Abstract Approximately 70% of breast cancer patients have estrogen receptor positive (ER+) tumors. Fulvestrant (FUL), a selective estrogen receptor degrader (SERD) able to ablate ER, was first introduced to clinical practice in 2002 as the second and third line treatment for ER+ metastatic breast cancer. In August 2017, FUL was added to first-line therapy in postmenopausal women by demonstrating superiority compared to first-line AI therapy, anastrozole, with improved progression-free-survival (PFS) and a 20% reduction in the risk of disease progression or death. However, acquisition of FUL-resistance in both first-line setting or as combination therapy with CDK4/6 inhibitor has been observed clinically, leaving patients and oncologists with chemotherapy as the only backup therapy. Little is known about resistance mechanisms to FUL alone or in combination with CDK4/6 inhibitors. Here, we showed that targeting epigenetic vulnerabilities in breast cancer using novel bromodomain and extra-terminal (BET) inhibitors can effectively inhibit growth of tamoxifen-, FUL- and CDK4/6- resistant breast cancer cells. We designed and developed a novel series of pyridinone-based BET inhibitors, optimized by growth inhibition in tamoxifen- and FUL- resistant breast cancer cell lines (MCF7:CFR) in both 2D and 3D cultures. Optimized compounds showed superior in vitro activity to BET inhibitors in clinical trials. Selected compounds were further evaluated for pharmacokinetics and shown efficacy in endocrine-resistant xenograft models. Citation Format: Rui Xiong, Yangfeng Li, Jiong Zhao, Debra Tonetti, Gregory Thatcher. Novel bromodomains and extra-terminal motif (BET) inhibitors developed against estrogen receptor positive, fulvestrant- and CDK4/6-resistant breast cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3830.