For the millions of adults living with chronic obstructive pulmonary disease, every exacerbation carries real risk — hospitalization, accelerated lung decline, and increased mortality. A persistent clinical puzzle has been whether patients whose eosinophil counts fluctuate over time can still reliably benefit from anti-eosinophil therapies, since variable biomarker readings often lead clinicians to withhold treatment.
This pooled analysis draws on individual patient data from three phase 3 randomized controlled trials — METREX, METREO, and MATINEE — examining mepolizumab (100 mg subcutaneous), a monoclonal antibody that neutralizes interleukin-5, in COPD patients stratified by blood eosinophil count (BEC) patterns in the 12 months preceding randomization. The key finding: mepolizumab reduced annualized rates of moderate-to-severe exacerbations by 21–36% across patients whose eosinophil counts were elevated at any single pre-randomization timepoint or varied substantially over time. Critically, reductions in exacerbations requiring emergency department visits or hospitalization were observed across all subgroups with BEC at or above 150 cells/µL. Patients with persistently low counts below 150 cells/µL showed no meaningful benefit.
The broader significance here is methodological as much as clinical. Eosinophil counts are notoriously labile — influenced by corticosteroid use, acute illness, diurnal rhythm, and seasonal variation — meaning a single measurement can misclassify patients. This analysis suggests that one elevated reading within a 12-month window may be sufficient to identify treatment-responsive patients, lowering the bar for eligibility in clinical practice. That said, several caveats apply: this is a post-hoc pooled subgroup analysis, and the trial populations were enriched for exacerbation-prone patients with existing type 2 inflammatory signals. Generalizability to less severe or recently diagnosed COPD cohorts remains untested. Mepolizumab has already received regulatory approval in some markets for eosinophilic COPD, and these findings are confirmatory rather than paradigm-shifting — but they meaningfully refine the biomarker thresholds clinicians should consider when evaluating treatment candidacy.