Abstract P6-10-12: Discovery and Development of Next-Generation Estrogen Receptor Mutant Inhibitors using DNA-Encoded Chemical Library Screening

Abstract

Abstract Background: Activating somatic ESR1 mutations Y537S and D538G occur more frequently in endocrine therapy-resistant metastatic breast cancer, which is associated with an aggressive phenotype and poor survival in breast cancer patients. These gain of function mutant receptors are constitutively active and allow resistance to first-line endocrine therapies. Therefore, the development of next-generation small molecule drugs targeting mutant estrogen receptor (ER) is an important priority. Here, we searched the small molecule inhibitors for Y537S and D538D ER mutants using DNA-encoded chemical library screening. Methods: Wild type (WT) and mutant ER ligand binding domain (LBD) proteins were expressed in E. coli. The soluble proteins were purified by Ni-NTA chromatography followed by anion exchange and size exclusion chromatography. Homogeneous time-resolved fluorescence (HTRF) and fluorescent polarization (FP) assays were performed in these purified proteins. We employed a DNA-encoded chemical library affinity selection using our in-house collection of 6 billion compounds. Hit compounds were resynthesized and validated in biochemical assays. Finally, we have performed functional studies in CRISPR-Cas9 knock-in of Y537S and D538G mutant MCF-7 breast cancer cells. Results: We have successfully purified microgram amounts of ERα LBD of WT, Y537S, and D538G proteins. To test whether the purified WT and mutant proteins are active, HTRF and FP assays were performed in the presence and absence of estradiol and 4OH tamoxifen. Steroid receptor coactivator 3 (SRC3) peptide binding to the WT ER protein occurred only in the presence of estradiol. However, Y537S and D538G proteins are recruited by the SRC3 peptide in the absence of estradiol, indicating that these mutants are constitutively active and bind to SRC3. Furthermore, an in vitro biochemical FP assay was also established for WT and mutants in the presence of estradiol and 4OH tamoxifen. The screen of our multibillion small molecule collection of DNA-encoded chemical libraries identified several hits in WT and mutant ER. To confirm the selection output, we synthesized off-DNA compounds and validated these in biochemical and cell-based studies. We have identified that the compounds, CDD-1272 and CDD-1274, are active in HTRF and FP assays. Furthermore, these compounds inhibit WT and mutant cell growth in the presence of estradiol. More importantly, CDD-1274 degrades ER mutant and cyclin D1 proteins. In addition, CDD-1274 induced p21 protein expression in WT and mutant cells. Conclusions: We have identified potent novel ER mutant binders by using our DNA-encoded chemical library platform. Our compounds are active in biochemical and ER mutant cell lines, suggesting these molecules are potential chemical probes to explore in in vivo models of breast cancer. Support: NIH/NCI R03 CA259664 and CPRIT RP220524 to MP. Citation Format: Murugesan Palaniappan, Kurt M. Bohren, Yong Wang, Damian W. Young, Suzanne A. Fuqua, Martin M. Matzuk. Discovery and Development of Next-Generation Estrogen Receptor Mutant Inhibitors using DNA-Encoded Chemical Library Screening [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P6-10-12.