Purpose: The HER2 oncogene is involved in the biology of many different tumor types and serves as a prognostic marker and a therapeutic target in breast cancer. In contrast to breast cancer, studies on Her2 overexpression and gene amplification in prostate cancer have yielded different results. The purpose of this study was to learn more on the prevalence and clinical significance of HER2 amplification and overexpression in prostate cancer.Experimental Design: A tissue microarray containing >2,000 prostate cancers with follow-up data was used. Tissue microarray sections were analyzed on protein and DNA level using two different antibodies (HercepTest, DAKO; Novocastra NCL-CB11) and fluorescence in situ hybridization.Results: Immunohistochemical analyses showed highly similar results for both antibodies. Detectable Her2 immunostaining was observed in 17.2% for the HercepTest and in 22.5% for the Novocastra antibody with the vast majority of cases showing 1+ or 2+ staining. For both antibodies (HercepTest/ Novocastra), significant associations were found between positive staining and high Gleason grade (P < 0.0001, both), advanced pT stage (P < 0.0001/P = 0.0015), rapid tumor cell proliferation (P = 0.0004/P = 0.0071), and tumor recurrence (P < 0.0001, both). HER2 amplification was only found in 1 of 2,525 analyzable cases (0.04%).Conclusions: Low-level Her2 overexpression occurs at relevant frequency in prostate cancer and in the absence of gene amplification. Increased Her2 expression may potentially lead to an aggressive behavior of tumor cells through the stimulation of tumor cell proliferation because Her2 staining was shown to be significantly associated with Ki67 labeling index. These data argue for reconsidering anti-Her2 therapy, possibly with modified approaches. Clin Cancer Res; 16(5); 1553-60. ©2010 AACR.The Her2 protein, a transmembrane tyrosine kinase growth factor receptor, is found in normal and malignant epithelial cells and is involved in regulation of cell proliferation and differentiation (1). Her2 is well known as a strong prognostic factor and a successfully used therapeutic target in breast cancer (2). In 15% to 20% of breast cancer cases, Her2 is overexpressed as a consequence of gene amplification (2-4). Overexpressed Her2 protein serves as the therapeutic target for trastuzumab, a humanized monoclonal antibody (5, 6), which is now used both in a metastatic setting and as an adjuvant therapy of Her2-positive breast cancer.Many other tumor types, including prostate cancer, express increased levels of Her2, and evidence accumulates that trastuzumab can also be effective in Her2-positive tumors other than breast cancer. In previous studies, widely divergent rates for Her2 expression and amplification in primary prostate cancers have been reported. Rates of positivity range from 0% to 100% in immunohistochemical studies (7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22) and from 0% to 53% in amplification analyses (18,(22)(23)(24). Most likely, these conflicting data are due...

ClinicalTrials.gov identifier NCT00732914, www.clinicaltrials.gov.

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