Discovery of Palazestrant (OP-1250), a Potent and Orally Bioavailable Complete Estrogen Receptor Antagonist (CERAN) and Selective Estrogen Receptor Degrader (SERD).

Metastatic breast cancer (mBC) is a leading cause of cancer death in women. Most breast cancer patients are administered estrogen-receptor-targeted endocrine therapies to treat or prevent progressive metastatic disease. Development of endocrine resistance through acquisition of mutations in the estrogen receptor gene, , that constitutively activate the estrogen receptor leads to relapse. Complete antagonism of both WT and mutant () with an oral therapeutic that persistently antagonizes ER-driven oncogenic transcriptional activities is a requirement for efficacy. Here, we describe our discovery of the investigational drug OP-1250 (palazestrant). OP-1250 is a potent complete estrogen receptor antagonist (CERAN) and selective estrogen receptor degrader (SERD) that is active in both WT and breast cancer tumors. OP-1250's effective induction of tumor regression either as a single agent or in combination with a CDK4/6 inhibitor has led to the rapid advancement of this compound into a Phase 3 clinical trial (OPERA-01).