Women with metastatic hormone receptor positive (HR+) breast cancer benefit from therapies targeting the Estrogen Receptor (ER), but the tumors eventually acquire resistance leading to disease progression. Endocrine resistance involves multiple mechanisms, including deregulation of ER signaling due to activating mutations in the ligand-binding domain of the ESR1 gene. In addition, recent reports describe specific contexts of ESR1 mutations in the preclinical setting where tamoxifen and fulvestrant have demonstrated limited activity. These data together with the frequency of ESR1 mutations (~20-60%) observed in mBC patients in the clinic suggest a need for more effective agents. The activity of elacestrant (RAD1901), an oral selective estrogen receptor degrader (SERD) with demonstrated activity against tumors harboring ESR1 mutations, both in preclinical models and in patients with previously treated advanced breast cancer, was assessed in ex vivo cultures of circulating tumor cells (CTCs) derived from women with metastatic HR+ breast cancer who had received multiple prior hormone therapies. Cultured breast CTCs underwent apoptosis following exposure to elacestrant, and CTCs harboring mutant ESR1 were significantly growth inhibited (IC50: 17.65 ± 3.32 nM) by elacestrant compared to the standard-of-care (SOC) agent fulvestrant. Furthermore, elacestrant eliminated the fulvestrant-resistant CTC populations in vitro. Elacestrant treatment led to significant suppression of ER target genes, PGR and GREB1. Elacestrant is currently being investigated versus standard of care endocrine monotherapy in a Phase 3 clinical trial (EMERALD) in patients with ER+/HER2- advanced/metastatic breast cancer. A co-primary endpoint of the EMERALD trial is evaluation of progression-free survival in patients whose tumors harbor ESR1 mutations.

Citation Format: Taronish Dorab Dubash, Aditya Bardia, Brittany A Reeves, Brian Chirn, Annamaria Szabolcs, Ben S Wittner, John Iafrate, David Ting, Hitisha K Patel, Teeru Bihani, Mehmet Toner, Daniel A Haber, Shyamala Maheswaran. Elacestrant (RAD1901) inhibits growth of ex vivo cultured circulating tumor cells derived from hormone receptor-positive metastatic breast cancer (mBC) patients including those harboring ESR1 mutations [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P4-01-06.