PI3K/AKT/mTOR pathway activation in primary and corresponding metastatic breast tumors after adjuvant endocrine therapy

Both preclinical and clinical data suggest that activation of the PI3K/AKT/mTOR pathway in response to hormonal therapy results in acquired endocrine therapy resistance. We evaluated differences in activation of the PI3K/AKT/mTOR pathway in estrogen receptor α (ERα) positive primary and corresponding metastatic breast cancer tissues using immunohistochemistry for downstream activated proteins, like phosphorylated mTOR (p-mTOR), phosphorylated 4E Binding Protein 1 (p-4EBP1) and phosphorylated p70S6K (p-p70S6K). For p-mTOR and p-4EBP1, the proportion of immunostained tumor cells (0–100%) was scored. Cytoplasmic intensity (0–3) was assessed for p-p70S6K. The difference between expression of these activated PI3K/AKT/mTOR proteins- in primary and metastatic tumor was calculated and tested for an association with adjuvant endocrine therapy. In patients who had received endocrine therapy (N = 34), p-mTOR expression increased in metastatic tumor lesions compared to the primary tumor (median difference 45%), while in patients who had not received adjuvant endocrine therapy (N = 37), no difference was found. Similar results were observed for p-4EBP1 and p-p70S6K expression. In multivariate analyses, adjuvant endocrine therapy was significantly associated with an increase in p-mTOR (p = 0.01), p-4EBP1 (p = 0.03) and p-p70S6K (p = 0.001), indicating that compensatory activation of the PI3K/AKT/mTOR pathway might indeed be a clinically relevant resistance mechanism resulting in acquired endocrine therapy resistance. WHAT'S NEW? Inhibitors of the PI3K/AKT/mTOR pathway can overcome the resistance to estrogen-depletion therapy that often develops in metastatic breast cancer. In this study, the authors compared primary and metastatic tumors; their results suggest that activation of the PI3K/AKT/mTOR pathway in patients who receive adjuvant endocrine therapy is a clinically relevant mechanism of acquired hormone resistance. For identification of companion diagnostics for PI3K/AKT/mTOR inhibitors, the authors conclude that analyzing primary tumor tissue may often fail to predict treatment response in metastatic breast cancer.