HER2--a discussion of testing approaches in the USA.

HER2 (neu, erbB-2), a receptor related to the human epidermal growth factor receptor, has now become more important as a predictive marker of treatment response. While the value and direction of the treatment/HER2 interaction may vary, depending on the agents, dose, or schedule of drug administration, there is little disagreement that HER2 testing is an important part of breast cancer evaluation. In 1998, trastuzumab (Herceptin) was approved for the treatment of HER2-positive metastatic breast cancer patients by the Food and Drug Administration of the USA. Patients with abnormal HER2 in their breast cancer cells (generally 2 or 3+ with the HercepTest, overexpression by other immunohistochemical assays or amplification by fluorescence in situ hybridization [FISH] assay) have demonstrated the greatest response to trastuzumab treatment. It is unclear which test (method, reagent, cut-off points, etc.) is best to use to evaluate HER2 for this purpose because parallel testing of the same cancers from patients who received trastuzumab has only recently been initiated and the data are limited. It is widely believed that breast cancers without HER2 alterations will not be responsive to trastuzumab, although a clinical trial to test this specific hypothesis has not been initiated. There are also concerns that clonal heterogeneity for HER2 within a tumor, or between primary and metastatic cancer foci, may affect treatment response; yet we do not currently evaluate these parameters. Consensus regarding the best methods, reagents, or cut-off points to define HER2 status for determining trastuzumab responsivity has not yet been reached. HER2 testing for other prognostic or predictive purposes, e.g. to determine whether patients are likely to respond to other agents, such as dose-intensive doxorubicin, may be less. Data from the Cancer and Leukemia Group B trial 8541 (companion 8869) suggest that, with proper controls in high-volume laboratories, many of the available methods produce comparable results.