7993 Thienopyrimidine-Based Estrogen Receptor Modulators Adopt Unique Ligand Binding Poses to Elicit Anti-Proliferative Activities in ER+ Breast Cancer Cells

Abstract Disclosure: E.C. Fink: None. S.L. Mateus: None. N. Meganathan: None. V.K. Sammeta: None. J.D. Norris: None. D.P. McDonnell: None. T. Willson: None. S.W. Fanning: None. One in eight women will be diagnosed with breast cancer in their lifetimes. Estrogen receptor alpha (ER) drives breast cancer pathology and is expressed in approximately seventy percent of tumors. Hormone therapies are used to treat and prevent the metastasis of ER+ breast cancers and these patients have favorable 5-year survivals. However, most breast cancer patient deaths are ER+. While the next generation of antiestrogens have shown promise in the advanced setting, more work needs to be done to fully address these mortalities. ER is a ligand-dependent master transcriptional regulator, whereby ligand-specific ER conformational ensembles redirect multiple programs to produce oncogenic or therapeutic endpoints. To better understand ER structure-activity relationships, we have developed new antiestrogenic small molecules based on a thienopyrimidine scaffold. Comprehensive structural, ER activity, and cancer endpoint profiling shows that these new molecules adopt a unique ligand binding pose, which perturbs new structural elements within the orthosteric hormone binding pocket. These molecules downregulate ER target genes and halt the proliferation of ER+ breast cancer cells. Presentation: 6/3/2024