A secondary analysis of the randomized STO-3 trial found that immune, endothelial, and fibroblast cell abundances within the tumor microenvironment significantly modulate long-term clinical benefit from tamoxifen in postmenopausal ER+/HER2− breast cancer. These findings implicate both immune and stromal compartments as determinants of endocrine therapy efficacy.
A secondary analysis of the Stockholm Tamoxifen (STO-3) randomized controlled trial, published in BMC Medicine, demonstrates that tumor microenvironment (TME) composition is a meaningful determinant of long-term tamoxifen benefit in postmenopausal women with ER+/HER2− breast cancer. Specifically, low immune cell abundance, intermediate endothelial cell abundance, and low-to-intermediate fibroblast abundance were each independently associated with greater therapeutic benefit from tamoxifen.
These findings extend the conventional understanding of endocrine resistance beyond tumor-intrinsic factors—such as ESR1 mutations or luminal subtype—by highlighting the stromal and immune architecture as co-regulators of treatment response. The interplay between immune infiltration and fibroblast density may influence estrogen receptor signaling pathways or alter drug accessibility within the tumor stroma, though the precise mechanistic basis requires further investigation.
From a clinical standpoint, TME profiling could eventually inform risk stratification and adjuvant endocrine therapy selection in postmenopausal ER+/HER2− disease. However, as a secondary analysis of a historical randomized trial, these results are hypothesis-generating and require prospective validation before integration into clinical decision-making frameworks.
Key points
- Secondary analysis of the STO-3 randomized trial examined TME composition as a predictor of tamoxifen efficacy in postmenopausal ER+/HER2− breast cancer.
- Low immune cell abundance within the TME was associated with greater long-term tamoxifen benefit.
- Intermediate endothelial and low-to-intermediate fibroblast abundances also correlated with improved tamoxifen outcomes.
- Both immune and stromal TME compartments appear to independently influence endocrine therapy response.
- Findings are hypothesis-generating; prospective validation is needed before clinical application.
Why it matters
TME profiling may offer a novel, tissue-based approach to predicting adjuvant tamoxifen benefit beyond standard receptor status, potentially refining endocrine therapy selection in postmenopausal ER+/HER2− breast cancer. Identifying patients unlikely to benefit could support earlier escalation to alternative or combination endocrine strategies.