Abstract
Purpose

mTOR inhibitor everolimus is used for hormone receptor-positive (HR+)/HER2-negative metastatic breast cancer
(mBC). No reliable predictive biomarker of response is available. Following evidences from other solid tumors, we aimed
to assess the association between treatment-associated immune system features and everolimus activity.

Methods

We retrospectively explored a correlation with the therapeutic activity of everolimus and tumor-associated immune
pathways with ingenuity pathway analysis (IPA), neutrophil-to-lymphocyte ratio (NLR), circulating lymphocytes, and
endothelial cells (CECs) in 3 diferent HR+mBC studies, including the BALLET phase IIIb study.

Results

The circulating levels of CD3+/CD8+, CD3+/CD4+, and overall T lymphocytes were higher in responders versus
non-responders at baseline (p=0.017, p<0.001, p=0.034) and after treatment (p=0.01, p=0.003, p=0.023). Reduced
CECs, a tumor neoangiogenesis marker, were observed in responders after treatment (p<0.001). Patients with low NLR
(≤4.4) showed a better progression-free survival compared to patients with high NLR (>4.4) (p=0.01). IPA showed that
the majority of immunity-related genes were found upregulated in responders compared to non-responders before treatment,
but not after.

Conclusions

Lymphocytes subpopulations, CECs and NLR could be interesting biomarkers predictive of response to everolimus-based regimens, potentially useful in daily clinical practice to select/monitor everolimus-based treatment in mBC.
Further studies to confrm such hypotheses are warranted.