In vitro activity of the mTOR inhibitor everolimus, in a large panel of breast cancer cell lines and analysis for predictors of response.

Everolimus (RAD001, Afinitor(®)) is an oral, selective mTOR inhibitor recently approved by the US-FDA in combination with exemestane for treatment of hormone receptor positive advanced breast cancer. To date, no molecular predictors of response to everolimus in breast cancer have been identified. We hypothesized predictive markers could be identified using preclinical models. Using a molecularly characterized panel of human breast cancer and immortalized breast epithelial cell lines, we determined sensitivity to everolimus alone or in combination with ER- or HER2- targeted therapy. Gene expression microarrays and comparative genomic hybridization were performed on the cell lines to identify predictors of response to everolimus. Among 13 everolimus-sensitive cell lines, 10/13(77 %) were luminal, while in 26 resistant cell lines, 16/26(62 %) were non-luminal, and 10/26(38 %) were luminal. Only 3/24 non-luminal lines were sensitive, two of which were HER2+. Everolimus enhanced the anti-proliferative effect of both tamoxifen (TAM) and fulvestrant (FUL) in ER+ breast cancer cell lines, as well as trastuzumab in HER2+ cell lines. Everolimus + FUL but not everolimus + TAM reversed acquired resistance to TAM. Everolimus inhibited mTOR in tested cell lines by decreasing S6 phosphorylation, mediating its anti-proliferative effect by G0/G1 cell cycle arrest and induction of apoptosis. Chromosomal amplifications of AURKA (p value = 0.04) and HER2 (p value = 0.03) were each associated with increased sensitivity to everolimus. Transcript expression microarrays identified GSK3A, PIK3R3, KLF8, and MAPK10 among the genes overexpressed in sensitive luminal lines, while PGP, RPL38, GPT, and GFAP were among the genes overexpressed in resistant luminal cell lines. These preclinical in vitro data provide further support for continued clinical development of everolimus in luminal (ER+ or HER2+) breast cancer in combination with targeted therapies. We identified several potential molecular markers associated with response to everolimus that will require validation in clinical material.