P3-01-02: EC 304: A Novel Progesterone Receptor (PR) Antagonist Inhibits Transcription and Induces Cell Cycle Arrest in T47D Breast Cancer Cells.

Abstract

Abstract Progesterone receptor (PR) is a ligand activated transcription factor which plays a crucial role in female reproduction. PR antagonists have been shown to repress estrogen dependent proliferation in the uterus and mammary gland. Even though PR antagonists are potent therapeutic candidates for breast cancer, partial agonism towards glucocorticoid receptor (GR) and androgen receptor (AR) lead to undesirable side effects. Evestra's rational design and development approach in the synthesis of EC304 was aimed at minimizing these side effects as well as to deliver more potent and selective antiprogestins. We used select screen® (Invitrogen) to determine relative binding affinity of EC304 with AR,GR and PR. The binding affinity of EC304 showed strong antagonism towards PR with mild or negligible agonism with AR or GR. PR transactivation was inhibited by EC304 with IC50 of 0.04nM in the presence of progesterone (5nM). EC304 showed superior cytotoxicity over known antiprogestins ZK230211, ORG33628, CDB2914, CDB4124 and RU486 in T47D cells. IC50 of EC304 was found to be 0.5nM in a 6-day cytotoxicity assay (MTT) in T47D cells stimulated with 1nM estrogen (E2). In a 21-day in vitro tumorigenicity assay, EC304 inhibited 80 and 100% of colony formation of T47D cells in the presence of E2 at 1 and 10 nM respectively. Further studies revealed that EC304 induced apoptosis and G1-phase arrest in cell cycle progression when co-incubated with E2 for 24 h. Treatment with EC304 down regulated PR target genes and up regulated cell cycle dependent kinase (CDK) inhibitor P21 in vitro. Ongoing studies address whether EC304 is specifically acting on PR-A or PR-B isoform at gene expression, protein levels and PR isoform specific knockdowns in T47D and other metastatic breast cancer cells. Considered together, these data indicate that EC304 might be a safer but efficacious novel antiprogestin for hormone sensitive postmenopausal breast cancer patients. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P3-01-02.