Eosinophil counts linked to recurrence outcomes in HR-positive, HER2-negative breast cancer

Researchers evaluated preoperative circulating eosinophil counts as a potential recurrence biomarker in hormone receptor–positive, HER2-negative breast cancer. The analysis included a multimodal discovery cohort of 472 patients and an expanded independent cohort of 23,170, with propensity score overlap weighting used for validation. The team also assessed eosinophil counts alongside 21-gene recurrence scores in 2,973 patients and examined their association with adjuvant CDK4/6 inhibitor benefit in 3,471 high-risk patients.

Higher eosinophil counts were associated with improved recurrence-free and distant metastasis-free survival in the discovery cohort, with the clinical associations validated in the expanded cohort. Counts provided prognostic information beyond the 21-gene recurrence score. Among high-risk patients, eosinophil levels were reported to identify a subset deriving greater benefit from CDK4/6 inhibition. Multiomic and spatial analyses linked higher counts to systemic and intratumoral immune activation; mouse experiments also found reduced tumor burden after eosinophil transfer.

These findings support further evaluation of eosinophil counts as a noninvasive biomarker, but the clinical cohorts establish associations rather than proving that the marker can guide treatment. The supplied abstract does not provide effect estimates or details of the treatment-benefit analysis, and the proposed biomarker requires further clinical validation before its role in decisions is established.