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Human epidermal growth factor 2 (HER2) is overexpressed in 15-20% of breast carcinomas. The overexpression of HER2 was previously associated with a poor prognosis until the development of the first anti-HER2 therapy, trastuzumab, which drastically improves the prognosis of HER2-overexpressing breast cancers. However, its mechanism of action remains not fully understood. Several studies have proposed that the behavior and mechanism of action of trastuzumab may be drastically altered in vitro and in vivo. The present study assesses the ability of trastuzumab to inhibit the phosphorylation of the key-proteins of phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT)/mechanistic target of rapamycin and Ras/Raf/mitogen-activated protein kinase (MAPK) signaling pathways in vitro, in breast cancer cell lines and in tumor biopsies obtained from patients treated with trastuzumab preoperative monotherapy as part of the Unicancer GEP04 RADHER phase II clinical trial. HER2-positive SKBR3 and HER2-negative MCF-7 cell lines were exposed to trastuzumab for 72 h. In total, 41 patients received trastuzumab alone for 6 weeks of preoperative treatment. Biopsies were collected at the baseline and at surgery. A total of 19 pairs of associated baseline and surgery tumor specimens were eligible for protein extraction and comparative phosphoprotein expression analysis, prior to and subsequent to treatment. The expression of phosphoproteins was quantitatively assessed using a multiplex immunoassay. In the SKBR3 cell line, a statistically significant decrease of the expression level of phosphorylated (p-)AKT, p-ribosomal protein S6 kinase B1, p-extracellular signal regulated kinase 1/2 and p-mitogen-activated protein kinase kinase 1 was observed after exposure to trastuzumab. In contrast, no statistically significant variations for levels expression of these phosphoproteins were observed in patients following treatment. The lack of downregulation of PI3K and MAPK pathways could probably be explained by the implementation of a predominant immunological mechanism of action for trastuzumab, a type of antibody-dependent cell-mediated toxicity, which has previously been reported in preoperative monotherapy settings. The present study confirms that trastuzumab involves various modes of action when assayed in vitro and used clinically. [ABSTRACT FROM AUTHOR]