Lower response to trastuzumab emtansine in metastatic breast cancer patients with human epidermal growth factor receptor 2 immunohistochemistry score of 2 and fluorescence in situ hybridization positive compared with immunohistochemistry score of 3: a retrospective study.

Tumor human epidermal growth factor receptor 2 (HER2) status is defined by either protein expression using immunohistochemistry (IHC) or gene amplification using fluorescence in situ hybridization (FISH). Approximately 20% of HER2-positive breast cancer is HER2 IHC 2+/FISH-positive. Unlike trastuzumab, it has not been studied whether the response to trastuzumab emtansine (T-DM1) differs according to HER2-positive status. We retrospectively identified and reviewed medical records of all patients with HER2-positive advanced breast cancer (ABC) who received T-DM1 in our hospital from October 2013 to December 2016. We compared the objective response rate (ORR) and progression-free survival (PFS) between patients in the HER2 IHC 3+ group and those in the HER2 IHC 2+/FISH-positive group. A total of 39 patients (IHC 3+: n = 32; IHC 2+/FISH-positive: n = 7) were analyzed. Nineteen (48.7%), 13 (33.3%), and 29 (74.4%) patients had received at least one prior chemotherapy, more than three lines of chemotherapy, and prior pertuzumab for ABC, respectively. ORR was significantly higher in the IHC 3+ group than in the IHC 2+/FISH-positive group (53.3% vs. 0%, P = 0.024). Median PFS was 7.9 months in the IHC 3+ group versus 3.9 months in the IHC 2+/FISH-positive group (hazard ratio 0.68; 95% confidence interval 0.28-1.69, P = 0.408). Among the HER2-positive ABC patients treated with T-DM1, ORR was significantly worse in HER2 IHC 2+/FISH-positive than in HER2 IHC 3+ patients. Median PFS tended to be shorter in patients with HER2 IHC 2+/FISH-positive.