Abstract 4376: OP-1250, a complete estrogen receptor antagonist (CERAN) that shrinks estrogen receptor positive tumors and exhibits favorable pharmacokinetics

Abstract Fulvestrant (FASLODEX), a Complete Estrogen Receptor ANtagonist (CERAN) is recognized as the most effective endocrine therapy for estrogen receptor positive (ER+) metastatic breast cancer. Unfortunately, fulvestrant must be injected i.m. and has poor drug exposure. Thus, an oral compound that shares fulvestrant's drug virtues with improved drug exposure might be both more effective and more convenient than fulvestrant. This realization has sparked intensive efforts to develop such a compound, however most efforts have focused on discovering novel selective estrogen receptor degraders (SERDS) most of which have also displayed agonist activities on gene expression and uterus, and are therefore are also selective estrogen receptor modulators (SERMs). We predict that CERANs that fully block the activities of estrogen receptor alpha (ERalpha) will have a clinical advantage over SERM/SERDs in shrinking breast tumors as well as preventing and treating endocrine resistance. We have developed a small molecule, OP-1250 that, like fulvestrant, is a CERAN that completely turns off both transcriptional activation functions of ERalpha, as can be seen in a uterine cell assay, and also prevents uterine weight gain in ovariectomized mice. In addition, it blocks degrades ERalpha and blocks the proliferation of a large variety of ER+ breast cells. Using RNAseq, we found many estrogen-regulated biomarkers that are also regulated by SERM/SERDs, however CERANS such as OP-1250 and fulvestrant have no, or even an inverse effect on many such genes. Unlike fulvestrant, OP-1250 gives high and stable oral drug exposure in multiple species. Consequentially, OP-1250 effectively shrinks ER+ tumors in multiple human tumor xenografts in mice at very low daily doses of 3 mg/kg, even in models containing activating ERalpha mutant receptors. Importantly, the pharmacokinetics and biodistribution of OP-1250 has a favorable profile that results in unusual potency of OP-1250 in vivo. Citation Format: Leslie Hodges-Gallagher, Cyrus L. Harmon, Richard Sun, David C. Myles, Peter Kushner. OP-1250, a complete estrogen receptor antagonist (CERAN) that shrinks estrogen receptor positive tumors and exhibits favorable pharmacokinetics [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4376.