onset were predictors of a better response, while predictors of a lesser response were the male sex and BMI ≥30. A subgroup of super-responders presented a more intense T2 high profile and fewer comorbidities [31].
Another study from the UK including 99 patients with severe eosinophilic asthma receiving mepolizumab determined the prognostic factors associated with response and super-response to treatment. In this study, response was defined as ≥50% reduction in exacerbations, or for patients requiring maintenance oral corticosteroids, ≥50% reduction in prednisolone dose. Super-responders were defined as exacerbation-free and off maintenance OCS at 1 year of treatment. Nasal polyposis, a lower BMI and a lower maintenance OCS requirement at baseline were associated with better outcomes. Twelve-month response was identifiable in over 90% of patients by week 24 of treatment [32].
One of the first real-life studies assessing the therapeutic effect of benralizumab in severe eosinophilic asthma included 13 patients who were evaluated at baseline and 4 weeks after drug administration. A rapid drop in blood eosinophil count (from 814.7 ± 292.3 cells/μL at baseline to 51.3 ± 97.5 cells/μL) was already noted 4 weeks after the first injection and this was associated with clinically significant improvements in ACT and FEV
, allowing the complete cessation of OCS [33].
Renner et al. showed that within a real life setting, benralizumab exerts quite a rapid and effective improvement in particular outcomes from the first 24 h of administration in patients with severe eosinophilic asthma. They recorded the response of 56 patients that were included in the study right after the first dosing, in 24 h and one week after treatment initiation and thereafter at each administration for up to 48 weeks. The median ACT score improvement was 7.0 points at 4 weeks and the median ACQ6 score improved by 1.17 points after one week. Furthermore, median FEV
significantly increased within the first 24 h, with further improvements after one week of administration. These improvements remained statistically and clinically significant until the end of the study. Although an immediate depletion of the peripheral blood eosinophil count was observed, there was no apparent trend in FeNO measurement despite FeNO being significantly reduced compared to baseline at specific timepoints (week 8, 16, 32 and 40). This study showed no differences in the effect of improving asthma control between non-smokers and former smokers and also between asthmatics with and without reversibility. The OCS sparing effect of benralizumab was not evaluated in this study because only 12 patients received OCS at baseline [34].
By suppressing eosinophilic inflammation, benralizumab not only decreases asthma exacerbations but at the same time improved airflow limitation and lung hyperinflation. This was shown in a study that enrolled 22 allergic patients with severe eosinophilic asthma. Benralizumab completely depleted peripheral eosinophils (from 810 to 0 cells/μL), with a concomitant significant decrease in both asthma exacerbation rate (from 4 to 0 exacerbations) and residual volume (from 2720 to 2300 mL) at 24 weeks of treatment [35].
The efficacy and safety of benralizumab was assessed in a real-world cross-sectional multi-center study of a cohort of 42 consecutive patients with severe refractory eosinophilic asthma who received benralizumab for at least 6 months. This study confirmed once again the findings of previous pivotal studies showing a rapid initial improvement in asthma control and lung function, along with a reduction in the number of emergency department visits and the use of inhaled and oral corticosteroids. This study also showed that patients may continue to improve during the first 6 months of treatment, and interestingly, to a greater degree than in pivotal RCT studies. Benralizumab was safe and well-tolerated in the real-life setting [36].
In a retrospective study, 215 patients were treated with reslizumab for at least ≥ 7 months. Reslizumab was associated with a significant reduction in the proportion of patients experiencing an exacerbation (from 86.0% to 40.5%) as well as in the number of exacerbations per patient (mean 2.84 vs. 0.94). Moreover, significant improvements were observed in FEV
1
and in ACT score after treatment initiation. Reslizumab showed an oral steroid sparing effect, since more than half of the asthmatics that were receiving maintenance OCS at baseline discontinued them by approximately 10 months after reslizumab initiation [37].
In another real-world study from Ireland including 26 patients, reslizumab was well-tolerated and was associated with a reduction of 79% in the exacerbation rate/year and improvement of ACQ-6 (3.5 to 1.7) at 1 year; 54% of patients were on OCS and 35.7% of them managed to discontinue them after 1 year of treatment [38].