Abstract LB005: Synergistic effect of tamoxifen and doxorubicin on a novel estrogen receptorβ-p53-p73 axis: A new therapeutic strategy against triple negative breast cancer cancer

Abstract Introduction: Triple negative breast cancer (TNBC) is an aggressive invasive cancer with fewer treatment options and worse prognosis. The current care of TNBC consists of chemotherapy regimens such as anthracyclines, and taxanes with an evolving role for immunotherapy, antibody-drug conjugates, and drugs targeting DNA damage and repair. However, these regimens are not very effective in majority of patients and have serious adverse effects. Therefore, there is an unmet need for rationally designed therapies. Although the importance of p53 and estrogen receptors in breast cancer is known for years, much less is known about functional integration between p53, estrogen receptors, and p73 signaling in TNBC. We have discovered a novel estrogen receptor beta (ERβ)-p53-p73 axis that can be exploited to develop new therapeutic strategies against TNBC. Binding and inactivating tumor suppressor p73 is a major gain-of-oncogenic function of mutant p53. Methods: We used MDA-MB468 and isogenic TNBC cell line (MDA-MB231-LM) models differing in ERβ and p53 expression status, cell line-derived xenografts (CDXs), and patient tumor-derived xenografts (PDXs) to analyze functional interactions between these proteins and their impact on cell proliferation, cell death, migration, invasion, and therapeutic response. Various experimental approaches including knocking down and overexpressing genes, Co-IP, chromatin immunoprecipitation (ChIP), ChIP-Seq, RNA-seq, quantitative real time PCR, reverse phase protein array (RPPA), proximity ligation assay (PLA), immunofluorescence, immunohistochemistry, flow cytometry, and PK/PD and synergy analysis were used for comprehensive mechanistic studies and for determining translational opportunities. Results: Conventionally, Tam has not been used for TNBC therapy. We show that tamoxifen (Tam) synergizes with doxorubicin (Doxo) to increase apoptosis and decrease cell proliferation of TNBC cells in vitro, and to inhibit growth of TNBC CDX and PDX models in vivo. These therapeutic responses are dependent on binding of ERβ to mutant p53. Mechanistically, Tam enhances ERβ-mediated sequestration of p73-bound mutant p53, thereby disrupting p73-mutant p53 complex resulting in reactivation of p73, leading to apoptosis and inhibition of proliferation in synergy with Doxo that induces DNA damage. Importantly, RNA-seq analysis showed Tam and Doxo combination targeted unique set of genes regulating pathways including metastasis and DNA damage repair (DDR). An IRB-approved phase II clinical trial is set to start to test this therapy in patients. Conclusion: As large percentage of TNBCs express both mutant p53 and ERβ, successful translation of our novel findings on repurposing Tam will have huge clinical impact with an effective and safe therapy without financial toxicity to the patients. Citation Format: Gokul M. Das, Chetan C. Oturkar, Junhoung Park, Christina Adams, Iqbal Aijaz, Utpal K. Upadhyay, Melissa Dolan, Michalis Mastri, Alexander Caradori, Masanori Oshi, Yoshihisa Tokumaru, Yalii Zhang, Jianmin Wang, Brent Boleslav, Donald E. Mager, Tagari Samanta, Sujkin Yang, Seung Y. Han, Richa Mishra, John Ebos, Kazuaki Takabe, Kristopher Attwood, Benny A. Kaipparettu. Synergistic effect of tamoxifen and doxorubicin on a novel estrogen receptorβ-p53-p73 axis: A new therapeutic strategy against triple negative breast cancer cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 2 (Late-Breaking, Clinical Trial, and Invited Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_2):Abstract nr LB005.