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Purpose: Although the clinical benefits of trastuzumab are well known, intrinsic or acquired resistance is a commonly encountered clinical condition. A potential resistance mechanism is aberrant downstream signal transmission due to loss of phosphatase and tensine homologue (PTEN). This study investigated the relationship between trastuzumab response and loss of PTEN in metastatic breast cancer patients., Methods: Patients with histologically confirmed human epidermal growth factor receptor 2 (HER2) positive metastatic breast cancer, who were treated with trastuzumab were enrolled into the study. PTEN expression was immunohistochemically evaluated., Results: The patient median age was 50 years. Of 38 patients, 6 (15.8%) showed PTEN loss. No statistically significant difference was found between trastuzumab response, overall survival (OS) and progression-free survival (PFS) and PTEN loss (p=0.538)., Conclusion: The activation of phosphatidylinositol 3-kinase (PI3K) pathway resulting from PTEN loss was not found to be correlated with trastuzumab response and survival. PTEN loss should not lead to exclusion of patients from the potential to benefit from trastuzumab administration.

Purpose: Unraveling the mechanisms underlying the resistance to trastuzumab is important for amending the prognosis of patients with human epidermal growth factor receptor 2 (HER2) positive metastatic breast cancer. Experimentally, it has been shown that p95-HER2 positive breast tumors are resistant to trastuzumab. The aim of this study was to investigate the predictive and prognostic importance of p95-HER2 expression by immunohistochemistry in HER2-positive metastatic breast cancer patients treated with trastuzumab., Methods: Only patients who had a histological diagnosis of HER2-positive metastatic breast cancer and who had received first line therapy containing trastuzumab were enrolled in the study. Immunohistochemistry was used to analyze p95-HER2 expression in the tissue blocks of the patients., Results: The study was performed on 38 patients aged between 30 and 84 years. In 14 patients (36.8%), p95-HER2 was positive, whereas it was negative in the remaining 24 patients (63.2%). There was no significant correlation between p95-HER2 expression and overall survival, response to trastuzumab, and progression-free survival (PFS)., Conclusion: Unlike previous reports, there was no correlation between the p95-HER2 expression and resistance to trastuzumab. It may be argued that an analysis using immunohistochemistry is inadequate for determining p95- HER2. In order to ascertain whether immunohistochemistry is an appropriate method, studies with larger patient groups are needed.