Efficacy of hormone therapy as first-line treatment in Patients With ER-LOW HER-2-metastatic breast cancer: real world data

Estrogen receptor (ER) status is a key predictive biomarker in metastatic breast cancer (mBC), yet the efficacy of endocrine therapy (ET) in ER-low HER2-negative subtypes remains unclear. This retrospective study analyzed 61 patients with ER-low HER2-negative mBC treated with first-line therapy (2019–2024) to compare progression-free survival (PFS) between ET (n=28; monotherapy or combined with CDK4/6 inhibitors) and chemotherapy (n=33). Median ER expression in both groups was 3%. Objective response was observed in 14.3% (n=4) of ET patients versus 39.4% (n=13) in the chemotherapy group, with disease stabilization in 42.9% (n=12) and 24.2% (n=8), respectively. Median PFS was 7.0±0.8 months (95% CI 5.5–8.6) for chemotherapy and 5.0±1.3 months (95% CI 2.4–7.6) for ET (p=0.52), with no statistically significant difference. Results demonstrate no superiority of ET over chemotherapy in first-line treatment of ER-low HER2-negative mBC. Despite higher objective response rates with chemotherapy, survival outcomes did not differ significantly, potentially due to study limitations: a small cohort (n=61) and the rarity of the ER-low HER2-negative subtype requiring centralized biomarker verification.

Conclusions: ET shows no statistically significant PFS advantage compared to chemotherapy in ER-low HER2-negative mBC. These findings highlight the need for further research to optimize treatment strategies, including evaluating combination regimens and identifying additional prognostic markers in this subpopulation.