Development of a novel HER2 test based on the therapeutic antibody trastuzumab

Abstract

Despite the use of immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH), the response rate of trastuzumab monotherapy remains approximately 50%. Here, we developed a novel test, Trastuzumab IHC, that uses unmodified trastuzumab and IHC procedures to identify patients whose tumors were found to be HER2-positive by the HercepTest or FISH analysis, but who did not respond to trastuzumab therapy. The Trastuzumab IHC test was performed on HER2-negative/-positive cell lines, including trastuzumab-sensitive (SK-BR-3)/resistant (JIMT-1) breast cancer cell lines. We also applied the Trastuzumab IHC on 133 consecutive primary breast cancer tumors and 12 needle biopsied specimens who received trastuzumab pre-operatively. Then, the results were compared with those obtained from the HercepTest and FISH analysis. The Trastuzumab IHC is the only test that could distinguish the JIMT-1 from SK-BR-3 cell lines in terms of HER2 expression. With 133 tumor specimens, only 4% (2/50) of the cases who were HercepTest 2+ and FISH (-) were positive on the Trastuzumab IHC test. In addition, 48.5% (16/33) of the FISH (+) cases were confirmed to be positive by Trastuzumab IHC. This result is consistent with the clinical benefit rate of trastuzumab in patients with FISH-positive results. The Trastuzumab IHC is 24.8% more accurate and 52.1% more specific at predicting HER2 gene amplification by FISH than positive staining with the HercepTest. Also, positive predictive value for Trastuzumab IHC (88.9%) was higher than the HercepTest (39.8%). Furthermore, among 12 patients who received trastuzumab pre-operatively, two patients who did not responded to trastuzumab therapy were negative for the Trastuzumab IHC test. In conclusion, the Trastuzumab IHC test has a high specificity and positive predictive value, which can be used to predict patient responses to trastuzumab therapy.