Figure 1.

Schematic drawing of IHC with DAB (IHC‐DAB) and IHC with quantum dots (IHC‐QDs). In conventional IHC‐DAB, HER2 proteins are immunostained with primary antibody and secondary antibody conjugated with HRP (A). The primary antibodies used for pathological diagnosis of trastuzumab therapy recognize intracellular domain of HER2 protein (A). On the other hand, trastuzumab recognizes the extracellular domain of HER2 (B). Therefore, epitope of trastuzumab differs from that of diagnostic antibodies. In IHC‐QDs, trastuzumab were monomerized and then conjugated with QDs (mean value, 2.5 of monomer trastuzumab fragments per single QD) (B). It have been reported that various truncated forms of HER2 lack the extracellular domain, like p95HER2 (C). Trastuzumab‐conjugated QDs cannot bind to the truncated forms of HER2 (C). In addition, overexpression of MUC4 is known to seal the surface of the HER2 protein. These effects on the extracellular domain of HER2 prevent the interaction of trastuzumab with HER2 but not the interaction of a diagnostic antibody and HER2 (C). The difference of epitope between antibodies leads to the gap between diagnostics and therapeutic efficacy. DAB, 3,3′‐diaminobenzidine; HER2, human epidermal growth factor receptor 2; IHC, immunohistochemical; HRP, horseradish peroxidase.