Quantitative diagnosis of HER 2 protein expressing breast cancer by single‐particle quantum dot imaging

Overexpression of HER 2 is one of the major causes of breast cancer, and therefore precise diagnosis of its protein expression level is important. However, current methods estimating the HER 2‐expression level are insufficient due to problem with the lack of quantification. This might result in a gap between diagnostics and therapeutics targeting HER 2. Therefore, a new effective diagnostic method is needed. We developed a new immunohistochemical ( IHC ) technique with quantum dots ( QD )‐conjugated trastuzumab using single‐particle imaging to quantitatively measure the HER 2 expression level. Tissues from 37 breast cancer patients with available detailed clinical information were tested by IHC with QD s ( IHC ‐ QD ) and the correlation with IHC with 3,3′‐diaminobenzidine ( DAB ), fluorescence in situ hybridization ( FISH ), and IHC ‐ QD was examined. The number of QD ‐conjugated trastuzumab particles binding specifically to a cancer cell was precisely calculated as the IHC ‐ QD score. The IHC ‐ QD score in 37 cases was correlated proportionally with the score of HER 2 gene copy number as assessed by FISH ( R  = 0.83). When HER 2 positivity was judged to be positive, the IHC ‐ QD score with our cut‐off level was exactly concordant with the FISH score with a cut‐off value of 2.0. Furthermore, IHC ‐ QD s score and time to progression ( TTP ) of trastuzumab therapy were well correlated in HER 2‐positive cases ( R  = 0.69). Conversely, the correlation between FISH score and TTP was not observed. We developed a precisely quantitative IHC method using trastuzumab‐conjugated QD s and single‐particle imaging analysis and propose the possibility of using IHC ‐ QD s score as a predictive factor for trastuzumab therapy.