Regarding allergic asthma, TSLP is believed to have a significant impact on both the pathophysiology and the level of exacerbations. As mentioned before, TSLP is responsible for the maturation and activation of DCs to promote the differentiation of naive T cells to Th2 cells (Figure 2). Furthermore, it stimulates Th2 cells to produce proinflammatory cytokines like IL-4, IL-5, and IL-13 that play a crucial role in orchestrating an allergic inflammation [51, 52]. IL-4 mediates the induction of IgE by B lymphocytes and upregulates the IgE receptors on cells’ membranes, enhancing the allergic reaction [53], whereas IL-5 activates the eosinophils and is involved in their maturation [52]. IL-13 is the dominant cytokine of Th2 mediated immune response, which regulates mucus production, bronchoconstriction, airway hyperresponsiveness (AHR), remodeling, and fibrosis [54]. It has been also observed that TSLP prompts basophil responses to function as antigen presenting cells in allergen induced Th2 response.

In non-allergic asthma, the antigens that stimulate TSLP production include viruses, parasites, bacteria, smoke, and others [55]. In this subtype, there is limited literature regarding the role of TSLP. It is known that in the non-allergic eosinophilic subtype, TSLP stimulates ILC2 to produce IL-5, which is responsible for the production of eosinophils. Studies also demonstrate the significant role of alarmins in ILC2 regulation and their correlation with steroid resistant asthma, thus it is suggested that, while dexamethasone can reduce the IL-33 driven inflammatory effect of ILC2s, TSLP may provide steroid resistance to ILC2s [40, 56].

On the other hand, in the neutrophilic type of asthma, TSLP drives DCs to stimulate the production of neutrophil activating Th17 lymphocytes [37]. More specifically, IL-17, a cytokine produced by Th17, is highly expressed in patients with moderate to severe asthma when compared to healthy individuals. It is suggested that a combination of Th2, Th1, and neutrophilic inflammation orchestrated by Th17 cells, is involved in the pathogenesis of severe asthma. A study conducted by Tanaka et al. [20] suggests that DCs activated by TSLP and Toll-like receptor 3 (TLR3) ligands, encourage the differentiation of Th17 cells, particularly in conditions that typically favor the development of Th2 cells. Additionally, these Th17 cells exhibit characteristics of central memory T cells [20]. Overall, TSLP is pivotal in initiating and intensifying allergic inflammation in asthma, as well as the development of non-allergic asthma by promoting airway inflammation, hyperresponsiveness, and remodeling, making it a promising target for therapeutic intervention in various subtypes of asthma including severe T2 low subtype.

PATHWAY (NCT02054130), a phase 2, multicenter, randomized, and double-blind study, compared the efficacy of subcutaneous (SC) tezepelumab administered at three dose levels over placebo during a 52-week treatment period [57]. The participants of the study were nonsmokers, 18 to 75 years of age, and had SUCA despite receiving medium or high doses of ICS along with LABA. Other criteria for the enrollment were the following: FEV₁ between 40–80%; a history of ≥ 2 asthma exacerbations in the past year that required the use of systematic glucocorticoids or hospitalization; a score on the six-item ACQ (ACQ-6) of at least 1.5 during screening [56]. The participants of the trial were randomly allocated (in a 1:1:1:1 ratio) to four subgroups to receive one of three different doses (70 mg, 210 mg, or 280 mg) of SC tezepelumab or placebo [57]. The primary endpoint of the study was to evaluate how tezepelumab affects the annualized asthma exacerbation rate (AAER) at week 52. Results showed that AAER was markedly lower in tezepelumab groups versus placebo by 62% (low-dose group), 71% (medium-dose group), and 66% (high-dose group) [57]. This reduction in AAER occurred independently of eosinophil blood counts. Thus, the drug’s action was demonstrated across T2 high and T2 low inflammation. In addition to this, data from post hoc analyses of the PATHWAY study on AAER revealed the following: tezepelumab decreased AAER by 66–78%, versus placebo, in patients with any perennial allergy [58]; the drug decreased AAER across all seasons compared with placebo [59]. As for secondary endpoints, the prebronchodilator FEV₁ was greater in the low-dose, medium-dose, and high-dose tezepelumab groups than in the placebo group by 120 mL, 130 mL, and 150 mL, respectively [57]. The drug also decreased significantly ACQ-6 score in all three tezepelumab groups and AQLQ[S] + 12 ([standardized] for patients 12 years of age or older), score only in the high-dose group, at the 52nd week of treatment [57].

