Abstract P6-04-23: HER2 up-regulation with neoadjuvant endocrine therapy in patients with hormone receptor-positive, HER2-negative breast cancer

Background Endocrine therapy provides significant disease-free survival (DFS) and overall survival (OS) benefit to patients with early stage hormone receptor positive (HR+) HER2 negative breast cancer. However, metastatic recurrence of endocrine-resistant disease develops in 20-25% of patients and remains a major cause of breast cancer mortality. Several mechanisms play a role in the progression and development of endocrine resistance. Understanding and characterizing the underlying molecular features of resistance for more accurate response prediction and optimal clinical management is of the utmost importance. Methods We designed an interventional study to assess tumor responses, changes in tumor proliferation and molecular biomarkers in early stage HR+ HER2- clinical node-negative breast cancer patients treated with neoadjuvant endocrine therapy for 4 weeks. The primary objective is to assess changes in HER protein expression (HER1-4) with neoadjuvant endocrine therapy; secondary objectives assess other molecular changes such as therapy-resistant CK5+ progenitor cells, iron-related proteins, Bruton’s tyrosine kinase (BTK), PD-L1, PD-L2 protein expressions, association with Ki-67 and tumor responses. Interim data on changes in HER2 protein in the first 16 patients is reported here. Optimized protocols for immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH) were used to assess HER2 status in the pre and post treatment tumor specimens. This is an ongoing investigator-initiated clinical trial funded by Rock River Foundation and the Medical College of Wisconsin Cancer Center (NCT03219476). Results Median age at diagnosis was 65 (42-81) and median BMI was 28.3 (19.3-50.1). Most patients were post-menopausal (81%). Median tumor size clinically was 1.3 cm (0.5-7.7). Most tumors were low (50%) or intermediate (37.5%) grade and invasive ductal histology (62.5%). Median tumor size at surgery was 1.1 cm (0-3.9). One patient had a complete pathologic response (pCR) at surgery. Four patients had pN1 (1 lymph node involved) and one patient had pN1mic disease at surgery. There was no significant change in ER-positivity between pre and post-treatment specimens. However, a significant decrease in PR-positivity was seen in post-treatment tumors consistent with functional ER pathway disruption. On HER2 protein assessment by IHC, most patients had IHC 0 (37.5%) or IHC 1+ (56%) at diagnosis. Up-regulation in HER2 protein was seen in post-treatment tumors with 36% (n=5) IHC 2+, with one case showing Her2 gene amplification by FISH. Conclusions HER2 was up regulated in 36% of tumors after short-term neoadjuvant endocrine treatment in patients with early stage HR+ HER2- breast cancer. Short-term neoadjuvant endocrine treatment holds potential for identifying emergent endocrine resistance mechanisms and pathways. Targeted therapy with HER2-directed antibodies for such HER2 up-regulated tumors after neoadjuvant endocrine treatment may benefit patients upon disease recurrence or possibly as adjuvant combination therapy in patients with high risk of recurrence. Citation Format: Lubna N Chaudhary, Julie Jorns, MaryBeth Gonyo, Amanda Kong, Amy R Peck, Yunguang Sun, Carmen Bergom, Anjishnu Banarjee, Christopher Chitambar, Hallgeir Rui. HER2 up-regulation with neoadjuvant endocrine therapy in patients with hormone receptor-positive, HER2-negative breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P6-04-23.