Abstract 474: Preclinical development of an oral selective estrogen receptor degrader (SERD) TFX06 for the treatment of ER+HER2− breast cancer

Abstract Breast cancer (BC) has recently surpassed lung cancer to become the most commonly diagnosed cancer in women worldwide. Approximately 70% of BC was ER+, in which estrogen receptor α (ERα, encoded by ESR1 gene) drives dysregulated cell proliferation. In clinic, ER+ BC has been effectively treated by endocrine therapy targeting estrogen or ER. As the first approved ERα degrader, fulvestrant has proven to be effective for locally advanced or metastatic BC. However, its clinical utilization is hampered by inconvenience of intramuscular injection, as well as its poor PK/PD profile and limited efficacy, especially in the patient population that developed ESR1 mutation-conferring drug resistance after the earlier line of treatment with an aromatase inhibitor. Thus, developing an oral and more effective therapy for ER+ BC remains an unmet medical need. We report herein discovery and preclinical investigation of a novel orally bioavailable SERD TFX06. TFX06 demonstrated potent binding affinity to ERα with a Ki of 0.10 nM and preferably antagonized ERα to ERβ with an IC50 value of 0.086 nM and 2.83 nM, respectively. TFX06 potently induced ERα degradation in ER+ BC cells and substantially inhibited cell proliferation in BC cell lines with ER wild type (WT), and MCF-7 cells expressing Y537S or D538G mutants. Oral administration of TFX06 demonstrated impressive antitumor efficacy in ER WT (TGI = 101%) and ER D538G-expressing (TGI = 99%) MCF-7 cell line-derived xenograft (CDX) models in mice. Similarly, in an ER+ BC patient-derived xenograft (PDX) model (HCI-013) with ER Y537S mutation, TFX06 also exerted excellent antitumor activity (TGI = 99%). Furthermore, TFX06 in combination with palbociclib achieved synergistic activity in MCF-7 CDX model. Moreover, TFX06 exhibited an excellent correlation between systemic/local drug exposure, pharmacodynamic modulation (i.e., ERα downregulation) and antitumor activity. Collectively, these findings demonstrate that TFX06 is a novel, orally bioavailable SERD. TFX06 demonstrates substantial antitumor activity in both in vitro and in vivo preclinical tumor models, including those expressing ESR1 mutations, through downregulation of ERα. The preclinical data warranted the clinical evaluation of TFX06 in human with an IND application submitted to the China NPMA. Citation Format: Jinping Wu, Ke An, Douglas D. Fang, Jianyu Lu, Lihong Hu, Jing Wang, Kaili Zhang, Yuanfeng Xia, Charles Ding, Shuhui Chen, Sha Wei. Preclinical development of an oral selective estrogen receptor degrader (SERD) TFX06 for the treatment of ER+HER2− breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 474.