NAVIGATOR (NCT03347279), a phase 3, multicenter, randomized, double-blind, placebo-controlled trial enrolled patients aged 12 to 80 years old, randomly assigned to receive 210 mg tezepelumab or placebo, SC, every 4 weeks (Q4W) for a 52 week treatment period. Patients were also diagnosed with asthma by a physician and were under treatment with a medium- or high-dose ICS (daily dose of ≥ 500 μg of fluticasone propionate or equivalent) for at least 12 months before screening, in combination with not less than one other controller medication, with or without OCS, for at least 3 months prior to their enrollment [60]. Moreover, asthma had been diagnosed ≥ 12 months, and patients had had a minimum of 2 exacerbations in the preceding 12 months, in advance of agreeing to participate [60]. The primary outcome measure in this study, consistent with that of the previous one, was AAER measured in patients with BEC both more and less than 300 cells per microliter [60]. Secondary endpoints included change in the prebronchodilator FEV₁ from the baseline (minimum clinically important difference 0.1 L), ACQ-6, AQLQ[S] + 12, and the weekly mean asthma symptom diary (ASD) score. Concerning the primary endpoint, the rate of AAER was notably lower in the tezepelumab group compared to the placebo group [60]. In a pre-specified exploratory analysis tezepelumab reduced the AAER versus placebo through all seasons in both baseline BEC subgroups (≥ 300 eosinophils/μL and ≤ 300 eosinophils/μL), whereas in the placebo group, there were seasonal variations in the AAER, with a peak in winter [61]. In addition, tezepelumab reduced AAER by week 52 by 58–68% in patients with proof of allergic inflammation (perennial aeroallergen sensitivity, confirmed symptomatic allergy, and omalizumab-eligible patients) [62]. Especially, among patients appropriate for receiving omalizumab, AAERs were decreased regardless of baseline BEC and FeNO levels [62]. Regarding secondary outcomes, a difference of 0.13 liters in the pre-bronchodilator FEV₁ was noticed between the groups from the first time of examination (week 2) and maintained until the end of the trial. As for the scores mentioned above, they were all remarkably improved in the tezepelumab group [60]. Patients both with and without perennial allergy, experienced improvement in lung function (FEV₁ increase) and patient-reported outcomes [62].

Clinical trials evaluating the efficacy of tezepelumab are shown in Table 2.

As mentioned previously, a portion of patients with SUCA receive daily maintenance OCS (mOCS), for a period longer than 6 months (chronic use), which is linked to adverse events (AEs) on many aspects of health. In the NAVIGATOR study, 100 of the 1,062 randomized patients were receiving mOCS [63]. In the mOCS subgroup, treatment with tezepelumab reduced numerically AAER and resulted in substantial progress in asthma symptoms, quality of life, and lung function compared to placebo [63].

SOURCE (NCT03406078) was an OCS-sparing, phase 3, multicenter, randomized, double-blind, placebo-controlled trial, that enrolled 150 participants from 18 to 80 years old, diagnosed with asthma by a physician, undergoing treatment with medium-dose or high-dose ICS and a history of ≥ 1 exacerbation the previous 12 months before the screening day [64]. After an 8-week OCS optimization, participants were randomly assigned to receive SC tezepelumab 210 mg or placebo, Q4W, for 48 weeks [65]. The study primarily aimed to investigate whether the medication would reduce and to what extend the daily mOCS dose from baseline to week 48 while maintaining asthma control. Although the decrease in daily mOCS did not reach the level of statistical significance, there was indeed a numerical reduction in the group of tezepelumab, in comparison to the placebo group, at week 48 [65].

Both UPSTREAM (NCT02698501) [66] and CASCADE (NCT03688074) [67, 68] aimed to assess tezepelumab effects on AHR. UPSTREAM was a randomized, double-blind, placebo-controlled, phase 2 trial. Patients included were adults, non-smokers, aged between 18 and 75 years, having SUCA and AHR to inhale mannitol [provoking dose of mannitol causing a 15% reduction in FEV₁ (PD₁₅) ≤ 315 mg] while receiving stable doses of ICS [66]. Patients were randomly allocated into two subgroups (1:1) to receive SC either 700 mg tezepelumab or placebo, Q4W for a 12-week treatment period [66]. Regarding the primary outcome, at week 12 tezepelumab did not significantly change PD₁₅ in doubling doses (DD) from baseline, indicating no remarkable reduction in AHR to mannitol. However, the proportion of patients without AHR to mannitol (at week 12) was significantly greater [66]. Alongside this, tezepelumab led to a remarkable reduction in the number of eosinophils in the sub-epithelium, as well as in the BAL and in the number of airway tissue mast cells (secondary outcome) [66].

