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History of Testing for HER2: Early Trials used Immunohistochemistry (IHC) testing

The early trials of trastuzumab in metastatic breast cancer enrolled patients after central testing using an IHC assay with the anti-HER2 antibodies 4D5 (the parent antibody of trastuzumab) and CB11 on formalin-fixed, paraffin-embedded tissue (FFPE). This clinical trial assay identified staining patterns for HER2 as:
  • Negative: 0 and 1+
  • Positive: 2+ and 3+
In these studies, only patients with 2+ or 3+ staining were eligible for treatment with trastuzumab. Retrospective analyses have suggested that only patients with IHC 3+ staining and/or HER2 gene amplification by fluorescent in situ hybridization (FISH) benefited from trastuzumab.