Lower response to T-DM1 in metastatic breast cancer patients with HER2 IHC score of 2 and FISH positive compared with IHC score of 3

Abstract

Background: Ado-trastuzumab emtansine (T-DM1) is the standard second line chemotherapy for HER2 overexpressed metastatic breast cancer (MBC). Tumor HER2 status is measured by either immunohistochemistry (IHC) or fluorescence in situ hybridization (FISH). A previous study showed no difference in objective response rate (ORR) with trastuzumab monotherapy between IHC 3+and IHC 2+/FISH positive groups. It is not known whether response to T-DM-1 differs between IHC 3+and IHC 2+/FISH positive patients. The aim of this study is to compare the efficacy of T-DM1 in IHC 3+ group to that of IHC 2+/FISH positive group. Methods: We retrospectively identified and reviewed the medical records of all patients with HER2 positive MBC who received T-DM1 in our hospital from October 2013 to December 2016. In the efficacy analysis, we excluded five patients who had HER2 negative tumors at metastatic sites. Results: A total of 44 patients were identified and 36 patients were available for efficacy analysis of ORR. Median age was 58 years old (range 28-80). 95.5% received prior trastuzumab. 45.5% received at least one chemotherapy for MBC, 29.5% received more than four lines of chemotherapy. 79.5% had IHC 3+ and 20.5% had IHC 2+/FISH positive. ORR was 16/30 (53.3%) in IHC 3+ group and was 0/6 (0%) in IHC 2+/FISH positive group (P=0.024). Median progression free survival (PFS) was 7.0 months (95% CI, 5.58 to 8.42) in IHC 3+ group and was 2.0 months (95% CI, 0.00-4.57) in IHC 2+/FISH positive group. Conclusions: ORR and PFS were significantly worse in HER2 IHC 2+/FISH positive patients compared with IHC 3+ patients. This is the first report to demonstrate the difference of T-DM1 efficacy by HER2 test results.