PI3K Pathway Activation Markers in ER-Positive, HER2-Negative Breast Cancer: A Clinicopathologic Study

Endocrine therapy is the cornerstone of treatment for estrogen receptor-positive breast cancer; however, a substantial proportion of patients develop resistance during the disease course. Activation of the PI3K/AKT/mTOR signaling pathway has been implicated as a key mechanism underlying resistance to endocrine therapy and disease progression. While genomic alterations in PI3K pathway-related genes are commonly assessed, they do not consistently reflect pathway activation or predict therapeutic response, highlighting the need for practical surrogate biomarkers. The aim of this study is to evaluate immunohistochemical markers of PI3K pathway activation in estrogen receptor-positive, HER2-negative invasive breast carcinoma and to explore their associations with endocrine therapy response and clinicopathologic characteristics. Specifically, the study examines whether levels of phosphorylated AKT (p-AKT) and phosphorylated S6 (p-S6) differ between endocrine-sensitive and endocrine-resistant cases and whether these markers correlate with histologic features related to tumor biology and the tumor microenvironment. This is a retrospective observational study including patients with estrogen receptor-positive, HER2-negative invasive breast carcinoma who received adjuvant endocrine therapy as part of routine clinical care. Archived formalin-fixed, paraffin-embedded (FFPE) tissue blocks from excisional surgical specimens will be retrieved, and clinicopathologic and follow-up data will be collected from medical records. Cases will be classified into endocrine-sensitive and endocrine-resistant groups based on international consensus definitions using the timing of disease recurrence relative to endocrine therapy. Immunohistochemical staining for p-AKT and p-S6 will be performed on FFPE tissue sections and evaluated using a semi-quantitative scoring system. Histopathologic assessment will include standard reporting parameters as well as additional features such as tumor-infiltrating lymphocyte density, tumor budding, lymphovascular invasion, perineural invasion, and tumoral necrosis. Associations between biomarker expression, clinicopathologic features, and follow-up outcomes will be explored.