Abstract B143: Establishment of primary tumor models of TNBC and ER+/HER2- breast cancer in humanized mice and validation of pembrolizumab efficacy

Background: Immunotherapy is an emerging treatment option for different subtypes of breast cancer. Pembrolizumab (Keytruda®) targets PD-1 expressing T cells in tumors and mediates anti-tumor effects. Targeting PD-1 has shown promising results in advanced breast cancer and is being evaluated in clinical trials in triple-negative breast cancer (TNBC) and in estrogen receptor positive, human epidermal growth factor 2 negative (ER+/HER2-) breast cancer patients. Immunotherapy is not yet an established treatment option for early breast cancer. The aim of this study was to establish preclinical primary tumor models for TNBC and ER+/HER2- in humanized mice, and to evaluate efficacy of pembrolizumab in the models. Materials and methods: Immunodeficient CIEA NOG female mice engrafted with human CD34+ hematopoietic stem cells (huNOG mice) were inoculated orthotopically into mammary fat pad with either MDA-MB-231(SA) (TNBC) or MCF-7 (ER+/HER2-) human breast cancer cells. The mice receiving MCF-7 cells were implanted with estradiol (E2) releasing pellets before cancer cell inoculation to support tumor growth. The mice were treated with pembrolizumab (5 mg/kg) or isotype control in a Q5D schedule. The treatments were started 3 or 14 days after cancer cell inoculation in the TNBC and ER+/HER2- models, respectively. Tumor growth was monitored by caliper measurements, and the study was terminated at 3 weeks (TNBC model) or 7 weeks (ER+/HER2- model). Tumors were collected and analyzed immunohistochemically for tumor-infiltrating lymphocytes (TILs), and hematological analysis was performed from the mice in the ER+/HER2- model. Results: Tumors responded to pembrolizumab treatment in the TNBC model. Three out of eight mice responded to the treatment and one of them had a tumor rejection. Pembrolizumab treatment was associated with decrease in CD8+ TILs and granzyme B+ cells in the tumor. In the ER+/HER2- model, E2 supplementation was essential for tumor growth. However, E2 caused a decrease in the clinical condition, including decrease in white and red blood cells, leading to early sacrifices. Only modest responses for pembrolizumab were observed in a few mice and the results did not reach statistical significance. No major changes were observed in TILs due to pembrolizumab treatment. Conclusions: In the established breast cancer models in humanized mice, pembrolizumab decreased tumor growth in the TNBC model but induced only minor tumor responses in the ER+/HER2- model. Estrogen was essential for ER+/HER2- tumor growth but induced severe anemia and had immunomodulatory effects potentially influencing the lack of pembrolizumab efficacy in the model. Citation Format: Tiina E Kahkonen, Mari I Suominen, Jenni HE Maki-Jouppila, Azusa Tanaka, Philip Dube, Michael Seiler, Jussi M Halleen, Jenni Bernoulli. Establishment of primary tumor models of TNBC and ER+/HER2- breast cancer in humanized mice and validation of pembrolizumab efficacy [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr B143. doi:10.1158/1535-7163.TARG-19-B143