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Purpose. When combined with anthracyclines, the humanized anti-HER2 monoclonal antibody trastuzumab (Herceptin™) provides significant clinical benefit for women with HER2-overexpressing metastatic breast cancer. However, its use is limited by severe cardiotoxicity. To clarify whether myocardial HER2 and HER4 expression in response to anthracycline exposure and cardiac damage contributes to cardiotoxicity, we assessed expression of HER2 and HER4 in pathologically altered myocardium.Experimental design. Cardiac biopsies from 60 patients with severe heart disease and cardiac tissue from 35 patients with breast cancer were obtained. Twenty-five of the patients with breast cancer had previously received anthracyclines. Three of 10 anthracycline-naïve patients with breast cancer had received trastuzumab. Expression of HER2 and HER4 was analyzed immunohistochemically (HER2: HercepTest™/A0485 (Dako), Cy3 detection (Dianova); HER4: Ab-4 (NeoMarkers)). FISH analysis (Ventana) was used to assess HER2 gene amplification.Results. Immunohistochemistry revealed weak HER2 membrane staining in six cardiac biopsies, appearing as dotted staining of the whole cell membrane and intensified HER2 signal using fluorescent Cy3 labeling. No HER2 membrane staining was detected in the remaining 54 cardiac biopsies or in the myocardium of the 35 patients with breast cancer. HER2 gene amplification was not observed. All specimens showed the mild cytoplasmatic HER4 staining of normal myocardium. No strong HER4 expression was detected.Conclusions. Cardiac alterations are not associated with an strong increase in HER2 and HER4 levels. IHC detects potential low-level HER2 expression in some samples. However, a more sensitive technique may be needed for studies of the role of HER2 in cardiac tissue. These data do not exclude a role for inhibition of cardiac HER2 expression by trastuzumab in the onset of heart failure in trastuzumab-treated patients. [ABSTRACT FROM AUTHOR]