Abstract P6-08-06: Neoadjuvant tamoxifen therapy synchronizes ERα binding and gene expression profiles (2016)

Abstract Background: The majority of breast cancer patients are diagnosed with ERα-positive breast cancer. Most ERα-positive patients are treated with adjuvant endocrine therapy — typically tamoxifen or aromatase inhibitors — to block cellular proliferation. Although these treatments are considered successful, resistance is common. Notably, cross-resistance between the two types of therapies is not always observed suggesting molecular heterogeneity and underlining the need for development of personalized treatments. The Anastrozole, Fulvestrant or Tamoxifen Exposure — Response in molecular profile study (AFTER study, NCT00738777) aims to investigate prospectively whether short-term treatment can induce molecular changes indicative of pre-operative therapy response. Study Design: ERα-positive breast cancer patients are included in this open-label multicenter study. Post-menopausal patients are randomized between tamoxifen, anastrozole and fulvestrant and pre-menopausal and male patients receive tamoxifen. Treatment occurs during the pre-operative window between diagnosis and surgery (4±2 weeks). Clinical characteristics collected are ERα/PR and HER2 status as well as lymph-node status. The primary endpoint is the decrease in tumor cell proliferation, as assessed by Ki67 gene expression and published cell proliferation gene expression signatures. All data are collected from both pre- and post-treatment samples. Additionally, we will compare the changes induced by treatment in gene expression, ERα/DNA binding interactions, DNA copy number, endoxifen and estradiol levels. Results: Among 67 patients currently enrolled, we examined the data from the subset of 28 tamoxifen treated patients. ERα and PR levels did not differ significantly between pre- and post-treatment. All tumors were HER2-negative. Proliferation examined by Ki67 (IHC and gene expression, MKI67) was significantly lower in post-treatment samples (P &amp;lt; 0.01). A significant association was identified with the change in gene expression proliferation signature score and change in MKI67 (rho = 0.7, P &amp;lt; 0.001). We identified two samples, which changed from MammaPrint (MP) low-risk to high-risk among 17 pairs with data. One sample's score was on the cutoff for high-risk definition. Interestingly, the second sample also had an increase in Ki67 gene expression and proliferation gene signature score in the post-treatment sample. Overall, ERα/DNA binding interaction regions overlapped significantly more among post-treatment samples as compared to pre-treatment samples (P &amp;lt;0.001). There were 3 samples that increased in MKI67 gene expression after drug exposure. Among these, only the MP low- to high-risk sample had an increase in proliferation gene signature and decrease in ERα/DNA binding interactions. Conclusions: Pre-treatment samples were more variable for both proliferation gene expression signatures and ERα/DNA binding interactions indicating the underlying molecular heterogeneity of the group prior to therapy. This inter-tumor heterogeneity appears to have been lowered by exposure to tamoxifen. Interestingly, not all samples were uniform in their response to tamoxifen exposure as measured by Ki67 and MP scores suggesting samples taken after treatment exposure may be useful for predictive biomarker discovery. Citation Format: Linn SC, Severson TM, Nevedomskaya E, Peeters J, van Rossum A, Kuilman T, Krijgsman O, Goossens I, Glas A, Koornstra R, Peeper D, Wesseling J, Simon I, Wessels L, Zwart W. Neoadjuvant tamoxifen therapy synchronizes ERα binding and gene expression profiles. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P6-08-06.