Abstract 1236: Preclinical characterization of LY3484356, a novel, potent and orally bioavailable selective estrogen receptor degrader (SERD)

Abstract Nearly 70% of newly diagnosed breast cancers are estrogen receptor alpha (ERα) positive, for which endocrine therapy is a primary treatment. However, approximately 40% of those patients on endocrine therapy develop resistance, which includes mutations in ERα (ESR1) that drive constitutive activation of the receptor. One approach to overcome this resistance is to develop a potent degrader and pure antagonist of the estrogen receptor, which can effectively suppress estrogen receptor signaling. Fulvestrant, a pure antagonist and selective estrogen receptor degrader (SERD) is approved for the treatment of HR+/HER2- metastatic breast cancer. However, fulvestrant is limited by poor pharmacokinetics properties, limited exposure and sub-optimal in vivo estrogen receptor degradation. Here we describe the preclinical profile of LY3484356, an oral SERD and pure estrogen receptor antagonist, with potent activity against the wild type and mutant estrogen receptor. LY3484356 has Ki values of 0.64 nM and 2.8 nM against wild type ERα and Y537S mutant ERα proteins, respectively. It is a potent and highly efficient degrader of wild type ERα and Y537N mutant ERα proteins in cells, with IC50 values 3.0 nM and 9.6 nM, respectively. LY3484356 is also a potent inhibitor of ERα-mediated transcription in vitro and in vivo. It inhibits cell proliferation in wild type ERα and ESR1 Y537N mutant breast cancer cell lines, with average IC50 values of 3 nM and 17 nM, respectively. In a panel of breast cancer cell lines, 11 out of 12 ER+ breast cancer cell lines were sensitive to LY3484356 (IC50 less than 100 nM), whereas all ER- cell lines tested were insensitive. LY3484356 has demonstrated sustained and prolonged target inhibition (>75% inhibition of PGR transcription up to 96h after last dose) in ESR1 wild type (MCF7) and ESR1 Y537S mutant (ST941/C) xenograft tumors. Consistent with its profile as a pure antagonist, immature rat studies demonstrated no significant effect on uterine wet weight. LY3484356 demonstrated significant tumor growth inhibition and tumor regressions in wild type ESR1 breast cancer xenograft models such as MCF7, T47D and ZR-75-1, as well as ESR1 mutant breast cancer PDX models. LY3484356 has shown synergy or additivity in combination with CDK4/6 inhibitor abemaciclib, mTOR inhibitor everolimus and PIK3CA inhibitor alpelisib in inhibiting cell proliferation in ER+ breast cancer cell lines in vitro and tumor growth inhibition in relevant xenograft or PDX models in vivo. The first-in-human Phase 1/2 clinical trial of LY3484356 (EMBER, ClinicalTrials.gov NCT04188548) is currently ongoing and a window-of-opportunity study evaluating the pharmacodynamic effects of LY3484356 in early stage breast cancer is expected to begin early 2021. Citation Format: Shripad V. Bhagwat, Baohui Zhao, Weihua Shen, Cecilia Mur, Robert Barr, Lisa J. Kindler, Almudena Rubio, Jolie A. Bastian, Jeffrey D. Cohen, Brian E. Mattioni, Eunice Yuen, Thomas K. Baker, Mark A. Castanares, Dongling Fei, Jason R. Manro, Maria Jose Lallena, Sheng-Bin Peng, Alfonso de Dios. Preclinical characterization of LY3484356, a novel, potent and orally bioavailable selective estrogen receptor degrader (SERD) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1236.