CASCADE (NCT03688074) was an exploratory, phase 2, randomized, double-blind, placebo-controlled trial, that evaluated how tezepelumab impacts inflammatory cells, remodeling, and hyperresponsiveness of the airways in patients with moderate-to-SUCA [67, 68]. Participants aged 18–75 years with moderate-to-SUCA under treatment with medium-to-high dose ICS with a minimum of one additional asthma controller medication, with or without OCS, and were randomized (1:1) to receive SC either 210 mg tezepelumab or placebo, Q4W, for 28 weeks [67]. The patients were sectioned into subgroups according to baseline BEC as follows: 26% had less than 150, 34% had 150 to less than 300, and 40% had 300 cells/μL or more [67]. The trial mainly investigated (primary endpoint) how the drug affected the levels of the airway submucosal inflammatory cells (i.e., T cells, mast cells, neutrophils, and eosinophils), compared with the baseline levels, at week 28, as observed in bronchoscopic biopsies [67]. Secondarily, it evaluated whether there was a reduction in the reticular basement membrane (RBM) thickening and whether epithelial integrity increased, by week 28 [67]. Another secondary objective was the exploration of the primary outcome measures in patients having any of the two endotypes (T2 high or T2 low). As for the primary objective, tezepelumab markedly reduced submucosal eosinophils versus placebo, regardless of baseline BEC [68]. Speaking of secondary outcomes, no significant differences were observed between the groups in RBM thickening and airway epithelial integrity. As mentioned in an exploratory analysis, AHR to mannitol was significantly higher reduced in the drug group, rather than in the placebo group [68].

As mentioned previously both UPSTREAM and CASCADE studies assessed the effectiveness of tezepelumab in AHR. Following that, a proteomics analysis elucidated the potential mechanisms of tezepelumab’s effects, by determining which proteins alter the drug in the serum of PATHWAY participants [69]. The analysis noted that at baseline, periostin and MMP-10 (which are indicators of matrix remodeling) were significantly related to BEC and FeNO levels [69]. Besides that, it was also noted that the levels of MMP-10 and periostin, IL-5Rα, pregnancy-associated plasma protein-A (PAPP-A, which are eosinophil-related), IgE, and thymus and activation regulated chemokine (TARC) proteins were remarkably reduced in tezepelumab group versus the placebo group, at week 52 [69]. These data suggest that anti-TSLP blockade has further mechanisms besides the impediment of airway inflammation. Exploratory findings of the NAVIGATOR study supported the previous ones, identifying a linear association between MMP-10, MMP-3, and BEC, as well as a significant decrease in MMP-10 and MMP-3 levels in tezepelumab arm compared to the placebo [70].

Regarding biomarkers in the PATHWAY trial, remarkable and enduring reductions in BEC and FeNO levels were observed from week 4 in all tezepelumab groups, after treatment initiation [57]. Decreases from baseline in BEC and FeNO levels were noticed by week 2 and maintained during the treatment duration in patients who received tezepelumab in the NAVIGATOR study [60]. In both PATHWAY and NAVIGATOR, total serum IgE gradually reduced in all drug treatment groups [57, 60]. The data mentioned above indicate that tezepelumab impacts the entire spectrum of asthma inflammation.

A post hoc analysis of the PATHWAY study revealed supplementary information on how biomarkers and AAER are related to each other [71]. This analysis indicated a positive correlation between the indicators of T2 inflammation (i.e., IL-5, IL-13, periostin, BEC, and FeNO) at baseline [71]. Additional results were the reduction of all biomarkers by baseline at week 52 in the tezepelumab versus the placebo group and a reduction by 55–83% in the AAER regardless of the baseline levels of BEC, FeNO or serum total IgE, IL-5, IL-13, periostin, TARC, and TSLP [71]. Furthermore, another post hoc analysis revealed that the baseline IL-5 and IL-13 were notably greater in patients with SUCA of the PATHWAY study than in the ones who participated in a separate cohort of age-matched healthy individuals [72]. This study also demonstrated that following treatment of 52 weeks, tezepelumab reduced IL-5 and IL-13 levels in patients having SUCA to the extent they were comparable to those observed in the healthy group [72]. As for NP profile and biomarker levels, participants treated with tezepelumab 210 mg demonstrated greater reductions in BEC and levels of FeNO, IL-5, and IL-13 than the ones treated with placebo, regardless of NP status [73]. In a post hoc analysis of the NAVIGATOR trial, the efficacy of tezepelumab on biomarker levels was assessed in groups of patients of different baseline biomarker levels [74]. Results showed that at week 52, tezepelumab reduced BEC, FeNO levels, and serum total IgE levels in those with elevated biomarkers at baseline, with more pronounced decreases observed in patients who initially had higher levels of the specific biomarker [74].

In a subgroup analysis of the SOURCE study, tezepelumab demonstrated a greater decrease in daily OCS dose in patients with baseline BEC of ≥ 150 cells/μL and ≥ 300 cells/μL, compared to the placebo group [65]. Conversely, in patient groups with ≤ 150 cells/μL and ≤ 300 cells/μL, a higher reduction was observed in the placebo group [65].

