Benralizumab Reduces Flare Risk in Hypereosinophilic Syndrome: Results from the Phase 3 NATRON Trial

Key Takeaways

• Benralizumab reduced the risk of hypereosinophilic syndrome (HES) flares by 65% compared to placebo in a phase 3 randomized trial.
• Treatment led to near-complete eosinophil depletion and significantly improved fatigue, a key patient-reported outcome.
• Safety outcomes were comparable to placebo, supporting its role as a potential add-on therapy in FIP1L1::PDGFRA-negative HES.

A phase 3 randomized trial published in Nature Medicine provides compelling evidence that benralizumab, an anti–IL-5 receptor α monoclonal antibody, can significantly reduce disease activity in patients with hypereosinophilic syndrome (HES), a rare and heterogeneous condition marked by eosinophil-driven organ damage.

The NATRON trial enrolled 133 patients with FIP1L1::PDGFRA-negative HES, a subgroup lacking effective targeted therapies. Participants were randomized to receive benralizumab 30 mg every four weeks or placebo, in addition to background therapy, over a 24-week double-blind period. As shown in the trial profile on page 2, the study achieved high completion rates, with over 90% of patients in both arms completing the study period.

The primary endpoint—time to first HES flare—demonstrated a clear and clinically meaningful benefit. Only 19.4% of patients receiving benralizumab experienced a flare, compared with 42.4% in the placebo group. This translated to a 65% reduction in flare risk (hazard ratio 0.35, 95% CI 0.18–0.69; P=0.0024), as illustrated in the Kaplan–Meier curve on page 5. The separation between treatment groups emerged early and was sustained throughout the 24-week period.

Secondary outcomes reinforced these findings. The proportion of patients experiencing a flare or withdrawing from the study was halved in the benralizumab group (22.4% vs 45.5%), and the annualized flare rate was reduced by 66% (0.41 vs 1.23 flares per year). Notably, no patients in the benralizumab arm experienced more than one flare, whereas multiple flares occurred in the placebo group, as detailed in the outcomes table on page 5.

The biologic effect of benralizumab—direct eosinophil depletion—was evident across hematologic endpoints. Patients receiving benralizumab were 92% less likely to experience hematologic relapse (defined as eosinophil counts ≥1,000 cells/µL), with divergence between groups observed as early as week 4 (page 6). Sustained eosinophil suppression was achieved in 91% of treated patients, compared with just 12% in the placebo arm.

Beyond objective disease measures, patient-reported outcomes highlighted meaningful symptomatic improvement. Fatigue, one of the most burdensome symptoms in HES, improved significantly with benralizumab, with differences emerging within the first month and persisting through week 24. Quality-of-life measures, including SF-36 physical and mental component scores, also showed favorable trends.

Importantly, these benefits were achieved without a significant increase in adverse events. As summarized in the safety table on page 7, adverse events occurred at similar rates in both groups (64.2% with benralizumab vs 66.7% with placebo), with no new safety signals identified. Serious adverse events were infrequent and not attributed to treatment, consistent with the drug’s established safety profile in other eosinophilic diseases.

The study population, detailed in the baseline characteristics table on page 3, reflected the clinical complexity of HES, with most patients exhibiting multi-organ involvement and requiring background corticosteroid therapy. This enhances the generalizability of the findings to real-world practice, though the relatively small sample size—an inherent limitation in rare disease trials—should be considered.

Taken together, the NATRON trial underscores the central role of eosinophils in HES pathophysiology and positions benralizumab as a promising therapeutic option for patients with limited alternatives. While longer-term data and head-to-head comparisons with existing biologics such as mepolizumab are still needed, these findings mark a meaningful step forward in the management of this challenging disorder.