Abstract P5-17-06: Immune modulation with humanized anti-EGFR antibody panitumumab in an immunocompetent mouse model for inflammatory breast cancer

Background: Inflammatory breast cancer (IBC) is the most lethal and aggressive form of breast cancer and there are no approved targeted therapies specifically for IBC. Targeting the epidermal growth factor receptor (EGFR) pathway is a promising therapeutic target for patients with triple-negative IBC (TN-IBC) with a reported pathological complete response rate of 42% (JAMA Oncology, 2018). The tumor microenvironment (TME) is a critical contributor to the aggressiveness of IBC. Delineating cross-talk between EGFR-targeted therapies and TME components, which define IBC, could inform more efficient combination regimens and novel clinical trial designs for IBC. However, such studies have not been conducted due to the lack of a syngeneic IBC mouse model. Here we report the establishment of an IBC immunocompetent mouse model and the effects of panitumumab (PmAb) on IBC tumor growth and the TME. Methods: TN-IBC cell lines, SUM149 or FC-IBC-02, were mixed with 50% Matrigel and inoculated into mammary fat pads of hu-NSG-SGM3 mice engrafted with hematopoietic stem cells (The Jackson Laboratory). SUM149 tumor growth in hu-NSG-SGM3 mice treated with either IgG2 (isotype control, 4 mg/kg) or PmAb (1 mg/kg and 4 mg/kg) was measured. The percentages of TME components, including human CD4+ T, CD8+ T, regulatory T (Tregs), and natural killer (NK) cells, and M1 or M2 macrophages, in the peripheral blood and tumor tissues treated with IgG2 and PmAb for 7 weeks were measured by flow cytometry. Results: Hu-NSG-SGM3 mice supported the growth of TN-IBC SUM149 and FC-IBC-02 xenografts. These humanized mouse models were named SUM149-huSGM3 and FC-IBC-02-huSGM3, respectively. Analysis of the blood cells showed that SUM149-huSGM3 mice display human CD4+ T, CD8+ T, Tregs, M1 and M2 macrophages. T cell infiltration and M1 and M2 macrophages were also detected in SUM149-huSGM3 tumors. NK cells were not detected in both peripheral blood and tumors. PmAb treated SUM149-huSGM3 mice had significantly reduced SUM149 tumor growth, compared with mice that received the IgG2 control. PmAb treatment increased the percentage of CD8+ T cells and reduced the percentage of Tregs in peripheral blood. A similar analysis of tumor infiltrating lymphocytes isolated from each group showed an increase in percent CD8+ T cells in mice treated with PmAb. There were no significant changes of M1 or M2 macrophages following PmAb treatment. These results suggest that the increase in percentage of CD8+ T cells in peripheral blood and IBC tumors, and the decrease in percentage of Tregs in peripheral blood may contribute to the therapeutic efficacy of PmAb. Conclusion: We established the first immunocompetent mouse model to study the TME and immune response in IBC, which provides the premise for conducting a diversity of novel preclinical therapeutic studies. The mechanism of how immune responses of TN-IBC xenografts mediates the therapeutic efficacy of PmAb in IBC tumors needs to be further investigated. Our study also suggests that combination therapy with immune checkpoint inhibitors may potentiate the efficacy of anti-EGFR therapy in IBC. The therapeutic efficacy of PmAb and anti-PD-L1 combination in SUM149 humanized mice is in progress. Citation Format: Wang X, Shao S, Pearson T, Cheng Y, Reuben JM, Tripathy D, Ueno NT. Immune modulation with humanized anti-EGFR antibody panitumumab in an immunocompetent mouse model for inflammatory breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P5-17-06.