Intermediate HER2 expression is associated with poor prognosis in estrogen receptor-positive breast cancer patients aged 55 years and older

Purpose: An antibody–drug conjugate targeting HER2, DS8201, has shown clinical activity against breast cancer with low-level HER2 expression. We aimed to evaluate the prognostic impact of intermediate HER2 expression in estrogen receptor (ER)+ early breast cancer (EBC) and metastatic breast cancer (MBC) cohorts. Methods: We analyzed prospectively collected data from EBC and MBC cohorts at Yonsei Cancer Center. Patients with HER2 immunohistochemistry (IHC) 0 ~ 1+ were assigned to the HER2-negative group, and patients with IHC 2+ and in situ hybridization (ISH)-negativity were assigned to the HER2-intermediate group. After the exclusion of HER2 IHC 3+ or ISH+ patients, a total of 2657 EBC and 535 MBC patients were analyzed. Results: In total, 654 (24.6%) EBC and 166 (31.0%) MBC patients were classified in the HER2-intermediate group. The HER2-intermediate patients more frequently tended to have progesterone receptor (PR)-negativity and higher nuclear grade in the EBC cohort, and showed a higher proportion of patients aged ≥ 55 years compared with the HER2-negative group in the MBC cohort. The HER2-intermediate patients showed significantly poorer recurrence-free survival (RFS) compared to the HER2-negative patients in the EBC cohort (p = 0.044). Notably, intermediate HER2 expression predicted poorer RFS in EBC patients aged ≥ 55 years (hazard ratio 1.95; p = 0.042) in multivariate Cox analysis but did not affect RFS in those aged < 55 years. In line with the EBC cohort results, intermediate HER2 expression predicted poorer overall survival (OS) in MBC patients aged ≥ 55 (hazard ratio 1.45; p = 0.044) without affecting OS of those aged < 55 years. Conclusion: Intermediate HER2 expression is an independent predictor of poor prognosis in both ER+ EBC and MBC patients aged ≥ 55 years. The clinical efficacy of new HER2-targeting antibody–drug conjugates needs to be validated in this high-risk subset of ER+ breast cancer patients.