Discovery of a novel small molecule degrader of wild type and mutant estrogen receptors using DNA encoded libraries.

Estrogen receptor α (ERα) variants with mutations in the ligand binding domain (LBD) are less sensitive than wild type to standard-of-care drugs that bind ERα directly. To identify novel small-molecule drugs that target ERα mutants, we screened our multibillion-compound DNA-encoded libraries against ERα LBD variants. CDD-1274, which was highly enriched with all three variants, blocked spontaneous coactivator peptide recruitment to mutant ERα LBDs and inhibited estradiol-driven proliferative markers in several ER-positive breast cancer cell lines, but not in ER-negative breast cancer cells. We demonstrated that CDD-1274 induced proteasomal degradation of ERα variants in breast cancer cell lines and caused Y537S ERα degradation more effectively than elacestrant in a palbociclib-resistant cell line. These findings establish that CDD-1274 potently blocks ligand-dependent and ligand-independent ER signaling in endocrine-resistant breast cancer cells and could be further optimized for developing a new class of ERα degraders for endocrine therapy-resistant breast cancer.