As discussed earlier, inflammatory biomarkers were additionally observed to decrease with the administration of tezepelumab in the CASCADE trial. Tezepelumab exhibited more substantial reductions compared to placebo in BEC, FeNO levels, IL-5, IL-13, and plasma eosinophil-derived neurotoxin at the end of the treatment period and during the follow up [68]. Similar results were revealed by the UPSTREAM study, as tezepelumab remarkably reduced FeNO and eosinophil levels in BAL, sputum, and blood, in comparison with the placebo [66]. All the data noted above, confirm the anti-inflammatory role of TSLP-blockade in patients with SUCA.

Combined safety information regarding the utilization of tezepelumab in patients with SUCA was established by the polling analysis of NAVIGATOR and SOURCE trials, as well as the long-term-extension (LTE) DESTINATION [75] and the NOZOMI [76] study. According to pooling estimates, tezepelumab and placebo arm did not exhibit significant differences in the percentage of AEs during the treatment (75% and 77% respectively) [77]. Nasopharyngitis, upper respiratory tract infection, headache, and asthma, were the most common AEs in both groups [77]. Pharyngitis, arthralgia, and headache were more common in tezepelumab group with an incidence of 3% or above [78], while sinusitis and asthma were reported with greater frequency among the placebo patients [77]. In recipients with arthralgia, severity/intensity was mild or moderate in all the patients and one individual receiving tezepelumab withdrew from the study, due to arthralgia [77]. At baseline, daily OCS use was reported in 24% of patients receiving tezepelumab and 0% of those receiving placebo, which is a possible explanation for the higher incidence in tezepelumab arm [77]. Among patients with back pain, daily OCS use was similar in both groups, while severity/intensity was mild or moderate in all recipients. Only one patient who was receiving placebo reported severe back pain, even though the study drug was not halted [77]. Overall, the proportions of patients reporting serious AEs (SAEs) during the treatment, were reported for 9% and 13% in the tezepelumab and placebo groups, respectively, with only 0.8% versus 0.3% reporting cardiac SAEs, respectively [77]. Additionally, tezepelumab and placebo group presented with similar incidences of severe infection and neoplasms (benign and malignant) and there were no reported anaphylactic reactions related to tezepelumab [77]. 4% of patients in tezepelumab group and 3% in the placebo group experienced injection-site reactions. In general, the discontinuation rates of treatment were about 2% for the tezepelumab arm and 3% for the placebo arm (1% and 2%, respectively, owing to SAEs) [77]. As for deaths, none was reported during the treatment period, whereas two deaths were reported during the safety follow-up period [77].

DESTINATION (NCT03706079) was a phase 3, LTE study in adults (18–80 years old) and adolescents (12–17 years old) with SUCA who completed the treatment period of NAVIGATOR and SOURCE studies [75]. The main outcome of this trial was to evaluate the tolerability and long-term safety of them in a 104-week treatment period (including the treatment period of the previous study) [75]. A secondary objective was the assessment of the continuous influence of tezepelumab on AAER [75]. Regarding both primary and secondary outcomes, DESTINATION revealed consistent results with those from NAVIGATOR and SOURCE, after treatment with tezepelumab for 2 years, showing safety, as indicated by its minimal side effects, and efficacy, indicated by prolonged reduction in AAER irrespectively of baseline T2 inflammation biomarkers [79].

NOZOMI (NCT04048343) was a phase 3, open-labeled, and single-arm study that mainly assessed the safety and tolerability of tezepelumab in 65 Japanese patients [76]. Participants aged 12 to 80 years old, diagnosed with SUCA and using medium to high doses of ICS and a minimum of one supplementary controller medication, with or without OCS, received 210 mg SC tezepelumab, Q4W, for a 52-week period. Among these 65 patients, 60% experienced 94 different AEs, most of them identified as mild or moderate [76]. The dominant ones were nasopharyngitis, pharyngitis, back pain, herpes zoster, and upper respiratory tract inflammation with an incidence of > 3%. Two individuals underwent AEs deemed to be drug related (injection site erythema of mild intensity) and four noted one SAE (atrial fibrillation, viral gastroenteritis, lung abscess, tonsilitis) [76]. All the SAEs were unrelated to the drug and did not lead to a fatal outcome. During the study, no cases of immune complex disease, anaphylactic reactions, or hypersensitivity were recorded [76]. In terms of exploratory outcomes, tezepelumab was associated with a low AAER (0.11/patient-year), a mean change of 0.075 L in pre-bronchodilator FEV₁, and a change in ACQ-6 score of −0.98. It is worth mentioning that tezepelumab demonstrated safety and tolerability in patients with T2 low inflammation at baseline, who constituted 60% of the patients in the NOZOMI study [76].

Mechanistic studies evaluating the efficacy, safety, and tolerability of tezepelumab in asthma are shown in Table 3.