HER-2 is a member of the c-erbB family of receptor tyrosine kinases and is overexpressed by 20-30% of human breast cancers. HER-2 overexpression is an independent adverse prognostic factor and may also predict for response to both chemotherapy and endocrine agents. Trastuzumab is a humanised monoclonal antibody that binds with high affinity to the extracellular domain of HER-2. In HER-2-overexpressing preclinical models trastuzumab has been shown to have a marked antiproliferative effect and demonstrates synergy with a number of cytotoxic drugs. Several phase II and phase III clinical trials have now been performed in patients with advanced breast cancer that overexpress HER-2. Trastuzumab was initially shown to be active and well tolerated as a single agent in heavily pretreated women. Subsequently, studies of first-line treatment for metastatic breast cancer have demonstrated an improvement in survival for trastuzumab when used in combination with either paclitaxel or an anthracycline-cyclophosphamide regimen compared with chemotherapy alone. Unexpectedly, the combination of trastuzumab and the anthracycline-containing regimen was associated with a significant incidence of cardiac dysfunction. The benefit of trastuzumab is generally confined to patients whose tumours have gene amplification as detected by fluorescence in situ hybridisation (FISH) and this is tightly associated with immunohistochemical (IHC) staining at the highest (3+) level. A small number of patients have IHC 2+ tumours together with FISH evidence of gene amplification and may also derive benefit from treatment. Trastuzumab has also been shown to be effective when used as first-line monotherapy for advanced breast cancer. Trials to date have employed trastuzumab in a weekly schedule, but there is emerging evidence that a three-weekly regimen may be as effective. Trastuzumab has shown encouraging activity when used with other agents including docetaxel and vinorelbine. The combination of trastuzumab, docetaxel, and platinum salts also appears to be very active. The role of trastuzumab as adjuvant therapy for early breast cancer is being tested in a number of large randomised trials.

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